Aircraft Energy Question
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Why not use a sailplane/hang glider etc. in a rising air mass as an example?
If you launch into the wind from a ridge, you can gain potential energy (i.e. climb) - just as if you were a powered aircraft. Ditto in a thermal, or wave soaring.
I even read an account of an old KC-97 with a pilot who was glider rated - he found a nice mountain wave, pulled power back to idle, and climbed!
Does the rising air mass affect the L/D? The answer is NO - even though your apparent L/D approaches infinity or beyond.
If you launch into the wind from a ridge, you can gain potential energy (i.e. climb) - just as if you were a powered aircraft. Ditto in a thermal, or wave soaring.
I even read an account of an old KC-97 with a pilot who was glider rated - he found a nice mountain wave, pulled power back to idle, and climbed!
Does the rising air mass affect the L/D? The answer is NO - even though your apparent L/D approaches infinity or beyond.
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As I picture it, an aircraft that weighs (for example) 200 tons, has to be creating 200 tons of lift every single instant to stay aloft. To climb it needs to create more than 200 tons, to descend less than 200.
It does this due to the effect of the wing angle of attack coupled with the shape*, which when combined force 200 tons of air downward every single instant, thus winning the battle against gravity, which is trying to pull the aircraft down at 9.81m/s at all times.
That's why work is being done even in straight and level flight.
Plus of course barging a plane shape's worth of air out the way, 'dragging' along a heap of air due to the boundary layer, creating all those wing vortices etc. Whole bunch of stuff going on when you get up close and personal with air at speed!
Same reason your arm aches - holding it out is actually resisting gravity which is trying to accelerate your arm downwards. Don't think there's a boundary layer or drag to consider here though.
*Ignoring body lift etc as it's easier to simply think of the wing as creating the lift.
It does this due to the effect of the wing angle of attack coupled with the shape*, which when combined force 200 tons of air downward every single instant, thus winning the battle against gravity, which is trying to pull the aircraft down at 9.81m/s at all times.
That's why work is being done even in straight and level flight.
Plus of course barging a plane shape's worth of air out the way, 'dragging' along a heap of air due to the boundary layer, creating all those wing vortices etc. Whole bunch of stuff going on when you get up close and personal with air at speed!
Same reason your arm aches - holding it out is actually resisting gravity which is trying to accelerate your arm downwards. Don't think there's a boundary layer or drag to consider here though.
*Ignoring body lift etc as it's easier to simply think of the wing as creating the lift.
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Not quite right, glum. Anytime the ship's in steady-state flight, be it cruise, climb, or descent, net lift must be 200 tons. (Note: net lift is the summation of wing and horizontal tail lift, plus any vertical component of engine thrust).
It is during the transition to climb or descent that lift changes. I.E. in cruise, ROC = 0. To transition to climb of 1000 ft/min, we must accelerate the aircraft to this vertical velocity. To do that takes a few additional tons of net lift. Once stabilized in climb, net lift is again 200 tons. Opposite is true in descent.
This is much more obvious in a highly maneuverable airplane; the change in net lift is read out on a G-meter.
It is during the transition to climb or descent that lift changes. I.E. in cruise, ROC = 0. To transition to climb of 1000 ft/min, we must accelerate the aircraft to this vertical velocity. To do that takes a few additional tons of net lift. Once stabilized in climb, net lift is again 200 tons. Opposite is true in descent.
This is much more obvious in a highly maneuverable airplane; the change in net lift is read out on a G-meter.
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I think that's what I meant! And from recent lectures (MSc Aircraft design), the tail isn't really creating lift, it's there to shift the centre of lift to a better place for stability taking the C of G into account, and generally has a negative angle of attack since the C of G is aft of the centre of pressure if you only had a wing - hence why flying wings are very very hard to make work...
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It's very easy to make a flying wing stable, IF it's a swept wing. A bit of washout in the wing makes a stabilizing couple - the outboard wing is aft, and behaves as a horizontal stabilizer.
Relatively efficient too, because the "wingtip" vortex is now inboard, and countered by the negative lift at the real tip.
I learned this lesson building model gliders as a pre-teen.
Relatively efficient too, because the "wingtip" vortex is now inboard, and countered by the negative lift at the real tip.
I learned this lesson building model gliders as a pre-teen.
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No energy, as such, is required for stable, level flight. Since the aircraft is not changing it's height or speed, no work (in the physical sense) is being performed. So that kind of makes the whole discussion nonsensical, doesn't it?
The wings produce lift, used to counter gravity. This is no different from you sitting on your chair right now - the chair is acting on you with a force equally big but in the opposite direction of gravity. That force prevents you from falling. However, if your chair is not lifting you up or letting you down, there is no work being performed.
If you now also decide to scoot your (rolling-)chair along in the corridor, according to Newton, it would want to continue rolling along the corridor with unaltered direction and speed. However, friction works to slow it down, so you use force (your feet) to maintain said speed. Again, no net work is being performed as long as you maintain speed. You do work, and so does friction, but the physical resultant work is zero. There is no net change in energy.
