Helicopters cannot be trimmed in flight

[bcoconni]

I'm submitting a ...

• [X] bug report
• [X] feature request
• [ ] support request => Please do not submit support request here, see note at the top of this template.

Describe the issue As per the discussion #1132, helicopters can't be trimmed in flight.

What is the current behavior? Currently, helicopters can be trimmed on ground by setting simulation/do_simple_trim to 2 (i.e. trim on ground).

However, helicopters cannot be trimmed in flight because helicopters produce lift by their rotor while the trim algorithm assumes that lift is produced by wings and hence adjusts the angle of attack to maintain constant altitude.

https://github.com/JSBSim-Team/jsbsim/blob/b41fcddc505baf041a3082ac3be2312e392c7792/src/initialization/FGTrim.cpp#L808-L809

What is the expected behavior? JSBSim should be able to trim helicopters in flight just as it does for aircraft.