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Learn more- This is the first in a series of three educational films entitled "High Speed Flight". This film produced in 1956 looks at the aerodynamics of flight at speeds approaching the speed of sound, using animated sequences to illustrate important physical principles.
Sound travels through the air in a series of waves of compression and expansion. Sound waves spread out from their source in all directions and at the same speed, the speed of sound. The speed of sound varies according to the temperature of the air in which it travels. The higher the temperature, the faster sound travels. Since the temperature falls with increasing altitude, so the speed of sound falls with increasing altitude.
At high speeds, the exact relationship between the speed of an aircraft and the speed of sound is very important. The ratio of the aircraft's true air speed to the speed of sound is called the aircraft's Mach Number (M). At high speeds it is essential for the pilot to know the Mach Number, so Mach meters are fitted to all high speed aircraft.
The behaviour of the air flow around the aircraft during flight is extremely complex. During flight, the air flow slows down at the nose to form what is called the stagnation region. The air then speeds up as it passes round the curvature of the wing and slows down again towards the trailing edge of the wing section. These changes in speed cause changes in air pressure. All the variations in pressure together produce lift and drag.
At speeds approaching the speed of sound, the air flow speeds up as it passes over the wing and reaches a maximum speed at a certain point on the wing. The Mach Number here will always be greater than that of the aircraft as a whole, called the flight Mach Number. An aircraft may be flying at less than the speed of sound, but the speed of the air flow at the point on the wing may be moving at the speed of sound. The flight Mach Number when this happens is called the Critical Mach Number of the aircraft.
When the wing exceeds its Critical Mach Number, a sudden sharp region of increasing density forms on the wing. This is called a shock wave. It is a very narrow region where the pressure waves caused by the moving aircraft meet the air flow moving in the opposite direction, causing a pile up of air. At speeds approaching the speed of sound, the most important result of the shock wave is to cause the air flow to spread from the wing's surface. This is called shock induced separation. It produces a large turbulent wake that alters the pressure distribution. Violent buffeting may occur or there may be a sudden loss of stability and reduced effectiveness of the controls.
Two main wing designs have been adopted to reduce the problems of extra drag and loss of control caused by shock waves. The first is to use relatively thin wings. The thinner the wing, the less the air accelerates, which delays the onset of the shock wave. The second is to incorporate sweep back in the wing design. A special kind of sweep back is the crescent wing. Another, the delta wing, combines a high degree of sweep back with great strength. Sweep back and thin wings bring problems at low speeds, so designers must compromise between high speed and low speed requirements.
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