Thought that I would chime in with a bit of interesting aeronautic history. MiloMorai, you pointed out the terminal velocity chart from NACA ACR L5G31. I believe ACR L5G31 is actually evidence of P-47's in vertical dives (and also the difficulty with them in situations prior to dive flaps being installed).
L5G31 references that the P-47C-1-RE data in Figure 17 (the terminal mach chart) were collected from dives done by Maj. Perry Ritchie (a Capt at the time). The P-47 experienced compressibility stability and control issues like many WW2 warbirds. At the time however they didn't know the cause. P-47 pilots reported that high speed vertical dives entered from a Split S at 35,000 ft exhibited a nose tuck beyond vertical (>90 degrees nose down) and the loss of elevator effectiveness (a "control freeze"). The dive appeared unrecoverable until around 15,000 ft when control suddenly returned and if the pilot didn't break the P-47 in the process due very high maneuver loads was able then to recover (usually with bent wings!).
The cause was hotly debated among leading NACA aerodynamicists with three theories explaining the cause. However the only way to see which theory was correct was to flight test the issue and record what was happening with the elevator. NACA and Wright Field had difficulty finding a contract pilot willing to do the tests. Instead they found Maj. P. Ritchie, apparently one of the physically strongests USAAF test pilots who volunteered to do the tests for nothing. Maj. Ritchie proceeded to do some 30+ hair raising high speed vertical dives in the P-47 and his flight tests proved that the cause was due to compressibility stability & control issues. He was awarded an air medal for his bravery.
The following chart is a time history of the P-47 vertical dives and what was happening due to compressibility:
In a vertical dive once the P-47 was beyond it's critical mach compressibility caused the nose of the P-47 to tuck away from pure vertical. Also the elevator lost effectiveness to change the attitude of the aircraft. As can be seen on the figure the elevator deflection angles ranged between +3 to -3 degrees. At the high airspeeds of the dive usual loads from those amount of elevator angles would have produced loads in excess of 20 to 30 g's, easily destroying the aircraft. As can be seen however due flow separation from sonic shock waves due to compressibility the elevator effectiveness only produced a 1/2 g instead which was pretty much was control freeze to a pilot diving at that speed. At about 15,000 ft the air got dense enough increasing the drag and slowing the airplane down enough to reduce or eliminate the compressibility shock separation so that elevator control came back. However if the pilot was continue to pull hard back or had the tab set to full trim up this would result in a violent recovery which could easily destroy the aircraft.
That Maj. Ritchie was able to recover the airplane said a lot about his strength, control and coolness. As Dr. Robert Gilruth describes
"Fortunately, when you get to lower and lower altitude, the drag goes up to the place where it forces your speed down below where that sharp separation takes place, and the flow reseats itself and then it's a regular airplane. But you're apt to pull the wings off because by that time you're just scared to death, afraid you're going to hit the ground. Perry Ritchie didn't do that. He did it just right."Sadly Maj. Ritchie died later in 1944 while doing dive and recovery testing on a new B-25.
