airfoil rising

[StringJunky]

I found this NASA article that starts with 3 incorrect theories of lift and then explains the correct one. The link to the next page is at the bottom of each. I've always thought it was the first example but it's wrong. The correct answer is rather complicated ...for me anyway.

[arc]

I found this NASA article that starts with 3 incorrect theories of lift and then explains the correct one. The link to the next page is at the bottom of each. I've always thought it was the first example but it's wrong. The correct answer is rather complicated ...for me anyway.

 

That was a very helpful explanation of the different ideas out there. Thank you for that. I've spent my fair share of time contemplating this subject. I have to simplify these concepts as much as I can to maintain even a basic picture in my mind as to what is going on.

 

I like the simple concept of a rudder on a boat or an airplane, both are rather useless unto themselves and only redirect flow and in turn provide movement to themselves when utilized in conjunction with a structural frame of some sort. They are at a basic level a levering device. Horizontal planes on submarines are wings but do not in themselves posses anything more than what a vertical rudder provides, an angled surface that redirects the flow of a force provided by movement between the planed surface and the fluid.

 

The silent partner in this is the structural frame that provides the fulcrum, so to speak, that the advantage provided by the simple planed surface or wing can be advantaged against. Whatever the actual glide that some of those early pre Wright Brothers contraptions accomplished, they were likely entirely dependent on the redirection of force that was balanced between two sets of planed surfaces (wings) separated by a long structural frame.

 

Compare this to a true flying wing that unto itself, without a separate fuselage structure, provides its own stability and lift. A true and accurate representation of the characteristics of lift albeit without the versatility and greater stability of the separate control surfaces that a conventional fuselage provides.

[cxxLjevans]

Ok lets simplify.

We consider two stationary airfoils with the fluid moving past both, except that one is tied down and one isn't.

The fluid has a certain amount of energy and it imparts some of it to the airfoils ( heating, lift, etc. ).

In the case where the airfoil is allowed to rise, an even smaller amount is imparted to the airfoil as a rise in gravitational potential.

In the case where the airfoil is constrained, the fluid is 'deflected' more strongly. In effect the fluid keeps more of its energy

 

This is more easily seen with a flat plate. If you introduce a flat plate orthogonally to a fluid flow, it will tend to fly away in the direction of the fluid flow. The plate gains some energy from the fluid which loses some. The only way to keep it stationary is to hold it, in effect, by giving it energy in the way of thrust.

From:

 

The fluid has a certain amount of energy and it imparts some of it to the airfoils ( heating, lift, etc. ).

 

I gather that the energy used to lift the airfoil a certain distance, d, comes from a decrease

in the fluids energy (via decrease in pressure or slowing down). OK, so how much comes from

slowing down the air speed and how much from decrease in pressure?

 

BTW, I've had the same questions as Zet and I applaud him in being persistent in getting

an answer he (and hopefully I) can understand.

 

Thanks Zet.

Edited January 22, 2015 by cxxLjevans

[studiot]

 

cxxlevans

 

The fluid has a certain amount of energy and it imparts some of it to the airfoils ( heating, lift, etc. ).

 

 

I gather that the energy used to lift the airfoil a certain distance, d, comes from a decrease

in the fluids energy (via decrease in pressure or slowing down). OK, so how much comes from

slowing down the air speed and how much from decrease in pressure?

 

Does it?

 

When the aircraft has passed by is the fluid any different from before the aircraft arrived?

In what way does it have less energy?

[cxxLjevans]

 

Does it?

 

When the aircraft has passed by is the fluid any different from before the aircraft arrived?

In what way does it have less energy?

 

Did I misinterpret MigL's statement? If so, could you clarify what MigL meant by:

 

The fluid has a certain amount of energy and it imparts some of it to the airfoils ( heating, lift, etc. ).

 

Also, if the energy does not come from the air, then does it only come from the thrust?

