David Gregory, age 16, of Kalispell, Mont., for his question:
CAN YOU EXPLAIN AERODYNAMICS?
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 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.
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.
Bernouilli's principle is used in aerodynamics to explain the lift of an airplane wing in flight. A wing 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.
The velocity of a wind that strikes the bluff surface of a building is close to zero near its wall. According to Bernouilli's principle, this would lead to a rise in pressure relative to the pressure away from the building, resulting in wind forces that the structures must be designed to withstand.
Another important aspect of aerodynamics is the drag, or resistance, acting on solid bodies moving through the 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. 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 minimum or high speed is primarily absorbed in overcoming air resistance.
The term "supersonics" was formerly used more broadly to include ultrasonics.
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, 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 Mac 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 760 miles per hour 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 or M 1.16.
By designating speeds by Mach number, rather than by miles per hour or kilometers, a more accurate representation of the actual conditions encountered in flight can be obtained.