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H
AERONAUTICS
EIGHTEENTH ANNUAL REPORT
OF THE
NATIONAL ADVISORY COMMITTEE
FOR AERONAUTICS
1932
INCLUDING TECHNICAL REPORTS NOS. 401 TO 440
UNITED STATES
GOVERNMENT PRINTING OFFICE WASHINGTON : 1933
For sale by the Superintendent of Documents, Washington, D. C.
Price $2.50 (Buckram)
LETTER OF SUBMITTAL
To the Congress oj the United, States:
In compliance with the act of March 3, 1915, which established the National Advisory Committee for Aeronautics, I submit herewith the eighteenth annual report of the committee for the fiscal year ended June 30, 1932.
It is noted that the committee reports material and gratifying improvements in aircraft performance and reliability, and that the steady advances in technical development have increased the relative importance of aviation as an arm of national defense and as an agency of transportation.
In the new phase of the industrial age upon which the country is entering, substantial achievements will rest largely on the stimulation given to scientific research. The remarkable progress of aeronautics since the war is a demonstration of the value and necessity of research.
The National Advisory Committee for Aeronautics is the governmental agency for coordinating and con¬ ducting fundamental research in aeronautics. I concur in the committee’s opinion that America should keep at least abreast of other nations in the development of aviation and believe that the best way to assure this is to provide for the continuous prosecution of organized scientific research.
Herbert Hoover.
The White House,
December S, 1932.
hi
LETTER OF TRANSMITTAL
National Advisory Committee for Aeronautics,
Washington, D. C., November 25, 1982.
Mr. President:
In compliance with the provisions of the act of Congress approved March 3, 1915 (U. S. C., title 50, sec. 153), I have the honor to transmit herewith the Eighteenth Annual Report of the National Advisory Com¬ mittee for Aeronautics for the fiscal year ended June 30, 1932.
Technical progress as a result of scientific research in the field of aeronautical development during the past year has been most gratifying. Scientific data resulting from well-planned and coordinated research have found application in the improvement of both military and commercial types of aircraft.
The committee, in coordinating its research programs, has covered the problems in the whole field of air¬ craft research, both military and commercial, and has placed special emphasis on the major problems of in¬ creased safety, reliability, and efficiency of aircraft. The placing in operation of the new full-scale wind tunnel and the new N. A. C. A. tank has made it possible to undertake problems which will have a very important bearing on future aeronautical development. The use of these new pieces of equipment in conjunction with the other excellent facilities of the committee’s laboratories makes it possible for the committee to carry out research programs which cover the needs of both military and commercial aviation and assure continued progress.
Attention is invited to Part IV of the report, presenting a summary of technical developments in aero¬ nautics accomplished under the committee’s direction during the past year.
Respectfully submitted.
Joseph S. Ames, Chairman.
The President,
The White House, Washington, D. C.
I
CONTENTS
Page
Letter of submittal _ in
Letter of transmittal _ v
Eighteenth annual report _ 1
PART I. ORGANIZATION ANI) GENERAL ACTIVITIES
Functions of the committee _ 3
Organization _ 3
The executive committee - 4
Subcommittees _ 4
The Langley Memorial Aeronautical Laboratory _ 5
The Office of Aeronautical Intelligence _ 5
Study of aircraft accidents _ 6
Consideration of aeronautical inventions _ 6
Cooperation with the Army and the Navy _ _ _ (5
Relations with the aircraft industry _ 7
Financial report _ 8
PART II. REPORTS OF TECHNICAL COMMITTEES
Functions of the technical committees _ 9
Report of committee on aerodynamics _ 9
Report of committee on power plants for aircraft _ 22
Report of committee on materials for aircraft _ 28
Report of committee on problems of air navigation _ _ _ 34
PART III. TECHNICAL PUBLICATIONS OF THE COMMITTEE
Summaries of technical reports _ 37
List of technical notes issued during the past year _ 43
List of technical memorandums issued during the past year _ 49
List of aircraft circulars issued during the past year _ 51
PART IV. SUMMARY OF PROGRESS IN AERONAUTICAL RESEARCH
Aerodynamics _ 53
Aircraft engines _ 56
Materials and structures _ 58
Airships _ 59
Summary _ 60
VII
TECHNICAL REPORTS
Pape
No. 401. Combustion in a High-Speed Compression-
Ignition Engine. By A. M. Rothrock _ 63
No. 402. Effect of Orifice Length-Diameter Ratio on Fuel Sprays for Compression-Ignition En¬ gines. By A. G. Gelalles _ 79
No. 403. Ice Prevention on Aircraft by Means of Engine Exhaust Heat and a Technical Study of Heat Transmission from a Clark Y Airfoil. By Theodore Theodorsen and William C. Clav_ 91
No. 404. The Effect on Increased Carburetor Pressure on Engine Performance at Several Compression Ratios. By Oscar W. Schey and Vern G.
Rollin _ 113
No. 405. Application of Practical Hydrodynamics to Airship Design. By Ralph H. Upson and
W. A. Klikoff _ 123
No. 406. Drop and Flight Tests on NY-2 Landing Gears Including Measurements of Vertical Veloci¬ ties at Landing. By W. C. Peck and A. P.
Beard _ 141
No. 407. The Characteristics of a Clark Y Wing Model Equipped with Several Forms of Low- Drag Fixed Slots. By Fred E. Weick and Carl J.
Wenzinger _ 155
No. 408. General Formulas and Charts for the Calcula¬ tion of Airplane Performance. By W. Bailey
Oswald _ 163
No. 409. The Elimination of Fire Hazard Due to Back Fires. By Theodore Theodorsen and Ira M.
Freeman _ 211
No. 410. The Theory of Wind-Tunnel Wall Interference.
By Theodore Theodorsen _ 219
No. 411. Theory of Wing Sections of Arbitrary Shape.
By Theodore Theodorsen _ 229
No. 412. The 7 by 10 Foot Wind Tunnel of the National Advisory Committee for Aeronautics. By
Thomas A. Harris _ 241
No. 413. A Method for Computing Leading-Edge Loads.
By Richard V. Rhode and Henry A. Pearson. 249
No. 414. The Effect on Airplane Performance of the Fac¬ tors that must be Considered in Applying Low- Drag Cowling to Radial Engines. By William H. McAvoy, Oscar W. Schey, and
Alfred W. Young _ _ 259
No. 415. Tests on Nacelle-Propeller Combinations in Various Positions with Reference to Wings.
Part I, Thick Wing — N. A. C. A. Cowled Nacelle — Tractor Propeller. By Donald H.
Wood _ 277
No. 416. The N. A. C. A. Variable-Density Wind Tun¬ nel. By Easman N. Jacobs and Ira H.
Abbott _ 305
No. 417. Pressure Distribution Tests on a Series of Clark Y Biplane Cellules with Special Ref¬ erence to Stability. By Richard W. Noyes. 315 No. 418. Preliminary Investigation of Modifications to Conventional Airplanes to give Nonstalling and Short-Landing Characteristics. By Fred E. Weick _ 343
Page
No. 419. Wind-Tunnel Research Comparing Lateral Control Devices, Particularly at High Angles of Attack- I — Ordinary Ailerons on Rectangular Wings. By Fred E. Weick and
Carl J. Wenzinger _ 357
No. 420. Aircraft Speed Instruments. By K. Hilding
Beij - 381
No. 421. Measurement of the Differential and Total Thrust and Torque of Six Full-Scale Ad¬ justable-Pitch Propellers. By George W. Stickle _ 437
No. 422. Wind-Tunnel Research Comparing Lateral Control Devices, Particlarly at High Angles of Attack. II — Slotted Ailerons and Frise Ailerons. By Fred E. Weick and Richard
W. Noyes _ 457
No. 423. Wind-Tunnel Research Comparing Lateral Control Devices, Particularly at High Angles of Attack. Ill — Ordinary Ailerons Rigged Up 10° When Neutral. By Fred E. Weick
and Carl J. Wenzinger _ 471
No. 424. Wind-Tunnel Research Comparing Lateral Control Devices, Particularly at High Angles of Attack. IV — Floating Tip Aile¬
rons on Rectangular Wings. By Fred E.
Weick and Thomas A. Harris _ 481
No. 425. The Effect of Nozzle Design and Operating Conditions on the Atomization and Dis¬ tribution of Fuel Sprays. By Dana W. Lee. 505 No. 426. The Effect of Humidity on Engine Power at Altitude. By D. B. Brooks and E. A.
Garlock _ 523
No. 427. The Effect of Multiple Fixed Slots and a Trad¬ ing-Edge Flap on the Lift and Drag of a Clark Y Airfoil. By Fred E. Weick and
Joseph A. Shortal _ 531
No. 428. Wind-Tunnel Tests of a Clark Y Wing with a Narrow Auxiliary Airfoil in Different Posi¬ tions. By Fred E. Weick and Millard J.
Bamber _ 537
No. 429. The N. A. C. A. Apparatus for Studying the Formation and Combustion of Fuel Sprays and the Results from Preliminary Tests.
By A. M. Rothrock.... _ _ 549
No. 430. Measurements of Flow in the Boundary Layer of a 1/40-Scale Model of the U. S. Airship
Akron. By Hugh B. Freeman - 567
No. 431. Characteristics of Clark Y Airfoils of Small
Aspect Ratios. By C. H. Zimmerman _ 581
No. 432. Force Measurements on a 1/40-Scale Model of the U. S. Airship Akron. By Hugh B. Free¬ man _ 591
No. 433. Rates of Fuel Discharge as Affected by the Design of Fuel-Injection Systems for In¬ ternal-Combustion Engines. By A. G. Gel¬ alles and E. T. Marsh - 607
No. 434. Lift and Drag Characteristics and Gliding Per¬ formance of an Autogiro as Determined in Flight. By John B. Wheatley - 621
VIII
TECHNICAL REPORTS
IX
Page
No. 435. Fuel Vaporization and its Effect on Combus¬ tion in a High-Speed Compression-Ignition Engine. By A. M. Rothrock and C. D.
Waldron _ 629
No. 436. Tests on Nacelle-Propeller Combinations in Various Positions with Reference to Wings.
II — Thick Wing — Various Radial-Engine Cowlings — Tractor Propeller. By Donald
H. Wood _ 653
No. 437. The Effect of Area and Aspect Ratio on the Yawing Moments of Rudders at Large An¬ gles of Pitch on Three Fuselages. By Hugh L. Dryden and B. H. Monish - 693
Page
No. 438. Experiments on the Distribution of Fuel in
Fuel Sprays. By Dana W. Lee - 703
No. 439. Wind-Tunnel Research Comparing Lateral Control Devices, Particularly at High Angles of Attack. V — Spoilers and Ailerons on Rectangular Wings. Fred E. Weick and
Joseph A. Shortal _ 719
No. 440. The Mechanism of Atomization Accompanying
Solid Injection. By R. A. Castleman, Jr — 735
)
i
NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
3841 NAVY BUILDING, WASHINGTON, D. C.
Joseph S. Ames, Ph. D., Chairman,
President, Johns Hopkins University, Baltimore, Md.
David W. Taylor, D. Eng., Vice Chairman ,
Washington, D. C.
Charles G. Abbot, Sc. D.,
Secretary, Smithsonian Institution, Washington, D. C.
Arthur B. Cook, Captain, United States Navy,
Assistant Chief, Bureau of Aeronautics, Navy Department, Washington, D. C. William F. Durand, Ph. D.,
Professor Emeritus of Mechanical Engineering, Stanford University, California Benjamin D. Foulois, Major General, United States Army,
Chief of Air Corps, War Department, Washington, D. C.
Harry F. Guggenheim, M. A.,
The American Ambassador, Habana, Cuba.
Charles A. Lindbergh, LL. D.
New York City.
William P. MacCracken, Jr., Ph. B.,
Washington, D. C.
Charles F. Marvin, Sc. D.,
Chief, United States Weather Bureau, Washington, D. C.
William A. Moffett, Rear Admiral, United States Navy,
Chief, Bureau of Aeronautics, Navy Department, Washington, D. C.
Henry C. Pratt, Brigadier General, United States Army,
Chief, Materiel Division, Air Corps, Wright Field, Dayton, Ohio.
Edward P. Warner, M. S.,
Editor, Aviation, New York City.
Orville Wright, Sc. D.,
Dayton, Ohio.
Bureau of Standards, Washington, D. C.
George W. Lewis, Director of Aeronautical Research John F. Victory, Secretary
Henry J. E. Reid, Engineer-in-Charge, Langley Memorial Aeronautical Laboratory, Langley Field, Va.
John J. Ide, Technical Assistant in Europe, Paris, France
THE EXECUTIVE COMMITTEE
Joseph S. Ames, Chairman David W. Taylor, Vice Chairman
Charles G. Abbot. Charles F. Marvin.
Arthur B. Cook. William A. Moffett.
Benjamin D. Foulois. Henry C. Pratt.
Charles A. Lindbergh. Edward P. Warner.
William P. MacCracken. Orville Wright.
John F. Victory, Secretary
EIGHTEENTH ANNUAL REPORT
OF THE
NATIONAL ADVISORY COMMITTEE FOR
AERONAUTICS
Washington, D. C., November 10, 1932.
To the Congress of the United States:
In accordance with the act of Congress approved March 3, 1915, which established the National Ad¬ visory Committee for Aeronautics, the committee submits herewith its eighteenth annual report for the fiscal year 1932.
Gratifying progress. — The committee is pleased to report that gratifying improvement has been con¬ tinued during the past year in the performance and reliability of airplanes. This is equally true for military and commercial types. This has been due to organized scientific research conducted by the com¬ mittee and to the practical application of the results by the Army, the Navy, and the aircraft industry.
Importance of speed. — Speed is the most important single factor in increasing the relative value of aircraft for national defense and in extending their use for commercial purposes. During the past year the speed of military bombers was increased to a rate greater than that of pursuit-type airplanes of two years ago, and the normal cruising speed of large commercial air transports was increased 20-40 per cent. This notable advance was made possible primarily by improvements in aerodynamic efficiency resulting in large measure from researches conducted by the com¬ mittee, including especially investigations of the cowling of engines and the determination of the best positions of engines with reference to the wings of multi-engine airplanes. An important factor in this advance was the development of more efficient and reliable engines sponsored by the Army and Navy. This advance in speed, coupled with the development of reliable schedules of passenger and express service at reasonable rates, is making air transportation more attractive to the public and enabling it to make prog¬ ress toward a self-sustaining status. Notwithstanding the difficulties of the period, air passenger and express transportation have materially increased during the past year.
Facilities for fundamental research. — With the far¬ sighted support of the Congress the committee’s laboratory, known as the Langley Memorial Aero¬
nautical Laboratory and located at Langley Field, Va., has become unsurpassed in the development of original and ingenious equipment and methods for fundamental research. The development of this laboratory repre¬ sents an accomplishment in which the Congress and the country can take pride, for the excellence of its product has gained for the United States an advanta¬ geous position among nations in the development of aeronautics.
Comprehensive researches serve all needs. — The
facilities of the committee’s laboratory are used largely for the conduct of researches requested by the Army and Navy air organizations, which depend upon the committee for the investigation and study of fundamental problems to enable them to improve the design of military and naval aircraft to meet their particular needs. The aircraft manufacturers and operators also rely upon the committee for fundamental data, and the committee frequently broadens its re¬ searches to obtain as much fundamental information as possible in order to meet the needs of commercial aviation. To determine these needs the committee holds at its laboratory an annual conference with representatives of the aircraft industry. As a result the research programs of the committee, formulated largely by the various technical subcommittees, are comprehensive in scope and embrace the problems deemed necessary for the further improvement of military, naval, commercial, and civil airplanes and also airships.
Coordination increases value of research. — The
research needs of aviation are thus effectively de¬ termined in cooperation with the Army, the Navy, and the aircraft industry. This policy and practice of coordinated planning of fundamental research and its continuous arid orderly prosecution under the single and direct control of the committee assure results of the greatest value to aeronautics, and at the same time prevent duplication and waste.
Special attention to safety. — The committee is devoting special attention to investigations of various possible means of obtaining greater control of an air¬ plane at low speeds incident to taking off and landing,
1
2
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
with a view to increasing safety and reliability, and in this connection is also investigating the possibilities of several autorotative-wing types of aircraft.
The committee’s new full-scale wind tunnel and towing tank for seaplane models have been developed into indispensable items of research equipment, per¬ mitting full-scale research hitherto not possible.
Transoceanic air transport. — The development of air transport service to South America is considered a helpful factor in promoting foreign trade. Such service, involving long over-water flights, will require the development of more efficient large seaplanes. The new N. A. C. A. tank for the testing of models of seaplane floats and flying-boat hulls will provide much-needed information. For trans-Atlantic air transport service to Europe, greater cruising range and carrying capacity are required than can be efficiently provided in heavier-than-air craft at the present stage of aeronautical development. Rigid airships at this time offer a prospect for air passenger service to Europe. They are already being used by a European nation in providing regular air passenger service across the South Atlantic. The Congress has done much to establish a rigid-airship industry in this country by appropriations for the construction of the naval airships Akron and Macon. The Navy’s ex¬ perience with lighter-than-air craft has provided valuable information and data for commercial airship development and operation. The committee believes that the United States should continue to encourage the development and use of rigid airships as a means of ocean transportation.
Continue research to keep abreast. — Because of its increasing importance for both military and com¬ mercial purposes it is essential that America strive constantly to keep at least abreast of other nations in the rapidly developing science of aeronautics. With the present disturbed conditions in the world it is vitally necessary to continue aeronautical re¬ search, experimentation, and development for the national defense. To fall behind would not only endanger our security but would retard definitely the progress of civilization.
Factors in increased importance of aircraft. — Since the Great Wrar gave to the world indications of the possibilities of aircraft, the material improvement from year to year in their performance and reliability has greatly increased their relative importance for national defense and also as an agency of transporta¬ tion. The scientific researches conducted by the committee have been the chief factor contributing to the technical development of aircraft. Other im¬ portant factors in the general development of aviation have been: (a) The engineering and experimentation activities of the Army and Navy in the development of efficient military aircraft and aircraft materiel;
(6) the regulation and encouragement of civil and commercial aviation by the Aeronautics Branch of the Department of Commerce, including especially the establishment and maintenance of airways and aids to air navigation unsurpassed in any country; (c) the effective assistance of the Weather Bureau in the field of meteorology and special weather report service in aid of air navigation; and ( d ) the policy of the Congress in enabling the Post Office Department to supply air mail service to all parts of the country, and, through contracts for the carriage of air mail, to give that indispensable support that has made pos¬ sible the development of air passenger transportation in the United States.
Effect of sound governmental policy. — The present advanced state of aeronautical development in America is largely the result of long-continued sound govern¬ mental policy that began with the act of Congress in 1915 when in establishing the National Advisory Com¬ mittee for Aeronautics “to supervise and direct the scientific study of the problems of flight,” the Congress laid the foundation for rapid and continuous progress in this new science. A part of recent governmental policy has been the maintenance of a nucleus of an aircraft industry capable of satisfactory expansion ot meet needs in an emergency. The volume of com¬ mercial sales of aircraft has been severely checked during the past three years, with the result that the Government is still the chief customer of the aircraft industry. With the completion of the 5-year aircraft procurement programs of the Army and Navy, a further legislative declaration of policy is necessary under present conditions to afford sufficient stability to maintain a satisfactory nucleus of an aircraft industry.