 

To put concrete numbers on this, assume:

 

1) force of gravity, g, is 1.0 meter/(second^2)

2) thrust, T, is 2.0 (kilogram*meter)/(second^2)

3) wt. of airfoil is 1.0 kilogram

4) the airfoil and rocket engine have no drag

5) the fluid is incompressible

6) density of fluid = 1

(in "appropriate" units

[ this needed to calculate the change in p

in the bernoulli equation A on:

http://en.wikipedia.org/wiki/Bernoulli%27s_principle

]

).

7) speed of airfoil at start time is 1.0 meter/sec

8) the airfoil cross section is an isosceles triangle

with width=1 meter and height of .1 meter.

9) the airfoil is within a wind tunnel

stetching 1 meter above and below the base

of the triangle (this data is needed to calculate the

speed up in fluid flow over top of triangle, AFAICT).

10) density of airfoil is 1.1 * density of fluid.

( this is needed in order for energy to be used

in lifting the airfoil. If the airfoil density = fluid density

then there would be no change in gravity potential

no matter what the height of airfoil.

)

 

What is the difference in height of airfoil after 1 sec.?

 

Since there's no drag, and since no energy comes from

the fluid, I guess part of the thrust is used to accellerate

the airfoil horizontally and part is used to lift the

airfoil vertically. But how much is used horizontally and

how much is used vertically?

Edited January 23, 2015 by cxxLjevans

[studiot]

 

Did I misinterpret MigL's statement? If so, could you clarify what MigL meant by:

 

The fluid has a certain amount of energy and it imparts some of it to the airfoils ( heating, lift, etc. ).

 

Also, if the energy does not come from the air, then does it only come from the thrust?

 

You will have to ask MigL exactly what he meant.

 

A body may, on occasion, extract temporary energy from the air, but all non powered 'heavier than air' bodies eventually fall back to the ground.

 

The path of a glider is basically a longer or shorter fall to ground. The fall may be extended by temporary 'borrowing' energy from some fortuituous local pressure difference and/or or air current.

A leaf may be blown up in the wind, but it returns the energy to the fluid by displacement when it falls back to earth.

 

For sustained, heavier than air, flight the power comes from the engines which develop thrust in some form.

 

You should be careful and not think in horizontal and vertical terms because the forces may well not be horizontal/vertical.

 

 

There are three main forces involved in a heavier than air craft.

 

Weight, which always acts vertically.

Thrust which always acts in the direction of motion (the craft may be climbing or descending)

The force exerted by the fluid on the craft.

This force is usually resolved into two components

 

The drag which acts in the same line as the thrust but in the opposite direction

The lift which is perpendicular to the drag, in the opposite sense to the weight.

In level flight only the lift is vertical and the drag is horizontal.

 

After resolution into lift and drag there four forces acting. This the usual description.

 

However the four forces do not, in general, meet at a point.

This means that there is also a residual moment to consider.

The moment is countered by the air force on the subsidiary planes, particularly the tailplane.

 

So there are five forces to consider.

Finally the weight is actually distributed and measures to pump fule and other fluids about are used to maintain trim.

 

As to your calculation, I have no idea from your sketchy figures and this site is not a do your calculations for you site.

[cxxLjevans]

 

You will have to ask MigL exactly what he meant.

 

A body may, on occasion, extract temporary energy from the air, but all non powered 'heavier than air' bodies eventually fall back to the ground.

 

The path of a glider is basically a longer or shorter fall to ground. The fall may be extended by temporary 'borrowing' energy from some fortuituous local pressure difference and/or or air current.

A leaf may be blown up in the wind, but it returns the energy to the fluid by displacement when it falls back to earth.

 

For sustained, heavier than air, flight the power comes from the engines which develop thrust in some form.

 

You should be careful and not think in horizontal and vertical terms because the forces may well not be horizontal/vertical.

 

 

There are three main forces involved in a heavier than air craft.

 

Weight, which always acts vertically.

Thrust which always acts in the direction of motion (the craft may be climbing or descending)

The force exerted by the fluid on the craft.

