Jack
Airliners routinely cruise above M crit. The development of the supercritical wing allows this with little drag increase, initially. And they do not become longitudinally unstable, if flown below max permissible mach. Mach trim systems allow aircraft to meet stick gradient certification requirements when hand flown. Not required for autopilots, nor FBW aircraft, as far as I know. Pilots do not know when they are at M crit. It depends primarily on AOA, or weight when at 1 G flight if you prefer. Although there could be charts published by Boeing.
Mach buffet occurs above mach crit, say .1 above or so, when flow separation affects the wing enough to cause vibration, and adverse handling characteristics, again, will not happen if the aircraft is kept within altitude, mach, weight and G envelope.
Subscale,
At high enough machs, intial aft movement of C of P means stick gradient not per certification. Even higher, and I’d guess stick gradient could reverse, and contribute to mach tuck. The dangerous mach tuck was caused by downwash on the tail, no longer allowing the elevator to provide it’s downforce. Aircraft then pitches nose down.
The “all flying” horizontal tail may have helped prevent this.
414, did the 707 have a fixed horiz stab and elevator? The L1011 an all flying horiz tail?
If so, this may explain it. Also, the tri star had one of the first super critical wings, IIRC
Capt Stable,
Not sure what you mean by “shock wave generate lift of their own”, but indeed, hi subsonic flight does produce a rear ward shift in C of P
I don’t fly airliners, and of course I could be wrong.
I’ll leave it to Keith Williams or Bookworm to shoot me down.
Hawk
Last edited by hawk37; 25th May 2004 at 23:12.