After of Updated the Rotorlib, Helicopter have Throttle/RPM Control.
Higher throttle(or RPM) is Gain the Altitude and lower is lose it.

but i'm a currently make some Fixed-Wing Aircraft Addons, i'm think about that point is always need it and its will be more great gaming experience for fixed-wing user in arma 3.

what is different of current system and Throttle control
(follow the arma 2 key binding its currently i'm used.)
Current : Q - Acceleration / Z - Deacceleration
Change to : Q - Throttle Up / Z - Throttle Down

current system is just Accelerate/Deaccelrate, so you need keep the press of key for maintain of speed.
but if we have throttle control, speed is match the current Throttle Percentage.
If current throttle is 80%, aircraft is accelerate until 700km/h and try maintain the speed automatically. aircraft is during maneuver, speed is loose it and recover on stable flight(not maneuvering).