My first post, I hope it can help somebody.
I have noticed many things when it comes to auto hover and trimming in DCS: BS, and I have made conclusions based on experience. I don't claim (though I wish I could) to have full understanding of the system it uses.
First of all, auto-hover cannot be used below 4 meters, and neither will the HUD display the airspeed provided by data from the INU, that's why the airspeed freezes for you. I assume it's because the designers of the helicopter assumed you wouldn't be stupid enough to try to use a failure-prone system at such a dangerous altitude. Your eyes and intellect should be good enough at 4 meters.
As for trimming, the method of holding down the trimmer while you make adjustments is the best way to do it. The autopilots enter corrections to the stick based on inputs from the trimmer. If you just move the stick to the position and then press and release the trim button, you're including the autopilot corrections and leaving more chance for error.
At least, that's how I think it is. :pilotfly:
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Mar