Research pays. — Although technical progress in the development of aircraft has been gratifying for a num¬ ber of years, there is yet an urgent need for greater safety and greater economy. The major problems are to increase the aerodynamic efficiency of aircraft, the horsepower and operating efficiency of engines, and the control of airplanes at low speeds. The com¬ mittee, in the exercise of its prescribed function under the law is investigating these and many other impor¬ tant problems which underlie progress in aeronautics. The committee’s work not only leads directly to greater safety, efficiency, and reliability of aircraft, but its researches yield results annually of economic value alone in excess of the cost thereof.
In submitting this, its eighteenth annual report, the committee invites attention to Part II presenting re¬ ports of activities of the various technical subcom¬ mittees, and also to Part IV presenting a summary of technical development in aeronautics accom¬ plished under the committee’s direction during the past year.
PART I
ORGANIZATION AND GENERAL ACTIVITIES
FUNCTIONS OF THE COMMITTEE
The National Advisory Committee for Aeronautics was established by act of Congress approved March 3, 1915 (U. S. C., title 50, sec. 151). The organic act charged the committee with the supervision and direc¬ tion of the scientific study of the problems of flight with a view to their practical solution, the determi¬ nation of problems which should be experimentally attacked, and their investigation and application to practical questions of aeronautics. The act also author¬ ized the committee to direct and conduct research and experimentation in aeronautics in such laboratory or laboratories, in whole or in part, as might be placed under its direction.
Supplementing the prescribed duties of the commit¬ tee under its organic act, its broad general functions may be stated as follows:
First. Under the law the committee holds itself at the service of any department or agency of the Government interested in aeronautics for the furnish¬ ing of information or assistance in regard to scientific or technical matters relating to aeronautics, and in particular for the investigation and study of funda¬ mental problems submitted by the War, Navy, and Commerce Departments with a view to their practical solution.
Second. The committee may also exercise its func¬ tions for any individual, firm, association, or corpora¬ tion within the United States, provided that such individual, firm, association, or corporation defray the actual cost involved.
Third. The committee institutes research, investi¬ gation, and study of problems which, in the judgment of its members or of the members of its various sub¬ committees, are needful and timely for the advance of the science and art of aeronautics in its various branches.
Fourth. The committee keeps itself advised of the progress made in research and experimental work in aeronautics in all parts of the world.
Fifth. The information thus gathered is brought to the attention of the various subcommittees for con¬ sideration in connection with the preparation of pro¬ grams for research and experimental work in this country. This information is also made available promptly to the military and naval air organizations and other branches of the Government and such as is not confidential is immediately released to university
laboratories and aircraft manufacturers interested in the study of specific problems, and also to the public.
Sixth. The committee holds itself at the service of the President, the Congress, and the executive depart¬ ments of the Government for the consideration of special problems which may be referred to it.
The act of Congress approved July 2, 1926, and amended March 3, 1927 (U. S. C., Supp. V, title 10, sec. 31 Or), which created and specified the functions of an aeronautical patents and design board, consist¬ ing of an Assistant Secretary of War, an Assistant Secretary of the Navy, and an Assistant Secretary of Commerce, provided that upon favorable recommen¬ dation of the National Advisory Committee for Aero¬ nautics the patents and design board should deter¬ mine questions as to the use and value to the Govern¬ ment of aeronautical inventions submitted to any branch of the Government. The legislation provided that designs submitted to the board should be referred to the National Advisory Committee for Aeronautics for its recommendation, and this has served to impose upon the committee the additional duty of consider¬ ing on behalf of the Government all aeronautical in¬ ventions and designs submitted.
ORGANIZATION
The act of Congress establishing the committee, as amended by act approved March 2, 1929 (U. S. C., Supp. V, title 50, sec. 151a), provides that the Na¬ tional Advisory Committee for Aeronautics shall con¬ sist of 15 members appointed by the President, as follows: Two members from the War Department, from the office in charge of military aeronautics; two members from the Navy Department, from the office in charge of naval aeronautics; a representative each of the Smithsonian Institution, the United States Weather Bureau, and the United States Bureau of Standards; and not more than eight additional persons acquainted with the needs of aeronautical science, either civil or military, or skilled in aeronautical engineering or its allied sciences. The law further provides that all members as such shall serve without compensation.
On December 19, 1931, Maj. Gen. James E. Fechet, United States Army, was relieved from membership on the committee because of his retirement from active duty in the Army and as Chief of the Air Corps. Under date of January 5, 1932, Maj. Gen. Benjamin D.
3
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REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
Foulois, United States Army, General Fechet’s suc¬ cessor as Chief of the Air Corps, was appointed by the President to succeed him on the committee.
On July 2, 1932, the committee lost one of its most valuable and devoted members in the death of Dr. George K. Burgess, Director of the Bureau of Standards. Doctor Burgess was appointed to membership by the President on May 26, 1923, and was for nine years chairman of the important subcommittee on materials for aircraft and for five years chairman of the sub¬ committee on metals. He also served as chairman of the committee on aircraft accidents from its organization in 1928 until his resignation from the chairmanship in 1931. Doctor Burgess’ successor on the committee has not yet been appointed.
The entire committee meets twice a year, the annual meeting being held in October and the semiannual meeting in April. The present report includes the activities of the committee between the annual meeting held on October 22, 1931, and that held on October 20, 1932.
The present organization of the committee is as follows :
Joseph S. Ames, Pli. D., chairman, president of Johns Hopkins University, Baltimore, Md.
David W. Taylor, D. Eng., vice chairman, Wash¬ ington, D. C.
Charles G. Abbot, Sc. D., secretary of the Smith¬ sonian Institution.
Capt. Arthur B. Cook, United States Navy, As¬ sistant Chief of the Bureau of Aeronautics, Navy Department.
William F. Durand, Ph. D., professor emeritus of mechanical engineering, Stanford University, California.
Maj. Gen. Benjamin D. Foulois, United States Army, Chief of the Air Corps.
Harry F. Guggenheim, M. A., American Ambas¬ sador to Cuba.
Col. Charles A. Lindbergh, LL. D. New York City.
William P. MacCracken, jr., Ph. B., Washington, D. C.
Charles F. Marvin, Sc. D., Chief of the Weather Bureau.
Rear Admiral William A. Moffett, United States Navy, Chief of the Bureau of Aeronautics, Navy Department.
Brig. Gen. Henry C. Pratt, United States Army, Chief of the materiel division, Air Corps.
Edward P. Warner, M. S., editor of Aviation.
Orville Wright, Sc. D., Dayton, Ohio.
THE EXECUTIVE COMMITTEE
For the purpose of carrying out the work of the advisory committee the regulations provide for the election annually of an executive committee. The present membership of the executive committee in¬
cludes all the members of the advisory committee with the exception of its two far-distant members, Doctor Durand and Mr. Guggenheim.
The executive committee has organized the necessary clerical and technical staffs for handling the work o{ the committee proper. The total paid personnel of the committee numbered 317 employees on June 30, 1931, comprising 44 in Washington, 270 at the Langley Memorial Aeronautical Laboratory, Langley Field, Va., and three at the office of the technical assistant in Europe, Paris, France. General responsibility for the execution of the policies and the direction of the activities approved by the executive committee is vested in the director of aeronautical research, Mr, George W. Lewis. He has immediate charge of the scientific and technical work of the committee, being directly responsible to the chairman of the executive committee, Dr. Joseph S. Ames. The secretary, Mr. John F. Victory, is ex officio secretary of the executive committee, directs the administrative work of the organization, and exercises general supervision over the expenditures of funds and the employment of personnel.
SUBCOMMITTEES
In order to facilitate the conduct of its work, the executive committee has organized a number of stand¬ ing committees, with subcommittees in some instances to cover the general field more effectively.
The committees of the executive committee, with their subcommittees, are as follows:
Aerodynamics —
Subcommittee on airships.
Power plants for aircraft.
Materials for aircraft —
Subcommittee on metals.
Subcommitte on aircraft structures —
Temporary subcommittee on research programs on monocoque design. Subcommittee on miscellaneous materials. Subcommittee on methods and devices for testing aircraft materials and structures.
Problems of air navigation —
Subcommittee on instruments.
Subcommittee on meteorological problems.
Aircraft accidents.
Aeronautical inventions and designs.
Publications and intelligence.
Personnel, buildings, and equipment.
The organization and work of the technical commit¬ tees on aerodynamics, power plants for aircraft, mate¬ rials for aircraft, and problems of air navigation are covered in the reports of those committees in Part II of this report, while the activities of the committee on aircraft accidents and the committee on aeronautical inventions and designs are included under the subjects of the study of aircraft accidents and the consideration of aeronautical inventions, respectively.
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
0
The organization of the administrative committees on publications and intelligence, and personnel, build¬ ings, and equipment is as follows:
Committee on Publications and Intelligence
Dr. Joseph S. Ames, chairman.
Dr. Charles F. Marvin, vice chairman.
Miss M. M. Muller, secretary.
Committee on Personnel, Buildings, and Equipment
Dr. Joseph S. Ames, chairman.
Dr. David W. Taylor, vice chairman.
•j *
John F. Victory, secretary.
In addition to its standing committees, the executive committee has established from time to time special work. On such conference, the special conference on aeronautical nomenclature, is at present engaged in the revision of the standard nomenclature for aeronautics, which was published by the committee in 1926 as Technical Report No. 240. On completion of this work the revised nomenclature will be issued by the committee to supersede the previous report.
THE LANGLEY MEMORIAL AERONAUTICAL LABORATORY
The Langley Memorial Aeronautical Laboratory is operated under the direct control of the committee. It is located at Langley Field, Va., on a plot of ground set aside by the War Department for the committee’s use. The laboratory was started in 1916 coincident with the establishment of Langley Field.
The laboratory is organized with six divisions, as follows: Aerodynamics division, power plants division, hydrodynamics division, physical research division, technical service division, and property and clerical division. The laboratory is under the immediate direction of an engineer-in-cliarge, Mr. Henry J. E. Reid, subject to the general supervision of the officers of the committee.
There are at present 11 structures comprising the Langley Memorial Aeronautical Laboratory, as follows :
1. A research laboratory building containing ad¬ ministrative offices, technical library, physics labora¬ tory, photographic laboratory, and headquarters of the various divisions.
2. An atmospheric wind-tunnel building, in which are housed a modern closed-return tunnel with an open rectangular throat 7 by 10 feet and a vertical tunnel with closed return and an open throat 5 feet in diam¬ eter. These two pieces of equipment are used in a comprehensive study of the problems of control and spinning characteristics of an airplane. In addition to the above equipment the building houses a 6-inch open-throat wind tunnel for instrument and wind- tunnel studies and a 6-inch refrigerated tunnel for the study of ice formation on aircraft.
3. A variable-density wind-tunnel building, housing the variable-density wind tunnel and a jet- type high¬ speed wind tunnel which utilizes the waste air from the variable-density wind tunnel.
4 and 5. Two engine dynamometer laboratories of a semipermanent type equipped to carry on investiga¬ tions in connection with power plants for aircraft. In addition to the usual dynamometer equipment and single-cylinder test engines for studying both carbu¬ reted and Diesel-oil engines, there is equipment suited to the study of superchargers and the cooling of engine cylinders, and special high-speed photographic equip¬ ment for the study of fuel sprays.
6. A service building containing an instrument laboratory, drafting room, machine shop, woodworking shop, and storeroom.
7. A propeller-research tunnel, in which tests may be made in a 20-foot air stream at 100 miles per hour. This equipment permits the full-scale testing of pro¬ pellers, fuselages, and landing gears.
8. An airplane hangar, 240 feet long by 110 feet wide, including office space for the staff of the com¬ mittee’s flight research laboratory, and a repair shop and facilities for taking care of the airplanes used in flight research.
9. A combination heating plant, storehouse, and garage.
10. A full-scale wind tunnel, having an oval-shape throat 60 by 30 feet, large enough for the testing of full-size airplanes, and particularly suitable for the stud}7 of the problems of airplane resistance or drag, stability, and control at low speeds, and spinning characteristics. The tunnel is operated by two 4,000- horsepower electric motors, generating an air stream of 115 miles per hour.
11. The N. A. C. A. tank, 1,980 feet long, 24 feet wide, and 12 feet deep, for the testing of large models of seaplane floats and flying-boat hulls. The carriage for towing the models through the water may be operated at speeds up to about 60 miles per hour.
Items 1, 2, 3, 4, 5, 6, and 9 are located on plot 16. Items 7, 10, and 11 are located within two blocks of the laboratory headquarters, and item 8 is located on the flying field.
The committee’s laboratory is well equipped and has the great advantage of being located on a flying field. The work of the laboratory is conducted with¬ out interference with military operations at the field. In fact, there is a splendid spirit of cooperation on the part of the military authorities, who by their helpful¬ ness in man}7 ways have aided the committee materially in its work.
THE OFFICE OF AERONAUTICAL INTELLIGENCE
The Office of Aeronautical Intelligence was estab- | fished in the early part of 1918 as an integral branch of the committee’s activities. Its functions are the col-
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REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
lection, classification, and diffusion of technical knowl¬ edge on the subject of aeronautics, including the results of research and experimental work conducted in all parts of the world, to the military and naval air organizations, aircraft manufacturers, educational institutions, and others interested. It is the officially designated Government depository for scientific and technical reports and data on aeronautics.
Promptly upon receipt, all reports are analyzed, classified, and brought to the special attention of the subcommittees having cognizance and to the attention of other interested parties through the medium of public and confidential bulletins. Reports are dupli¬ cated where practicable and distributed upon request. Confidential bulletins and reports are not circulated outside of Government channels.
To handle efficiently the work of securing and exchanging reports in foreign countries, the committee maintains a technical assistant in Europe, with head¬ quarters at the American Embassy in Paris. It is his duty to visit the governmental and private laboratories, centers of aeronautical information, and private indi¬ viduals in European countries and endeavor to secure for America not only printed matter which would in the ordinary course of events become available in this country but more especially advance information as to work in progress and technical data not prepared in printed form, which would otherwise not reach this country. John Jay Ide, of New York, has served as the committee’s technical assistant in Europe since April, 1921.
STUDY OF AIRCRAFT ACCIDENTS
A standard procedure for the analysis of aircraft accidents as recommended by the committee in Tech¬ nical Report No. 357 is followed in the War, Navy, and Commerce Departments. Uniformity in inter¬ pretations of definitions and explanations has been secured through meetings of the committee on air¬ craft accidents, which includes representatives of the War, Navy, and Commerce air organizations.
The work of the committee was materially furthered by the cooperation of medical officers of the War, Navy, and Commerce Departments who met, not only with the committee, but with each other, with a view to evolving a plan for determining the effect of the physical, mental, and temperamental characteristics of pilots on the number and nature of accidents. Statistical information is needed on the underlying physiological and psychological causes of aircraft accidents. During the past year that committee cooperated with representatives of the Actuarial Society of America, the United States Aviation Under¬ writers (Inc.), and other insurance interests in con¬ nection with problems relating to the determination of a basis for rating pilots’ risks.
The present membership of the committee on air¬ craft accidents is as follows:
Hon. Edward P. Warner, editor of Aviation, chairman.
Maj. J. W. Jones, United States Army, Air Corps,
George W. Lewis, National Advisory Committee for Aeronautics.
W. Fiske Marshall, Aeronautics Branch, Depart¬ ment of Commerce.
Lieut. Commander A. C. McFall, United States Navy, Bureau of Aeronautics.
Lieut. H. B. Temple, United States Navy, Bureau of Aeronautics.
Lieut. Lyman P. Whitten, United States Army, Air Corps.
The following are the medical officers of the Gov¬ ernment who cooperated with the committee on air¬ craft accidents during the past year:
Col. Glen I. Jones (M. C.), United States Army.
Dr. R. F. Longacre, Aeronautics Branch, Depart¬ ment of Commerce.
Lieut. Commander John R. Poppen (M. C.), United States Navy.
Lieut. Commander Joel J. White (M. C.), United States Navy.
CONSIDERATION OF AERONAUTICAL INVENTIONS
In accordance with act of Congress approved July 2, 1926, as amended by act approved March 3, 1927, the National Advisory Committee for Aeronautics passes upon the merits of aeronautical inventions and designs submitted to any branch of the Government for an award. Under the law an aeronautical patents and design board, consisting of the Assistant Secretaries for Aeronautics in the Departments of War, Navy, and Commerce, is authorized, upon the favorable recom¬ mendation of the committee, to “determine whether the use of the design by the Government is desirable or necessary and evaluate the design and fix its worth to the United States in an amount not to exceed $75,000.”
During the past year the committee received approx¬ imately 800 new submissions, most of which, being addressed direct to the committee and not meriting favorable recommendation, were settled by direct cor¬ respondence, without reference to the aeronautical patents and design board. The committee, through its committee on aeronautical inventions and designs, submitted reports to the board on 91 cases during the past year.
COOPERATION WITH THE ARMY AND THE NAVY
Through the personal contact of the heads of the Army Air Corps and of the Bureau of Aeronautics of the Navy serving as members of the National Advis¬ ory Committee for Aeronautics and of their chief sub¬ ordinates acting as members of the subcommittees,
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
7
there is first-hand discussion of the technical problems of these organizations, and this has served as a most effective means of coordination of aeronautical research.
Many of the committee’s fundamental researches have resulted from requests received from the Army and Navy for the study of particular problems.
The Army and Navy have placed at the committee’s disposal airplanes and engines required for research purposes. The committee desires to give special recog¬ nition to the splendid spirit of cooperation shown by the Army and Navy, and to acknowledge in particular the many courtesies extended by the Army authorities at Langley Field, where the committee’s laboratories are located, and by the naval authorities at the Hampton Roads Naval Air Station.
The investigations conducted by the committee dur¬ ing the past year for the Army and the Navy are as follows :
Study of loads on wings of military type airplanes.
Investigation of pressure distribution on bomber airplane in dives.
Calculation of pressure distribution for various air¬ foils.
Study of water-pressure distribution on seaplane floats and hulls.
Effect of variations in dimensions and form of hull on performance of flying boats during take-off.
Investigation of design of floats for racing seaplanes.
Determination of maximum negative thrust coeffi¬ cients of propellers.
Effect of propeller pitch settings on dive speed and engine revolutions.
Study of design factors for metal propellers.
Investigation of unsatisfactory spinning character¬ istics of F4B-2 airplane.
Investigation of autorotation.
Investigation of flight path characteristics.
Investigation of methods of improving wing charac¬ teristics by control of the boundary layer.
Investigation of effect of sags in wing surface.
Investigation of aerodynamic loads on U. S. airship Akron.
Study of the forces on an airship at large angles of yaw.
Investigation of windshields and fairings for protec¬ tion from air currents.
Development of cowling combining cooling, accessi¬ bility, and low drag, for radial air-cooled engine in¬ stalled in nacelle.
Investigation of cowling and cooling of 2-row radial air-cooled engine.
Study of wing-fuselage interference in airplane of gull-wing type.
Investigation of pressure distribution on observation- type airplane.
Investigation of tail surface loads in maneuvers.
Investigation of means of silencing airplane pro¬ pellers.
Investigation of effect of slipstream on airplane tail surfaces.
Determination of standard design characteristics for certain airfoils.
Pressure distribution on ring cowling.
Investigation of wing flutter.
Development of solid-injection type of aeronautical engine.
Investigation of application of compression ignition to air-cooled engine cylinders.
Performance of airship engine using hydrogenated safety fuels.
Effect of supercharging highly heated inlet air on performance of carburetor-type engine.
Study of effect of different degrees of supercharging with several compression ratios.
