|Mach tuck is an aerodynamic effect whereby the nose of an aircraft tends to downward pitch 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 caused by a rearward movement of the centre of pressure in transonic flight. As an aerofoil passes through the air, the air flowing over the top surface accelerates to a higher local speed as part of the mechanism for generating lift. 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 as a whole 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 bigger and moves rearwards, creating high lift further back along the wing. This rearward movement of lift causes the aircraft tail to rise and the nose to pitch down or "tuck."
The severity of Mach tuck on any given design is affected by the choice of aerofoil, the sweep angle of the wing, and any resulting change in airflow over the tailplane.
The camber and thinness of the aerofoil affect the critical Mach number, with a more highly curved upper surface having a lower value.
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.
The changing airflow over the wing can affect the downwash over a conventional tailplane, further aggravating the effect and making it difficult to recover.
Another problem with a separate horizontal stabiliser is that it can itself achieve local supersonic flow with its own shock wave. This can affect the operation of a conventional elevator control surface.
Aircraft without enough elevator authority to maintain trim and stay level can enter a steep, sometimes unrecoverable dive. Until the aircraft is supersonic, the faster top shock wave can reduce the authority of the elevator and horizontal stabilizers.
All transonic and supersonic aircraft experience Mach tuck.