And now you wish to bring in engine thrust and all? I am sticking to OP, there is no generalization. Bullet shot in downwards direction from a flying aircraft. You are bringing all these additional stuff that under a special aerodynamics even if horizontal speed is non zero the vertical terminal speed is achieved. You know thrust can create any kind of system, I can make an object fly in any direction at constant speed by sufficiently controlled thrust. Are we discussing that??