RELATIONS WITH THE AIRCRAFT INDUSTRY
The committee cooperates closely with representa¬ tives of the aircraft industry in planning research programs which are of particular interest and impor¬ tance to commercial aeronautics. The general pro¬ gram of research as formulated by the committee for the cowling and cooling of radial air-cooled engines was circulated to engineers of the industry and their com¬ ments obtained, and the result of this investigation was the development of the N,. A. C. A. cowling. During the past, year the results obtained by the committee in the study of propeller efficiency and the interference drag of combinations of engine nacelle and wing, and made available to the airplane manufacturers, have been reflected in the improved performance of recent designs of airplanes, particularly the large transport types. The committee takes advantage of every opportunity to obtain the comments and suggestions of the industry in formulating its research program on problems of general interest, such as the cowling and cooling of air-cooled engines and the best location of engine nacelles for any wing arrangement.
One problem in connection with which there has been active cooperation with the industry is the study of load factors in flight due to atmospheric disturbances. A number of air transport companies have cooperated with the committee in this investigation. The N. A. C. A. accelerometer, which was developed by the committee for this study, is now being superseded by a new and improved instrument known as the N. A. C. A. V-G recorder. This instrument registers both applied load factors and corresponding air speeds, thus greatly facilitating the task of obtaining the information needed in the study of this problem. Records from these instruments have been or are being obtained for the committee’s investigation by the Boeing Air Transport, American Airways, Ford
149900—33 - 2
8
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
Motor Co., Pan American Airways, United Air Lines, and Varney Speed Lines, on airplanes in regular operation.
Annual research conference. — In 1926 the National Advisory Committee for Aeronautics established the policy of holding at its laboratory at Langley Field, the Langley Memorial Aeronautical Laboratory, annual conferences with representatives of the manu¬ facturers and operators of aircraft. The purposes of these conferences are to give to the representatives of the industry an opportunity to become acquainted with the facilities for aeronautical research at the com¬ mittee’s laboratory and to afford them an opportunity to make suggestions to the committee as to aeronautical research problems of interest to the industry which in their opinion the committee is especially equipped to solve.
In accordance with this policy, the Seventh Annual Aircraft Engineering Research Conference was held at the committee’s laboratory on May 25, 1932. The committee was represented by its officers, members of the main committee, and the members of its committees on aerodynamics and power plants for aircraft. The conference was presided over by Dr. Joseph S. Ames, chairman of the National Advisory Committee for ; Aeronautics.
At the morning session the principal investigations under way at the laboratory, both in aerodynamics and power plants, were explained by the engineers in charge of the work and charts were exhibited showing some of the results obtained.
At the close of the session the representatives of the industry were conducted on a tour of inspection of the committee’s laboratories and the research equipment was shown in operation. On this occasion the com¬ mittee exhibited for the first time a specially con¬ structed smoke-flow wind tunnel which shows visually by means of smoke streamers the character of the air flow over wmg models, streamhne forms, and other shapes.
The afternoon session was detoted to the discussion of the problems of commercial aeronautics. Prior to the conference, the committee had requested the rep¬ resentatives of the industry to submit suggestions in writing, and a large number were received. In addi¬ tion, several such suggestions were presented and dis¬ cussed at the conference. Among these were the in¬ vestigation of the interference effects of engine nacelles in various positions in relation to biplane wings; in¬ vestigation of flying boat hulls at various displace¬
ments and with various assumed wing loadings; further study of interference effects, with particular reference to the effect of obstructions on airplane wings study of the interference and of the cowling and cooling of in-line type air-cooled engines; further study of cooling fins on air-cooled engine cylinders; and fur¬ ther investigation of the two-stroke-cycle engine for aircraft use.
All the suggestions for investigation presented in connection with the conference, totaling 37, including both those presented in the discussion in the afternoon session and those submitted in correspondence, have been carefully studied and classified, and each has been considered by the committee on aerodynamics, the committee on power plants for aircraft, or the subcommittee on aircraft structures. Many of these problems were already a part of the committee’s re¬ search program, and in a number of other cases the program has been modified to include the problems suggested.
FINANCIAL REPORT
The general appropriation for the National Advisory Committee for Aeronautics for the fiscal year 1932, as carried in the independent offices appropriation act approved February 23, 1931, was $1,028,070. In re¬ sponse to the President’s plea for economy, the com¬ mittee early in the fiscal year pledged a definite saving of $53,500 and by sustained pressure managed to effect a saving of $68,552 leaving the amount expended and obligated $959,518, itemized as follows:
Personal services _ $670, 858. SO
Supplies and materials _ 40, 433. 5S
Communication service - 1, 829. 05
Travel expenses _ 13, 972. 1C
Transportation of things _ 1, 843. 20
Furnishing of electricity _ 23, 103.0’
Rent of office (Paris) - 960. OC
Repairs and alterations - 28, 210.41
Special investigations and reports - 49, 500. OC
Equipment _ 128,808.31
Expenditures _ 959, 518. 22
Unobligated balance _ 68, 551.7!
Total, general appropriation _ 1, 028, 070. OC
The appropriation for printing and binding for 1932 was $23,000, of which $22,963.59 was expended.
The appropriations for the current fiscal year 1933 total $920,000, of which approximately $51,500 will be impounded in the Treasury pursuant to the Economy Act approved June 30, 1932.
PART II
REPORTS OF TECHNICAL COMMITTEES
FUNCTIONS OF THE TECHNICAL COMMITTEES
To facilitate the conduct and coordination of the scientific study of the problems of aeronautics in its various phases the National Advisory Committee for Aeronautics has established under the executive com¬ mittee four main technical committees, with subcom¬ mittees for certain specific subdivisions of the work. The main technical committees are the committee on aerodynamics, the committee on power plants for air¬ craft, the committee on materials for aircraft, and the committee on problems of air navigation.
The functions of the technical committees are as follows:
1. To determine what problems in their respective ; fields are the most important for investigation by gov- , ernmental and private agencies.
2. To coordinate by counsel and suggestion the re- i search work involved in the investigation of such problems.
3. To act as a medium for the interchange of infor¬ mation regarding investigations in their respective i fields and developments in progress or proposed.
4. To direct and conduct research in their respective fields in such laboratory or laboratories as may be placed either in whole or in part under their direction.
5. To meet from time to time on call of their chair¬ men and report their actions and recommendations to the executive committee.
The technical committees, by reason of the repre¬ sentation of the various organizations interested in aeronautics, are in close contact with all research work in their respective fields being carried out in the United States. The current work of each organization is therefore made known to all, duplication of effort being thus prevented. Also all research work is stimu- j lated by the prompt distribution of new ideas and new results, which add greatlv to the efficient conduct of aeronautical research. The committees keep the re¬ search workers in this country supplied with informa¬ tion of European progress in aeronautics through the work of the foreign representative of the National Advisory Committee, who is in close touch with aero¬ nautical activities in Europe. This direct information is supplemented by the translation and circulation of copies of the more important foreign reports and articles.
The committees on aerodynamics and power plants for aircraft have direct control of the aerodynamical and power plant research, respectively, conducted at
Langley Field, and of a number of special investiga¬ tions conducted at the Bureau of Standards. The major part of the research under the supervision of the committee on materials for aircraft is conducted by the Bureau of Standards. The experimental investi¬ gations in aerodynamics, aircraft power plants, air¬ craft materials, and air-navigation problems under¬ taken by the Bureau of Aeronautics of the Navy, the Army Air Corps, the Bureau of Standards, and other Government agencies are reported to the committees on aerodynamics, power plants for aircraft, materials for aircraft, and problems of air navigation, respec¬ tively.
REPORT OF COMMITTEE ON AERODYNAMICS
ORGANIZATION
The committee on aerodynamics is at present com¬ posed of the following members:
Dr. David W. Taylor, chairman.
Dr. L. J. Briggs, Bureau of Standards.
Lieut. Commander W. S. Diehl (C. C.), United States Navy.
Dr. H. L. Dryden, Bureau of Standards.
Capt. Albert C. Foulk, United States Army, mate¬ riel division, Air Corps, Wright Field.
Richard C. Gazley, Aeronautics Branch, Depart¬ ment of Commerce.
Maj. C. W. Howard, United States Army, mate¬ riel division, Air Corps, Wright Field.
George W. Lewis, National Advisory Committee for Aeronautics (ex officio member).
Dr. Charles F. Marvin, Weather Bureau.
Hon. Edward P. Warner, editor of Aviation.
Commander W. W. Webster (C. C.), United States Navy.
Dr. A. F. Zahm, division of aeronautics, Library of Congress.
Under the committee on aerodynamics there has been organized one subcommittee, the subcommittee on airships, for the supervision and coordination of investigations in connection with lighter-than-air craft.
SUBCOMMITTEE ON AIRSHIPS
The present organization of the subcommittee on airships is as follows :
Hon. Edward P. Warner, editor of Aviation, chairman.
Starr Truscott, National Advisory Committee for Aeronautics, vice chairman.
10
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
Dr. Karl Arnstein, Goodyear-Zeppelin Corpora¬ tion.
Commander Garland Fulton (C. C.) United States Navy.
Maj. William E. Kepner, United States Army, materiel division, Air Corps, Wright Field.
George W. Lewis, National Advisory Committee for Aeronautics (ex officio member).
Ralph H. Upson, Ann Arbor, Mich.
The subcommittee on airships has kept in close touch with the airship investigations under way during the past year at the Langley Memorial Aeronautical Laboratory. At a meeting of the subcommittee held on February 5, 1932, the results obtained in tests of a 1/40-scale model of the airship Akron in the propeller- research tunnel and in tests of the full-size airship in flight, conducted at the request of the Bureau of Aeronautics of the Navy Department, were discussed. At this meeting the subcommittee approved an investi¬ gation of airship forms to be conducted in the variable- density wind tunnel to determine the effect on the drag of variations in the nose fullness, tail fullness, and tail angle.
At a meeting held on October 10, 1932, a program of investigation of airship problems to be conducted by the Langley Memorial Aeronautical Laboratory was approved. This included model representation in the full-scale wind tunnel of ground handling of the Akron , as requested by the Bureau of Aeronautics of the Navy ; an investigation in the N. A. C. A. tank of the drag of a model of the Akron , for comparison with the results obtained in wind-tunnel tests; further study in the j 20-foot propeller-research tunnel of the boundary layer and of the pressure distribution on an airship model; and an investigation in the propeller-research tunnel of the effect of surface roughness on the drag of the airship model. At both these meetings there was general discussion of problems of airship development and operation.
LANGLEY MEMORIAL AERONAUTICAL LABORATORY
Equipment. — Improvements recently completed in the variable-density wind tunnel have been described in a report which incorporates also for ready reference the standard methods of reducing the airfoil data obtained in this tunnel. (Technical Report No. 416.)
A report has also been prepared during the year de¬ scribing the 7 by 10 foot tunnel, with its balance. (Technical Report No. 412.) The high-speed tunnel has been in operation for several months, and a report is in preparation describing the tunnel and presenting the results of tests of a number of airfoils at air speeds approaching the velocity of sound. The special spinning balance for the 5-foot vertical tunnel has been placed in operation, and a report is in prepara¬ tion describing this balance and presenting the results
of an investigation of the forces and moments on a model of an NY-1 airplane in spinning attitude.
The full-scale tunnel has been calibrated during the year, and after adjustment of the angular setting of some of the guide vanes a velocity distribution was obtained in which the variation over that portion ol the jet occupied by an airplane under test is within plus or minus one-half per cent. A report describing the tunnel and the methods of operation is in prepara¬ tion. The N. A. C. A. tank, for the study of hydro- dynamic problems connected with aeronautics, is also in regular operation on a carefully considered program of research. Several new instruments have been developed during the year for use in flight investiga¬ tions, and methods have been developed for the use of smoke to indicate the flow about the parts of an airplane in flight (Technical Note No. 425) and to show the flow about bodies in a small tunnel.
Controllability and Stability. — The past yeai has seen a substantial improvement in safety on com¬ mercial airlines and in military flying. This improve¬ ment is largely the result of careful training of pilots and well-regulated flying. Satisfactory stability and controllability have not yet been attained throughout the entire range of speed or of angle of attack of con¬ ventional airplanes, and as indicated by the accident records for unorganized flying, there is great need foi further research on control and stability.
During the past year considerable progress has been made in an extensive investigation on lateral con¬ trollability. In this investigation the relative merits of various forms of ailerons and other lateral control devices are tested and compared in regard to their effect on lateral controllability and lateral stability at high angles of attack, and also their effect on airplane performance. The comparisons are based on wind- tunnel test data, all the control devices being fitted to model wings having the same span, area, and airfoil section, and being subjected to the same series of force and rotation tests. The investigation is being carried out mainly in the 7 by 10 foot tunnel, but certain of the more interesting devices are also being tested nr flight.
The wind-tunnel tests have been completed for the following arrangements : (1) Ordinary ailerons of three different proportions, all mounted in the usual position at the trailing edge of the wing, the ailerons being tested both locked to the wing and arranged to float with respect to the wing (Technical Report No. 419); (2) slotted ailerons and Frise ailerons (Technical Report No. 422); (3) ordinary ailerons rigged up 10c when neutral, to eliminate the adverse yawing-moment characteristics of the ailerons with the usual rigging in line with the wing (Technical Report No. 423); (4) full-chord floating tip ailerons on rectangular wings (Technical Report No. 424); (5) spoilers and coinbi-
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
11
nations of spoilers and ailerons on rectangular wings (Technical Report No. 439); (6) skewed ailerons on rectangular wings (Technical Report No. 444); (7) Handley Page tip slots and full-span slots with ailerons and spoilers (Technical Report No. 448); (8) ordinary ailerons on wings with rounded tips; (9) ordinary ailerons on tapered wings. Technical reports are in preparation covering items (8) and (9).
Square tips. — On rectangular wings with ordinary or skewed ailerons the direct rolling control at high angles of attack was best with short, wide ailerons rigged up 10° when neutral and having an extreme differential movement from the rigged-up position, but the control forces required were exceptionally high. With the ailerons neutral, the autorotational tendency was found to be less with the ailerons rigged up 10° than when they were rigged even with the wing.
Rounded tips.- — On wings with rounded tips reason¬ ably satisfactory rolling moments at all angles of attack that can be maintained by average airplanes were obtained by short, wide ailerons, and particularly by short wide skewed ailerons on a wing with a long and slender tip. In general, the yawing moments were somewhat smaller for the ailerons on the wings with rounded tips than for the rorresponding ailerons on rectangular wings.
Tapered wings. — Two wings tapered in plan, one tapered 5:3 and the other 5:1, were tested with ordi¬ nary ailerons. The ailerons on tapered wings gave better lateral control below the stall in regard to all three of the important factors, rolling, yawing, and hinge moments, than corresponding ailerons on rec¬ tangular wings, but with the 5 : 1 taper the rolling moments fell off almost completely just above the stall and adverse yawing moments of great magnitude occurred.
Floating tip ailerons. — Full-chord floating tip ailerons on rectangular wings gave definitely poorer general performance, particularly in regard to speed range and climb, than that for a wing with the same over-all dimensions and ordinary ailerons. At the higher angles of attack well above the stall, the control with the wing-tip ailerons was found to be greater than the assumed satisfactory value, whereas the ordinary ailerons failed almost completely. The floating tip ailerons gave no appreciable adverse yawing moments (body axes), but gave large favorable ones at high angles of attack. The autorotative tendency was not as bad as for the wings with ordinary ailerons.
Spoilers.—' The investigation of spoilers showed that when spoilers and ailerons are used together, the full effect of both is not obtained if the spoilers are located directly in front of the ailerons. With the proper combination of spoiler and aileron, however, it is possible to obtain satisfactory rolling control up to high angles of attack (several degrees above the stall), together with favorable yawing moments and small
control forces. A moderate amount of rolling control with favorable yawing moments and small control forces was obtained with a large spoiler alone. The results of these tests indicated that the spoilers might give improved lateral control over that obtained by means of ordinary ailerons, and they are therefore being tested also in flight, both as the sole means of lateral control, and coupled with short, wide ailerons. The flight tests are being carried out on a small parasol monoplane.
Wing slots. — Wind-tunnel experiments were made on a wing with various lengths of Handley Page tip slot to find the best length of slot for the prevention of autorotation. (Technical Note No. 423.) The best length was found to be 50 per cent of the semispan, or slightly greater, for the particular arrangement tested. Control tests were then made on a wing model with tip slots of this length, and also on a model with full-span slots. The model with the tip slots and certain combinations of ailerons and properly located spoilers was found to have satisfactory damping in roll and satisfactory rolling control with no adverse yawing moments at all angles of attack up to 30°. With the full-span slot, the conventional ailerons alone did not give rolling control of the assumed satisfactory amount at angles of attack above 10° (maximum lift occurred at 26°), but when combined with spoilers satisfactory rolling moments were obtained with no adverse yawing moments. Large spoilers tested as the sole means of lateral control on both the wing with tip slots and that with the full-span slot gave a moder¬ ate amount of rolling control at all angles of attack, together with favorable yawing moments which were extremely large, possibly too large.
Flight tests have also been made on an airplane equipped with Handley Page slots and flaps, to deter¬ mine the effect of those devices on the lateral control, on the lilt and drag, and on take-off time and distance. (Technical Note No. 398.) The airplane used was a low-wing monoplane having full-span automatic slots divided in such a manner that they could be used as tip slots only, if desired, and tr ailing-edge flaps which were confined to the portion of the span inboard of the ailerons. A tentative analysis of the results of the tests indicates that there is an approximately lineal variation of rolling moment with lift coefficient for the unstalled portion of the range of angle of attack, the moments decreasing with increased lift coefficient. The full-span slots merely extend the unstalled range without altering the rolling moments in the range covered by the normal wing. This condition is in accordance with what would be expected from the wind-tunnel tests with Handley Page slots. With oniy the tip portions slotted, however, the rolling control is definitely decreased, indicating an increased damping in roll. A report covering this work is in preparation.
12
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
Separate airfoils as ailerons. — Wind-tunnel tests are now under way on ailerons consisting of small sym¬ metrical airfoils separated from the main wing. They are being tried in a large number of positions with respect to the main wing. In each location the aileron airfoils are deflected for control in the usual manner, and also when allowed to float.
Full-scale stability tests. — Flight tests on longitudinal stability have been carried out with a small parasol monoplane. The results of these tests showed that the dynamic stability theory, based on small deviations from the steady state, is satisfactory for the conditions encountered in flight. (Technical Report No. 442.) j In an attempt to calculate the stability of the airplane from the type of basic data that are available at the outset of a new design, it was found that very satis¬ factory results could be obtained for the power-off condition but that more data are required concerning slipstream effects for the power-on condition.
Landing. — One obvious means of increasing the safety of aircraft is to reduce the minimum flying speed and increase the angle of glide, making possible landing and taking off in smaller areas. A report has been published covering the investigation on modifica¬ tions to conventional airplanes giving nonstalling and short-landing characteristics. (Technical Report No.
4 IS.) Such landings require that the landing gear be able to absorb a greater shock than the conventional gear would withstand. Some measurements have been made of glide landings with an airplane equipped with long-stroke shock-absorber struts. Further tests will include a study of the behavior of the airplane during glide landings in gusty air. A method has been per¬ fected for an accurate detailed study of the motion of an airplane for an approach in landing. A motion- picture camera on the ground is employed to record the various items of interest in connection with the study of landings and flight close to the ground. The results of an investigation made at the request of the Bureau of Aeronautics on the relative shock-absorbing properties of rubber-cord and oleo landing gears have been published. (Technical Report No. 406.)
