Originally Posted by Niallo
for an aircraft in cruise at high altitude and weight, the allowable range (margin) of air speed is small, to avoid stall at the lower limit or Mach overspeed at the higher limit.
A typical figure for a typically-loaded long-haul aircraft at cruise altitude (high-thirty-thousands of feet ISA) is 70-80 kts TAS.
Typically, Mach overspeed is not a problem leading to upset. During certification, the airplane is flown at Maximum (Mach) Operating Limit Speed in level flight, then the nose is pitched down 7.5° for twenty seconds, followed by a 1.5g pull-up. The speed reached is V_D (by definition), and the aircraft controls are cleared for flutter on paper to 120% of this. You can check out the criterion at CS 25.335. CS 25 is available from the EASA WWW site under Certification.
There is a huge drag increase at some transonic point. This is known to aerodynamicists as the Drag-Divergence Mach number, M_DD. For many airfoils, the difference between the critical Mach number, M_Crit, the point at which some of the airflow over the wing reaches Mach 1, and M_DD, is small or zero. Supercritical airfoils are designed for this difference to be large, so that the airplane can be flown at speeds between them.
This drag increase is sufficient to inhibit large increases in speed, even if you put the nose down. However, getting to M_D can be somewhat unpleasant, not to speak of nerve-wracking, so I am told, since there is a huge increase in buffet as more of the airflow over the wing goes sonic and supersonic. I haven't talked to anyone who has actually performed such tests.
Upsets at cruise altitude and speed are hard to come by: first, because of drag divergence; second, because the dynamic pressure is low enough to restrict the reaction in attitude of the aircraft to control displacements, although controls are sensitive - large excursions in altitude and acceleration may be caused by relatively small control inputs.
The U2 is an example of an aircraft point-designed to fly in a very narrow range between stall and M_DD: only about 7mph separates the two (John D. Anderson, Jr., Introduction to Flight, 6th Edition, McGraw-Hill 2008, p 338).
PBL