Originally Posted by MustangFlyer
I was once asked this question a few years ago and I replyed with the usual answers. After thinking about it a bit more and read further (including a running debate in New Scientist) I realised that there is a major failing in the standard answers, as regards giving a basic understanding what is actually going on.
Firstly, except for some military planes, no plane has the power to fly by engine power alone.
Simple test, take a 747, put it on its a*s*, firewall the throttles ... nothing happens.
So where is the free energy coming from?
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The inverse happens with a plane. You go fast (the energy from fossel fuels) and you close the energy gap and (with the right angle of wing to create unbalanced forces ) you lift.
Summarising: You get free energy from the momentum and repulsive forces of each air molecule. Normally these are evenly balanced. But, if you add energy (speed) and unbalance these forces they will lift you, even though the energy you expend in going fast is not enough to overcome gravity. Basically you get free energy from the atmosphere.
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What free energy are you talking about?
In level flight, no energy AT ALL is required for the plane to hold altitude - it is neither gaining nor losing potential energy. The only energy involved is the drag (induced + parasitic drag) x velocity, and this energy is provided (exactly) by the engines.
In a climb, to gain potential energy, more propulsive thrust is used. In descent, the opposite applies. But in level fight, the potential energy is constant, and there's no "free energy".
And your treatise completely disregards the viscous forces in the air, which would lead you to an appreciation of Reynold's number...