If you could create a chair with frictionless, or low-friction wheels, that would make YOU perform less work to maintain speed, because the chair is doing less work the other way. So it would be beneficial to you that the chair drag was lower for the same chair-up-force (the one that keeps you sitting), and you could calculate a ratio for that, and call it....
Edit: and Barit1: The net lift on a 200-ton airplane is not 200 tons in climb or descent. Lift is defined as the components of forces acting normally to the flight path, and so if the flight path is not normal to the force of gravity, lift and gravity is not 180 degrees apart, and for them to equal out, they must be different, and have a lift/drag component as well. This is easily provable by asking how much lift is required for a 90 degree (vertical) climb? None! But a whole lot of thrust. Again, in a vertical descent, all the lift in the world would not make you decrease your rate of descent, only drag would.
Edit: perhaps I didn't make myself clear (perhaps I don't have it figured all out, that's probably it, actually). I was starting the post out as a discussion on work being performed along the normal axis, ie vertically, and the thrust/drag component aside. And then I obviously went away from my original thoughts. I haven't edited the contents above thogh, because of the posts below.
The wings produce lift, used to counter gravity. This is no different from you sitting on your chair right now - the chair is acting on you with a force equally big but in the opposite direction of gravity. That force prevents you from falling. However, if your chair is not lifting you up or letting you down, there is no work being performed.
If you now also decide to scoot your (rolling-)chair along in the corridor, according to Newton, it would want to continue rolling along the corridor with unaltered direction and speed. However, friction works to slow it down, so you use force (your feet) to maintain said speed. Again, no net work is being performed as long as you maintain speed. You do work, and so does friction, but the physical resultant work is zero. There is no net change in energy.
If you could create a chair with frictionless, or low-friction wheels, that would make YOU perform less work to maintain speed, because the chair is doing less work the other way. So it would be beneficial to you that the chair drag was lower for the same chair-up-force (the one that keeps you sitting), and you could calculate a ratio for that, and call it....
Edit: and Barit1: The net lift on a 200-ton airplane is not 200 tons in climb or descent. Lift is defined as the components of forces acting normally to the flight path, and so if the flight path is not normal to the force of gravity, lift and gravity is not 180 degrees apart, and for them to equal out, they must be different, and have a lift/drag component as well. This is easily provable by asking how much lift is required for a 90 degree (vertical) climb? None! But a whole lot of thrust. Again, in a vertical descent, all the lift in the world would not make you decrease your rate of descent, only drag would.
Edit: perhaps I didn't make myself clear (perhaps I don't have it figured all out, that's probably it, actually). I was starting the post out as a discussion on work being performed along the normal axis, ie vertically, and the thrust/drag component aside. And then I obviously went away from my original thoughts. I haven't edited the contents above thogh, because of the posts below.
Last edited by bfisk; 12th Nov 2010 at 21:30.
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The wing as it moves through the air has two components of drag:
induced drag - is the drag which results from the fact that the wing produces lift. There will always be a component of induced drag. It is impossible (even theoretically) to design a wing which would have 0 induced drag.
parasitic drag - is the drag resulting from everything else - designers strive to reduce this.
Now as the plane moves through the air the power is simply P=f*v (force to overcome induced drag * velocity). The fact that neither the kinetic nor potential energy of the aircraft are changing makes no difference. (In reality - as the plane 'ploughs' through the air the air molecules will be accelerated - heated - hence their kinetic energy is increasing).
So what is the magnitude of this induced drag? I think, assuming some kind of 'ideal' wing, that the power expended to overcome induced drag will be be equal to the power required to overcome the constant accelerating force of gravity on the plane. I guess we could try to imagine this as the wing 'deflecting' a certain mass of air downwards. The mass of this air * the acceleration (i.e. force) will need to be equal to the force exerted on the aircraft by gravity - 9.8 N/kg.
Regards,
Gregory
induced drag - is the drag which results from the fact that the wing produces lift. There will always be a component of induced drag. It is impossible (even theoretically) to design a wing which would have 0 induced drag.
parasitic drag - is the drag resulting from everything else - designers strive to reduce this.
Now as the plane moves through the air the power is simply P=f*v (force to overcome induced drag * velocity). The fact that neither the kinetic nor potential energy of the aircraft are changing makes no difference. (In reality - as the plane 'ploughs' through the air the air molecules will be accelerated - heated - hence their kinetic energy is increasing).
So what is the magnitude of this induced drag? I think, assuming some kind of 'ideal' wing, that the power expended to overcome induced drag will be be equal to the power required to overcome the constant accelerating force of gravity on the plane. I guess we could try to imagine this as the wing 'deflecting' a certain mass of air downwards. The mass of this air * the acceleration (i.e. force) will need to be equal to the force exerted on the aircraft by gravity - 9.8 N/kg.