There have also been conducted investigations of the air conditions that an airplane must encounter in land¬ ing. For this purpose measurements have been ob¬ tained with a device that indicates the approximate magnitude and direction of the wind simultaneously at velocities from the ground up to 50 feet. These measurements have extended over a period of about six months so as to include various representative air conditions.
In connection with the general study of landings, an analysis of the directional stability while taxying (with particular reference to ground looping) has been made and a report is in preparation.
Landing Speed and Speed Range. — In an effort to provide means for obtaining lower landing speeds and
greater speed ranges, many devices have been de¬ veloped for increasing the maximum lift without exces¬ sive increase of the minimum drag. These devices include pilot planes, slots, flaps, etc., most of which have movable parts entailing a certain amount of complication. In this field, wind-tunnel investigations have been made recently on Clark Y basic airfoils with various of these devices. In addition, to avoid the obvious objection to movable parts, tests were made on fixed slots and fixed auxiliary airfoils.
Fixed slots. — Aerodyamic force tests were made in the 5-foot vertical tunnel on a Clark Y wing equipped with several forms of fixed slot (Technical Report No. 407), and with four fixed slots and a trailing-edge flap. The slots were arranged to act singly and in combina¬ tion to determine the optimum combination with the flap neutral and also with it deflected. (Technical Report No. 427.) On the basis of the maximum lift coefficient and the speed-range ratio CLmax/CDmin with the flap neutral no appreciable improvement was found with the use of more than a single slot at the leading edge. On the same basis but with the flap down 45° the optimum combination was obtained with only the two rearmost slots, the maximum lift coeffi¬ cient in that case being 2.44 as compared with 1.29 for the plain Clark Y.
Flap modifications. — An investigation was made in the 7 by 10 foot wind tunnel on a model wing with split trailing-edge flaps of three different sizes. (Technical Note No. 422.) The flaps were formed of the lower rear portion of the wing, and were rotated downward about the axes at their front edges. The axes could be moved back in even steps to the trailing edge of the main wung, giving in effect an increase in area. The split flaps when deflected about their original axis lo¬ cations gave slightly higher maximum lift coefficients than conventional trailing-edge flaps, and the lift coefficients were increased still further by moving the axes toward the rear. The highest value of CLrnax, which was obtained with the largest flap hinged at 90 per cent of the main wing chord from the leading edge, was 2.52 as compared with 1.27 for the basic wing.
Another high-lift device tested in the 7 by 10 foot tunnel was the Hall high-lift wing, which is essentially an airfoil having a split flap and also an internal air passage. (Technical Note No. 417.) The air enters the passage through an opening in the lower surface somewhat back of and parallel to the leading edge, and flows out through an opening made by deflecting the rear portions of the under surface downward as a flap. For ordinary flight conditions, the front opening and the rear flap can be closed, providing in effect a con¬ ventional airfoil. The tests were made with various flap settings and with the entrance to the passage both open and closed. The highest lift coefficient found, CL = 2.08, was obtained with the passage closed.
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
13
Wind-tunnel tests were also made on a model of the Fowler variable-area wing (Technical Note No. 419), which consists of a combination of a main wing and an extension surface, also of airfoil section. The extension surface can be entirely retracted within the lower rear portion of the main wing, or it can be moved to the rear and downward. The tests were made with the nose of the extension airfoil in various positions near the trailing edge of the main wing, and with the surface at various angular deflections. The highest lift co¬ efficient obtained was 3.17, as compared with 1.27 for the main wing alone.
Airfoils of low aspect ratio. — Because of the inter¬ esting high-lift characteristics of flat plates of very low aspect ratio, tests were made in the 7 by 10 foot wind tunnel on Clark Y airfoils having aspect ratios varying from 0.5 to 3, the force, moment, and autorotational characteristics being determined. The tests revealed a marked delay of the stall and a decided increase in the values of maximum lift coefficient and maximum re¬ sultant force coefficient for aspect ratios of the order of 1, as compared with the values for aspect ratios of 2 and 3, and higher. The results, when reduced to infinite aspect ratio by conventional formulas, indicate that such formulas are not applicable for aspect ratios of less than 1.5. The plan form and tip shape were found to be of major importance among the factors affecting airfoil characteristics at aspect ratios of 1.5 and smaller. (Technical Report No. 431.)
Fixed auxiliary airfoil.- — -Another investigation made in the vertical wind tunnel consisted of a series of tests on a Clark Y wing fitted with a narrow fixed auxiliary airfoil. (Technical Report No. 428.) The results of the wind-tunnel tests were so favorable that flight tests were also made with a similar auxiliary airfoil mounted on a small parasol monoplane. The addition of the auxiliary airfoil increased the lift capacity of the air¬ plane 45 per cent, and the drag for the high-speed condition only 4 per cent. The minimum gliding speed was decreased 19 per cent, and the minimum gliding angle was unaffected. The maximum gliding angle at low speed, however, was increased from 8.6° to 16.8°, a change that tends to facilitate landing in restricted areas. The flight tests are in good agree¬ ment with the results predicted from the wind-tunnel tests. A report of this work is in preparation.
This investigation of fixed auxiliary airfoils is now being continued in the vertical tunnel to determine the best size and section of the auxiliary airfoil when combined with a main wing of Clark Y section. The original auxiliary airfoil used in both the wind-tunnel and flight tests was a highly cambered section (N. A. C. A. 22) and had a chord about 15 per cent that of the main wing. At present, auxiliary airfoils with this section and also with a symmetrical section (N. A. C. A. 0012), and with the Clark Y section, are being tested in several different sizes each. The results
obtained so far (N. A. C. A. 22 section only) indicate that the chord of the auxiliary airfoil has little effect on the characteristics of the combination except that when the chord of the auxiliary is increased to 25 per cent of the wing chord the characteristics are slightly inferior. The optimum locations of the auxiliary air¬ foils, however, are considerably different for the differ¬ ent sizes. The effect of the auxiliary airfoil on the main wing appears to be due to the “scrubbing” action of the air over the upper surface of the main wing and the resultant suppression of the boundary layer, and not to the downwash from the auxiliary wing.
Rotating- Wing Aircraft. — One obvious disadvan¬ tage of the conventional airplane with respect to safety is the comparatively high minimum flying speed, making necessary large smooth areas for landing and taking off. The rotating-wing type of aircraft is attractive in this respect, as it makes possible the maintenance of lift with little, if any, motion of the body supported by the wings. With a view to study¬ ing the real advantages offered by the autogiro and accumulating knowledge regarding similar types, the committee has in progress investigations on three types of rotating-wing aircraft.
Glide tests have been completed and lift and drag characteristics determined on an autogiro. (Technical Report No. 434.) Preparations have been made to determine the distribution of load between the fixed and moving wings of the autogiro by means of pressure- distribution measurements in flight. The results will be of importance in providing quantitative confirmation of the present theory of the autogiro. The theory of each of the other types of rotating-wing systems men¬ tioned above has also been studied.
Spinning. — Investigations are being continued in flight and in the wind tunnel to add to the present knowledge of spinning and to make it possible to design airplanes that will spin satisfactorily and recover quickly and surely. The results of full-scale spin tests with an airplane in which the mass distribution and several items pertaining to the geometric arrangement of the airplane were varied have been published. (Technical Report No. 441.) In conjunction with the flight results, there is also given a strip-method analy¬ sis of data obtained with a wind-tunnel model of the airplane. The comparison between the predicted and actual conditions of spinning equilibrium for that air¬ plane has indicated that the method of analysis gives fairly satisfactory qualitative results. The method has consequently been applied to a study of the effect of various factors that enter into the equilibrium con¬ ditions, and a paper on the results of this study is now being prepared.
With another airplane the effects of control position and the effects of mass distribution have been investi¬ gated. Also, owing to the reports of success with
14
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
sharp leading edges in the prevention of bad spins, tests were made with the wings of the airplane modi¬ fied to form sharp leading edges. This modification resulted in a marked and probably favorable effect on the spin, but caused objectionable decreases in the maximum speed and the maximum lift coefficient. A paper on the results of the tests with the sharp leading edges is being prepared.
Several factors have recently tended to focus atten¬ tion on the yawing moments developed in spins as exerting a very important influence on the character of the spins. Analysis has indicated that the resultant yawing moment in spins is relatively small, and smoke- flow pictures obtained during the last year in steady spins demonstrate that the vertical tail surfaces are blanketed to a large extent. (Technical Note No. 421.) The tests have shown, however, that recovery is effected largely through the use of the rudder. It seems evident, therefore, that the conditions for equi¬ librium are upset by a small variation in the resultant yawing moment. Some recent tests have shown that a small change in the alignment of the very small verti¬ cal fin on the airplane used in the previous investiga¬ tion changes the character of the spin to a marked extent.
At the present time, flight tests are being made with an airplane possessing reputedly vicious spinning characteristics, the primary purpose of which is to find if possible a simple means of improving the char¬ acter of the spin of this particular airplane. The method developed for measuring in flight the motion of, and forces acting on, an airplane during a steady spin is now used regularly in all flight tests. A study is now in progress with a view to extending this method to include the measurement of the same elements during the accelerated portion of the spin, the entry, and the recovery.
The construction of a spinning balance for the 5-foot vertical tunnel has been completed. Pre¬ liminary calibration tests indicated that considerable damping was required on the balance arms, and after this was applied satisfactory operation was obtained. This balance measures all six components of the air forces and moments about the center of gravity of a model airplane when it is being turned at the partic¬ ular rate, radius, and attitude corresponding to an actual steady spin as measured in flight. Variations from the actual spin can aiso be made as desired. The first series of tests has been completed on a model of the NY-1 airplane mounted on the spinning balance. The results agree with flight tests in show¬ ing that the rudder is much more effective in giving a moment opposing the spin when the elevator is up than when it is down. In addition, the results indicate that nothing would be gained with the particular air¬ plane tested by putting the elevator below neutral in coming out of a spin, and that the elevator is most
effective in producing a diving moment when the rud¬ der is neutral. A report describing these tests is in preparation.
Structural Loading. — The rapid strides made in airplane performance in the past year and the broad¬ ened scope and increased severity of the tactical ma¬ neuvers of military and naval airplanes have pointed to the necessity of placing the structural design of air¬ planes on a sound and rational basis. Studies by the flight research section of the basic problem of the structural loads occurring on the various types of air¬ planes have been extended along several lines.
Load, factors. — Special investigations of the ap¬ plied load factors, or total loads, on military and naval airplanes during tactical maneuvers have been made and the information obtained has furnished, in large part, the basis of extensive revisions to struc¬ tural design procedure for the types of airplanes in¬ volved. Further investigations of this type are in progress, including a study of the relation between the control force and the applied load factor in pull¬ ing out of a fast dive. Statistical data have been accumulated on the applied load factors experienced by transport airplanes on several routes during all seasons of the year. Although these data are not yet considered complete, enough information has been obtained to establish fairly reliable indications of the load factors to be expected daring transport operations.
Inasmuch as these data have been obtained on sev¬ eral types of airplanes under a variety of conditions of speed and loading, the results have been reduced to corresponding “effective” gust velocities on the basis of the assumptions of sharp-edge gusts and no pitching displacement. This procedure makes possible the plotting of curves of gust probability which show the expectancy of encountering “effective” gusts of given intensity during unit flying time. It has been found from data obtained in several thousand hours of flying that effective gusts of the order of 25 to 30 feet per second occur about three times per 100 hours of flying, and that they may act either upward or down¬ ward with equal expectancy. Effective gusts of greater intensity than 30 feet per second have not yet been encountered.
A special type of instrument which records the acceleration and air speed has been devised to aid in the accumulation of statistical data on load factors for airplanes. An advantage of this instrument is that it can be left unattended in an airplane for any desired period of time, and for this period it yields, in directly usable form, the desired data. Several such instruments are nowT in service.
The use of the vertical dive as a bombing maneuver has required that special consideration be given to the applied load factors involved in pulling out of dives at terminal velocity and to means for reducing the terminal velocities. An investigation of the in-
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
15
fluence of the propeller on the diving speed has been completed, the results indicating that an adjustable- pitch propeller can be used to great advantage in controlling diving speeds for special purposes.
Load distribution. — In addition to investigations of the applied load factor or total load, studies have been continued on problems of load distribution. A report presenting a method of calculating the design load for the leading-edge portion of the wing has been pub¬ lished (Technical Report No. 413) and the method has been incorporated in Navy design speciAcations. The theoretical basis of this method has been extended and practical methods have been devised therefrom for con¬ venient application of accurately simulated aerody¬ namic loads to wing-rib tests for any desired condition of load. These methods are shortly to be incorporated in design speciAcations.
A study of the lift distribution between biplane wings has been made and a report is in process of pub¬ lication. (Technical Report No. 445.) The working charts presented in this report are based partly on theoretical considerations but largely on empirical data, and make possible the rapid determination of the lift distribution for the principal combinations of the important biplane variables.
An investigation of the load distribution on wing tips has been completed, and results have been pub¬ lished from time to time during the progress of the tests. (Technical Notes Nos. 347, 360, 379, 387, and 433.) A comparison of the results on all tips tested, which range from slender elliptical to square in plan form, indicates that the distribution of lift coefficient and moment coefficient along the span is independent of the plan form. It is believed that these results will enable a simple and exact method to be devised for applying wing-tip loads to any particular design.
Further work on structural loading is in progress, the most important of which is a complete series of meas¬ urements of the total load, load distribution, structural deformations, and stresses on a Navy diving bomber. This work is of a highly specialized character, but should provide some information of general interest regarding the true interrelations of structural loads, stresses, and deformations as compared with calculated relations from present design methods.
Tail loads. — Studies of tail loads have been con¬ tinued on an Army observation-type airplane and several new aspects of the problem have come to light. In conjunction with the study of tail loads, some tests have been made in the variable-density wind tunnel to determine the infiuence of airfoil thickness and camber and slight amounts of decalage such as might inadvert¬ ently be present on the moment coefficient of biplane cellules of varying stagger. The results indicate a negligible infiuence of the airfoil thickness and camber, but show that a slight amount of accidental decalage, particularly at the large staggers, infiuences the pitch¬
ing moment of the cellule, and hence the tail load, so greatly that a precise predetermination of the tail load for such cases is probably not possible.
Prevention of Ice Formation. — The study of a method of prevention of ice formation on aircraft was completed and reported. (Technical Report No. 403.) This report shows that there is ample heat in the exhaust of an airplane engine to prevent ice from form¬ ing on the wings of the airplane. The report presents information for the design of such equipment and outlines the results obtained with one device for utiliz¬ ing heat from the engine exhaust in heating the leading edge of an airplane wing. The study of the formation of ice on gasoline-tank vents has been reported (Tech¬ nical Note No. 394), and shows that with a vent tube of suitable size pointing downstream the danger of stoppage by ice formation is eliminated.
Aerodynamic Interference and Drag. — The mutual aerodynamic effects of the component parts of aircraft have become increasingly important as the parts have been refined and as flying speeds have been increased. For example, the arrangement of a pro¬ jecting gasoline filler cap was of relatively little impor¬ tance when it projected from a fuselage of crude form over which the flow of air had been thoroughly dis¬ turbed by an uncowled air-cooled engine. The tur¬ bulence set up by the same object and the drag associated with it demand consideration, however, when it projects from a modern fuselage of fine aero¬ dynamic form, particularly if the engine in front is fitted with a cowling. To insure continued increases in the speed and efficiency of aircraft, it becomes necessary to give more consideration to the subject of aerodynamic interference.
The committee has in progress in the variable- density wind tunnel a series of investigations dealing with the aerod37namic interference between simple bodies including fuselage-wing combinations. A new piece of equipment, known as a smoke tunnel, has been added during the past year to assist in the interference investigations. It permits the flow of air about inter¬ fering bodies to be observed directly and photographed. Investigation of the interference effects between engine nacelles and wings arranged in different ways, including the effects of the propeller slipstream, has been con¬ tinued throughout the year in the propeller-research tunnel. The arrangements investigated include vari¬ ously cowled nacelles with tractor propellers in com¬ bination with monoplane wings and biplanes; tandem propellers with monoplane wings; and pusher propeller nacelles with monoplane wings.
Interference of small protuberances. — The interfer¬ ence investigations that have been carried out in the variable-density tunnel during the past year have dealt largely with the interference and drag of small objects protruding from the surfaces of bodies. An examination of present-day airplanes, both military
16
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
and commercial, indicates that a considerable part of their drag arises from small projecting objects such as fittings, tubes, wires, rivet heads, lap joints, butt straps, filler caps, and many other objects projecting from the main surfaces that may be classed together as protuberances. A systematic investigation of protuberances differently formed and variously lo¬ cated on both streamline bodies of revolution and on airfoils was therefore undertaken.
The additional drag resulting from a small flat plate protruding at various positions on the surface of a streamline body of revolution was measured over a wide range of values of Reynolds Number. The re¬ sults showed that the variation of the drag with the po¬ sition of the protuberance could be entirely accounted for by considering the velocity variations near the bare hull at the protuberance positions as determined from boundary-layer surveys made in the propeller- research tunnel on a 1 /40-scale model of the U. S. airship Akron. Protuberances of streamline form faired into the body surface were shown to cause only slight increases in drag at large values of Reynolds Number. A report is in preparation giving the re¬ sults of this investigation.
The effects on the aerodynamic characteristics of an airfoil of variations of the position of a protuber¬ ance, and in the height and shape of the protuberance extending along the entire span of the airfoil, were measured. The results indicate that the forward por¬ tion of the upper surface of the airfoil is most sensi¬ tive to the addition of protuberances. (Technical Report No. 446.) The drag of any bluff object pro¬ truding from the airfoil surface should, however, in no case be considered negligible, inasmuch as rec¬ tangular protuberances from any part of the airfoil surface caused drag increases of the order of the prod¬ uct of the protuberance frontal area and the dynamic pressure. Rectangular protuberances having a height exceeding one-half of 1 per cent of the airfoil chord produced marked interference effects when located on the forward portion of the upper surface. Such pro¬ tuberances should, of course, be particularly avoided unless an increased drag is sought.
The characteristics of wings as affected by pro¬ tuberances of short span were also investigated. These protuberances were of one height and located at one position on the airfoil profile but of various lengths and variously distributed along the span.
The results indicate that the central sections of a rectangular wing are more sensitive to the addition of protuberances than the outer sections and that a very short protuberance in the midspan position may cause a disproportionately large reduction in maximum lift. Consideration is given in the analysis of the results to induced interference effects that may cause disproportionately large drag increases at the higher lift coefficients. At lift coefficients corresponding to
high-speed flight the drag due to the protuberances is shown to increase approximately as the total length of the protuberances. The adverse effects are shown to be greatly reduced by the application of simple fairings ovrr the protuberances. A technical report on this work is in process of publication. The investigation of interference is being continued in the variable- density tunnel to include the effects of a wing in vari¬ ous positions with reference to the fuselage.
Wing-nacelle-propeller effects. — The results from the investigation of the relative merits of various arrange¬ ments of nacelles with tractor propellers in the pro¬ peller-research tunnel have been published during the year. (Technical Reports Nos. 415 and 436.) The effects of idling and locked propellers and of nacelle cowlings on the landing speed of tractor mono¬ planes have also been studied in connection with this investigation, and the results published. (Technical Note No. 420.)
An investigation of the net efficiency of a nacelle with tractor propeller located in various positions with reference to the wings of a biplane indicated that if the nacelle were placed in the leading edge of the upper wing the net efficiency was practically the same as in the case of the monoplane wing, and when located in the leading edge of the lower wing the results were only slightly inferior. Positions other than in the leading edge of one of the wings proved to be decidedly inferior and a position between the wings, commonly used in practice, gave a net efficiency of 66 per cent, as compared with 75 per cent for the optimum position.
