Mach tuck

Mach tuck is an aerodynamic effect whereby the nose of an aircraft tends to pitch downward as the airflow around the wing reaches supersonic speeds; the aircraft will first experience this effect at significantly below Mach 1. This speed is known as the critical Mach number of the wing.

Mach tuck is usually caused by two things, a rearward movement of the centre of pressure of the wing and a decrease in wing downwash velocity at the tailplane both of which cause a nose down pitching moment. For a particular aircraft design only one of these may be significant in causing a tendency to dive, delta-winged aircraft with no foreplane or tailplane in the first case and, for example, the Lockheed P-38 in the second case. Alternatively, a particular design may have no significant tendency, for example the Fokker F28 Fellowship.

As an aerofoil generating lift moves through the air, the air flowing over the top surface accelerates to a higher local speed than the air flowing over the bottom surface. When the aircraft speed reaches its critical Mach number the accelerated airflow locally reaches the speed of sound and creates a small shock wave, even though the aircraft is still travelling below the speed of sound. The region in front of the shock wave generates high lift. As the aircraft itself flies faster, the shock wave over the wing gets stronger and moves rearwards, creating high lift further back along the wing. This rearward movement of lift causes the aircraft to tuck or pitch nose-down.

The severity of Mach tuck on any given design is affected by the thickness of the aerofoil, the sweep angle of the wing, and the location of the tailplane relative to the main wing.

The camber and thickness of the aerofoil affect the critical Mach number, with a more highly curved upper surface causing a lower critical Mach number.

On a swept wing the shock wave typically forms first at the wing root, especially if it is more cambered than the wing tip. As speed increases, the shock wave and associated lift extend outwards and, because the wing is swept, backwards.

...
Wikipedia