Comment by Ajedi32

The air this thruster gathers isn't used as fuel, only as propellant. The energy needed to accelerate the propellant and generate thrust still has to come from somewhere.

Aircraft don't need propellant; they can simply "push off" of the air (via propellers, turbofan engines, etc); so this mechanism is useless for them. They still need fuel to run their engines, which this device doesn't provide.