An investigation of tandem arrangements of en¬ gines in connection with a monoplane wing revealed that the tandem type in all cases gave lower net efficiencies than the tractor type, and that the high¬ est efficiency was obtained when the nacelles were located below the wing with the forward propeller some distance back of the leading edge. The tan¬ dem arrangement in its best position gave a net effi¬ ciency of 68 per cent as compared with 75 per cent for the tractor. Increasing the distance between the tandem propellers had a negligible effect on the net efficiency. A report covering the results of this investigation is now in preparation.
A third series of tests on a nacelle with pusher propeller on a monoplane wing is now in progress. With the completion of the present investigation it is thought that the question of wing-nacelle-propeller interference will have been answered in a very com¬ prehensive manner. The large scale ol the models (4-foot propellers) and the careful reproduction of the actual airplane parts are particularly noteworthy. The suggestions of manufacturers have been of great help in the preparation of the research programs with a view to practical application of the results.
Airships. — At the request of the Bureau of Aero¬ nautics, Navy Department, tests were made in the
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
17
propeller-research tunnel on a 1 /40-scale model of the U. S. airship Akron , and various measurements were taken on the airship itself during the performance trials. The wind-tunnel tests included not only the lift, drag, and moment measurements, but also the elevator hinge moments (Technical Report No. 432) and pressure-distribution measurements over the en¬ tire hull and control surfaces (Technical Report No. 443). A survey of the boundary layer over the hull was also made, and theoretical computations based on the data thus obtained show very good agreement with force measurements of the drag. (Technical Report No. 430.) Predictions from the results of the wind-tunnel measurements of the hinge moments of the tail surfaces were borne out in the flight tests of the full-scale airship.
The flight tests included pressure-distribution meas¬ urements, deceleration tests, turning trials, and the rates of ascent and descent, and were made at the request of the Navy Department and in cooperation with the Goodyear-Zeppelin Corporation. The results of the pressure-distribution measurements and other data have been evaluated and forwarded to the Bureau of Aeronautics. The drag coefficient for the airship deduced from deceleration tests was found to be slightly less than any previously obtained. The lon¬ gitudinal distribution of pressure along the hull at zero pitch in steady flight was found to be in fair agreement with the measurements made on the model in the propeller-research tunnel. The results also agree fairly well with the theoretical distribution for the hull without the fins.
Maneuverability. — An investigation of maneu¬ verability of airplanes, requested by the Bureau of Aeronautics, Navy Department, has been continued. The purpose of this investigation is to study the fac¬ tors that influence maneuverability and to develon a satisfactory criterion of maneuverability. The quan¬ titative data previously obtained have been aug¬ mented by the completion of tests with a Navy observation-type airplane. During these tests the pilot performed various selected maneuvers designed to show the separate and combined effects of the factors that contribute to maneuverability. A report on these tests is now being prepared.
Propellers. — A large number of model tests of propellers have been made in connection with the wing-nacelle-propeller investigation, a 4-foot propeller being used. The results of these tests are included in the various reports on the efficiency of the various wing-nacelle-propeller combinations. In some cases the measurements have been extended to low pitch
settings and to values of
n D
bevond zero thrust and
power to obtain data on negative thrust character¬ istics, which are of some importance in estimating the landing speeds of airplanes and the speeds of
airplanes in dives. As mentioned in the discussion of structural loading, it has been found possible to reduce the speeds of airplanes in vertical dives by adjusting the propeller pitch. The results of a num¬ ber of tests have been assembled in a report now in preparation, which will provide data on metal pro¬ pellers at negative thrust analagous to those of Durand and Lesley on wooden propellers published some years ago.
During an investigation of propeller efficiencies at various tip speeds, data were obtained on the direc¬ tion and velocity of the air leaving the propeller. From these data the distribution of thrust and torque along the blade was computed, and a report giving the results has been published. (Technical Report No. 421.)
Airfoils — Tests in the variable-density tunnel. — The investigation of a wide variety of related airfoils in the variable-density wind tunnel has been completed, and the results published in preliminary form as Technical Notes Nos. 385, 391, 392, 401, and 404. Meanwhile, all the data have been assembled and the results corre¬ lated to show the trend of aerodynamic characteristics with changes of airfoil shape. This information will be made available in a technical report now in prepara¬ tion.
The main investigation has been extended to include studies of the effects of certain additional changes in the airfoil shape. One series of tests was made to de¬ termine the relation between nose shape and the shape of the lift curve at maximum lift. For this purpose, five airfoils were developed from the N. A. C. A. 2412 section by reducing the thickness of the airfoil forward of the maximum-thickness station. A technical note is in preparation giving these results. Another series of tests dealt with the effect of major changes of shape aft of the maximum ordinate. The N. A. C. A. 2412 section was again used as the basic airfoil and the trail¬ ing edge was upturned various amounts to produce a stable center of pressure. The results will be incor¬ porated in the report on the main investigation men¬ tioned above.
Several other airfoil studies have been conducted in the variable- density tunnel. Two airfoils of Gottingen 398 section modified by the addition of sharp leading edges were tested in connection with a study of the spinning characteristics of the wings, as described more in detail in the discussion of spinning. The results were published as Technical Note No. 416. Another investigation was made to determine the effects of fabric sag on the aerodynamic characteristics of airplane wings. Two airfoils simulating wings with sagged fabric between the ribs were tested and the character¬ istics compared with those of the wing without sag. It was concluded that the usual sagging of the wing covering has for practical purposes a negligible effect on the aerodynamic characteristics of the wing. (Tech-
18
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
nical Note No. 428.) The effect of surface texture on airfoil characteristics has also been further investigated during the year. These data are now being analyzed for publication. They indicate that varying surface conditions have marked effects, and that certain of these effects are not likely to be predicted from low- scale tests.
Several routine tests of nonrelated airfoils have been made at the request of the Army and Navy to provide data for immediate application to practical problems. Tests of a series of well-known airfoils have been car¬ ried out through the range of angles of attack from negative maximum lift to positive maximum lift, to provide data needed in the structural design of certain types of airplanes. The results have been published. (Technical Notes Nos. 397 and 412.)
Tests in high-speed wind tunnel. — The results of pre¬ liminary investigations in the high-speed wind tunnel have indicated that airfoils of 2-inch chord may be satisfactorily tested and may be considered as of infi¬ nite aspect ratio. The airfoils are built of steel for adequate stiffness and the tests are carried through air velocities up to 96 per cent of the velocity of sound. Tests of a series of six commonly used propeller sec¬ tions are now in progress, and the results thus far have indicated large detrimental effects at speeds well below the velocity of sound. A technical report giving the description of the tunnel and an account of this work is in preparation. One result of this investigation that may find immediate application outside the field of propeller design is the indication of an important com¬ pressibility effect on the airfoil pitching moment. Pitching moments considerably greater than those for which an airplane is designed may act on the wings in a dive of high velocity. A related series of symmetri¬ cal airfoils having systematic changes in their geo¬ metrical characteristics is now under construction as part of a more comprehensive study of the compressi¬ bility phenomena, to make possible the correlation of aerodynamic characteristics with changes of airfoil shape.
Theory oj wing sections. — A report has been prepared (Technical Report No. 411) on the theory of wing sections of arbitrary shape. This work is being con¬ tinued and a report is now being prepared which pre¬ sents an exact treatment of the problem of determining the 2-dimensional potential flow around the wing sec¬ tions of any shape. The problem is condensed into the compact form of an integral equation capable of yielding numerical solutions by a direct process. An attempt has also been made in this paper to analyze and coordinate the results of earlier studies relating to properties of wing sections. The existing approximate theory of thin wing sections and the Joukowsky theory with its numerous generalizations are reduced to special cases of the general theory of arbitrary sections, per¬
mitting a clearer perspective of the entire field. The method not only permits the determination of the ve¬ locity at any point of an arbitrary section and the asso¬ ciated lift and moments, but furnishes also a scheme for developing new shapes of predetermined aerody¬ namical properties. The theory applies also to bodies that are not airfoils, and is of importance in other branches of physics involving potential theory.
Wind-Tunnel Wall Interference. — A theoreti¬ cal study of the wind-tunnel wall interference has been reported in Technical Report No. 410. This work has been carried on further to check experimentally this theoretical study by tests in the model of the full-scale tunnel, and the results of this work are found to be of considerable interest to wind-tunnel investigators. Not only was the maximum lift found to depend on j the nearness to the adjacent wall, but it was also dis¬ covered that in some cases the true tunnel axis did not remain entirely fixed. A report on this work is in progress.
Sources of Noise in Aircraft. — During the past year the laboratory has continued its investigation of the sources of noise in aircraft. One phase of the work has been the study of the propeller noise, the research being almost exclusively concentrated on problems of a fundamental nature. Efforts are being made to create a satisfactory and consistent theory of propeller sound f capable of explaining the real origin and nature of the very complex disturbances produced by a revolving propeller. In this study the new type of sound ana¬ lyzer developed last year is being used.
Another phase of the research on sources of noise has been the study of the vibration of aircraft struc¬ tures. In this work an airplane was suspended in a floating position and excited by a sinusoidal force ap¬ plied in the plane of the cylinder bank. Test results indicated that the airplane wing system possessed three critical frequencies in which the response became ex¬ cessively large. Two of these frequencies were within i the operating range of the engine speed. Flight tests confirmed the fact that an unusually large vibration amplitude existed. The objects of this research are to accumulate knowledge regarding the essential causes of vibration and noise, to devise efficient methods by which the manufacturer may detect the nature of the difficulty, and to determine means of prevention and elimination.
Work in the Full-Scale Wind Tunnel. — A num¬ ber of investigations have been made in the full-scale wind tunnel during the year. The aerodynamic char¬ acteristics of four different types of airplanes were in¬ vestigated and compared with the results obtained in flight tests. Satisfactory agreement was obtained, a correction factor for the jet boundary previously de¬ termined from tests of a series of airfoils in a model of the full-scale tunnel being used. Several other air-
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
19
planes were tested at the request of the Army or Navy to answer specific questions of design. Tests of a series of airfoils are in progress at the present time.
Several projects may be mentioned as showing the usefulness of the full-scale wind tunnel in answering questions of practical design that have arisen in con¬ nection with the work of the military and naval serv¬ ices. At the request of the Army Air Corps the com¬ mittee investigated the effect on the take-off charac¬ teristics of wheel wells in the wing of a Lockheed Altair airplane. The airplane was installed in the full-scale tunnel and the lift and drag measured with the landing gear retracted and extended and with the wheel wells both open and closed when the wheels were extended. The cockpit was covered on one of the tests to ascertain whether the open cockpit affected the drag appreciably. It was found that neither the open wheel wells nor the open cockpit had an appreciable influence on the drag or take-off characteristics.
In connection with the design of a large flying boat (the P3M-1), the committee was requested by the Bureau of Aeronautics, Navy Department, to investi¬ gate the cooling characteristics of the engines located in nacelles between the biplane wings and to recom¬ mend, if possible, means for reducing the drag of the nacelle. For these tests a complete engine assembly with a stub wing of 14-foot span was furnished. By an alteration of the tail of the nacelle and the choice of a suitable engine cowling, the drag of the nacelle was appreciably reduced and at the same time satisfactory cooling was obtained.
The Bureau of Aeronautics, Navy Department, also requested an investigation on a Fairchild F-22 air¬ plane to determine the comparative advantages of two wing sections, the N-22 and the N. A. C. A. 2412. It was found that there was practically no difference be¬ tween the two sections, the principal difference being that the N. A. C. A. 2412 section showed a sharper break in the lift curve at the stall than the N-22.
At the request of the Department of Justice, an in¬ vestigation was made to test the validity of the claims of Mr. Esnault Pelterie in connection with his suit for infringement of patents on the stick control for air¬ planes. A Vought Corsair airplane was fitted with the ring and bird type tail surfaces and measurements made to determine the resulting pitching and yawing moments for each.
As a final determination of the jet-boundary correc¬ tions for this tunnel, a series of airfoils of the same profile and aspect ratio but of different spans is being tested at air speeds chosen to give the same Reynolds Number for all. The airfoils are of Clark Y section and have spans of 12, 24, 36, and 48 feet. To obtain data on scale effect the tests are being extended to cover a range of Reynolds Numbers from 350,000 to 5,600,000. A marked scale effect on maximum lift is indicated thus far by the tests.
The optimum position of a wing with reference to a fuselage has never been determined for high Reynolds Numbers. At the request of the Army Air Corps, an investigation of this question has been started. A YO-31A airplane with gull wing has been chosen for the investigation. The airplane has been tested, and a mock-up fuselage of the same airplane is under con¬ struction to permit testing with the wings in different positions. For some of the tests the gull-wing roots are replaced by a straight center section, permitting the position of the wing to be changed. The program includes also a thick cantilever wing at the bottom of the fuselage. In addition to the force measurements, pressure-distribution measurements will be made on the wings for each of the various wing positions and also a survey will be made to determine the direction and velocity of the air at all points about the tail surfaces.
Seaplanes. — The N. A. C. A. tank, or seaplane channel, for the study of hydrodynamic problems con¬ nected with aeronautics is now in regular operation. The program for the tank includes several items of direct interest to the aircraft industry, a number of investigations specifically requested by the Bureau of Aeronautics, Navy Department, and also certain items which may be classed as fundamental information.
Effects oj variations in dimensions avid jorm of hull on the take-off of flying boats. — The forms given to the hulls of flying boats differ radically according to the designer and the country. It seems hardly possible that all of the five distinct types, which can be easily recognized, can be of equal merit. The determination of the effect of the form and the dimensions of the hull on the take-off of a flying boat is therefore of immediate interest. In this problem are included not only the resistance to propulsion along the water, but also the attitude assumed and the spray thrown.
The effects of variations in dimensions are being studied by tests of a series of models derived from a common form by systematic variations in dimensions. The effects of variations in fundamental form are being studied by tests of models representing several of the distinct types.
The nature and amount of spray thrown out during the take-off is of special interest. If it is heavj^ and is thrown into the propellers these may be damaged. If it strikes the wings or other surfaces damage may be done or resistance increased. One of the simpler methods of reducing the spray and changing its direc¬ tion is to fit narrow strips along the side of the hull, usually projecting from the chine at an angle, which will deflect the rising sheets of water.
A model of a hull which was known to have good performance was selected, and to it were applied suc¬ cessively spray strips having various angles and various widths. Tests of these showed that the wider strips at sharper downward angles gave the better suppression
20
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
of the spray and that the resistance was affected but little. A technical note describing these tests is being prepared.
Floats for seaplanes. — At the request of the Bureau of Aeronautics, the committee is investigating the manner in which variations in the form and dimensions of the twin floats used in high-speed seaplanes affect the performance during take-off. This investigation is conducted with model floats which are varied in dimensions and form much as in the case of the flying- boat hulls.
Fundamental information regarding planing sur¬ faces. — For a large part of the take-off run of a seaplane that part of the weight of the craft not supported by the wings is supported by the hydrodynamic reaction of the water on the bottom of the float or boat. In the actual craft, or complete model, the various phe¬ nomena connected with this stage are so entangled that the effects from the bottom alone can not be studied separately. However, by testing surfaces that skim along the top of the water simulating only the bottom of a float, much valuable fundamental information can be obtained, and by its use it will be possible to understand more perfectly what happens under the bottoms of actual floats.
Surfaces for such tests have been procured and tests are planned to begin shortly.
Frictional resistance of boat surfaces. — The frictional resistance of those surfaces of a boat hull which are exposed to the passing water has been determined quite accurately for speeds up to about 15 miles per hour. For speeds of from 30 to 60 miles per hour, or higher, the frictional resistance can only be estimated. Furthermore, the effect on this resistance of slight structural roughnesses produced by rivet heads, plate butts, etc., is practically unknown.
Friction plates which are to be towed at relatively high speeds to determine the frictional resistance and the effect of roughnesses have been procured and are being prepared for testing. The information obtained from these tests should make possible a decision as to how smooth a boat bottom need be or how large an increase in resistance must be accepted if rivet heads project. It should also help in the more accurate estimation of the resistance of model and full-size hulls.
BUREAU OF STANDARDS
Wind-Tunnel Investigations — The aerodynamic activities of the Bureau of Standards have been con¬ ducted as heretofore in cooperation with the Aero¬ nautics Branch of the Department of Commerce and the National Advisory Committee for Aeronautics. The work completed and in progress may be summar¬ ized under the following heads :
Improved apparatus for measuring turbulence.— A report has been submitted for publication describing recent improvements in the design of the equipment
associated with the hot-wire anemometer for the measurement of fluctuating air speeds in turbulent air flow. The design of amplifiers and compensating circuits is discussed in some detail and a description is given of attempts to measure the distribution of the energy of the fluctuations with respect to frequency. With the improved equipment, fluctuations of air speed with frequencies up to 5,000 cycles per second may be studied.
Increasing the aerodynamic efficiency of jets. — A report has been completed on the studies of the effects of deflectors, guide vanes, ring-shape airfoils, and Venturi tubes on the thrust reaction of free jets. The augmentation of the thrust obtained was not sufficient to make jet-propulsion practical at ordinary airplane speeds.
Computation of boundary-layer flow. — The air flow in the boundary layer of a plate subjected to sinusoidal variations of pressure along the length of the plate has been computed by an approximate method for the case of laminar flow. If one imagines the sinusoidal pres¬ sure variation to move slowly along the plate, the computed fluctuation of the air speed at a fixed point varies with the distance of the point from the plate in a manner similar to that measured experimentally by the hot-wire anemometer. The mean velocity is nearly the same as it would be if there were no pressure fluctuations present. It has thus been concluded that the large speed fluctuations in that part of the boundary layer which is inferred to be laminar from the character of the distribution of mean speed are forced by the turbulence of the external stream. The amplitude of the fluctuations within the layer is much greater than the amplitude in the external stream.
Aerodynamic characteristics of conventional control surfaces. — Measurements of the yawing moments due to rudders of several spans and chords have been described in Technical Report No. 437, “The Effect of Area and Aspect Ratio on the Yawing Moments of Rudders at Large Angles of Pitch on Three Fuselages."
Two short papers have been prepared dealing with ailerons, in which questions raised in the discussion of Technical Reports Nos. 298, 343, and 370 have been answered. These papers deal with the mutual inter¬ ference between ailerons on opposite wings and the effect of aileron displacement on wing characteristics.
Reduction of Airplane Noise. — Measurements have been completed on the reduction of noise and of engine horsepower due to several mufflers intended for use on airplane engines. It was concluded that it is not profitable to use mufflers unless the propeller noise is less than the noise of the unmuffled engine, which means in general that the propeller tip speed must be as low as 650 to 700 feet per second; that the use of a manifold system in itself gives a considerable muffling effect; that it is possible to reduce the noise of the unsilenced engine (open-port condition) in the absence
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
21
of the propeller by 15 to 20 decibels by means of a muffler with an added weight of 30 or 40 pounds and a reduction in engine horsepower of less than 3 per cent. The detailed results will be published in an aeronautics bulletin of the Aeronautics Branch of the Department of Commerce.
