Under controls>aim up/down/left/right set these to mouse up/down/left/right.
(Have helicopter controls also assigned to the mouse)
Fly a quadcopter - you can't use the mouse to control it. Take over the turret - you can use the mouse to move the turret.
Go to controls>aim up/down/left/right and delete the mouse bindings.
Now you can control the quadcopter with the mouse, but no longer move the turret using it (since you deleted the bindings).