IN 1911 Professor Herbert Charles Sadler (Glasgow '93, D.Sc. hon. ibid. '02, LL.D. ibid. '27), chairman of the Department of Naval Architecture and Marine Engineering, attended the first International Aviation Meeting at Boston, Massachusetts. Many of the great contemporary flyers, including the Wright brothers, Glenn Curtiss, Louis Bleriot, and Graham White, took part in the airplane races and exhibition flights held on that occasion.
Interest in aeronautics was traditional in Sadler's family, for his great-granduncle, James Sadler (1751-1828), of Oxford, was the first English balloonist, and the two sons of James Sadler, Windham and John, followed their father's hobby and career. J. E. Hodgson in his voluminous History of Aeronautics in Great Britain devoted an entire chapter to the Sadlers (Chap. VI, "The First English Aeronaut, James Sadler and His Sons").
Moreover, during his early teaching career at the University of Glasgow, Sadler had as his colleague Percy S. Pilcher, who was Otto Lilienthal's follower and the British pioneer in modern glider flying.
Upon his return from the aviation meeting in Boston Sadler reorganized the University of Michigan Aero Club. In the year 1911-12 the students built a small wind tunnel in the "mold loft" of the West Engineering Building and experimented with various kinds of craft. They also built a glider, patterned to some extent on the Wrights' biplane, and flew it as a kite in the hilly country surrounding Ann Arbor. Since they had had no aeronautical experience, they concentrated on controlling the rise and descent of their craft and on maintaining its longitudinal equilibrium, depending for the more difficult lateral equilibrium on two helpers standing on the ground and holding ropes attached to the wing tips. The ground helpers were often lifted into the air by sudden gusts of wind or by the pilot's abrupt use of the elevator control. Sadler helped and advised, repeating the warning given him by Wilbur Wright: "If you will advise them [the students] to build a glider and to fly it, do not let them build it too light."
In 1910 Felix Wladyslaw Pawlowski (Paris '10, M.S. Michigan '14), who had taken the first course in aeronautical engineering ever given, that of Lucien Marchis at the University of Paris, arrived in this country. He spent two years in Chicago as a designer for the automobile industries. In 1911-12 he wrote to a number of engineering colleges and technological institutes requesting an opportunity to develop courses in aeronautics. He received negative replies from most of them on the grounds that aviation "very likely" would never amount to anything. But he had two encouraging answers, one from the Massachusetts Institute of Technology expressing interest in the proposal, although declining it "for the present" because of lack of funds, and another from Dean Mortimer E. Cooley, which resulted in Pawlowski's appointment to the University of Michigan in 1913 as Teaching Assistant in Mechanical Engineering, with the understanding that later he would be permitted to introduce courses in aeronautical engineering. He became Instructor in 1914.
In view of the times and the stage of Page 1182development of aviation, it is easy to find justification for the negative attitude of the various deans approached, but the farsightedness of Dean Cooley as well as his confidence in the future possibilities of this new mode of transportation is evident. When he presented the matter to his standing committee, Sadler and Ziwet expressed their approval.
After Pawlowski's arrival at the University interest in aviation increased rapidly. For a time, partly because of his insufficient command of the English language and partly because aeronautical engineering was not yet considered important (Cooley, Scientific Blacksmith), Pawlowski's official duties were limited to drawing-room work in mechanical engineering. He took a leading part in the activities of the Aero Club, however, thus relieving Sadler of that responsibility. Once a week the club held regular meetings, during which principles of aerodynamics and aviation were discussed, and another larger, but not a better, biplane glider was built. This glider also was flown as a kite, and, probably for the first time in the history of aviation, an automobile was used to tow it. Two students, Flavius E. Loudy ('16e [Ae.E.], Ae.E. '38) and Lewis C. Wilcoxen ('16e), distinguished themselves particularly in connection with the construction and the handling of this glider. Loudy later had a successful career in the aeronautical industries and in the Bureau of Aeronautics of the Navy Department.
The enthusiasm following a series of lectures delivered at the University in the fall of 1913 by Professor Lucien Marchis contributed to the establishment of the first regular courses in aeronautics in February, 1915. Professor Marchis throughout his earlier career had been interested primarily in the industrial or practical applications of physics; thus he lectured successively on the theory of steam engines, internal combustion engines, automobiles, and on aeronautics.