Aeronautic-Instrument Investigations. — The work on aeronautic instruments was conducted in cooperation with the National Advisory Committee for Aeronautics and the Bureau of Aeronautics of the Navy Department and included the investigations and the development of particular instruments outlined below:
Temperature coefficient of elastic moduli. — The experi¬ mental work has been completed on the determination of the temperature coefficient of the elastic moduli of elastic materials for aircraft instruments. The tem¬ perature coefficient was measured in the temperature range —50° to +50° C. and data were obtained on the effect of heat treatment, strain hardening, and external stress. It was found that, except for tungsten, the temperature coefficients of two elastic moduli are practically the same, and are independent of external stress within the range applied. The coefficients generally increase with increase in temperature and vary considerably with the heat treatment or cold work of a given material. A report has been prepared on the investigation, and is now being edited.
Reports on aircraft instruments. — -A report on current methods of measuring air speed and ground speed, emphasizing descriptions of, and performance data on, the instruments, was completed and has been pub¬ lished as Technical Report No. 420. A second draft of a report on power plant instruments was practically completed. Considerable progress was made on re¬ ports similar in scope on altitude instruments and on compasses.
Friction of pivots. — Attention was given to improving the apparatus for making the measurements, modifi¬ cation of which has been completed.
Airship thermometers. — Four electrical resistance thermometers of the type previously developed for use in the flight testing of airplanes were constructed for use on airships in measuring the free-air temperature. The instruments are of the unbalanced Wheatstone- bridge tj^pe. The temperature-sensitive resistance element is mounted on the airship so as to be in the free air, and the indicator and other parts are installed in the control car. This type of instrument has the advantage that it can be built to have a much smaller time lag in indication than liquid-in-glass thermome¬ ters.
Strut thermometer. — Distant-indicating thermome¬ ters now available are far from satisfactory for service use in airplanes to measure free-air temperature. An experimental thermometer of the bimetal type, which was designed to be mounted as a unit on a strut of the airplane, was constructed. The thermometer indicates
on a dial sufficiently large so as to be easily read from the cockpit.
WASHINGTON NAVY YARD
The wind-tunnel equipment at the Washington Navy lard consists of an 8 by 8 foot closed-circuit type, built in 1913, and a new 6-foot closed-circuit type, recently completed. The 6-foot tunnel has been con¬ tinuously employed on current design and related problems during the past year. The early completion of the balance for the 6-foot tunnel will enable such testing to be made under more favorable conditions. Testing to date in the 6-foot tunnel has been confined to air-flow and simple drag measurements. Some vibration difficulties due to the type of construction were found and eliminated in the preliminary tests.
The Washington Navy Yard tests are concerned largely with complete model airplanes, the routine tests covering the measurement of lift, drag, pitching moments, and yawing moments, and the moments being obtained for a series of control positions. Rolling moments are not considered a part of routine tests, although measured on unconventional designs. The nonroutine tests for the greater part consist in studies of fairing and slight changes for drag reduction and in tests for improvement of control and stability.
In accordance with established policy the wind- tunnel work of the Washington Navy Yard is confined to problems relating directly to the design of naval aircraft. All research investigations of a fundamental aerodynamic nature are requested from the National Advisory Committee for Aeronautics.
MATERIEL DIVISION, ARMY AIR CORPS
The present wind-tunnel equipment at Wright Field consists of two wind tunnels, a 5-foot tunnel, and a high-speed tunnel 14 inches in diameter, both of the N. P. L. type.
The wrork in the 5-foot wind tunnel is concerned largely with design problems and routine tests on air¬ plane models of designs submitted to the Air Corps. Models of the XP-936, XP-900 XP-25, and YO-31 airplanes were tested for lift, drag, and stability.
Special vnnd-tunnel, tests. — Tests were made to de¬ termine the aerodynamic effects of various changes in fairings, cowlings, wing shapes, tail shapes, cockpit enclosures, and landing-gear designs. Airplane acces¬ sories such as Venturis, impellers, radiators, and tail wheels were tested also.
Wing location and root shape. — A study based on wind-tunnel tests was made of the effects of the vertical location of a monoplane wing. Several methods of attaching and fairing the wung into the fuselage were employed.
Tail design.— It has been necessary to make a large number of wTind-tunnel tests of models wfith modified tail assemblies. The necessity for changes in tail de¬ sign has usually arisen from changes wfflich have been
22
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
made in the cockpit enclosures, engine nacelles, or wing center sections.
Performance estimates. — Estimates of the perform¬ ance of many of the existing and proposed airplanes with alternative power-plant installations were made. Consideration was given to engines under development as well as to standard types.
Autogiro. — An autogiro wTas tested in flight and the data thus obtained formed the basis for a study wffiich was made to determine the practicability of adapting such a machine to the military service.
Controllable propellers. — Considerable progress has been made in the development of mechanisms for con¬ trolling the pitch setting of propellers while in flight.
REPORT OF COMMITTEE ON POWER PLANTS FOR
AIRCRAFT
ORGANIZATION
The present organization of the committee on power plants for aircraft is as follows: N
Hon. William P. MacCracken, jr., chairman.
George W. Lewis, National Advisory Committee for Aeronautics, vice chairman.
Henry M, Crane, Society of Automotive Engineers.
Prof. Harvey N. Davis, Stevens Institute of Technology.
Dr. H. C. Dickinson, Bureau of Standards.
Carlton Kemper, National Advisory Committee for Aeronautics.
Capt. Clements McMullen, United States Army, materiel division, Air Corps, Wright Field.
Commander C. A. Pownall, United States Navy.
Prof. C. Fayette Taylor, Massachusetts Institute of Technology.
LANGLEY MEMORIAL AERONAUTICAL LABORATORY
Compression-Ignition Engines. — The continued improvement in the safety and performance of air¬ craft is dependent upon a continued increase in the reliability, power output, efficiency, and safety of aircraft power plants. The low specific fuel consump¬ tion obtained with the compression-ignition engine for a wide range of propeller loads and the reduced fire hazard when Diesel fuel oil is used are the two out¬ standing advantages of this type of engine which warrant its continued development as a power plant for aircraft. These advantages are of such, importance for commercial rigid airships that compression- ignition engines are already being used as power plants for airships in spite of their decreased power output per cubic inch of displacement, which makes them unsuitable at present as power plants for airplanes. The researches of the committee on the compression- ignition engine have been directed toward increasing the specific power output of this type of engine by the use of improved fuels and by the accumulation of new
knowledge concerning the injection and combustion characteristics of fuel sprays.
Fuel spray characteristics. — The design of suitable combustion chambers for compression-ignition engines is influenced by the distribution of fuel sprays. Re¬ search has been conducted to determine the relative distribution of the fuel throughout the spray during injection. The methods employed at the start of this investigation consisted of injecting the fuel against targets of Plasticine and of directing normal to the fuel spray a jet of air of small diameter having an extremely high velocity. The targets showed that the greater part of the fuel in the spray was concen¬ trated in the spray core. The spray core could not be disrupted by the air flow unless the velocity of the fuel jet was considerably less than that usually employed in compression-ignition engines. Further tests in which fuel sprays were directed counter to each other showed that the core was discontinuous, since the two sprays would apparently penetrate through each other with little interference if the point of impingement was at least 1 inch from the nozzle.
The process by which a solid jet of fuel issuing from an orifice is disintegrated to form a fuel spray has been determined from photomicrographs, made at a magni¬ fication of 10 diameters, of various sections of fuel sprays during injection. The photomicrographs showed clearly the progress of the jet disintegration under varying conditions of jet velocity, air density, fuel viscosity, and discharge-orifice diameter. A comparison of the results obtained with fuel sprays injected into a vacuum with those of sprays injected into dense air showed that, although the turbulence of the fuel flow assisted the disintegration of the fuel jet, the greater effect was obtained from the interaction of the liquid fuel with the air into which the fuel was sprayed. The results showed that the rate of jet disintegration could be increased by increasing the relative velocity between the fuel and the air, by increasing the air density, or by decreasing the fuel viscosity and the orifice diameter. The results of the research on spray distribution have been published as Technical Report No. 438.
The research on the hydraulics of fuel-pump in¬ jection systems has been extended to include the determination of the rates of fuel injection when a cam-operated fuel-injection pump and an automatic . injection valve are used. The data showed that the rate of fuel discharge from a given pump injection system could be varied considerably by variations in the length and diameter of the injection tube, the in¬ jection valve-opening pressure, and the discharge- ! orifice diameter. The data also showed that in the design of a fuel-injection system, the system from the pump to the discharge orifice should be considered a unit. The results of this investigation are described in Technical Report No. 433.
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
23
Combustion in compression-ignition engines. — The necessary improvement in the performance of com¬ pression-ignition engines that will warrant their use in airplanes is dependent upon increased knowledge concerning the various stages in the combustion proc¬ ess. Outstanding progress has been made with the N. A. C. A. fuel-spray combustion apparatus in deter¬ mining the rates at which present fuels vaporize under conditions of air density, temperature, and air flow comparable to those occurring in the engines during the injection period. A description of the apparatus and the results of preliminary tests have been published in Technical Report No. 429.
An investigation to determine the effect of air flow velocities from 0 to 800 feet per second when di¬ rected counter to and normal to fuel sprays has been made with the combustion apparatus. The tests showed that although the air flow has considerable effect on the spray envelope and on the whole spray after injection is completed, the main core of the spray is only slightly deflected during injection.
A series of five fuels covering a range of boiling points as measured under atmospheric conditions from 150° to 375° F. has been used in investigating the effect of engine speed, temperature of the combustion- chamber walls, injection advance angle, and fuel quantity on the vaporization of fuel sprays. During injection the fuel vaporized at a rate that far exceeded the rates at which the fuel had previously been thought to vaporize. The results showed that the vaporiza¬ tion was materially affected b}7 the boiling tempera¬ ture of the fuel and by the temperature of the combus¬ tion-chamber and cylinder walls. The flame records showed that unless combustion by auto-ignition was forced to take place before all of the fuel was injected, the burning started almost simultaneously throughout the combustion chamber and was accompanied by violent combustion shock. The diffusion of the fuel vapors was found to be more rapid and uniform than the diffusion of the atomized fuel spray. Technical Report No. 435 has been published giving the results of this investigation.
The increasing interest in the development of fuels for high-speed compression-ignition engines and the differences in the combustion characteristics of several fuels indicated that it would be desirable to have a method of determining by simple engine tests the suita¬ bility of a fuel for the operating conditions of an indi¬ vidual engine, and which one of several fuels is most suitable. Tests made with an engine having no effec¬ tive air flow in the combustion chamber were found to be of more value in rating fuels than those made with combustion chambers having effective air flow.
The engine tests led to the conclusion that a fuel that will allow injection to be advanced beyond the op¬ timum injection advance angle without encountering excessive knock or excessive cylinder pressures is a 149900—33 - 3
satisfactory fuel for that particular engine and its operating conditions. A comparative rating of several fuels may be obtained by determining the consistent operating range of the injection advance angle. This range is directly dependent upon the ignitibility of the fuel, but involves no question of the method of measuring ignition lag. Fuels with longer consistent operating ranges are more satisfactory for engine oper¬ ation. The results of these tests have been published in Technical Note No. 418.
The reduction of combustion shock in compression- ignition engines depends to a great extent on the reduc¬ tion of the time interval between the injection of fuel into the engine cylinder and the start of combustion. During this time interval the fuel absorbs sufficient heat from the high-temperature air to raise it to its auto-ignition temperature. The addition of heat to the fuel before its injection into the combustion cham¬ ber should reduce this time interval. Tests have been made to determine the effect on ignition lag and com¬ bustion of preheating Diesel fuel oil before injecting it into the combustion chamber. As the addition of heat to the fuel results in decreased spray penetration, the tests were conducted with engines having precombus¬ tion and quiescent-combustion chamber shapes. The results showed that preheating the fuel to 330° F. reduced the ignition lag and combustion shock. The greater reduction in ignition lag was obtained with the combustion-chamber shape having high-velocitj7 air flow.
The process of combustion in the engine cylinder can be followed by a chemical analysis of the gases taken from the engine cylinder. A stroboscopic valve has been designed and constructed so that samples of the charge in the combustion chamber may be removed for chemical analysis at any point in the cycle. This gas-sampling valve is inertia-operated and the mecha¬ nism is driven through a flexible shaft. The effective opening period is less than 0.0003 second, although the valve stem leaves the seat for slightly more than 0.0004 second. These values are independent of operating speed.
The valve has been used in the integral combustion chamber to determine whether its contents remain in a quiescent state during combustion. Gas samples taken during the combustion period with the fuel injected through a standard 6-orifice nozzle showed the in¬ crease in carbon dioxide content of the gases almost co¬ incident with the rise of pressure in the cylinder. When a 2-orifice nozzle was used with the gas¬ sampling valve located in a part of the combustion chamber not served with fuel, no carbon dioxide could be detected in the samples obtained, indicating that combustion does not materially affect the quiescent condition of the combustion chamber.
The accumulation of engine exhaust gases in the unventilated cockpits of airplanes powered with car-
24
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
buretor engines is dangerous, because of the percentage of carbon monoxide contained in the exhaust. An investigation was made to determine the quantity of carbon monoxide in the exhaust gases from spark- ignition and compression-ignition engines for a range of operating conditions. An analysis of the samples of exhaust gas taken at the exhaust valves of a single¬ cylinder universal test engine operating with aviation gasoline at two-thirds and full load and with both a carburetor and a fuel-injection system showed 5 per cent by volume of carbon monoxide in the exhaust. A 500-horsepower multicylinder radial engine operating under service conditions showed an average of 7 per cent carbon monoxide in the exhaust. When the com¬ pression-ignition engine was operated with a clear exhaust, the carbon-monoxide content of the exhaust gases varied from 0.03 to 0.50 per cent. With exces¬ sive smoke and flame in the exhaust the percentage of carbon monoxide increased to 0.75 per cent. The results indicate that the use of compression-ignition engines will considerably reduce the danger of carbon- monoxide poisoning.
Combustion-chamber investigation — Integral type. — The basic requirements for injection-valve nozzles for quiescent combustion chambers as formulated last year assumed that the effective pressure on each individual orifice was the same. As the total discharge area was increased to take care of the increased fuel required for supercharged operation, it became appar¬ ent that the required equal pressures were not being maintained. A method was devised for catching and weighing the discharges of the individual orifices and increasing the size of orifices where necessary to main¬ tain the proper proportional distribution throughout the combustion chamber.
The engine performance tests of a quiescent com¬ bustion chamber with supercharging have been continued and have included an investigation of the effect of scavenging the combustion space by using valve overlap. The results of this investigation have shown that an increase in power from the additional air available for combustion is obtained and that the removal of the residual products of combustion has a beneficial effect on the combustion of the fuel and allows greater quantities of fuel to be burned without smoke or flame appearing in the exhaust. The brake mean effective pressure obtainable with clear exhaust increases repidly with increase of inlet pressure up to 3 inches of mercury above atmospheric pressure, indi¬ cating that this pressure will effect the complete scavenging of the combustion space. The increase in performance with clear exhaust under these conditions is 33 per cent greater power and 16 per cent less specific fuel consumption. The results of these tests have been prepared for publication.
Combustion chamber investigation — Auxiliary -cham¬ ber type. — The engine performance tests of the auxiliary
chambers containing 20, 35, 50, and 70 per cent of the total clearance volume have been completed and an analysis of the results showed that variation of clear¬ ance distribution had little effect on control of com¬ bustion, although maximum pressures and rates of pressure rise were least with the 35 per cent chamber. A slight gain in performance was obtainable with the larger chambers, but it was not commensurate with the increase in maximum pressures and rates of pressure rise. The results of these tests have been published in Technical Note No. 435.
The work on this type of combustion chamber has included an investigation of the effect of variation of the diameter of the connecting passage between the cylinder and the auxiliary chamber. This change was in effect a variation in the velocity of the air entering the auxiliary chamber. The results of this investiga¬ tion showed that a passage size determined for maximum engine speed would permit satisfactory performance at lower speeds. Passage sizes so small as to cause large pressure differences between cylinder and chamber were too small to allow good performance and were responsible for large throttling losses as evidenced by high friction mean effective pressure. With the larger sizes the air velocity was so reduced that the mixing of the fuel and air was not effective, and the engine performance suffered from incomplete and delayed combustion. Between these two extremes there was a satisfactory operating range in which the passage size was not very critical. Technical Note No. 436 pre¬ sents the results of this investigation.
Fire Hazard in Aircraft — Carburetor intake- system backfires. — The investigation on confining carburetor intake-system backfires has been com¬ pleted. The results are reported in Technical Report No. 409.
Hydrogenated safety fuels. — The use of fuels of low volatility having flash points at atmospheric condi¬ tions of 105° F. in conventional spark-ignition engines continues to be an attractive method for reducing the fire hazard in aircraft. The antiknock characteristics of these fuels have been improved by the manufac¬ turer, so that they can now be used at a maximum compression ratio of 8.5 without the addition of tetraethyl lead or other fuel dope. The investigation of the engine performance obtained with these safety fuels injected into the engine cylinder has been con¬ tinued Progress has been made in reducing the specific fuel consumption with safety fuels by improv¬ ing the distribution of the fuel spray and by increas¬ ing the temperature of the engine coolant from 150° to 250° F. With direct fuel injection a brake mean effective pressure of 144 pounds per square inch and a specific fuel consumption of 0.49 pound per brake horsepower per hour have been obtained at a com¬ pression ratio of 7.5 and an engine speed of 1,500 revolutions per minute.
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
25
The performance characteristics of these safety fuels at higher speeds and compression ratios are being determined for the Bureau of Aeronautics of the Navy Department. Alterations have been made to an N. A. C. A. universal test engine to permit the maxi¬ mum rotative speed of the engine to be increased from 1,800 to 2,400 revolutions per minute.
The increase in the buo3Tancy of rigid airships as the fuel supply is consumed is counteracted by the condensation of water vapor from the engine exhaust. Because of the lower hydrogen content of the safety fuel as compared with gasoline, the airship industry is interested in knowing the quantity of water recover¬ able from the exhaust of an engine using safety fuels. The results obtained from a single-cylinder test engine operated with a carburetor and gasoline and with a fuel-injection system and safety fuel indicated that the water recovered from the exhaust when the safety fuel was used was 80 per cent of that obtained when gasoline was used.
Increase in Engine Power — Two-stroke-cyde inves¬ tigation. — The use of the 2-stroke cycle instead of the 4-stroke cycle with internal-combustion engines is a method of obtaining increased power output per unit of weight and displacement. The committee is conducting investigations to determine the effect of the various factors influencing the performance of an engine of this type having gasoline injection and elec¬ tric ignition. A new water-cooled cjdinder and cjdin- der head have been constructed to continue this inves¬ tigation at higher speeds and power outputs than were possible with the air-cooled 2-stroke-cycle engine. The new engine can be operated at a maximum speed of 2,000 revolutions per minute. Preliminary tests have been conducted at a compression ratio of 5.0, which indicate exceptional promise for this type of engine on the basis of increased horsepower per cubic inch of displacement.
Large valve overlay. — An appreciable gain in engine performance can be obtained by completely scavenging the clearance volume of a 4-stroke-cycle engine. This improved scavenging can be obtained efficiently by using a large valve overlap and a low boost pressure in the inlet manifold. With a fuel-injection system the time of fuel injection can be controlled to prevent the loss of fuel with the scavenging air. The results of tests made with a single-cylinder engine for a range of compression ratios from 5.5 to 8.5, a valve overlap of 112°, and an engine speed of 1,500 revolutions per minute, showed that complete scavenging of the clear¬ ance volume could be obtained with a pressure differ¬ ence of 2 to 5 inches of mercury across the intake and exhaust valves. The results showed that with a com¬ pression ratio of 6.5, a boost pressure of 2 inches of mercury, and a fuel quantity giving 98 per cent of maximum power, the brake mean effective pressure
developed was 182 pounds per square inch and the corresponding fuel consumption 0.44 pound per brake horsepower per hour. A comparison of the brake mean effective pressure obtained in operation with valve overlap and fuel injection as compared with that obtained in operation with normal valve timing showed a gain of 18 per cent in brake mean effective pressure obtained by improved scavenging.
