PPRuNe Forums - View Single Post - Mach Crit, Mach Number Buffet, Mach Tuck, and Mach Trimmers
Old 23rd May 2004, 18:29
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LOMCEVAK
 
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You need to consider the complete airframe and not just the wing.

Mcrit is the lowest Mach number at which the local airflow somewhere over the airframe reaches 1.0M.

Mach buffet is caused by flow separation due to the compressible nature of the airflow and I believe could occur at less than Mcrit on straight wings with relatively high thickness:chord ratios. Remeber that compressibility has a major effect on the aerodynamics of a wing before a shockwave actually forms.

Mach tuck is due to the rearwards movement of the centre of pressure of an airframe (mainly the wing) at high transonic Mach numbers. The severity of this will be affected by the CP shift over the whole span, the wingsweep angle, and the change in downwash angle at the tailplane. It occurs over a speed range and is basically a region in which the aircraft exhibits apparent longitudinal static instability.

A Mach trimmer is a device which varies the pitch trim automatically as a function of Mach number to oppose Mach tuck and thus restore apparent longitudinal static stability.

Therefore, only Mach tuck and Mach trimmers are directly related. Mcrit and Mach buffet are both characteristics due to the compressible nature of the airflow, but the complex 3D nature of an airframe makes any direct relationship difficult to predict.
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