268 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1955 



To succeed Dr. Lewis, the Committee in 1947 chose the Associate 

 Director of the National Bureau of Standards, Dr. Hugh L. Dry den. 

 He was no stranger to the NACA. Trained in physics and mathe- 

 matics by Dr. Ames at Johns Hopkins University, he had gone to the 

 Bureau of Standards in 1917, where he soon earned an international 

 reputation by his aerodynamic researches in turbulence and boundary 

 layer. His new task at the NACA was extremely difficult, yet it was 

 vital to the Nation that a "new look" at the postwar situation be taken, 

 and new objectives defined in terms of supersonic jet-propelled ve- 

 liicles potentially available for the worldwide exercise of air power 

 and, eventually, for civil air transportation. 



At the end of World War II, the most urgently sought goal was 

 attainment of practical flight at supersonic speed. It was realized 

 that success in this effort required new knowledge which could be 

 obtained only with new tools and new techniques. Even before the end 

 of the war efforts were made to acquire needed data. Efforts to de- 

 velop useful transonic aerodynamic theory had failed and it was neces- 

 sary to resort to direct experimentation at velocities passing through 

 the speed of sound. The fact that the principal tool of aerodynamic 

 research, the wind tunnel, was subject to "choking" phenomena near 

 the speed of sound forbade its use for the critical experimentation. 

 Entirely new techniques had to be devised. The NACA's attack was 

 broadened to include all approaches whicli offered promise. 



The earliest attempt used especially instrmnented aerodynamic 

 bodies dropped from a high altitude, but it was not until late in 1943 

 that advances in radar and radiotelemetering equipment made it pos- 

 sible to obtain reliable data by this method. Even then, the velocity 

 of a free-falling body seldom went much beyond a Mach number of 1 

 (M==l equals the speed of sound). 



Other attempts sought to use the acceleration of airflow above a 

 curved surface. Small model wings were mounted near the leading 

 edge of the wing of an airplane. In this way, lift, drag, and other aero- 

 dynamic characteristics of the model were measured. The method was 

 employed also to study stability and trim of airplane shapes in the 

 transonic speed range. The same principle of accelerating airflow was 

 tried with small models positioned over a "hump" in the test section of 

 a subsonic wind tunnel, but scale effects complicated the interpretation 

 of test results for use in design. 



Use of rocket-propelled models fired from the ground followed the 

 first work with free-falling bodies by about a year. As instrumenta- 

 tion has been improved, this technique has become a valuable tool for 

 transonic research. By the addition of powerful booster rockets, 

 models of this kind are being used to study aerodynamic problems at 

 speeds ranging up to a Mach number of 10 and higher. The fact that 