Hub dynamometer. — The flight testing of airplanes is handicapped because there are no means available for measuring the power delivered by an engine in flight. The committee has developed a hydraulic dynamometer that may be attached to the hub of a propeller and that records photographically the torque developed by an engine. The power developed may be calculated from the known torque and engine speed. During the year the dynamometer has been adapted to permit measuring the torque developed by engines equipped with steel propellers. The necessary temperature corrections for the variation in volume of the liquid in the dynamometer with change in air temperature have been determined. The dynamometer has been flight-tested in a Vought 03U-1 airplane for 20 hours and found to operate satisfactorily. The precision of the propeller-hub djmamometer will be determined by tests to be made in the full-scale wind tunnel with a liquid-cooled engine mounted in a special test fuselage fitted with a torque dynamometer.
Cowling and Cooling of Aircraft Engines — Cooling properties of finned surfaces. — The develop¬ ment of air-cooled engines having increased power output is dependent upon an improvement in the efficiency with which the waste heat from the engine cylinder can be dissipated to the cooling air. The effect of fin pitch, width, thickness, and shape on the quantity of heat dissipated by finned cylinders has been determined from tests made with a con¬ stant heat input and a range of air speeds from 30 to 150 miles per hour. The results of these tests in¬ dicate that for tapered fins having widths of 1 inch or less there is no interference with the air flow be¬ tween the fins providing the fin pitch is greater than 0.15 inch.
Patterns of the air flow around a cylinder mounted in the wind tunnel showed that approximately one- half the cooling area of the cylinder does not come in contact with the cooling air stream, because of the breakaway of the air stream from the cylinder. To determine the temperature distribution when the entire cooling area was used, a finned cylinder was enclosed with a cowling and the cooling air forced past the cylinder by a Roots blower. This method of cooling resulted in a greater quantity of heat being dissipated and more uniform temperature distribu¬ tion around the cylinder being obtained for the same
26
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
tunnel air speed. An investigation to determine the minimum quantity of air required to cool present air-cooled engines under operating conditions is being conducted with a single-cylinder air-cooled test engine.
Cowling oj engine nacelles. — Comparative drag and cooling tests were made for the Bureau of Aeronau¬ tics, Navy Department, with a P3M-1 engine nacelle. Three antidrag rings of various shapes were tested in combination with modifications made to the nose and tail of the nacelle to improve the cooling at air speeds of 60, 80, and 100 miles per hour. A sub¬ stantial reduction in cylinder and oil temperatures was obtained by the use of deflectors between the cylinders and an oil cooler placed in the inlet mani¬ fold between the carburetor and the engine.
BUREAU OF STANDARDS
Altitude tests of aircraft engines. — A report is in preparation on the distribution of fuel heat in the Curtiss D-12 engine, supercharged and unsuper¬ charged, under a variety of controlled operating con¬ ditions and at various altitudes. Study of formulas proposed for estimating the altitude performance of aircraft engines from sea-level tests indicates the need of additional experimental data (1) on the variation of horsepower with carburetor air tempera¬ ture when there is a supercharger between the car¬ buretor and the engine, (2) on the influence of initial temperature and pressure conditions upon the com¬ pression ratio of a supercharger aud upon the power required to drive it, and (3) on the effect of engine compression ratio upon the variation of horsepower with exhaust pressure.
Phenomena o/ combustion. — A comprehensive in¬ vestigation of the effect of inert diluents (argon, helium, and nitrogen) on the gaseous explosive re¬ action between equivalent amounts of carbon mon¬ oxide and oxygen has been completed. The gases in each case were saturated with water vapor. The preparation of a report on this work for publication by the National Advisory Committee for Aeronau¬ tics was interrupted by the sudden death of Doctor Stevens on May 17. It is now almost ready for re¬ lease, after revision by his successor in accordance with the suggestions of Doctors Kassel, Lewis, Von Elbe, and Tolman. The immediate program con¬ templates a similar study in the presence of smaller quantities of water vapor.
Combustion in an engine cylinder . —The effect of combustion-chamber shape and number and location of ignition points on flame movement and pressure development in an engine cylinder are being studied with 4 different combustion chambers, 7 different locations of a single ignition spark, and 2 different arrangements of twin ignition. A small camera is
used in photographing, through the stroboscope, the many small windows symmetrically distributed over the engine head. The protographs check satisfac¬ torily with visual observations and provide a perma¬ nent record of the progress of the flame.
Improved equipment has been developed for meas¬ uring infra-red radiation from the explosion in the engine cylinder. The apparatus is sufficiently sensi¬ tive to permit observations with a stroboscope open¬ ing of only 1° or 2° of engine crank-angle while using a selected series of filters to indicate roughly the spectral distribution of the radiant energy over the region from the visible to about ll/i. Studies are being made to determine to what extent such data can be interpreted in terms of charge temperature and chemical composition.
Pressures and temperatures in aircraft engines. — A special engine head was prepared and used for direct comparisons of diaphragm-type indicators, and for studying the effect of indicator location and of adapters which interpose a passage between the engine and the pressure element. Experiments indicated that, in the absence of detonation, pressure readings were inde¬ pendent of the location of the indicator in the cylinder head, but that long, narrow connecting passages caused a marked increase in the measured pressures during explosion, and magnified greatly the variations from cycle to cycle. These errors introduced by the con¬ necting passage become more prominent as maximum explosion pressures increase.
Mechanism of atomization. — A review of available data, both theoretical and experimental, on the atomi¬ zation accompanying solid injection indicates quite definitely that such atomization, like air-stream atomi¬ zation, is a surface phenomenon. Both can be ex¬ plained by the formation of ligaments at the liquid-gas interface, under the influence of the relative motion of gas and liquid, followed by the collapse of these ligaments under the influence of surface tension. The results of this study are being published as Technical Report No. 440.
Effect of spark character on ignition ability. — The use of the cathode-ray oscillograph to measure the voltage and current in the spark discharge without appreciably altering the character of the spark presented unex¬ pected difficulty, owing to the low inductance and capacitance of the ignition circuit. A report on the Theory of Voltage Dividers and Their Use With Cathode-Ray Oscillographs will be found in the Bureau of Standards Journal of Research for July, 1932 (RP 460). A report on the measurement of current in the spark discharge is in preparation. Further study of the effectiveness of ignition sparks, as measured by the amount of chemical reaction occurring when different sparks are passed through lean mixtures of oxygen and hydrogen at atmospheric pressure, was
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
27
made necessary by the discovery that changes in the length and location of the high-tension leads may alter the character of the spark.
The ignition laboratory is studying also the reli¬ ability of spark ignition and the phenomena of auto¬ ignition at the request of the Bureau of Aeronautics. In connection with the latter problem, the effects of pressure and temperature on the ultra-violet absorp¬ tion spectra of various hydrocarbon vapors with and without air have been determined with a quartz spectrograph. Preliminary attempts to detect pre¬ flame oxidation of fuel-air mixtures in the engine have been made by passing light from an iron arc through a pair of quartz windows in the combustion chamber and through a stroboscope to the spectro¬ graph. In order to isolate the effect of temperature, fuels which show no change in absorption with pres¬ sure have been employed in the engine tests.
Radio-shielded ignition equipment. — As a basis for preparing Army-Navy specifications on shielded igni¬ tion harnesses, tests were conducted jointly at the Naval Aircraft Factory in Philadelphia and at the Bureau of Standards on available commercial types of shielded harnesses applicable to radial aircraft en¬ gines. The five types tested were found equally satisfactory from the standpoint of radio-interference shielding blit they differed with respect to accessi¬ bility, strength, shielding efficiency, and waterproof¬ ing. Tests of the Curtiss D-12 engine with and with¬ out a radio-shielded ignition system were made in the altitude laboratory at pressures corresponding to alti¬ tudes from sea level to 20,000 feet. No significant effect of the shielding on the altitude performance of the engine was found.
Effect of air humidity on engine performance. — As a result of the tests reported in Technical Note No. 309 and Technical Report No. 426 of the National Advi¬ sory Committee for Aeronautics, the National Advi¬ sory Committee has adopted standard dry-air pres¬ sures as a basis for correcting sea-level and altitude tests of aircraft engines. A corresponding revision of the power-correction formula in the S. A. E. Handbook has been proposed. Preliminary work on the injection of water spray into the combustion chamber of a vari¬ able-compression engine at compression ratios from 4:1 to 10:1 verified earlier results, but funds were not available for completing the test program.
Tests at subfreezing temperatures on the modified psychrometer developed during this research showed that it functioned satisfactorily when supercooled water was used. It was also found that, for low air temperatures, humidity determinations with a psy¬ chrometer in which a thermometer covered with a thin film of ice replaced the wet bulb were more reliable than those obtained with the customary frozen-wick wet bulb.
Gumming characteristics of gasolines: — The investi¬ gation of the gumming characteristics of gasolines, undertaken at the request of the Air Corps, deals with the determination of the gum contents of gaso¬ lines and the evaluation of their tendency to form gum during storage. It is obviously necessar}^ to have a significant method for the determination of gum content before any limit can be set on the per¬ missible gum content for satisfactory engine operation and before the increase in gum content in storage can be followed. Since evaporation in the engine manifold takes place in a current of air, work in the laboratory was concentrated on an evaporation method using an air jet. Each of the variables affect¬ ing evaporation in the manifold was investigated over a wide range in connection with the laboratory evaporation method. It was found that changes in the conditions during evaporation affected the abso¬ lute values for gum content, but that the relative gum contents of a series of gasolines were independent of the particular conditions used in the evaporation, so long as these conditions were held constant during the series. Accordingly, any convenient and repro¬ ducible air-jet method is equally significant for the determination of gum content, and evaporation from a bath maintained at 200° C. was found to be the most convenient.
The investigation of gum stability presents con¬ siderable difficulty, for it is desirable to develop a method which will tell in advance how much gum a gasoline 11411 contain when stored for periods of six months to two years. Preliminary work has shown that the gum stability of gasolines decreases very rapidly as the temperature is increased. Accordingly, storage experiments are being made with several gasolines over a wide range of elevated temperatures with the idea of extrapolating to the stability at ordinary storage temperatures. It is hoped that this work will lead to a test method in which the measurement of stability at about 100° C. will per¬ mit an accurate prediction of stability at ordinary temperatures.
Type testing of commercial aircraft engines. — During the year, 27 type tests and 4 calibration tests were undertaken at the Arlington laboratory, involving 18 engine types submitted by 11 manufacturers. As a result, 14 engines passed, 14 failed, and 3 were with¬ drawn. Only 3 of the 14 engines which failed ran more than halfway through the 50-hour endurance test and 6 failed to complete the first 5-hour period. Four engine types failed two or three times each during the year, whereas five improved types passed on retest. Valves were the most common source of major failure, four cases being noted. Piston seizure, crankshaft breakage, and cylinder-head cracks were responsible for two failures each. The following
28
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
parts each caused one major failure: Cylinder-head j studs, thrust-bearing-plate studs, connecting-rod lock rings, and knuckle pin. At the end of June, 84 engines had received approved-type certificates from the Department of Commerce.
The department’s revised aircraft-engine require¬ ments (to be effective January 1, 1933) call for a 50-hour preliminary test by the manufacturer at the proposed rated speed and increase the severity but not the duration of the present type test. The revised regulations include special requirements for altitude engines, i. e., engines which are not designed for operation at full throttle below a specified alti¬ tude. In the case of such engines, the rated altitude and the maximum altitude power to be assigned to the engine will be based on an approximate altitude test. Equipment for making dynamometer calibra¬ tions and approximate altitude tests of air-cooled aircraft engines is being installed at the bureau.
REPORT OF COMMITTEE ON MATERIALS FOR
AIRCRAFT
ORGANIZATION
During the past year the committee on materials for aircraft has suffered the loss of its chairman, Dr. George K. Burgess, by his death in Washington on July 2, 1932. Doctor Burgess had served as chairman of the committee on materials for aircraft since 1923 and had been a member of that committee since 1917. He rendered invaluable assistance in the coordination of the activities of the Army Air Corps, the Bureau of Aeronautics of the Navy, the National Advisory Committee for Aeronautics, and other agencies, especially in connection with the investigation of problems relating to aircraft materials. Both the committee on materials for aircraft and the executive committee at their meetings following the death of Doctor Burgess recorded their sorrow at his death and their sincere appreciation of his long and faithful service.
The vacancy in the chairmanship of the committee on materials for aircraft has not yet been filled, and in the meantime the activities of the committee are being conducted under the leadership of the vice chairman, Prof. H. L. Whittemore.
The committee on materials for aircraft is at present composed of the following members:
Prof. H. L. Whittemore, Bureau of Standards, vice chairman and acting secretary.
Lieut. Commander R. S. Barnaby (C. C.), United States Navy.
S. K. Colby, American Magnesium Corporation.
Warren E. Emley, Bureau of Standards.
Commander Garland Fulton (C. C.), United States Navy.
Dr. II. W. Gillett, Battelle Memorial Institute.
C. H. Helms, National Advisory Committee for Aeronautics.
Dr. Zay Jeffries, Aluminum Company of America.
J. B. Johnson, materiel division, Army Air Corps, Wright Field.
George W. Lewis, National Advisory Committee for Aeronautics (ex officio member).
Capt. Alfred J. Lyon, United States Army, materiel division, Air Corps, Wright Field.
H. S. Rawdon, Bureau of Standards.
E. C. Smith, Republic Steel Corporation.
G. W. Trayer, Forest Products Laboratory, Forest Service.
Starr Truscott, National Advisory Committee for Aeronautics.
Hon. Edward P. Warner, editor of Aviation.
In order to cover effectively the large and varied field of research on aircraft materials the following subcommittees have been established under the com¬ mittee on materials for aircraft:
Subcommittee on metals.
Subcommittee on aircraft structures — temporary subcommittee on research program on Mono- coque design.
Subcommittee on miscellaneous materials.
Subcommittee on methods and devices for testing aircraft materials and structures.
SUBCOMMITTEE ON METALS
The present organization of the subcommittee on metals is as follows:
H. S. Rawdon, Bureau of Standards, chairman.
Dr. II. W. Gillette, Battelle Memorial Institute.
Dr. Zay Jeffries, Aluminum Company of America.
J. B. Johnson, materiel division, Army Air
Corps, Wright Field.
George W. Lewis, National Advisory Committee for Aeronautics (ex officio member).
E. C. Smith, Republic Steel Corporation.
Starr Truscott, National Advisory Committee for Aeronautics.
Prof. H. L. Whittemore, Bureau of Standards.
A number of investigations on metals and the factors which may limit their usefulness in aircraft have been conducted at the Bureau of Standards with the cooperation of the Bureau of Aeronautics of the Navy Department, the materiel division of the Army Air Corps, and the National Advisory Committee for Aeronautics.
Inter crystalline embrittlement of sheet duralumin.— This investigation has been of several years’ duration. In 1927 an extensive series of light alloys available in sheet form were exposed to the weather. Triplicate exposure test panels were set up at the naval air sta¬ tions at Coco Solo, Canal Zone; Hampton Roads, Ya.; and at the Bureau of Standards. At stated intervals,
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
29
twice a year, specimens have been brought into the laboratory to determine the change in properties resulting from the corrosive influences to which the metal had been subjected. The experimental part of this program has been completed. As mentioned in previous reports, a number of important facts have been established concerning the intercrystalline type of corrosion of duralumin and related aluminum alloys ; its inherent nature, the rate at which the deterioration may occur, methods for overcoming the attack by proper treatment of the alloy and by surface protective coatings. A full report of the investigation is in preparation. The salient features as set forth in last year’s report have not been changed in any important respect by the results of the past year.
Because of the marked advances and changes in the light-alloy industry during the past few years, a con¬ tinuation of the program appeared extremely desirable and a series of tests of recently developed materials will be undertaken. Manufacturers have cooperated in furnishing typical new commercial materials, both aluminum-base and magnesium-base alloys, and the assembled exposure panels are being exposed to the weather at the same three locations as in the first tests. The exposed specimens are in two forms, bars machined to size and strips from which test specimens can be machined after corrosion. New and promising coat¬ ing methods are also represented in the test program. The list of materials includes 19 aluminum-base alloys and 9 magnesium-base alloys. Over 7,000 individual test specimens are included in the program.
Protection of duralumin , anodic oxidation. — A tech¬ nical note was issued during the year summarizing the results of the study made of the anodic oxidation process as a preliminary basic treatment of aluminum alloys before coating them in any other way. It is entitled “Advantages of Oxide Films as Bases for Aluminum-Pigmented Coatings for Aluminum Alloys.”
Further work has been carried out on the various methods used for anodic treatment. One of the meth¬ ods studied appears very promising and a patent to protect the Government’s interest is being considered. A study is being made of the various factors which may contribute to the limited life of the electrolytic baths used in the commercial process of anodic treatment. The progressively increasing inferior character of the treated surfaces resulting from changes in composition which tend to “build up” in the solution used in the process constitute a serious drawback to the use of this very important method for the protection of aluminum alloys.
Exposure tests of magnesium and magnesium alloys. — Thirty-six months’ continuous exposure to the weather at the Bureau of Standards has resulted in no marked decrease in tensile properties of a series of magnesium alloys, although none of the coatings remained intact on the specimens for as long as 24 months. These re¬
sults for these materials in this location are in striking contrast to those obtained in the tropical location and it was considered especially desirable to include mag¬ nesium alloys in the new series of exposure tests for aluminum alloys.
High-frequency endurance tests of light alloy sheet materials. — Experimental work on this subject was completed during the year. The results given in the last annual report for the materials in this investiga¬ tion may be accepted with confidence as the endurance limit of the various aluminum and magnesium alloys reported for the test conditions employed, that is, the high-frequency vibration of a strip maintained free from any constraint due to mechanical supports. No difference was found in the endurance limit of heat- treated duralumin sheet tested in the plain condition and after being given the anodic oxidation treatment. The value of 15,000 pounds per square inch was found in each case.
There is some indication that slight atmospheric corrosive attack in this method of testing occurs simul¬ taneously with the stressing. This factor, however, influences all service results in like manner. This should be borne in mind in the interpretation and use of the values determined. A full report of the investi¬ gation is nearly ready.
Identification of chromium-molybdenum steel. — A technical note entitled “Rapid Chemical Test for the Identification of Chromium-Molybdenum Aircraft Tubing” was issued during the year. The chemical test for molybdenum described is relatively simple and rapid and though not so simple as a “spot test” is the closest approach to this that has yet been developed.
SUBCOMMITTEE ON AIRCRAFT STRUCTURES
The subcommittee on aircraft structures is at present organized as follows :
Starr Truscott, National Advisory Committee for Aeronautics, chairman.
Lieut. C. E. Archer, United States Army.
Capt. Howard Z. Bogert, United States Army, materiel division, Air Corps, Wright Field.
C. P. Burgess, Bureau of Aeronautics, Navy Department.
Richard C. Gazley, Aeronautics Branch, Depart¬ ment of Commerce.
Charles Ward Hall, Hall- Aluminum Aircraft Corporation.
Lieut. Lloyd Harrison (C. C.), United States Navy.
