40 YEARS OF AERONAUTICAL RESEARCH — HUNSAKER 257 



method now in universal use is to suspend the model from suitable balances in 

 a stream of air ... at a velocity of 60 mph . . . The balances register the 

 forces and moments acting on the model. From the results of such measure- 

 ments one decides whether the original design is good or not. But is one justified 

 in making such a decision? Why should the same laws apply to a little model 

 inside the wind tunnel, as it is called, and to the actual airplane flying freely 

 through the air? Evidently there is ground for grave uncertainty. The Com- 

 mittee has perfected a method for obviating this. It has been known from 

 aerodynamic theory for some time that tlie change in scale, from airplane to its 

 model, could be compensated by compressing the air from ordinra-y pressure 

 to 20 or 25 atmospheres: as the structure moving through the air is reduced in 

 size from 50 feet to 2 feet, tlie molecules of the air are brought, by comparison, 

 closer and closer together until their distance apart is one twenty-fifth of what 

 it was originally. The effect of scale is thus fully compensated and experiments 

 upon a model in this compresed air have a real meaning. The Committee has 

 constructed a large steel tank, 34 feet long and 15 feet in diameter, inside which 

 is placed a wind tunnel with its balances, etc.. and in which the air may be kept 

 in a state of high compression. The information to be obtained from the appa- 

 ratus will be the most important ever given airplane designers. 



Experience with simple airplane models without propellers in the 

 variable-density tunnel encouraged the NACA, in June 1925, to con- 

 struct a wind tunnel large enough to test full-scale airplane propellers 

 under conditions of flight. This was a costly decision, but the cost 

 was repaid manyfold by improved airplane performance. 



The propeller research tunnel was put into operation in 1927. It had 

 a circular test section 20 feet in diameter and was powered by two 

 Diesel engines rated at 1,000 hp. each. Its air speed was 110 mph. 

 and, at the time, it was the largest wind tunnel in the world. Almost 

 from the beginning of its use, the PRT provided information leading 

 to design changes which resulted in dramatic improvements in air- 

 plane performance. 



The first and most spectacular of these productive researches 

 brought about the development of what became known as the NACA 

 cowling for air-cooled radial engines. In its 1928 report, the Com- 

 mittee said that "by the application of the results of this study to a 

 Curtiss AT-5A Army pursuit training plane, the maximum speed was 

 increased from 118 to 137 mph. This is equivalent to providing 

 approximately 83 additional horsepower without additional weight or 

 cost of engine, fuel consumption, or weight of structure. This single 

 contribution will repay the cost of the Propeller Research Timnel 

 many times." 



The Collier Trophy, awarded annually "for the greatest achieve- 

 ment in aviation in America, the value of which has been thoroughly 

 demonstrated by actual use during the preceding year," went to the 

 NACA for the development of this form of cowling. President 

 Hoover made the presentation on January 3, 1930 (for the year 1928), 

 and after the reading of the citation Dr. Ames responded that "a 

 scientist receives his reward from his own work in believing that he 



