In previous versions, as recently as Aug 28 devbranch (126953), it was possible to control the helicopter using the mouse in order to perform cyclic inputs -- turn left, turn right, cyclic forward, and cyclic back -- under the standard flight model.
In devbranch versions including and above Sep 09 (127101), tested recently with latest devbranch 1.29.127167, the mouse controls the pilot's view instead, and there is no possibility of disabling this mouse view that I have observed. It is now specifically impossible to use the mouse to control the aircraft in standard flight model.
The behaviour is similar to the old locked Freelook function that is triggered by double-tapping 2x Alt.