I think that some of the confusion here is a good illustration of why thinking of problems in terms of energy is not always the best way to solve a problem. Other times, it is much simpler than other "models". Energy models work best when you just think in terms of energy...PE=mgh and KE=1/2 m(v)squared.
If you want to think of the problem in terms of work, then work = force x distance. In level, unaccelerated flight, the "work" done by the "thrust" is exactly equal to the work done by the drag, thus no overall work to the system. There is still "energy" being put into the system by the engines but stepping back and looking at the whole thing, no excess work is being done anywhere.
The lift can be thought of the same way with the induced drag being what keeps the whole lift/drag thing related. Thinking in terms of energy for level, unaccelerated flight doesn't really help you with too much. Now, when you get into climbing flight is where energy models become really useful. In climbing, unaccelerated flight, you are changing the potential energy of the aircraft. As your lift still equals weight and thrust in the direction of flight still equals drag, the thrust over and above drag becomes the excess "energy" in the system that results in changing your energy...either potential (height) or kinetic (you speed up). If you draw all kinds of vectors (curved?) with angles and stuff and are able to keep track of it, you will come out with the same answers that you come up with by using the much simpler energy equations.
Last edited by Gillegan; 16th Nov 2010 at 10:07.