A difference in kinetic energy -- E(outflow)-E(inflow) -- of air would result in net force pointing forward (a decelerating force, if we consider a plane). Basically, we'd need the air to get accelerated forward more than rearward, producing net force.

Kinetic energy is E = (mV^2)/2

Obviously the mass of inflow is equal to the mas of the outflow. We can achieve a difference in speeds if the effective cross-section of the (forward-pointing) outlet was smaller than cross-section of the inlet.

That's very off-topic, anyway :^)