This force is usually resolved into two components

 

The drag which acts in the same line as the thrust but in the opposite direction

The lift which is perpendicular to the drag, in the opposite sense to the weight.

In level flight only the lift is vertical and the drag is horizontal.

 

After resolution into lift and drag there four forces acting. This the usual description.

 

However the four forces do not, in general, meet at a point.

This means that there is also a residual moment to consider.

The moment is countered by the air force on the subsidiary planes, particularly the tailplane.

 

So there are five forces to consider.

Finally the weight is actually distributed and measures to pump fule and other fluids about are used to maintain trim.

 

As to your calculation, I have no idea from your sketchy figures and this site is not a do your calculations for you site.

 

Could you please provide a very simple example (hopefully by adding to my

sketchy example ) which is complete enough to calculate an answer?

 

Also, maybe you could at least outline the calculation steps needed in this

example to get an answer. I'd be happy to do the calculations myself then.

 

TIA.

 

-regards,

Larry

[studiot]

 

8) the airfoil cross section is an isosceles triangle

with width=1 meter and height of .1 meter.

 

Given the shape of your 'foil' I suggest you look at standard wind loading codes for buildings, as it is like a pitched roof.

This will give you both the uplift and drag forces on your shape.

 

 

I'm sorry I don't know these for Texas, only England.

 

https://www.google.co.uk/search?hl=en-GB&source=hp&q=wind+load+calculator&gbv=2&oq=Wind+l&gs_l=heirloom-hp.1.0.0l7j0i10j0l2.1531.3953.0.7468.6.6.0.0.0.0.110.594.4j2.6.0.msedr...0...1ac.1.34.heirloom-hp..0.6.594.sz0Y8pfMcgA

Edited January 23, 2015 by studiot

[MigL]

If you read the first line of my post it clearly states that we are considering the simplified case of STATIONARY airfoils in a MOVING medium ( air ? )

 

Its really not that hard to scroll up and re-read my post.

Edited January 24, 2015 by MigL

[studiot]

 

If you read the first line of my post it clearly states that we are considering the simplified case of STATIONARY airfoils in a MOVING medium ( air ? )

 

If the airfoil is truly STATIONARY what form of energy do you think it gains?

It obviously can't be kinetic or potential energy.

 

True it may gain some heat due to friction, but it may loose heat by convective cooling.

[MigL]

Why can it not gain potential energy compared to the case where both the airfoil and medium are stationary ?

 

But seriously, so I don't have to repeat or use the quote function ( which I hate because it forces you to read posts several times ), why not just read what I wrote in post #19, page 1 ?

[studiot]

Because if it is truly STATIONARY it moves neither up nor down, nor at all.

[MigL]

Again, read my post .

If the medium is moving past the stationary airfoil, then there is a force acting on the airfoil, and if that airfoil is constrained, there is a potential associated with that force.

Once the constraints are removed, the potential is exchanged for kinetic, i.e. remove the hold-downs and the airfoil moves backwards and upwards.

[studiot]

OK let us examine what you said in post19 more closely.

 

 

 

This is more easily seen with a flat plate. If you introduce a flat plate orthogonally to a fluid flow, it will tend to fly away in the direction of the fluid flow. The plate gains some energy from the fluid which loses some. The only way to keep it stationary is to hold it, in effect, by giving it energy in the way of thrust.

 

 

I have underlined the bit which does not make sense.

 

Any body which is stationary under the opposing action of two (or more) forces neither gains nor looses energy.

 

Now you said that both airfoils (the free one and the constrained one) gain energy.

 

 

Ok lets simplify.

We consider two stationary airfoils with the fluid moving past both, except that one is tied down and one isn't.

The fluid has a certain amount of energy and it imparts some of it to the airfoils ( heating, lift, etc. ).

In the case where the airfoil is allowed to rise, an even smaller amount is imparted to the airfoil as a rise in gravitational potential.

