High-speed Airplanes Research Articles

Predetermination of Aircraft Engine Cooling Requirements for Specific Flight Conditions

F the installation of air-cooled aircraft engines, the design of cooling passages and cowlings follows, in general, a rational procedure, the principles of which are now fairly well understood.' The installation designer so proportions the resistance of his air entrance and exit passages to the resistance of the cooling passages of the engine itself that when his particular assumed value of dynamic head is applied across all three resistances in series, a suitable pressure drop across the engine cooling passages results. I t has become quite important to achieve the right design of cooling system, for if the value of baffle pressure drop thus obtained is too low, insufficient cooling results and the installation must be revised. On the other hand, if the baffle pressure drop is substantially greater than necessary, the power consumed in forcing the cooling air through the system becomes excessive and some high speed airplane performance is thereby sacrificed. Also, if an adjustable cowl skirt is contemplated, the designer is anxious to so proportion it that the position of the skirt giving lowest external drag corresponds with the high performance cooling requirements. For the past several years, the engine manufacturer has provided the designer with the equivalent orifice area of the engine baffle passages of his various models, which have been established by flow testing; and to facilitate the designer's calculations, direct reading charts relating cowl exit gap size to baffle pressure drop and cooling drag for any indicated air speed and altitude have been provided. Such a chart is illustrated in Fig. 1. A brief inspection of it will illustrate the wide variation in exit gap size required for maintaining a given baffle pressure drop at various air speeds, and also the severe cost in drag which results if an exit gap size excessive for the speed is selected. Until recently, however, one link in the chain of rational cooling system design procedure has been missing, namely, a quantitative readily workable correlation of the principal variables influencing engine cooling performance under stabilized conditions. This is required so that the airplane designer may select the ' 'suitable'' baffle pressure drop which he must provide under his assumed conditions of allowable maximum cylinder temperature, brake horsepower, r.p.m., specific fuel consumption, pressure altitude, and prevailing air temperature. Such a correlation, in direct reading chart form for the airplane designer's convenience, is explained herein, and some important relations which become evident from its application to specific assumed conditions are briefly discussed.

Effects of Compressibility on High Speed Flight

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.

Engine Cowl Rings

Abstract The radial air-cooled engine is a popular power plant. Its reputation with the public can be said to date from the summer of 1927, when it demonstrated its reliability and ruggedness in the hazardous long-distance flights of that year. Since then many refinements and improvements have been brought forth, but the drag of the radial engine has always been high. Large frontal areas are inherent with the radial engine, and as the power output of the present radial has increased, so has its frontal area. Until recently it did not seem likely that the radial engine could ever successfully compete with the liquid-cooled type as the power plant for use in high-speed airplanes. However, with the development of the N.A.C.A. type of cowling and the engine cowl ring by various aerodynamical laboratories, the big gap existing in actual ship’s performance between these two widely divergent power plants has been eliminated.