I have a nice Trex-600 nitro and I am also interested in this some day. I would feel better about flying it if I had a good, quick GPS Position Hold (which of course does auto level too) or similar.
A few comments for a larger audience, and assuming a CP heli:
- LP doesn't have a complete full featured solution for helis and there is experimentation / research / study and needed by the user. Big helis are dangerous and expensive when there are problems. I certainly intend to start with a smaller cheap electric heli that I already have a few of; then maybe an old .30 nitro.
- "Quickness" is proportional to head speed. The higher the head speed the quicker the heli responds. That means ideal PIDs are (inversely) proportional to head speed; it will oscillate if you increase the head speed without reducing the PIDs. That means you need to retune PIDs if you change the head speed. You could run "idle up" with a fixed set of PIDs, although running "normal" and tuning it at max head speed (fast forward flight) would work. Be aware that simply leaning the motor out a bit increases the head speed, so even weather can change things.
- We should use TPS on the Stabilization->Advanced page to automatically adjust the PIDs based on head speed. You would think that throttle would be a good indicator of head speed, but if you automatically increase collective with throttle, you can even get reduced head speed with increased throttle. So your TPS curve will not be a straight line and will have to be different in "idle up" vs. "normal" (use different PID banks). Be careful to understand whether you are using throttle or collective for your thrust control.
- There is a setting whether to use throttle or collective for thrust control in GPS modes and that may cause confusion as related to TPS? What we really need is a firmware enhancement to use a single transmitter channel and do the throttle / collective mixing in the FC with a simple 5-7 point curve and put it in the selectable PID banks. Without that, switching from normal to idle up needs you to flip two transmitter switches (idle up switch and FMS). Also, an enhancement to read an RPM sensor and allow that as a source in TPS would allow for nearly perfect PIDs even with rich/lean motor changes. OK, it should use baro altitude too since heli PIDs tuned at 2000m altitude will probably oscillate at sea level.
- I wouldn't assume that a bigger heli always needs different PIDs than a smaller one. A lot depends on how much damping the flybar does. If you change the flybar damping you will have to retune PIDs. Look at the flip acceleration (twitchiness) of the two helis to get an idea of the relative PIDs of each. A less damped flybar needs lower PIDs to avoid oscillation.
- Generally, any time you increase the responsiveness (higher head speed, flybar weights closer to center, increased cyclic servo throw, etc.) you should consider retuning (just the affected controls); decreasing the PIDs to avoid potential oscillation.