all the findings here are compliments of 609_Kahuna, i have merely copy n pasted his posts from (the old) CWOS's "Lockheed Syndicate" forum
NACA research on critical mach/compressibility:
http://history.nasa.gov/SP-4219/Chapter3.html
The general aeronautics community was suddenly awakened to the realities of the unknown flight regime in November 1941, when Lockheed test pilot Ralph Virden could not pull the new, high-performance P-38 out of a high-speed dive, and crashed. Virden was the first human fatality due to adverse compressibility effects, and the P-38, shown below, was the first airplane to Suffer from these effects. The P-38 exceeded its critical Mach number in an operational dive, and penetrated well into the regime of the compressibility burble at its terminal dive speed, as shown by the bar chart on page 80 .35 The problem encountered by Virden, and many other P-38 pilots at that time, was that beyond a certain speed in a dive, the elevator controls suddenly felt as if they were locked. And to make things worse, the tail suddenly produced more lift, pulling the P-38 into an even steeper dive. This was called the "tuck-under" problem. It is important to note that the NACA soon solved this problem, using its expertise in compressibility effects. Although Lockheed consulted various aerodynamicists, including Theodore Von Kármán at Caltech, it turned out that John Stack at NACA Langley, with his accumulated experience in compressibility effects, was the only one to properly diagnose the problem. The wing of the P-38 lost lift when it encountered the compressibility burble. As a result, the downwash angle of the flow behind the wing was reduced. This in turn increased the effective angle of attack of the flow encountered by the horizontal tail, increasing the lift on the tail, and pitching the P-38 to a progressively steepening dive totally beyond the control of the pilot. Stack's solution was to place a special flap under the wing, to be employed only when these compressibility effects were encountered. The flap was not a conventional dive flap intended to reduce the speed. Rather, Stack's idea was to use the flap to maintain lift in the face of the compressibility burble, hence eliminating the change in the downwash angle, and therefore allowing the horizontal tail to function properly. This is a graphic example of how, in the early days of high-speed flight, the NACA compressibility research was found to be vital as real airplanes began to sneak up on Mach one
http://history.nasa.gov/SP-4219/4219-084.jpg
http://findarticles.com/p/articles/m..._n9283659/pg_2
Flight testing the P-38 disclosed that whenever the airflow over the wing exceeded Mach 1.0, compressibility effects were encountered.
This result was soon predictable when this slippery fighter accelerated in excess of 0.65 Mach in dive angles greater than 45 degrees at altitudes above 15,000 feet. Cockpit-installed Mach meters had yet to be invented. George W. Grey, in his history of NACA, departed from strict engineering terms when he described compressibility effects in the P-38, saying, "The behavior of the P-38 was new to pilots, terrifying, baffling. Several men putting this two-engine fighter through its diving maneuvers experienced a sudden violent buffeting of the tail accompanied by a lunging and thrashing about of the airplane, as though it was trying to free itself of invisible bonds, and then the maddening immobility of the controls, the refusal of the elevators to respond to the wheel control." The only element he left out was the most horrifying: the nose-down pitching. Even a strongly applied aft wheel force couldn't stop the problem.
The NACA High Speed Wind Tunnel team under John Stack's direction had been working on this problem and had devised a small pair of 6x40-- inch, electrically operated dive-recovery flaps to be installed on the P-38 wing's underside and outboard of the engine nacelles; they could be extended to 40 degrees. That action would rapidly pitch the aircraft up to 4G and enable the pilot to regain full control. Although Lt. Kelsey evaluated and approved this dive-recovery flap in February 1943, Lockheed did not incorporate it into production for another 14 months! By that time, 5,300 P-38s-more than half the number eventually produced-had been delivered to the USAAF.
In 1943, I experienced compressibility in a Hellcat; I wonder how many of those P-38 pilots in the pursuit of the enemy dived too steeply-well beyond the critical Mach limit and into compressibility-in the heat of combat and disappeared into oblivion. At the Joint Army/Navy Fighter Conference on October 16, 1944,
I tested the P-38L dive-recovery flap well in excess of its 0.65 Mach-number limit. Upon actuation, they instantly provided a smooth, 4G recovery without pilot effort. Immediately after I evaluated these "jewels," they were installed on all Grumman 17817-1 Bearcat fighters.
http://findarticles.com/p/articles/m..._n9283648/pg_4
At about 0.65 Mach, the P-38 developed heavy buffeting with a strong negative pitch ("Mach tuck"). Ordinarily, that was enough to warn the pilot of impending control lock. If the dive persisted into the transonic regime (around 0.72 Mach), the condition could become irrecoverable. Consequently, dive flaps were installed in the last 210 J models and in all Ls, and they provided a much needed speed brake. Essentially, they returned controllability to the elevators.
William H. Allen flew with the 55th Fighter Group and recalls P-38 dive-- bombing missions. "Dive-bombing depended on the fuse setting; sometimes, they were three-second delays, which meant a higher release altitude, and they went up to 19-second delays, where we would drop from 10 feet in a level attitude and let the bomb skip up to the target.
We would normally start our run at 8,000 to 10,000 feet and roll over, point at the target and drop when we got nervous. Dive speeds were no problem with the P-38 below 12,000 to 15,000 feet."
the 45 degree dive quoted by Corky Meyer, is should be noted, only refers to
sustained dive...
now if we can only get the 38's climb, engine power/top speed, and low speed handling, and DM (the tail booms share ONE hitbox, meaning control surfaces can go out despite the 38's redundant control runs) correct....