In the case where the airfoil is constrained, the fluid is 'deflected' more strongly. In effect the fluid keeps more of its energy

 

 

 

Again I have underlined the part which is at variance with basic mechanics, for the same reason.

 

It is quite simple.

If the body does not move its energy does not alter, apart from any heating or cooling which is not determinate.

 

This exemplifies the difficulty that you can get into with an energy analysis, rather than a force analysis, or mixing the two.

It can be done but you have to be careful.

[MigL]

Oh I see what you mean.

You're right, it doesn't 'gain' energy. My wording is often not very accurate.

 

The stationary airfoil has more potential in the moving fluid than a stationary airfoil in a non-moving fluid.

 

Just like you would have more gravitational potential energy standing stationary on the roof of a building than stationary at ground level. You can demonstrate this by stepping off the roof and exchanging some of the gravitational potential for kinetic, such that when you have the same amount of potential as at ground level, you have quite a bit of kinetic and it'll probably hurt.

 

Actually , now that I re-read it, I only say 'gains' in the case of an unconstrained flat plate, which gains energy at the expense of the moving fluid; kinetic in this case.

Anything subjected to a force has an associated potential, if it is allowed to move, some/all potential is exchanged for kinetic.

I thought this was simple mechanics.

[studiot]

 

Anything subjected to a force has an associated potential

 

I don't think it's as simple as that.

 

 

Just like you would have more gravitational potential energy standing stationary on the roof of a building than stationary at ground level

 

When you are standing on the roof you are subject to a reaction force from the roof, of opposite direction to gravity.

Applying your logic above, you would be subject to a negative potential equal and opposite to the gravitational potential., making your net potential zero.

 

Potential and potential energy are not necessarily the same thing.

Just as the reaction force and the gravitational field are not the same.

Edited January 25, 2015 by studiot

[MigL]

Hey, don't blame me, it's Mr. Lagrange's fault.

 

Throw a ball up in the air with a certain amount of kinetic energy.

At some point the ball stops moving, having transformed all its kinetic energy to gravitational potential energy. It is motionless and possesses the maximum gravitational potential energy.

Since the ball is not constrained, the force of gravity takes over and accelerates it back downwards. Once it reaches ground level it has regained its kinetic ( minus any lost due to drag ) at the expense of the previously gained gravitational potential energy.

 

I don't see how it could be simpler than that.

 

Alternatively, if you step off a roof, you invariably hit the ground with an amount of kinetic energy large enough to kill or at least break bones.

Where does this energy come from if not from the gained potential energy provided by you climbing the stairs to get on the roof ?

Edited January 25, 2015 by MigL

[studiot]

Again I didn't say that the man on the roof has no potential energy.

 

What I said was that applying your statement means he does not.

 

So let us re-examine your statement.

[MigL]

Should we re-examine my use of physics ?

Or my use of language ?

[studiot]

 

MigL

Anything subjected to a force has an associated potential

 

I am suggesting you reconsider this statement and supplied an example of when it is untrue.

 

As a matter of interest, you need the force to exist continuously between at least two distinct points for there to be a potential.

 

Potential theory is about fields.

Edited January 27, 2015 by studiot

[Zet]

This new conversation between cxxLjevans, studiot, and MigL leads right back to my original issue with the conservation of energy analysis of an airfoil. I’ll try to present my problem again in light of this conversation and in a different way.

 

 

If a solid is submerged in a more dense fluid then the force from the pressure pushing down on the top of the object plus the weight of the object will be less than the force from the pressure pushing up on the bottom of the object.

 

This is energy. Energy is the ability to do work. This is gravitational potential energy.

 

And if the solid is allowed to move then the less dense solid will rise and the more dense fluid will fall. There will be an increase in kinetic energy and an equal decrease in gravitational potential energy.

 

---

 

 

If the air moves horizontally over and under a stationary airfoil, it moves more quickly over the top and less quickly under the bottom. Moving air along a solid means less pressure from the air on that solid; and the faster the air the lesser the pressure. So, the pressure from the air on the airfoil decreases and it decreases more on the top than on the bottom.

