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What happens to the strength, the length, and the location of a normal shock wave when the Mach number (M) is increasing at positive lift? The strength of the shock wave...
• A
decreases, its length decreases, and the shock wave moves downstream.
• B
increases, its length increases, and the shock wave moves upstream.
• C
decreases, its length decreases, and the shock wave moves upstream.
• D
increases, its length increases, and the shock wave moves downstream.

Refer to figure.
An airplane does not have to be flying at the local speed of sound for the airflow over the wings to be supersonic. A normal shockwave can develop at any time the airplane is in the transonic speed range. The function of the wing is to accelerate the air passing over its upper surface. The airflow may be subsonic at the leading edge of the wing but can be accelerated to become supersonic by the trailing edge of the wing, despite the fact that the airplane is flying at a speed of less than Mach 1.0. This is transonic flight, which usually occurs between Mach 0.75 and Mach 1.2.

At speeds less than Mach 0.75 all the airflow over the wing is subsonic and as the airplane accelerates the first shockwave will form on the upper surface at the wing root.
When the air over the upper surface of the wing becomes sonic, a shockwave forms because the pressure waves over the rear of wing at the wing root are attempting to move forward and meet the air moving aft from the leading edge at exactly the same speed but in the opposite direction. The meeting point is usually just aft of the point of maximum camber because this is where the air over the upper surface of the wing is accelerating at the greatest rate.

The meeting point of the two air-pressure waves is the shockwave and is where the airflow changes from being sonic or supersonic to become subsonic. Air passing through the shockwave experiences an increase of pressure, temperature, and density. If the airplane speed continues to increase the area of supersonic air on top of the wing spreads backward towards the trailing edge, resulting in the shockwave moving aft.

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