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 aeroplane through the air, the wind forces exerted on a structure, and the operation of a windmill are all examples of aerodynamic action.

II

Bernoulli's Principle

One of the fundamental laws governing the motion of fluids is Bernoulli's principle, which relates an increase in flow velocity to a decrease in pressure and vice versa. Bernoulli's principle has traditionally been used in aerodynamics to explain the lift of an aircraft's wing in flight. A wing, or aerofoil, is so designed that air flows more rapidly over its upper surface than its lower one, leading to a decrease in pressure on the top surface as compared to the bottom. The resulting pressure difference provides the lift that sustains the aircraft in flight. Recent explanations, however, have focused on the momentum of air displaced by the aircraft wing as being the key factor in lift and sustaining flight.

The velocity of a wind that strikes the bluff surface of a building is close to zero near its wall. According to Bernoulli's principle, this leads to a rise in pressure relative to the pressure farther away from the building, resulting in wind forces that the structure must be designed to withstand.

Bernoulli's principle also governs the wind forces affecting other aerodynamically shaped surfaces. The sail of a yacht in motion, for example, forms an aerofoil (see sailing). Racing cars are low-slung, so that air travels fast through the narrow space between the body and the ground. This reduces the pressure beneath the car and pulls it firmly downwards, improving its road-holding. There is also an aerofoil, shaped like an upside-down aircraft wing, mounted on the rear of the car to increase the downward force.

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 aeroplane, for example, must be overcome by the thrust force developed by either the jet engine or the propellers. 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 a car 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 speed of a solid body exceeds the speed of sound in the medium, usually air, in which it is travelling. The speed of sound in the atmosphere varies with humidity, temperature, and pressure. 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 relative 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 1,220 km/hr (760 mph) represents a Mach number of 1. The same speed in the stratosphere, because of differences in density, pressure, and temperature, would correspond to a Mach number of 1.16. By designating speeds by Mach number, rather than by kilometres per hour or miles per hour, a more accurate representation of the actual conditions encountered in flight can be obtained.

A

Shock Waves

Studies of artillery projectiles in flight, by means of optical observations, disclose the nature of the atmospheric disturbances encountered in flight. At subsonic speeds, that is, below Mach 0.85, the only atmospheric disturbance is a turbulence in the wake of the projectile. In the transonic range, from Mach 0.85 to Mach 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 Mach 1, shock waves arise from the nose and tail and are propagated from the projectile in the form of a cone. The apex angle is smaller the greater the speed of the projectile. Thus at Mach 1 the nose wave is essentially a plane; at Mach 1.4 (1,712 km/hr, or 1,060 mph at sea level) the angle of the cone is about 90°; and at Mach 2.48 (about 3,030 km/hr, or about 1,885 mph), the shock wave trailing from 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 aeroplanes, in which the wings are swept back at angles as great as 60° to avoid the shock wave from the nose of the plane.