Mach Tuck

[Duffer2007]

Mach Tuck - how it develops?

This has probably been asked or discussed loads of times but im looking for a one or two line plain english answer, as one would give in an interview! If someone who knows exactly how to describe it as such please do. If you are going to show me a link to something else please don't post.

Thanks

Ian

[bombayhues]

Mach tuck can be described as the downwards pitching movement of the nose of an aircraft once it reaches the critical Mach number, due to the rearward movement of the Center of Pressure (towards the wing tip) which in turn is due to the progressive stalling on a sweptback wing starting from the wing root

Hope this helps

[Keith.Williams.]

If you want the explanation to mean much it needs to be more than one or two lines. The following is a bit longer but covers the essentials.

As an aircraft accelerates through the transonic speed range, shock waves form on the upper and lower surfaces of its wings. A shock wave is a sudden increase in static pressure. As the air flows through the shock waves, it is compressed by this higher pressure. This sudden pressure increase causes an equally sudden decrease in the velocity of the air. This in turn causes the boundary layer to separate and become turbulent. The separation of the boundary layer causes a large reduction in lift on the area of wing directly behind the shock waves. The turbulence caused by this separated airflow also cause buffeting.

The shock waves form first at the points at which airflow velocity is greatest. This occurs at the wing roots because this is the area where the thickness of the wing and hence acceleration over it is greatest. This means that the shock induced separation and loss of lift will occur on the upper surface at the wing roots. In the case of swept-back wings, the roots are ahead of the tips, so the loss of lift is on the forward part of the wing. This causes the overall centre of pressure of the whole wing to suddenly move aft, causing the aircraft to pitch nose down.

Prior to the onset of shock waves, the lift generated at the wing roots will
produce downwash, some of which will flow over the tailplane. This increases the down force generated by the tailplane and in so doing contributes to the correct trimming of the aircraft. But the sudden loss of lift caused by the formation of shockwaves on the wing roots greatly reduces this downwash. This in turn reduces the down force generated by the tailplane, thereby upsetting the trim of the aircraft. The combined effects of the rearward movement of the C of P and the loss of downwash over the tailplane cause the aircraft to pitch nose down.

[Duffer2007]

thanks guys 2 great answers