this is from an interview w/ Dr. Robert Gilruth who worked at Langley Memorial Aeronautical Laboratory '36 - '45 (later he was director of the Manned Spacecraft Center '61 - '72):
...WOLKO: This is again getting back now to Bell Aircraft, on the P-39, did you ever get the P-39 down at Langley to test it?
GILRUTH: Yes.
WOLKO: Was there a tumble problem with that aircraft?
GILRUTH: People thought it did.
WOLKO: That was one of the rumors that went around with the P-39.
GILRUTH: Yes, people tended to think it did. I don't think we were ever that adventurous with it. But it would dig in, and the force per G was pretty nominal until you got to high Gs and then the force per G went to about zero, and it would pull itself in. I know that. I remember that. And that was bad.
WOLKO: Was that because of the engine in the rear?
GILRUTH: I have no idea why. I don't think so. I think it had something to do with the fabric on the elevators. There are all kinds of possibilities. For so many years we just took ailerons and other control surfaces and put canvas over them and dope and when you got to high speed, you had no idea what the shape of that aileron was. It depended so much on the internal pressure, the tension on the fabric and the dynamic pressure.
WOLKO: It could balloon out on you.
GILRUTH: It could balloon out or it could suck in, depending on how it was vented. And similarly with the elevators. So in those days, you could get all kinds of things that you wouldn't understand. It was the British that finally said, "Isn't it time we quit fooling around with the fabric on something as sensitive as that?" My goodness, sure.
WOLKO: So the British were first to put metal skin on their...
here is a pic of Dr. Gilruth showing JFK a small model of the Apollo Command Module
the following is from a section of an autobiography of Robert E. Riddle, at the time flying w/ the 28th squadron in Panama (Later Lt. Riddle became an 11 victory ace w/ the 31st Figther Group, 307th Squadron in the MTO)
".... Whatever its faults, I actually enjoyed flying the machine. Besides having a center of gravity far back, little servo tabs on ailerons and elevators made the controls extremely sensitive. An uncoordinated maneuver could cause the ship to snap roll and in some cases tumble end-over-end until dropping into a flat spin. The snap roll was much more likely at slow speeds and took it's toll on pilots landing out of a tight circular approach. But I found that at speed, the P-39 was as easy to hold in a three-G shuddering high speed stall as any other plane I ever flew. After many complaints and some fatalities attributed to the stalled "tumbling" action, an experienced Army test pilot was assigned to investigate. His report stated that he was unable to induce such a reaction from the ship in any attitude. But I have seen it happen and a friend of mine twice experienced it while attempting to perfect a vertical circular loop..."
and of course theres this song sung by USAAF pilots
Don't give me a p-39,
With the engine mounted behind,
It'll tumble and roll,
And dig a deep hole
Don't give me a P-39.
It'll stall out and spin,
And soon auger in,
Don't give me a P-39
