For a slow-moving airplane, the pressure waves travel out ahead of the airplane, traveling at the speed of sound. Shock WavesSound coming from an airplane is the result of the air being disturbed as the airplane moves through it, and the resulting pressure waves that radiate out from the source of the disturbance. If an airplane flies faster than Mach 5, it is said to be in hypersonic flight. Supersonic speed is from Mach 1.20 to 5.0. At this speed, the shock wave which formed on top of the wing during transonic flight has moved all the way aft and has attached itself to the wing trailing edge. When an airplane is flying at supersonic speed, the entire airplane is experiencing supersonic airflow. Transonic speed is typically between Mach 0.80 and 1.20. The speed at which the shock wave forms is known as the critical Mach number. The shock wave also causes the center of lift to shift aft, causing the nose to pitch down. Stability problems can be encountered during transonic flight, because the shock wave can cause the airflow to separate from the wing. The shock wave forms 90 degrees to the airflow and is known as a normal shock wave.
Over the top of the wing, probably about halfway back, the velocity of the air will reach Mach 1 and a shock wave will form. When an airplane is flying at transonic speed, part of the airplane is experiencing subsonic airflow and part is experiencing supersonic airflow.
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