I have run into a problem with my JMP T-33/RAM-1000 setup that is baffling. The fuselage gets really warm with the engine running on the ground. My test pilot noticed that the trim on the elevator had changed. Instead of being at neutral with no stick input, it had a fair amount of up elevator without any stick input. After cooling down, the elevator went back to it's normal trim position.

I cannot pinpoint why this is happening. The obvious answer would be that the pushrod is expanding but that is definitely not it since the servo pulls on the rod for up elevator. Expansion of the rod would cause down elevator trim.

Could it possibly be the servo losing its center due to the relatively high heat? The servo is a JR3411. The top of the fuselage was pretty hot but not hot enough to soften or damage the fiberglass.

Has anyone ever experienced this phenomenon? How was the problem solved?