The XP-86 that Welch was flying had a fixed tail-plane
Was trimmable
con, as can be seen in the video of the aircraft's first flight.
Re supersonic 262, quote from Heinrich Beauvais, Me 262 Test Pilot, Rechlin Test Centre 1935 - 1945, "
One pilot, a man called Mutke thought he had passed the speed of sound. But there were problems at high speed. The plane was damaged by buffeting. The rivets popped out and the skin on the fuselage buckled. So I don’t believe it and neither do the the experts".