Aerodynamics

I

Introduction

Aerodynamics, branch of fluid mechanics that deals with the motion of air and other gaseous fluids, and with the forces acting on bodies in motion relative to such fluids. The motion of an airplane through the air, the wind forces exerted on a structure, and the operation of a windmill are all examples of aerodynamic action (see Airplane).

II

Lift

One of the fundamental forces studied in aerodynamics is lift, or the force that keeps an airplane in the air. Airplanes fly because they push air down. The leading edge of an airplane wing is slightly higher than the trailing edge when the plane is maintaining altitude. As the wing moves through the air, it deflects the air that flows underneath it downward. Air flowing over the top of the wing follows the surface of the wing and is also deflected downward. The third law of motion formulated by English physicist Sir Isaac Newton states that every action causes an equal and opposite reaction (see Mechanics: The Third Law). As the wing pushes the air down, the air pushes the wing up. Lift is also often explained using Bernoulli’s principle, which relates an increase in the velocity of a flow of fluid (such as air) to a decrease in pressure and vice versa. The pressure on the upper side of an airplane wing is lower than that on the lower side, and many engineers use equations derived from Bernoulli’s principle to design aircraft.

Another important aspect of aerodynamics is the drag, or resistance, acting on solid bodies moving through air. The drag forces exerted by the air flowing over the airplane, for example, must be overcome by the thrust force developed by either the jet engine or the propellers (see Propeller). These drag forces can be significantly reduced by streamlining the body. For bodies that are not fully streamlined, the drag force increases approximately with the square of the speed as they move rapidly through the air. The power required, for example, to drive an automobile steadily at medium or high speeds is primarily absorbed in overcoming air resistance.

III

Supersonics

Supersonics, an important branch of aerodynamics, concerns phenomena that arise when the velocity of a solid body exceeds the speed of sound in the medium, usually air, in which it is traveling. The speed of sound in the atmosphere varies with humidity, temperature, and pressure (see Sound). Because the speed of sound, being thus variable, is a critical factor in aerodynamic equations, it is represented by a so-called Mach number, named after the Austrian physicist and philosopher Ernst Mach, who pioneered the study of ballistics. The Mach number is the speed of the projectile or aircraft with reference to the ambient atmosphere, divided by the speed of sound in the same medium and under the same conditions. Thus at sea level, under standard conditions of humidity and temperature, a speed of about 1220 km/h (about 760 mph) represents a Mach number of one, that is, M-1. The same speed in the stratosphere, because of differences in density, pressure, and temperature, would correspond to a Mach number of M-1.16. By designating speeds by Mach number, rather than by kilometers or miles per hour, a more accurate representation of the actual conditions encountered in flight can be obtained.

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A

Shock Waves

Studies of artillery projectiles in flight, by means of optical observations, disclose the nature of the atmospheric disturbances encountered in supersonic flight. A series of such photographs discloses the following characteristics of flight. At subsonic speeds, that is, below M-0.85, the only atmospheric disturbance is a turbulence in the wake of the projectile. In the transonic range, from M-0.85 to M-1.3, shock waves appear as speed increases; in the lower part of this speed range shock waves arise from any abrupt breaks in the smooth contour of the projectile. As the speed passes M-1, shock waves arise from the nose and tail and are propagated from the projectile in the form of a cone, which has an apex angle inversely proportional to the speed of the projectile. Thus, at M-1, the nose wave is essentially a flat plane; at M-1.4 (1712 km/h, or 1064 mph at sea level) the angle of the cone is about 90°; and at M-2.48 (about 3060 km/h, or about 1900 mph), the shock wave preceding the projectile has a conical angle of slightly less than 50°. This line of research has already made possible the design of modern high-speed airplanes, in which the wings are swept back at angles as great as 60°, to avoid the shock wave from the nose of the plane.

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