Effects of Compressibility on High Speed Flight

Abstract

T T has been known tha t the air forces exerted -** on aerodynamic bodies follow the so-called square law except for the effects of scale and compressibility. The effects of dynamic scale, or Reynolds number, have become rather well known during the last 10 years, largely because of research conducted in the N.A.C.A. variabledensity wind tunnel. The effects of compressibility have commonly been neglected because until the relatively recent development of the last Schneider trophy aircraft the speeds have been low as compared with the velocity of sound, and the consequent local pressures over the surfaces of high speed airplanes have differed but slightly from atmospheric pressure. At the present time, however, the speeds associated with the fastest airplanes approach 60 percent of the velocity of sound, and the induced velocities over their exposed surfaces lead to local pressures tha t differ appreciably from the pressure of the atmosphere. When this condition exists air can no longer be regarded as an incompressible medium. The effects of compressibility on the aerodynamic characteristics of airfoils have been under investigation by the N.A.C.A. in the high speed wind tunnel, and it is the purpose of this paper to examine the possibility of further increases in speeds in the light of this relatively recent research. The following computations are made for a hypothetical airplane which, however, is not beyond the limits of possibility. The airplane is a scaled-up model which has been tested in the variable-density wind tunnel as part of an extensive investigation of wing-fuselage interference, and represents one of the best wingfuselage combinations thus far produced. The geometric scale factor applied to the model dimensions was determined from the ratio of the model fuselage diameter to the diameter of a fuselage sufficiently large to house a 2300horsepower Rolls-Royce R type engine. The model from which the airplane is derived is a mid-wing cantilever monoplane having a fuselage of circular cross section, and a straight tapered wing having the N.A.C.A. 0018 section at the center and the N.A.C.A. 0009 section at the tip. The principal full-scale airplane dimensions are given in Table I. An outline drawing of the airplane for which computations will be made is given in Fig. 1.