These first courses at the University, which were under the direction of Pawlowski, proved to be so popular that it was necessary to extend and correlate them as one of the regular groups of electives. This instruction, however, was for members of the Aero Club and was offered without credit. John H. Ledebaer's translation of Duchene's Flight Without Formulae was used as a textbook. The regular courses in aeronautical engineering, leading to a professional degree, were organized as a group of electives in the Department of Naval Architecture and Marine Engineering. Only junior and senior engineering students were eligible. The curriculum was much like that for naval architecture and marine engineering students and was similar also to the course in mechanical engineering. The aeronautical subjects were added partly at the expense of the electives.
The first course, Theory of Aviation, introduced in 1914-15 for two hours of credit, dealt with the principles of aerodynamics and the mechanics of flight. Students who were members of this first class included D. M. Bavly, M. L. Goldstein, K. W. Heinrich, A. Horbaszewski, Yocham Hu, F. E. Loudy, and Chien Hsün Sung.
In 1915-16 two new courses in aeronautics were added, Propulsion of Aeroplanes, which dealt with propeller design and the principal features of the various types of motors, and Aeroplane Design, which consisted of lectures and drawing room work. The details of the actual construction of an airplane were discussed, and a design was made to fulfill a given set of conditions. Sixteen students were enrolled in the three courses during the year.
Page 1183A 1912 model "B" hydroplane, manufactured by the Wright brothers, was presented to the University Aero Club in 1915 by Russell Alger, of Detroit, president of the Michigan Aero Club, and Frederick W. Alger, of Clarkston. Shortly afterward, the machine was destroyed in a trial flight on Barton Pond, fortunately without any harm to the pilot.
In 1916-17 a complete four-year program of study, Program VI, leading to the bachelor's degree in aeronautical engineering was arranged. The department was included in the then renamed Department of Naval Architecture, Marine Engineering and Aeronautics. Pawlowski, as Assistant Professor of Mechanical Engineering, still taught certain courses in mechanical engineering. During this year the following aeronautical courses were offered: General Aeronautics, Theory of Aviation, Theory and Design of Propellers, Aeroplane Design, Aeronautical Laboratory, Design of Aeronautical Motors, Theory of Balloons and Dirigibles, Design of Balloons and Dirigibles, Theory and Design of Kites, Design of Aerodromes and Hangars, Advanced Stability, Aeronautics — Advanced Reading and Seminar, Aeronautics — Advanced Design, and Aeronautics — Advanced Research. General Aeronautics was added as an introductory course dealing with the fundamentals underlying the design and performance of both the lighter-and the heavier-than-air craft.
Of the fourteen courses proposed and listed, only the first six were required as a minimum qualifying the student for the degree in aeronautical engineering; the remainder were offered as electives in accordance with the needs of senior and graduate students. During the first semester General Aeronautics was taught by Sadler, and Theory and Design of Propellers, Aeroplane Design, Advanced Reading and Seminar, and Advanced Aeroplane Design were offered by Pawlowski. At the end of the first semester of 1916-17 Pawlowski was granted a leave of absence to accept a position in Washington, D.C., as aeronautical engineer for the United States Army.
During the second semester the following courses were offered: General Aeronautics (Sadler), Theory of Aviation (Gerhardt), Aerodynamic Laboratory (Sadler), and Design of Aeronautical Motors (Fishleigh).
In May, 1917, the degree of bachelor of science in engineering (aeronautical engineering) was established, and in June, 1917, William Frederick Gerhardt was the first student to receive it.
Owing to the important role of aviation in World War I, the War Department began to organize design and construction of airplanes for the United States Army, and for that purpose drafted engineers who possessed some knowledge of aviation. After the declaration of war, however, and upon the arrival of the Balfour-Viviani Mission, the War Department accepted the advice of these experts of our Allies and abandoned attempts to develop original airplane design, concentrating instead on the utilization of the enormous manufacturing facilities of the country for quantity production of aircraft of Allied design. Consequently, in the fall of 1917, Pawlowski returned to the University to assist in conducting a special course, Principles of Aviation, which permitted students drafted into the Army to qualify or to claim preference for Air Corps service. The courses given during this year were: General Aeronautics, Theory of Aviation, Propeller Design, Airplane Design, Aeronautical Laboratory, and Aeronautical Motor Design. A two-hour course in "ground" instruction for future flyers was open to students from other schools and colleges Page 1184on the campus. Because of the large enrollment in Theory of Aviation, it was taught in two sections. In June, 1918, four students qualified for the degree of bachelor of science in engineering (aeronautical engineering), and W. F. Gerhardt, the first student to qualify for an advanced degree in aeronautics, was granted the degree of master of science in engineering (aeronautical engineering). Pawlowski was promoted to an associate professorship of aeronautical engineering and was relieved of further teaching duties in mechanical engineering.
