I think the original question contains a better explanation than anything posted afterwards, if I could be so bold.
1/ I know that an efficient aircraft has a high L/D ratio. Thus thrust provided by the engine is significantly less than lift that acts upon the aircraft. However from an overall energy perspective where does the energy for lift come from. Is it that burning fuel provides a certain amount of energy, this energy is used to create a total aerodynamic reaction from the aircraft (the result of pressure and shear distribution), the total aerodynamic reaction is then resolved into two forces lift and drag. However we consider lift as not taxing the aircraft because it does not impede horizontal motion? I hope I am being clear. Surely the energy for lift has to come from somewhere!
Except maybe for the last sentence. Lift is defined as being normal to the vector of flight, so cannot do "work" in this frame of reference. Energy is required to move the drag force along the flight path, so that's where the engines come in (and/or a change in mgh or 1/2mv^2).
2/ I know that work = force x distance. Therefore if I push on one side of an object with force A and move is distance B the work done will be AB. However! If someone else pushes with an equal force on the opposing side there will be no movement and thus no work done. Therefore my question is, in level flight all the forces on an aircraft are balanced, there is no net force and thus no acceleration. How is it then that the aircraft can be said to be doing work and using energy? Is this something to do with frame of reference?
See above.
