Although to further expound on the whole “coffin corner” thing, even though the force exerted by the air on the airplane is much lower at high altitude, the air still travels over the airframe at close to the ground speed. When an airplane approaches the speed of sound, the air going over some of the aerodynamic surfaces starts to go supersonic which will damage an airplane not designed for supersonic flight. “Coffin corner” is when you’re at an altitude at which achieving a fast enough indicated airspeed to generate enough lift to keep the plane in the air requires a ground speed fast enough to damage the airframe. On the high altitude U-2 missions they’d be flying just below that altitude, at a point in which like Morgenstern said there were only a few knots between the stall speed and the “never exceed” mach speed.
I’m going a bit off the deep end now and splitting hairs at the same time but…
I don’t know the particulars of the U-2, but the Coffin Corner is where stall speed and the onset of compressibility on a particular aircraft meet, however the later and the Never Exceed speed are not necessarily the same.
The never exceed speed is when the plane begins to shed bits and pieces, and that can happen at speeds way bellow the point were supersonic flow develops anywhere around the airframe.
Of course in practical terms, for a relatively fragile plane like the U-2, a sudden loss of control at high speed could end up with more Gs than the airframe can take, but other aircraft may not have the same problem and it wouldn’t result in structural damage.
For example, IIRC, some of the earliest flights of the X-1 run into the problem of loss of control due to supersonic shockwaves developing on the wings and tailplane when approaching Mach 1, but that didn’t cause the airplane to be torn apart. So in the case of those flights the airplane (as it was configured then) reached its Mach limit (one of the angles of the Coffin Corner) , but not the never exceed speed (which of course was well past Mach 1).
I don’t think it happened, at least not in the sense that he was swinging like a weight on a string while being bashed against the plane. IIRC he climbed partially out of the cockpit and got stuck, so his legs were still inside and he wasn’t just flopping around loose.
It is not widely appreciated that the speed of sound changes almost not at all with pressure, but is strongly dependent on temperature.
Why is that the case that it so changes?
Pretty good explanation here: Speed of sound - Wikipedia
The real short answer is that increased density causes a slowdown but increasing pressure causes a speed up. In a free gas those two effects offset exactly under the PV=nRT and density equations.
Which leaves just temperature and the molecular makeup of the gas as live variables. In the case of our atmosphere the gas mixture is assumed constant and you end up with sound speed as a function of temperature only
Minor aside: The above applies to gasses. In liquids or solids the live variables differ.