more on compressibilty:
http://yarchive.net/mil/p38.html
One problem the P-38 had in dealing with the Me-109, but not the FW-190
(which was more of a low and mid-altitude fighter) was the Me's high
altitude performace superiority. Above 25,000 ft., cooling or
supercharger impeller or turbine speeds became limiting for the Lockheed,
and high speed capability started to fall off. At low altitudes, the
plane could max out at about 330-340 mph. This rose to well above 400 mph
between 25,000 to 30,000. As the plane approached 30,000 ft, speeds over
Mach 0.60 could be sustained in level flight. Thus, manuevering could
quickly give the plane compressibility problems. At Mach 0.65 (290 mph
IAS, 440 mph TAS at 30,000 ft.; 360 mph IAS, 460 mph TAS at 20,000 ft.)
drag began to soar as the plane began to encounter compressibility. At
Mach 0.67 shock waves began forming and buffeting began at Mach 0.675. At
Mach 0.74 tuck under began. Buffeting developed at a lower Mach number in
any maneuver exceeding 1 g.
In contrast, the P-51, had far fewer compressibility problems at speeds
normally encountered in combat, including dives from high altitude. The D
model was placarded at 300 mph IAS (539 mph TAS, Mach 0.81) at 35,000 ft.
In a dive, the P-51 was such an aerodynamically clean design that it could
quickly enter compressibility if the dive was continued (in reality, a
pilot could, as a rule, catch any German plane before compressibility
became a problem). But, say, in an evasive dive to escape, as the P-51's
speed in the dive increased, it started skidding beyond what the pilot
could control (this could be a problem in a dive onto a much lower-flying
plane or ground target--couldn't keep the plane tracking on the target if
speed was too high). As compressibility was entered, it would start
rolling and pitching and the whole plane would begin to vibrate. This
began about Mach 0.72. The pilot could maintain control to above Mach
0.80 (stateside tests said 0.83 (605 mph) was max safe speed--but
structural damage to the aircraft would result).
The P-51's quirk that could catch the uprepared service pilot by surprise
was that as airspeed built up over 450 mph, the plane would start to get
very nose heavy. It needed to be trimmed tail heavy before the dive if
speeds over 400 mph were anticipated. However, in high speed dives, the
plane's skidding changed to unintended snap rolls so violent that the
pilot's head was slammed against the canopy. Depending on how much fuel
was in the fuselage tank, on pull-out stick force reversal could occur, a
real thrill that could totally flummox a low-time service pilot diving
earthward at close to 1,000 ft per second trying to escape a pursuer.