During the first semester of 1918-19, work in aeronautical engineering was conducted by Pawlowski alone. Most of the thirty-four students in the first course wished to qualify for service in the Air Corps. Ten students enrolled in the courses offered in the second semester, only two of whom qualified for the bachelor's degree. The two built a dynamometer for the small wind tunnel.
Pawlowski was granted leave of absence during the first semester of 1919-20 to organize aeronautical research for the Polish army. In 1920-21 he taught all of the courses except Dynamic Stability, which was given by Professor Ziwet, of the Mathematics Department. Thirty-seven students were registered in aeronautical engineering during this year, and eight were graduated with the bachelor's degree.
Edward Archibald Stalker ('19e[Ae.E.], M.S.E. '23), after two years of experience in airplane design with the Stout Engineering Laboratories, Dearborn, Michigan, was appointed Instructor in Aeronautical Engineering in 1921 to relieve Pawlowski of part of his teaching load in order that he might have time for original investigations.
Plans for the East Engineering Building provided for a new wind tunnel, and in 1924 Pawlowski visited Europe to study the development of aeronautics and to obtain information to aid in the installation of the wind tunnel, which was finally completed with the aid of a gift of $28,000 from the Guggenheim Fund in 1926. An additional amount of $50,000 from the same fund was also given to provide a professorship of applied aeronautics for ten years. Lawrence Vincent Kerber ('18e [Ae.E.], A.E. '36), who was appointed to this position, came to the University in February, 1927. He assisted with the instruction in design and developed a course in aerial transportation, the first to be given by a university. One project undertaken in 1928-29 led to Professor Stalker's discovery that sucking off the boundary layer (the thin layer of air at the surface of a wing) is an effective method of delaying wing "stalling."
In January, 1927, the staff, assisted by members of other departments and by officers of the Naval Reserve Corps of Detroit, conducted a newly approved course in naval aviation to train men for the Naval Reserve Air Force. In 1928 the department purchased a discarded plane from the government for use in this course.
In 1929-30 the department, with 254 students, had the largest enrollment in the College. A total of 129 bachelor's degrees, eighteen master's degrees, and one doctor's degree had been granted by the end of 1929. Walter Francis Burke (Massachusetts Institute of Technology '29, M.S. Michigan '32) was appointed Instructor in this year. A committee composed of Professors Pawlowski and Stalker, Assistant Professor Frank N. M. Brown ('28e[Ae.E.], M.S.E. '32), who had come in 1928, and W. Burke administered the work in aeronautical engineering. Pawlowski was appointed to the Guggenheim professorship formerly held by Professor Kerber, who had resigned in 1929, following a year's leave of absence Page 1185with the Department of Commerce.
During the year a new balance, which required only one man for operation, was constructed. This improvement facilitated various research projects. The facilities of the Aerodynamics Laboratory were used by a number of companies, including the Goodyear Tire and Rubber Company, the General Tire Company, and the Stout Engineering Laboratories.
According to the Regents' Proceedings of September, 1930, almost twenty years after Dean Sadler and Professor Pawlowski had roused interest in aviation at the University of Michigan, "The Department of Aeronautical Engineering was established as an organization separate from the Department of Marine Engineering with which it has to this time been merged." In November, Professor Stalker was appointed head of the department.
With the depression enrollment began to drop until by 1933-34 it had reached a low of 176 students. A few changes in staff took place in these years. Milton John Thompson ('25e [Ae.E.], M.S.E. '26, Sc.D. Warsaw '30) was appointed Assistant Professor of Aeronautical Engineering in 1930, and in 1933 Burdell Leonard Springer ('32e [Ae.E.], M.S. '33) replaced Burke, who requested a leave of absence to accept a position with the General Aviation Corporation.
