7 ms·

> 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) goe

> 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.

abakker 10y ago
I am not a physicist, but, doesn't fighting gravity by way of lift mean that at cruise altitude the plane is still fighting gravitational acceleration constantly? Drag is friction, right?
- solidsnack9000 10y ago
  It's true that at least some energy must be expanded to lift the plan, even were there no drag; otherwise planes would lift themselves. Drag is more like vacuum than friction.
  - kd5bjo 10y ago
    Keeping an object at a constant altitude requires force, but it doesn't necessarily cost any energy. For example, there's no energy being expended keeping my coffee cup elevated, just a table exerting a force. 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.
    - jasonwatkinspdx 10y ago
      There's more too it than that. The airfoil causes a net downwash, and via newton we know the acceleration of that mass of air will cause an upward force. The finer details of this are something a lot of textbooks get wrong. Wikipedia's article about it is pretty good. 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.
- aardvark291 10y ago
  The power required to fly at constant altitude and speed is velocity multiplied the force of drag. 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.
  - LeifCarrotson 10y ago
    Consider a helicopter nearly hovering, but moving forward at a walking pace. 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.
  - tgb 10y ago
    Are you actually claiming that a helicopter doesn't use any energy to hover in place? Your argument describes the work done on the helicopter (or plane), but not the work the helicopter does on the air, which it pushes downward considerably. So, too, does an airplane's wings 'push' air downwards. Contrast to the vehicle sitting on the ground, where the ground is incompressible and no work is done on it.
- nbadg 10y ago
  The relationship between drag and lift is complicated. They're ultimately both the same phenomenon: pressure differentials introduced by dynamic fluid flow. You cannot have lift without drag, and you cannot have drag without lift [1]. 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.
notlefthanded 10y ago
Short answer: that is not correct for heavier than air aircraft. https://www.grc.nasa.gov/www/k-12/airplane/forces.html 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.
- skelsey 10y ago
  Yes it is correct. Lift always creates drag. You can think of drag as the work necessary to gain lift. When lift is generated, it produced what is called induced drag[0]. Induced drag and parasitic drag, which is the drag generated from the aircraft structure itself, are where all the energy goes in unaccelerted flight. [0] https://en.wikipedia.org/wiki/Lift-induced_drag https://en.wikipedia.org/wiki/Lift-induced_drag
  - notlefthanded 10y ago
    I think we're interpreting op differently. No, not all the energy added to a heavier than air aircraft (thrust) in straight-n-level flight* is used to counteract drag. Yes, where there is lift, there is induced drag. But the kinetic energy being added to maintain velocity is also being used for lift. 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.
    - teraflop 10y ago
      No, that's a non sequitur. An aircraft that doesn't produce lift requires less energy input, but that's not because lift requires energy -- it's because when there is no lift, the aircraft is gaining kinetic energy by losing potential energy. If lift requires energy, then where would that energy go?

> 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.

abakker 10y ago

I am not a physicist, but, doesn't fighting gravity by way of lift mean that at cruise altitude the plane is still fighting gravitational acceleration constantly? Drag is friction, right?

solidsnack9000 10y ago
It's true that at least some energy must be expanded to lift the plan, even were there no drag; otherwise planes would lift themselves. Drag is more like vacuum than friction.
- kd5bjo 10y ago
  Keeping an object at a constant altitude requires force, but it doesn't necessarily cost any energy. For example, there's no energy being expended keeping my coffee cup elevated, just a table exerting a force. 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.
  - jasonwatkinspdx 10y ago
    There's more too it than that. The airfoil causes a net downwash, and via newton we know the acceleration of that mass of air will cause an upward force. The finer details of this are something a lot of textbooks get wrong. Wikipedia's article about it is pretty good. 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.
aardvark291 10y ago
The power required to fly at constant altitude and speed is velocity multiplied the force of drag. 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.
- LeifCarrotson 10y ago
  Consider a helicopter nearly hovering, but moving forward at a walking pace. 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.
- tgb 10y ago
  Are you actually claiming that a helicopter doesn't use any energy to hover in place? Your argument describes the work done on the helicopter (or plane), but not the work the helicopter does on the air, which it pushes downward considerably. So, too, does an airplane's wings 'push' air downwards. Contrast to the vehicle sitting on the ground, where the ground is incompressible and no work is done on it.
nbadg 10y ago
The relationship between drag and lift is complicated. They're ultimately both the same phenomenon: pressure differentials introduced by dynamic fluid flow. You cannot have lift without drag, and you cannot have drag without lift [1]. 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.

notlefthanded 10y ago

Short answer: that is not correct for heavier than air aircraft. https://www.grc.nasa.gov/www/k-12/airplane/forces.html 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.

skelsey 10y ago
Yes it is correct. Lift always creates drag. You can think of drag as the work necessary to gain lift. When lift is generated, it produced what is called induced drag[0]. Induced drag and parasitic drag, which is the drag generated from the aircraft structure itself, are where all the energy goes in unaccelerted flight. [0] https://en.wikipedia.org/wiki/Lift-induced_drag https://en.wikipedia.org/wiki/Lift-induced_drag
- notlefthanded 10y ago
  I think we're interpreting op differently. No, not all the energy added to a heavier than air aircraft (thrust) in straight-n-level flight* is used to counteract drag. Yes, where there is lift, there is induced drag. But the kinetic energy being added to maintain velocity is also being used for lift. 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.
  - teraflop 10y ago
    No, that's a non sequitur. An aircraft that doesn't produce lift requires less energy input, but that's not because lift requires energy -- it's because when there is no lift, the aircraft is gaining kinetic energy by losing potential energy. If lift requires energy, then where would that energy go?