My flight-sim experiment started to behave strange after I added some 2D-UI. I figured out why:
In a scene with gravity (-9.81) and a single counter-force of same strength nothing should move.
In fact there seems to be an additional downforce accelerating impostors downwards.
The effect gets worse if time consuming stuff (like UI updates in my case) is done during
onBeforePhysicsObservable up to multiple G.
The effect does not show up when starting the physics engine with zero gravity and adding G to each impostor.
Hope for help,