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Aviation History
1945
1945 - 0497.PDF
MARCH T5TH, 1945 FLIGHT Compressibility An Analysis of Basic Factors : Some Practical Experiences with the Republic P-47 in Dives : Sources of Buffeting Bv COSTAS ERNEST PAPPAS NATURE appears determined to prevent man from travelling fast in the air. In the upper levels where the air is " thin " and where consequently the resistance to forward mstion is low, jet propulsion affords an opportunity for very high speeds. But unfortunately it is also a law of nature that the airflow around the components of an aircraft changes when the speed of sound is reached or even approached, and until we have learned how to design for ths altered flow, troubles are encountered, as almost every aircraft designer has discovered. To make matters worse, the speed of sound falls off with altitude, so that the attractions of high-speed travel at great heights are difficult of attainment. The present article is based on a paper read by the author before the Albany Society of Engineers and is reproduced here by courtesy of our New York contemporary Aviation. The author is chief of ths aerodynamics department of the Republic Aviation Corporation. • —/ns/a PROBLEMS associated with high-speed flight aremanifested because air is compressible—and fiieseproblems are, in truth, very complex. Compressibility is the term which aeronautical engineers spiise for a large class of aerodynamic effects associated with high speed. This designation has been adopted because changes which occur in fluid flow at high speeds .are' chiefly produced by the ilas: ticity or compressibility of the air. If air were an incompressible medium, these effects would not be present. Associated with compressibility is the density of air, which changes rapidly, in turn causing the airflow to change over the surface of the body. At speeds approaching that of sound- approximately 760 m.p.h. at sea level—the aerodynamicist is confronted with the problem of the airflow changing in char- acter due to rapid changes in density resulting from, compressibility properties of the atmosphere. The practical significance of compressi- bility effects in aeronautics lies in the fact that the lift and drag of supporting surfaces, the effective operation of control surfaces, and the operation of the air- screw with respect to efficiency and capacity, are all affected since the actual fluid is compressible. The lilt, drag, and moment of a body are functions of the ratio of speed of the body to the speed of sound at the altitude in question. This ratio is called the Mach number after the Austrian physicist, Ernest Mach, who specialised in the study of high-speed phenomena. neous position of particle of air flow streamline /incompressible flaw streamline Fig. Path of particle Comparative position of streamlines for compressible and incom- pressible flow. A rathtr abstract appreciation of the foregoing is afforded by a consideration of the elementary forces acting on a. fluid particle. For our purposes it suffices to name prin- cipal forces. First is the well-known D'Alembert force of acceleration. This force is proportional to the square of the velocity with which the particle executes its motion, and the mass of the particle. Evidently the force is very small unless the velocity is high, but the force increases very rapidly as the velocity is increased. Second is the force of pressure acting differently on oppo- site sides of the particle. This force servos to balance the D'Alembert reaction. It is therefore called into being bv the motion, and it disappears when the motion ceases. Let us agree that viscosity forces are small and may be neglected within the first order approximation. Our picture now is of a particle of fluid executing an irregular motion in such a way as to avoid an airfoil which moves through the region initially occupied by the particle Probable path of particle End pointy of particle Fig. i. Diagram showing probablepath of a particle of air over the top surface of an aerofoil. At rightis shown the developed motion of the particle relative to the aerofoildatum (Figs. 1 and 2). Since the particle is accel- erated from rest and follows an irregular path thereafter, a complicated system of inertia forces appears on the moving par- ticle. These forces are balanced by pressure differences which appear between different points of the fluid. Since inertia forces are small with small acceleration, we expect small pressure differences at low speeds However, as the speed increases, the pressure forces required increase according to the square of the velocity. At sufficiently high speed these
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