Research became increasingly important in the development of the department. The wind tunnel was used to conduct tests for industrial concerns, and a study of the downflow of gases behind power plant gas stacks and other experiments were carried out as a result of the acquisition of a multiple-tube manometer and a darkroom for photographic work. New equipment was acquired as the need for it arose. A suction blower and auxiliary equipment were installed to test the resistance of automobiles by streamlining. With the aid of C.W.A. labor in 1933-34, a propeller dynamometer, a small portable smoke tunnel, and a dynamic stability dynamometer were completed. In 1934 a small wind tunnel was constructed with the aid of F.E.R.A. and N.Y.A. labor. This tunnel was a single-return type with a cross section twenty-one inches by thirty-two inches at the experimental chamber.
In 1935-36 both Professor Pawlowski and Professor Stalker were granted leave of absence because of illness, and Edward Irwin Ryder ('33e [M.E.], M.S.E. '34), of the Hammond Aircraft Company of Ypsilanti, was appointed Teaching Fellow in order to lessen the teaching load. Milton J. Thompson was made acting chairman of the department in the absence of Stalker.
The Army and Navy in 1935-36 began to send graduate officers to the University for instruction in aeronautics. A larger staff was needed in order to improve the courses offered and to develop a program of research. Charles S. J. MacNeil, Jr. (Massachusetts Institute of Technology '33e[Ae.E.]), was engaged as Lecturer during the second semester of 1936-37 to handle advanced work in aircraft propellers and performance. The student branch of the Institute of the Aeronautical Sciences was organized during the year with Springer serving as honorary chairman.
Research projects were completed by the staff for such groups as the National Advisory Committee for Aeronautics, Beech Aircraft Corporation, United Electric and Manufacturing Company, the Bell Aircraft Corporation, and the Wincharger Corporation. Extensive commercial tests of a one-tenth scale model of the "Electra," America's first all-metal transport airplane, were conducted for the Lockheed Aircraft Company of Burbank, California. These tests were later substantiated by flight tests of the Page 1186production airplane. One student, Clarence L. Johnson, who had helped with the tests, later became chief research engineer for the Lockheed Aircraft Company. He also received the Sperry Award in 1938.
In 1937-38 Emerson Ward Conlon (Massachusetts Institute of Technology '29e [Ae.E.]) was appointed Instructor to fill the vacancy caused by Springer's resignation, and Stalker resumed his duties as chairman of the department.
Two new design courses, Airplane Structures Laboratory and Applied Aerodynamics, were added to the curriculum in 1938-39. Upon completion of a pilot-training course which was set up in 1939 under sponsorship of the Civil Aeronautics Authority, the student received a civilian pilot's license. The program proved to be very successful, branching into primary and secondary courses. In 1942, because it was conducted in connection with the war effort, the name of the course was changed from Civilian Pilot Training to War Training Service. It was discontinued in August, 1943. At this time Conlon began research on the application of magnesium to aircraft structures. The department co-operated with the Dow Chemical Company in producing a wing Model SNJ-1 airplane for the Bureau of Aeronautics. This was the first magnesium aircraft structure to demonstrate a definite weight saving over the corresponding aluminum alloy structure.
In 1940 Associate Professor Thompson resigned to accept a position in charge of the aeronautical courses at the University of Texas. Arnold Martin Kuethe (Ripon '26, Ph.D. California Institute of Technology '33) succeeded him in 1941, and Franz Russell Steinbacher (New York '38e [Ae.E.], M. S. Michigan '42) was appointed Instructor. During World War II more changes occurred in the staff. Conlon, who had been promoted to Associate Professor of Aeronautical Engineering was called to active duty with the United States Naval Aeronautical Reserve in 1942, and Stalker resigned to enter industry. Professor Kuethe became acting chairman, and Associate Professor Edgar James Lesher (Ohio State '37, M.S.E. Michigan '40), who had served on the staff in 1939-40, returned in the fall of 1942.
The department participated in the training of aircraft inspectors for the Army Air Force under the E.S.M.W.T. program from 1942 to 1944. Steinbacher taught an extension course, Airplane Structures, and Lesher taught one listed as Aerodynamics. Under the same program Kuethe gave a course in Dynamics of Compressible Fluids, and Jacque Houser (Alabama Polytechnic Institute '42e [Mech. Eng.], M.S.E. Michigan '44), Teaching Fellow in Aeronautics, taught courses in aircraft inspection.
