Aerodynamics Essay, Research Paper

Used in Miami Florida, Palmetto Middle School, Mr. Sams.

Aerodynamics is a branch of fluid mechanics that deals with the motion of air and other gaseous fluids, and with the forces acting on bodies in motion. The motion of an airplane through the air, the wind forces exerted on a building, and the operation of a windmill are all examples of aerodynamic action.

Bernoulli’s Principle

One of the fundamental laws about 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 is used in aerodynamics to explain the lift of an airplane wing in flight. A wing is designed so that air flows more rapidly over its upper surface than its lower one, leading to a decrease in pressure on the top surface. The pressure difference provides the lift that keeps the aircraft in flight. The velocity of a gust of wind that strikes the surface of a building is close to zero near its wall. According to Bernoulli s principle, this would lead to a rise in pressure relative to the pressure away from the building, resulting in wind forces that the buildings must be designed to withstand. Another important aspect of aerodynamics is the drag, or resistance, on solid bodies moving through air. The drag forces exerted by the air flowing over the airplane, must be overcome by the thrust force from either the jet engine or the propellers. These drag forces can be significantly reduced by streamlining the body. On objects 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.

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 because the speed of sound is a critical factor in aerodynamic equations, it is represented by a Mach number, named after the Austrian physicist and philosopher Ernst Mach, who pioneered the study of ballistics. The Mach number is the speed of a projectile or aircraft with reference to the atmosphere, divided by the speed of sound under the same conditions. At sea level, under standard conditions of humidity and temperature, a speed of about 1220 km/hr (about 760 mph) represents a Mach number of one. The same speed in the stratosphere, because of differences in density, pressure, and temperature, would cause 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.

Shock waves

Studies of artillery projectiles in flight reveal the nature of the atmospheric disturbances encountered in supersonic flight. A series of such photographs discloses the following characteristics of flight. At subsonic speeds, below M-0.85, the only atmospheric disturbance is 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. At M-1, the nose wave is essentially a flat plane; at M-1.4 (1712 km/hr, or 1064 mph at sea level) the angle of the nose cone is about 90.; and at M-2.48 (about 3060 km/hr, or about 1900 mph), the shock wave in front the projectile has a cone-like 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.

Maximum efficiency

Other factors determined by research in the supersonic range of speeds of artillery projectiles include the shape of the projectiles and the rate of gas flow. The tear-drop shape, which is the ideal streamlined shape for subsonic speeds, is extremely uneconomical in the supersonic range. If gaseous flow occurs through a constricted tube, for example the nozzle of a rocket, at subsonic speeds, the speed of the flow increases and the pressure decreases in the throat of the constriction. The opposite phenomena take place at supersonic speeds, and speed of flow increases in a divergent tube. The exhaust gas of a rocket, increasing to sonic speed in the throat of a rocket nozzle, further increases its speed and thrust in the flare of the nozzle, and it multiplys the efficiency of the rocket system. Another factor, long known to rocket designers, is the direct influence of atmospheric pressures on the efficiency of the flight of planes in supersonic speed ranges. That is, the closer the surrounding atmosphere is to a perfect vacuum, the more efficient is the power plant of the plane. The range of the supersonic plane can also be increased by reducing the area, or cross section, displacing atmosphere. Increasing the weight by increasing the length, but at the same time making the plane more slender and equipping it with a needle nose, are necessary features of design for planes operating in the supersonic range in the atmosphere. In the years following World War II, the U.S. Air Force and the U.S. Navy established research institutions that included among their facilities wind tunnels capable of testing plane models and airplane parts in currents of air traveling at supersonic speeds.

Area rule

A major development in aeronautics resulting from wind-tunnel research was the discovery by the American physicist Richard Travis Whitcomb of the area rule, a new principle for the design of supersonic aircraft. According to this principle, the sharp rise in drag that occurs at transonic speeds results from the distribution of the total cross-sectional area at each point along the airplane. By pinching in the fuselage where the wings are attached, the reduction in the combined cross-sectional area of the fuselage and the wing produces a decrease in the drag characteristics of the aircraft. Whitcomb s so-called wasp-waist design made possible an increase of 25 percent in the supersonic-speed range without requiring any additional engine power.