As per the title, apologies for bringing this up again, but all previous threads were closed. Now, I can accept the answer and experimentation done, however as both a pilot and mechanical engineer, I just can't get my head around it. It's been bugging me for years, and created the account just for this : ) While I can happily agree things fly due to pushing air down and 3rd law, why wouldn't part of all of that energy eventually be dissipated into heat through the fluid's viscosity? To me it appears like a 1st law of thermo thing. Plane does work on air -> Becomes heat + less work on floor. Adjacently, to the accepted solution that the air gets pushed to bottom regardless of how high it is, does a propeller forcing air horizontally simply keep on going around the earth forever then? The water tank analogy doesn't do it for me either since different mechanisms are at work (buoyancy vs down wash/pressure differential) Finally, another way I picture this, is if instead of blowing air molecules down, lets say you had a machine gun which fired pingpong balls. They're fired out at some huge speed and rate, enough that if you're firing downward, its enough force to keep you aloft. By the time they reach the bottom, they will have slowed considerably due to friction, and though the mass flow rate will be the same, the velocity, and subsequent force on the bottom of the box when they hit will be considerably diminished.