> It goes to show how much energy just deals with drag in the current system.
All of the energy used during cruise flight (straight and level, accelerated) goes to drag.
All of the energy used during cruise flight (straight and level, accelerated) goes to drag.
https://www.grc.nasa.gov/www/k-12/airplane/forces.html
Lift requires energy, usually kinetic, like the forward motion of an aeroplane being converted to lift (and drag) via the wings, or the blades of a helicopter pushing air down, or real hot and fast gases pointed downwards (think harrier jump jet).
A blimp, on the other hand, relies on buoyancy for lift, so yeah, in that case, given an altitude at which it's stable, to maintain velocity it only needs to add enough thrust to counteract the drag created by its forward movement.
Imagine if the airfoil on an aeroplane were replaced with a symmetrical airfoil mounted with no angle of incidence. Thrust could be reduced because there's less drag from no lift. No lift, no induced drag, only parasitic drag, and the plane starts to lose altitude. Would you agree that not all the energy added to straight and level flight goes towards counteracting drag?
*where a' and v' are zero, and where for argument's sake, the thrust vector is perfectly horizontal
edit: by a' I mean change in vertical airspeed, by v' i mean change in true airspeed.
If lift requires energy, then where would that energy go?
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.
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 ;)
Drag is more like vacuum than friction.
From my admittedly limited understanding of aerodynamics, a plane's engines are only fighting against drag to keep the airspeed up, and it's the airspeed passing by the wings that generates lift -- if engines are necessary to generate lift, gliders and kites wouldn't be able to work at all.
With a kite the wind is the engine, the string allows the kite to use it. Or you can run on a windless day.
With a glider the tow plane or ground tow rope provides the initial energy to get to altitude, giving the glider potential energy. As it glides that potential energy is converted to kinetic energy. The pilot uses their knowledge and skill to glide to places where they can gather more energy from updrafts of various sorts. I think it's really amazing how after that initial injection of energy, it's just all just skill and ambient energy.
Energy is consumed to exert a force over a distance, but we're not moving against gravity ("constant altitude"), so no energy is directly expended to fight gravity.
Now, that argument cheats a little, because there is a relationship between lift and drag: compare induced drag (drag created as a result of producing lift) to parasitic drag.
To maintain the hover and prevent the helicopter from falling out of the sky, the engines are consuming large amounts of power. To move it forward at a sedate pace only requires a small expenditure of energy to overcome drag.
If it was on wheels, a human could push it across a hangar with little effort. A human definitely could not hold a conventional helicopter in the air by lifting or by pedaling to turn the rotors.
The reactive force lifting the helicopter should not be thought of as drag.
That being said, this is fluid dynamics, where nothing is simple. Some of drag could be loosely described as friction, but not all of it. Think of what you feel while you're swimming, or sticking your hand out of a car window. It's like something is actively pushing against you, like you're catching a ball or something -- which you wouldn't normally call "friction". On top of this there are temperature effects, turbulence, ... and so on. And, most of these are actually at least somewhat coupled to each other.
Anyways, though at the end of the day it may be technically accurate (in certain contexts) to say that all of the power consumed at level flight is going to drag, it's also disingenuous; a bunch of that drag is the direct result of needing to generate lift to fight gravity.
[1] Admittedly this is a somewhat loose interpretation of the word "lift" but when you get down to the nitty gritty details like this I don't think "lift" is any more than a semantic construct to denote "useful drag". But the lift created to help control Apollo command capsules during reentry is a good example of this: by altering the angle of attack, thereby introducing highly asymmetric drag, the capsules "generated" lift to ease reentry angles.