[notlefthanded]

I think ppl are equating energy with force. Airspeed, altitude, and fuel are forms of energy, kinetic, potential, and chemical, respectively. Lift, drag, thrust, and weight are forces. We're talking about a heavier than air aircraft in cruise right now, and the contention is over whether all the energy added to the aircraft if used to counter drag.

Simple example: consider a helicopter in cruise. Fuel is burned to produce thrust. There is an insignificant component of that thrust vector pointed orthogonal to the vector of velocity. Since drag by definition acts along the same vector as velocity, not all the energy is being used to counteract drag.

Back to an aeroplane in straight and level, since that's a more interesting example. Let's assume that the direction of travel of the aircraft is normal to the plane of the propeller, so thrust is acting on the same plane as drag, in this idealized situation. Energy is added to the system in the form of thrust created by the prop. Said thrust is used to maintain the amount of kinetic energy of the aircraft. At the same time, this kinetic energy is being transformed into both lift and drag by the wings (and elevators, depending on how far aft the cog is) ergo not all the energy added to the system is used to counteract drag.

[iliis]

To explain it in yet another way: As long as you maintain your height no energy is used for lift, as energy is equal to force integrated over distance. Like as standing on a table requires no energy ;) However, an airplaine isn't standing on anything and the lift force is generated by pushing air downwards and this is what consumes energy.

A car or a train that drives with a constant velocity has constant kinetic and potential energy (assuming level ground). Therefore all energy that is consumed to maintain the status quo is spent to counteract drag.

A plane however pushes down on air instead of solid ground and accelerates it downwards. So not only does the fuel heat up the system due to drag, some of the energy accelerates quite a chunk of air.

Now you can argue that 'moving air' is nothing else than turbulence that takes a bit longer to dissipate and is therefore just another form of drag ;)