According to this document the porpoising with fabrics could be encountered also at lower speeds at low altitude and the problem was remedied by metal covering up to 0.8 M. After that the plane still "wallows" in dive. There is no mention whether or not the porpoising phenomenon existed above 0.8M.
http://www.wwiiaircraftperformance.org/mustang/mustangIV-divetest.htmlWhat I found out from P-51 D/K pilot manual that once you enter the compressibility there is not much you can do in a P-51 but to wait for the control to come back while you will lose at least 8000 feet of altitude before you have decelerated enough to begin to come out of compressibility. Mind you that P-51 does not have a flying tail where as 109 does, so it's pilot can control when he want's to come out of dive. It was quite common that these situations came as a surprise to 109 pilots and they did not always realize that all that was needed to come out of dive was a few rotations of elevator trim but rather tried to pull the stick with both hands. On one instance the indicated airspeed of a diving 109 was 850 kph so the absolute maximum dive speed was well exceeded. It was essential in the 109 that the trim was not used too much as the aircraft would tighten the pull out by itself when it decelerated, effectively ripping the elevators off the plane. I recall that at least one such incident is known.
Also from different data these aircraft seemed to share the tendency for the trim changes in different phases of the dive even if the wing profiles would suggest otherwise. It seems that the configuration of control surfaces affected this at least in P-51 which needed down trim in high speed due to fixed angle of its elevator plane (the effects of which were evident in the case of Galloping Ghost).
It actually seems that it was safer to dive faster than 0.8M in a P-51 than in a Bf109 but the problem was that in a P-51 you were a passenger during the deceleration phase where as in a 109 the aircraft could safely exceed the safety margins and you could control your exit from the dive better.
Anecdotal evidence is OK in many cases but it has to be kept in mind that in the heat of the battle the initial conditions are not known. So it somebody says that flying the X he caught the Y easily in a dive you have no way of knowing what was the initial speed of the Y to begin with. Anecdotes may give an idea of relative performance if there is no usable test data at hand.
-C+