Regards,
Gregory
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You Nuggets!
Why the F... did I answer this????
"no work (in the physical sense) is being performed. So that kind of makes the whole discussion nonsensical, doesn't it?"
Nonsense? That means we need no fuel right? Free energy?
Another poster who drives a UFO!!
FGS sake get yourselves educated..PLEASE
You are embrrassing yourselves!
"no work (in the physical sense) is being performed. So that kind of makes the whole discussion nonsensical, doesn't it?"
Nonsense? That means we need no fuel right? Free energy?
Another poster who drives a UFO!!
FGS sake get yourselves educated..PLEASE
You are embrrassing yourselves!
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DERG seems to me to be a devoted student of Saul Alinsky. We haven't yet seen his theory of aviation, but rather plenty of his expert invective.
I'm willing to listen, however. Go ahead, sir.
I'm willing to listen, however. Go ahead, sir.
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Barit and all other wannabes
The only reason you folks hang around here is to suck the blood outta people like me. If you think I am going to give you a 101 lecture on Newtonian mechanics you are wrong. As for the other WANNABE no comment.
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When we talk about energy, we talk about energy transfers.
Steady level flight, constant speed:
The state of energy of the airplane remains constant (if we neglect the mass reducion due to fuel burn).
However, to maintain that state of energy you need to burn fuel, right? Otherwise you would slowdown or descend. This is because drag removes energy from the airplane, transfering it to the air in several forms (moving it and heating it, mainly). You replace that energy with the energy coming from the engines (much of which is wasted in the air as well, by the way).
There are many energy transfers, between the airplane and the air, and in both senses. The fuel burnt allows you to counter the drag caused by your airspeed. Your airspeed makes the lift that keeps your altitude constant.
The net result, in this case, I think it would be the fuel burnt. That is what keeps you up there for as long as you want.
In the case of a glider maintaining constant altitude and speed, the energy lost by drag has to come from the rising air, entirely.
nice thread
Steady level flight, constant speed:
The state of energy of the airplane remains constant (if we neglect the mass reducion due to fuel burn).
However, to maintain that state of energy you need to burn fuel, right? Otherwise you would slowdown or descend. This is because drag removes energy from the airplane, transfering it to the air in several forms (moving it and heating it, mainly). You replace that energy with the energy coming from the engines (much of which is wasted in the air as well, by the way).
There are many energy transfers, between the airplane and the air, and in both senses. The fuel burnt allows you to counter the drag caused by your airspeed. Your airspeed makes the lift that keeps your altitude constant.
The net result, in this case, I think it would be the fuel burnt. That is what keeps you up there for as long as you want.
In the case of a glider maintaining constant altitude and speed, the energy lost by drag has to come from the rising air, entirely.
nice thread
I think the original question contains a better explanation than anything posted afterwards, if I could be so bold.
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).
See above.
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!
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?
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Hi full wings
As I mentioned, the state of energy of the airplane remains constant. This does not mean that there are no energy transfers ocurring between the airplane and the air.
They occur. The airplane gives mechanical energy to the air. The airplane moves the air about it, creating a circulation around the wings, the reaction of which is Lift, Drag and aerodyinamic moment. Lift helps to maintain the state of energy. Drag does not. It would reduce it, but the Thrust from the engines counteracting Drag helps to maintain airspeed and thus the energy state constant.
The air is not giving energy to the airplane in the form of Lift. Lift is just a force (a much more misterious magnitude than energy, in my opinion). It gives energy to the airplane in the form of heat, vibration and others, but not in the form of potential or kinetic energy. These remain constant.
In the case of the glider in an updraft flying at constant altitude and speed, the air isn't giving potential or kinetic energy either. But it supplies the energy wasted by drag.
My conclusion is that an external supply of energy is needed for sustained flight. It can be energy from fuel or energy from rising air, or from a towing airplane, or any other. The amount of such energy is proportional to Drag.
As I mentioned, the state of energy of the airplane remains constant. This does not mean that there are no energy transfers ocurring between the airplane and the air.
They occur. The airplane gives mechanical energy to the air. The airplane moves the air about it, creating a circulation around the wings, the reaction of which is Lift, Drag and aerodyinamic moment. Lift helps to maintain the state of energy. Drag does not. It would reduce it, but the Thrust from the engines counteracting Drag helps to maintain airspeed and thus the energy state constant.
The air is not giving energy to the airplane in the form of Lift. Lift is just a force (a much more misterious magnitude than energy, in my opinion). It gives energy to the airplane in the form of heat, vibration and others, but not in the form of potential or kinetic energy. These remain constant.
In the case of the glider in an updraft flying at constant altitude and speed, the air isn't giving potential or kinetic energy either. But it supplies the energy wasted by drag.
My conclusion is that an external supply of energy is needed for sustained flight. It can be energy from fuel or energy from rising air, or from a towing airplane, or any other. The amount of such energy is proportional to Drag.