the interior of the cube in your example is a closed system, so ultimately there is no net force in any direction. if the cube is large, the drone might create a slight pressure differential accelerating the cube downwards, but it would eventually be canceled out when the drone hits the top of the cube (I'm assuming zero g, so the drone immediately starts accelerating when it turns on its props). if the cube is sufficiently small, the drone might immediately create too much turbulence to generate sustained thrust in any particular direction.