In research, Kuethe supervised the construction of special equipment for the Wright Field Wind Tunnel at Dayton, Ohio. Airplane model tests were conducted for the Ford Motor Company and the Lee Wendt Company of Chicago; wind pressure and rain penetration tests were carried out for the Celotex Corporation of Chicago; and an investigation of the pressure distribution over a steel hut for Army use was completed for the Stran Steel Division of the Great Lakes Steel Corporation. Several confidential projects were undertaken for the Army Air Force in conjunction with the Engineering Research Institute.
In 1944-45 Steinbacher was granted a leave of absence to do work in structural research for the Douglas Aircraft Company. Lesher was also granted leave to work on special projects for the Page 1187Stinson Division of the Consolidated Vultee Aircraft Corporation of Wayne, Michigan.
Julius David Schetzer ('39e, M.S. '44) was appointed Assistant Professor in 1944. In October, 1945, Conlon was released from duty as Commander with the United States Naval Reserve and returned to the University as Professor and chairman of the department. In the following year Professor Wilbur Clifton Nelson ('35e [Ae.E.], M.S.E. '37) and Associate Professor Myron Hiram Nichols (Oberlin '36, Ph.D. Massachusetts Institute of Technology '39) joined the staff. Lawrence Lee Rauch (Southern California '41, Ph.D. Princeton '49) came to the staff as Assistant Professor in 1949. Hans Peter Liepman (Dipl. Ing. Swiss Institute of Technology '37, M.S. Harvard '39) was appointed Lecturer and given charge of the supersonic wind tunnel at Willow Run. While Conlon was on leave of absence in 1950 he served as Technical Director of the Air Force project (Arnold Engineering Development Center) at Tullahoma, Tennessee, and Kuethe again served as acting chairman of the department.
During this period the visiting teachers in the department included: Professor David J. Peery, head of the Aeronautical Engineering Department at Pennsylvania State College, Professor K. D. Wood, chairman of the Department of Aeronautical Engineering at the University of Colorado, Sir Richard V.H. South-well, of the Imperial College of London, Professor F. R. Shanley, of the University of California, and Dr. Leslie S. G. Kovasznay, Johns Hopkins University. In 1951, preparatory to a revision of the curriculum in aeronautical engineering, Associate Professor Schetzer visited various aircraft companies to study their needs. He obtained valuable information on dynamics of the airplane at the Douglas Aircraft Company in Santa Monica, California, which he presented in a series of lectures to the Douglas engineers.
In 1953 the professorial staff included: E. W. Conlon, A. M. Kuethe, W. C. Nelson, M. H. Nichols, J. D. Schetzer, and L. L. Rauch; E. J. Lesher; John William Luecht ('42e [Ae.E.]) and Robert Milton Howe (California Institute of Technology '45, Oberlin '47, Ph.D. Massachusetts Institute of Technology '50).
More than thirty years after he had first kindled an interest in aviation at the University of Michigan, Professor Pawlowski retired in 1946 to live at Pau, France. Members of the department and all others who knew him experienced a feeling of great loss when news was received that he had died on February 17, 1951.
Much-needed space for the department was provided in 1946-47 when the addition to the East Engineering Building was built. The staff moved into its quarters in the new wing in November, 1947. Offices, two instrumentation laboratories, and several classrooms were on the first floor. The Aerodynamics, Design, Transportation, and Propulsion laboratories were on the fourth floor, and the Structures Laboratory was in the basement of the new wing. The small supersonic wind tunnel was moved to the Aerodynamics Laboratory on the fourth floor, and a 120,000-pound Tinius Olsen testing machine was installed in the new Structures Laboratory. The subsonic wind tunnel was modernized and converted to a closed-throat type. A six-component two-parameter strain gage type of balance system was designed, built, and installed, and a new fan having eight adjustable blades was installed.
On January 24, 1947, the Regents accepted the deed to the Willow Run Airport and Army Air Base from the War Assets Administrator. In doing so the University planned to improve facilities Page 1188for instruction and research in the field of aeronautics. It was also intended that so valuable a property should not be abandoned, but should be maintained for public airport purposes and future emergency. A plan was devised whereby landing field and hangar facilities were made available to the airlines through lease; the University retained space for a new and enlarged postwar program of aeronautical instruction and research.