George W. Lewis (ex officio member).
Lieut. Commander R. D. MacCart (C. C.), United States Navy.
Charles J. McCarthy, Chance Vought Corporation.
Prof. J. S. Newell, Massachusetts Institute of Technology.
Dr. L. B. Tuckerman, Bureau of Standards.
30
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
Airship girders and airship structural members. — The tests on small preformed wire terminal loops, made by the Goodyear-Zeppelin Corporation by the same method as was employed in making the loops for the ZRS-4 and ZRS-5, have been continued. The bending stresses have been measured on loops formed from wire of 0.092-inch diameter. Over 3,000 measurements of curvature were made on specimens up to loads of 800 pounds, in which they were bent around pins equal in diameter to the forming pin to a grommet of the largest diameter which could be in¬ serted in the loop. When the loops were much larger than the largest grommet or were unsvm metrical, tests were also made about a pin of as large size as could be inserted in the loop without deforming it. For these loops maximum-stress differences, calculated according to the Winkler curved-beam theory, of over 900,000 and 1,500,000 pounds per square inch corre¬ sponding to 450 and 800 pounds load or 28 and 50 per cent of the specified breaking load of the wire, respec¬ tively, have been found in bending a loop nominally 0.4-inch diameter about a pin 0.312-inch diameter.
The investigation is proceeding in an attempt to confirm the existence of these high-computed stresses by independent methods. X-ray studies have been made of the wire by the Naval Research Laboratory. Stress-strain curves of the wire have been made by means of the Tuckerman optical strain gage. The residual stresses in the wire are being studied by suc¬ cessively removing segments of the wire and computing the residual stress from measurements of change in curvature. As these high-computed bending stresses suggest the possibility of failure in fatigue at relatively low loads, flexural fatigue tests are being made to determine a Goodman diagram which may furnish some criterion as to the extent of the extreme fiber stress which can be safely used.
Specimens of the original German duralumin lattices and channels from the U. S. airship Los Angeles have been submitted at intervals throughout the year.
The results of the tensile tests on the channel mate¬ rial do not suggest that there has been any appreciable progress in corrosion in the chord members of the airship during the year. No specimen was found which bad a tensile strength below design values.
The samplings from the thin lattice material seem to indicate a definite but slow progress of the corrosive attack since the last sampling. However, no specimen was found in which the strength had diminished sufficiently to lessen the strength of the girders.
A number of specimens of thin sheet Alclad, cut from the hull plating of the metal-clad airship MC-2 , have been tested in tension. The specimens from the hull plating do not show a measurable deterioration as a result of corrosion. No hull specimens showed ultimate strengths of less than 54,800 pounds per square inch or elongations less than 15.5 per cent.
Flat plates under normal pressure.— Forty-one rec¬ tangular plates of 17 ST aluminum alloy have been tested under normal pressure, ranging in size from 2.5 by 2.5 to 7.5 by 7.5 inches and in thickness from 0.01 to 0.10 inch. Only six of the plates were tested with supported edges while the rest had fixed edges, the second condition of edge restraint being preferred because it is more clearly defined and is also closer to that actually prevailing in the bottom sheeting of seaplane floats.
Both the total deflection of the plates at the center and the permanent set at the center were plotted against the pressure for each plate, and it was observed that both curves approached straight lines for high pressure in all cases except those in which it was not possible to avoid slipping at the edges in spite of the use of knurled bars to hold down the plate. The inter¬ section of the straight-line asymptote of the permanent- set curve with the pressure axis was taken as a measure of the beginning of plastic yielding in the plate.
A plot of this yielding pressure versus the ratio of
h
thickness to width, — , for square plates results in a
£th
single curve for the 2.5 by 2.5 and 5 by 5 inch plates; the points for the 7.5 by 7.5 inch plates fall on this curve except for the very thin plates, which show lower yielding pressures, possibly because of slipping at the edges. The yielding pressure reaches a minimum value of about 20 pounds per square inch for a ratio
— = 0.000; yielding begins at a higher pressure for
both thicker and thinner plates of the same size. It seems possible, therefore, to increase the strength of such a plate and at the same time save weight by
decreasing its — ratio from 0.006 to, say, 0.003. This
ZiQ/
peculiar property of the square plate with fixed edges was found in qualitative agreement with calculations based on FoeppPs theory of the plate of medium thick¬ ness. The quantitative agreement was poor, however.
A detailed survey of the state of deformation in a 5 by 5 by 0.02 inch duralumin plate with fixed edges has been made, a special device being used to measure deflections at points 0.05 inch apart and a 1-inch Tuckerman optical strain gage to measure the surface strains at various points on the plate as the pressure was increased from 0 to 60 pounds per square inch. The empirical data thus obtained will enable us to compute both the bending stresses and the median- plane tensile stresses developed in the plate, and may provide an empirical basis for testing the as¬ sumptions made in FoeppPs theory and for interpret¬ ing the curves of permanent set versus pressure so far obtained.
Similar tests on rectangular plates of 18-8 stainless steel are being prepared. The tensile properties of
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
31
14 specimens, ranging in thickness from 0.011 to 0.039 inch have already been obtained with a 2-inch Tuckerman optical strain gage.
Strength of welded joints in tubular members for aircraft . — The welded joints of the second series of tests in this investigation have been completed and are now being tested.
All the tubes and joints were inspected by passing a magnetic flux through the specimens and dusting them with a specially prepared iron powder, the technique developed by A. V. DeForest being used. The accumulation of the powder which clung to the edges of flaws and cracks showed defects which were not otherwise visible under careful visual inspection even with a hand lens.
No difficulty was encountered in welding the joints made in chromium-molybdenum tubing of 1.5-inch outside diameter by. 0.058-inch wall. It was not found possible to avoid heat cracks in the thin-wall chromium-molybdenum tubing of 1.5-inch outside diameter by 0.020-inch wall in welding this tubing by the ordinary method using low-carbon welding rod. Numerous expedients were tried but without success. Finally a special technique which has recently been developed was used. This involves the use of a re¬ ducing flame and a special welding rod melting at a lower temperature than the base metal. No cracks could be found in any of the thin-wall tubes welded by this method.
Inelastic behavior of duralumin and alloy steels in tension and compression. — The compression tests on short lengths of thin-wall channel and tubing have been continued. Even when buckling has been re¬ strained in the free length of the material, difficulty is still found in restraining the secondary buckling of the ends of the specimen adjacent to the heads of the testing machine. A large number of experiments have been carried out in an endeavor to prevent this, so far with only partial success. The compressive stress-strain curves so far obtained on these thin-wall shapes are therefore definitely affected by secondary stresses. Additional refinement in the technique of testing is expected to reduce these further.
End fixation of struts. — The investigation has been continued with the testing of duralumin, of stainless steel, and of two more sizes of chromium-molybdenum tubes. A series of duralumin tubes 1.5 inches in diameter by 0.058 inch thick, covering the range from “short ” columns to “long” columns, was tested, and check tests were made with tubes 2 inches in diameter by 0.058 inch thick and tubes 1 inch in diameter by 0.035 inch thick. Stainless-steel tubes having five different sizes of cross section and cover¬ ing the range from “short” to “long” columns were tested. Check tests were made of chromium-molyb¬ denum tubes 1.125 inches in diameter by 0.049 inch thick and 1 inch in diameter by 0.035 inch thick.
In view of the applicability of the Consid^re- FJngesser theory to struts with elastically restrained ends, as found experimentally last year, the main body of the tests this year has been carried on with freely supported ends. In all cases, however, check tests have been made with restrained ends. It appears now that for practical purposes the strength of chro¬ mium-molybdenum steel and duralumin columns can be predicted by means of the Johnson parabola and the Euler hyperbola when the yield points, the moduli of elasticity, and the conditions at the ends are known. For some specimens of chromium-molybdenum tubing somewhat higher values have been previously found for columns in the transition region but the type of stress-strain curve which gives these higher values is not found in all the chromium-molybdenum tubing meeting present specifications. The work has not proceeded far enough to draw definite conclusions with respect to stainless-steel columns.
Torsional strength of tubing. — Torsion tests have been completed on 60 chromium-molybdenum steel tubes, ranging in length from 19 to 60 inches, in out¬ side diameter from 0.75 to 2.5 inches, in thickness from 0.03 to 0.125 inch, and in diameter-thickness ratio from 12 to 74.
There was a large scatter of values for the critical shearing stress, i. e., the shearing stress at which the tube failed by buckling. This scatter seemed to be related to the equally large scatter in the yield-point stress and ultimate strength in tension, as determined from tests on tensile specimens cut from the tubes. The results were therefore replotted in terms of the three dimensionless variables a, ju, A involving these proper¬ ties. a, n, A are defined as
a = -\/dcy/(TY.p.) U^Dft 5 A = cruitl<rY.p.
where ay = critical shearing stress;
aY.p.= yield-point stress in tension;
Cm » = ultimate strength in tension;
Z? = outside diameter of tube; tf = wall thickness of tube.
The method of least squares was then used to fit an empirical relation as closely as possible to the observed points. The empirical formula
a = 15.27 — 1 + 0.981
obtained in this way reduced the scatter from about 30 per cent to around 10 per cent. From it a set of curves of critical torque T divided by aY. p. TT‘ versus D/t was computed for different values of A. From these curves it is easy to calculate the size of chromi¬ um-molybdenum steel required to transmit a given torque provided the yield point of the material and the ratio A of its ultimate strength to its yield point are known and provided the tube falls within the range of dimensions and of properties of those tested.
32
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
A report of this work has been written and will soon be ready for publication.
Strength of riveted joints in aluminum, alloy. — This investigation was undertaken in cooperation with the National Advisory Committee for Aeronautics for the purpose of providing more fundamental and detailed data on the strength of riveted joints in thin alumi¬ num-alloy sheet than are now available.
A program has been worked out for this investiga¬ tion, sheet and rivets have been purchased for making test specimens, and a number of testing fixtures are under construction in the shop. It. is expected that tests will be started in a short time.
The first part of the investigation will be confined to static tests on single rivets. Later on it is hoped to extend the investigation to multiriveted joints and to joints under alternating stress. Plate thickness rang¬ ing from 0.010 inch to 0.125 inch and rivets from 0.0625 inch to % inch in diameter in six head types will be used.
The present program includes tests to determine the proper driving pressures, amount of upsetting pro¬ duced by driving, effect of amount of upsetting on the strength, allowable bearing and shearing stress in single shear and in double shear for a complete range of plate thickness and rivet diameters, effect of marginal distances on the strength of riveted and pinned joints, and tensile strength and frictional resistance of various head types.
TEMPORARY SUBCOMMITTEE ON RESEARCH PROGRAM ON MONOCOQUE DESIGN
The temporary subcommittee on research program on monocoque design is at present composed of the following members:
George W. Lewis, National Advisory Committee for Aeronautics, chairman.
Capt. Howard Z. Bogert, United States Army, materiel division, Air Corps, Wright Field.
Richard C. Gazley, Aeronautics Branch, Depart¬ ment of Commerce.
Eugene E. Lundquist, National Advisory Commit¬ tee for Aeronautics.
Lieut. Commander R. D. MacCart (C. C.), United States Navy.
Dr. L. B. Tuckerman, Bureau of Standards.
Stressed-Skin Structures. — As a result of the present trend toward all-metal airplane construction the strength of stressed-skin, or monocoque, structures is being given, perhaps, more consideration and study by aeronautical engineers than any other single air¬ craft structural problem. Early studies made of the strength of stressed skins indicated that the highest strength-weight ratios would probably be obtained with corrugated material. Accordingly, a relatively large amount of test data were obtained on the strength of corrugated sheet by the Army Air Corps and others. However, the difficulties encountered in
the fabrication of complete structures of corrugated material, the fact that corrugations stiffen the skit in one direction only, and the possibility of adversf aerodynamic effects from the use of corrugated skin; have caused designers who wish to employ the stressed, skin, or monocoque, type of construction to use, it many cases, smooth skin reinforced by longitudinal and transverse stiffeners. The efforts of the tempo- rary subcommittee on research program on monocoqu; design have therefore been directed toward studk which, it is hoped, will lead to rational methods o strength calculation for this type of structure.
Research on thin-wall duralumin cylinders. — An in vestigation at the Langley Memorial Aeronautics Laboratory on thin-wall cylinders of duralumin k been completed, including tests on cylinders of differen radii and tests on elliptic cylinders in practically al \ of the conditions of loading, compression, torsioi < bending, and combined loading. Approximately 40 f specimens have been tested during the past year an- * the data are now being summarized in both curve am j equation form by nondimensional coefficients whici c should apply to all materials in the range of elasti r failure. A preliminary report on the strength of thii t wall cylinders in torsion (pure shear) has been put }] lished (Technical Note No. 427) and other reports at 0 in preparation. p
Besides providing data on the strength of tri monocoque fuselage specimens, perhaps the mo: 0 valuable results of the investigation on thin-\n n cylinders are the data being obtained on the type ( t] failure which occurs in unreinforced skin. Tl ci wrinkles form in regular patterns, diagonal folds ft n< shear and “ diamonds" spaced in longitudinal an st circumferential rows for compression and bendift us As one of the objects of reinforcement is to pre?« st failure in the skin, the data obtained on the wrinklt re should be of great value in connection with the spacii d€ of longitudinal and transverse reinforcement to profit re maximum stiffening of the skin. re*
Some of the general conclusions which have be* drawn from the results of this investigation are tin the length-radius ratio, in addition to the radii thickness ratio, is a very important factor in the deft mination of the shear strength of true monocot] fuselage specimens, but that it is not an imports! orf factor in the determination of the compressive bending strength; that workmanship in fabricate concentrations of stress, and eccentricity in the elf ments of the skin are all factors which affect the coi pressive and bending strength very appreciably ; tl the stress developed on the extreme fiber in bendir as calculated by the ordinary theory of bending definitely greater than the stress developed in dir? compression; and that the bending strength of an liptic cylinder is very nearly equal to the bendi strength of the circumscribed cylinder of circulars tion with the same skin thickness.
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
33
Research on reinforced skin. — As a first study of the strength of skin in combination with stiffeners, a report is being prepared in which three methods of calculating the compressive strength of fiat sheet and stiffeners are compared. In this report it is concluded that a method based upon a mutual action of skin and stiffener gives the best general agreement between observed and predicted loads. Methods based on the assumption of only partial action of sheet and stiffener as a unit do not always give predicted loads that agree with loads observed in tests. Accordingly, restric¬ tions are given in the report which define the range of conditions over which these methods may be applied.
Coordination oj research on stressed-skin structures. — One of the duties of the subcommittee on research program on monocoque design is to suggest problems and keep informed regarding research on stressed-skin structures in other than Government laboratories so that no unnecessary duplication of effort will result and to insure that all organizations will be working progressively toward the development of methods for designing, rationally if possible, stressed-skin, or monocoque, structures for aircraft. In this connec¬ tion several problems were recommended by the com¬ mittee during the past year for study in other lab¬ oratories and close contact has been maintained with these organizations.
However, in an effort to insure that the data being obtained will be of fundamental and lasting value a memorandum is being prepared for circulation to these organizations in which special attention is called to the presentation of structural test data in nondimensional form correlated with the shape of the stress-strain curve for the material as well as with the usual material properties. Unless the shape of the stress-strain curve is taken into account and the results of tests reduced to nondimensional form, in¬ dependent of the materials used, there is little hope of reducing the large number of structural tests now required when a new material is adopted.
SUBCOMMITTEE ON METHODS AND DEVICES FOK TESTING AIRCRAFT MATERIALS AND STRUCTURES
The subcommittee on methods and devices for test¬ ing aircraft materials and structures is at present organized as follows:
Henry J. E. Reid, Langley Memorial Aeronautical Laboratory, chairman.
Capt. Howard Z. Bogert, United States Army, materiel division, Air Corps, Wright Field.
Lieut. Lloyd Harrison (C. C.), United States i: Navy.
George W. Lewis (ex officio member). f Eugene E. Lundquist, Langley Memorial Aero- i nautical Laboratory.
R. L. Templin, Aluminum Company o ^America.
Dr. L. B. Tuckerman, Bureau of Standards.
In the construction of aircraft of proved types and in the development of new types of construction, a large amount of testing is required to obtain quantita¬ tive data on the properties of materials and struc¬ tures, both for inspection and research purposes. The testing of aircraft materials and structures is therefore a subject of great importance. The accurate testing of materials and the study of the strength properties of structures require great care and an accurate knowl¬ edge of testing machines, devices, and methods. Many tests are worthless because the operator has used methods and equipment incapable of measuring the stress or strain accurately and oftentimes other factors unknown to the operator have existed and very seriously affected the results. While there are a great number of devices for use in testing materials and structures, many must be used with special pre¬ cautions and others are actually unsuited to the study of aircraft materials and structures.
Because of this difficulty the subcommittee on methods and devices for testing aircraft materials and structures was established. Under the auspices of this new subcommittee, a survey has been made of the methods and devices now in use, including those used by the Army, Navy, Bureau of Standards, and a num¬ ber of aircraft manufacturers. It also includes infor¬ mation obtained from various European sources through the Paris office of the committee. A number of manufacturers of testing machines have been visited and the devices available studied and corre¬ lated with respect to their use in testing aircraft materials. The subcommittee has had one meeting to study this survey and especially to discuss the causes of errors in testing. The survey is now being organ¬ ized with a view to the publication of the information by the committee as a report or a series of reports dealing with certain phases of the testing of aircraft materials and structures.
SUBCOMMITTEE ON MISCELLANEOUS MATERIALS
The present organization of the subcommittee on miscellaneous materials is as follows:
Charles II. Helms, National Advisory Committee for Aeronautics, chairman.
Dr. W. Blum, Bureau of Standards.
C. J. Cleary, materiel division, Army Air Corps, Wright Field.
Warren E. Einley, Bureau of Standards.
George W. Lewis (ex officio member).
J. E. Sullivan, Bureau of Aeronautics, Navy Department.
G. W. Trayer, Forest Products Laboratory.
P. II. Walker, Bureau of Standards.
G. P. Young, materiel division, Army Air Corps, Wright Field.
Mercerization oj cotton for aeronautical uses. — A sys¬ tematic study of the mercerization of cotton yarn for
34
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
aeronautical uses, where maximum strength for a given weight is important, has been completed. This study demonstrated that the optimum conditions of mercerization for increasing the strength of yarns are different from those of ordinary commercial merceriza¬ tion, where improvement in luster is the primary object. The elongation of yarn mercerized for maxi¬ mum strength is usually low. It was found that the elongation can be increased by a second mercerization without tension. An increase of 50 per cent in strength with an elongation of 10 per cent at break was found to be possible. A detailed report has been prepared for publication.
Substitute for silk cloth for parachutes. — The two cotton cloths of different construction which were finished commercially were not made into parachutes because the properties of the laboratory-finished cloths could not be duplicated satisfactorily on the commercial scale.
In view of the promising performance of the para¬ chute made from K K cloth and in view of the objec¬ tion to the bulkiness of a parachute made from a 2-ounce cotton cloth, experiments were undertaken to develop a cotton cloth for parachutes which would weigh 1.5 ounces per square yard and meet the air- permeability requirement, the strength and tear resist¬ ance to be as great as possible. Considerable progress has already been made. Sufficient cloth will be woven for a parachute which will be given performance tests in comparison with the standard parachute made from silk.
Gas-cell fabric of high tear resistance and outer-cover cloth for airships. — The experimental weaving of fab¬ rics of high tear resistance has been completed. A 2-ounce cloth was developed with a tear resistance