The Department of Aeronautical Engineering, through the Department of Engineering Research, negotiated the contract, known as the Wizard Project, with the Air Materiel Command in April, 1946, for an engineering study of a defensive guided missile. Another project, initiated for the Signal Corps in 1946, involved the measurement of atmospheric temperatures up to forty miles' altitude and the determination of the relative amounts of helium, neon, argon, and nitrogen in the upper atmosphere. To house these projects and provide the necessary laboratory facilities, the Aeronautical Research Center was developed at Willow Run Airport. Hangar No. 1, Bay 1, and parts of Bay 2, were remodeled into offices, stock room, drafting rooms, machine shop, electronics laboratory, and a model shop. A laboratory for upper-atmosphere research was also installed; this project, however, was moved to the University campus in November, 1947.
The large supersonic wind tunnel in the Warm-up Hangar at the Aeronautical Research Center was completed and put into operation in 1948. Laboratories for photographic work, turbulence, and propulsion were also constructed in the Warm-up Hangar. In Building 22 space was utilized for propulsion test equipment.
The department was relieved of its connection with Project Wizard in 1950, when the Engineering Research Institute became responsible for it; the Aeronautical Research Center became the Willow Run Research Center. The supersonic wind tunnel and most of the propulsion facilities at Willow Run were retained under the technical supervision of the department.
In addition to the various government contracts, research has been completed during recent years for such aircraft companies as Goodyear Aircraft, the Consolidated Vultee Aircraft, Bendix Aviation, Grumman Aircraft Engineering, and Boeing Airplane, and for such automotive concerns as Ford Motor, Studebaker, Kaiser-Frazer, and General Motors. Contracts have also been completed for the following companies: Palmer Bee, the Dow Chemical, Askania Regulator, the Pullman Standard, Outdoor Advertising, and the Stalker Development Company, which was founded by Edward A. Stalker, chairman of the department from 1930 to 1942.
Although enrollment in the department has fluctuated with the economic situation of the country and with the needs of industry it rose to an all-time high of 358 undergraduate and nineteen graduate students in 1940-41. During World War II, in 1943-44, there were only 181 undergraduates and six graduates, but the number increased to 270 undergraduates and seventy graduates in 1947-48. The tremendous increase in graduate students was due not only to the expansion of graduate courses and qualified staff but to the special post-graduate Pilotless Aircraft course for the specialized training of officers for the Air Force. This course, first offered in February, 1946, has been changed to the Guided Missiles Program for Air Force personnel, which covers two calendar years and one summer session.
In 1946-47, graduate courses in propulsion Page 1189and instrumentation, particularly in guided missiles, were added to the curriculum and later expanded. Courses such as Advanced Experimental Aerodynamics, Dynamics of Perfect Fluids, Dynamics of Compressible Fluids, Dynamics of Viscous Fluids, Nuclear Energy for Aircraft Propulsion, and Guidance of Pilotless Aircraft were introduced or expanded to cover new material. The nuclear energy course has been discontinued in favor of a more extensive nuclear engineering program in the College, directed by a committee under the chairmanship of Professor Rauch.
Each year the number of doctor's degrees awarded by the department has increased. Seven were granted in 1952. In 1948-49 the department attempted to standardize the requirements for the master's degree according to the student's option (aerodynamics, structures, propulsion, or design). Of the thirty hours required for the master's degree twenty were prescribed and the remaining ten were elective.
Early in 1951, at the request of the Air Force Institute of Technology, the guided missiles curriculum for Air Force officers was revised by Kuethe and Rauch to permit greater concentration of effort in the field of guidance and control. Professor Nichols was made director of the program upon his return to the department.
Members of the staff have been active in engineering associations, such as the Institute of the Aeronautical Sciences, The American Society for Engineering Education (formerly S.P.E.E.), The Aero Club of Michigan, and the Michigan Society for Professional Engineers. Several have served on the advisory or editorial boards of the Journal of the Aeronautical Sciences and the Applied Mechanics Reviews.
Books by members of the staff include Stalker's Principles of Flight (1931); Thompson's Fluid Mechanics (1937) (with Russell A. Dodge, Professor of Engineering Mechanics), Nelson's Airplane Propeller Principles (1944), and Kuethe and Schetzer's Foundations of Aerodynamics (1950).