 

It’s as if the airfoil has been moved into another fluid of greater density. (The denser the fluid the greater the pressure gradient.)

 

Is this energy?

 

It must be. Energy is the ability to do work. If the airfoil is free to rise, it will rise if the force from the pressure pushing down on the top of it plus its weight is less than the force from the pressure pushing up on the bottom of it. There will be an increase in kinetic energy (vertical motion).

 

This increase in kinetic energy must come from some other form of energy. And it comes from (... I don’t know what else to call it ...) the “simulated gravitational potential energy” created by the different decreases in pressures on the airfoil from the moving air.

 

And where does this energy (the “simulated gravitational potential energy”) come from?

 

 

In order to get this energy (from the different decreases in pressures on the top and bottom of the airfoil) the air (or the airfoil) must be in horizontal motion. This motion is kinetic energy. And so, for energy to be conserved, there must be a slowing of this (a decrease in the velocity of the moving air and so a decrease in kinetic energy) to offset the increase in “simulated gravitational potential energy” (which then, in turn, becomes vertical kinetic energy, which then, in turn, becomes an increase in actual gravitational potential energy).

 

And if this analysis is right, then energy is conserved.

 

 

-----

 

 

 

Here is the issue.

 

In the one case the wind blows and the airfoil is not allowed to rise, and in the other case the wind blows and the airfoil is allowed to rise.

 

The blowing wind then passes on and the airfoils remain at whatever height they are at. So, in the end, the one airfoil is up in the air (fixed to a pole) while in the other case the other airfoil remains near the ground.

 

In the one case there is more gravitational potential energy in the end and in the other case there is less gravitational potential energy in the end. And so in the one case there must be a decrease another form of energy that does not also occur in the other case. When the airfoil rises there must be a decrease in another form of energy that does not occur when the airfoil is prevented from rising.

 

(As well as to also offset the increase in vertical kinetic energy of the rising airfoil and the vertical kinetic energy of the air displaced downwards.)

 

 

The only energy change option in this system is the motion (the kinetic energy) of the wind. So, for energy to be conserved, when the airfoil rises there must be a decrease in the horizontal velocity of the wind (a decrease in kinetic energy) that does not occur when the airfoil remains in place near the ground.

 

Is there?

 

Why would there be a decrease in the horizontal motion of the moving air when the airfoil rises that does not occur when the airfoil remains in place?

 

More drag?

 

?

[MigL]

Just got back to this crummy weather in Canada..

 

Yes, the answer to your question is more drag.

 

There is a difference between bouyant ptential energy and gravitational potential energy.

An object less dense than the surrounding fluid experiences an upward force, its potential is highest at sea level and decreases as the density of the surrounding fluid decreases with altitude. Gravitational potential is, by definition, due to a downward force and is highest at infinity ( gravity is a long range force ) decreasing a as you approach the ground.

A bouyant force causes unconstrained motion upwards while gravity, if unsupported, causes you to fall down.

At any height the two potentials subtract suchb that the NET potential causes unconstrained movement in the direction of lesser net potential.

 

Studiot is a technical writer so, i suppose, he demands extreme accuracy in his wording.

Myself, I'm not so strict.

[Zet]

 

Yes, the answer to your question is more drag.

 

 

 

Yep.

 

The only energy change option possible in this scenario is the wind must be slower in the case where there is more gravitational potential energy in the end.

 

And it’s not enough to just assert that the wind will be slower.

 

There must be a mechanical reason why the wind is slower.

 

And, so, the only option here is “there is more drag when the airfoil rises than when that same airfoil remains in place.”

 

 

And this additional drag must slow the wind down by the precise amount so that the decrease in kinetic energy is exactly equal to the increase in gravitational potential energy (... for energy to be conserved).

 

When I first reached this point in this analysis I thought I had resolved the conservation of energy analysis.

 

But ... then I kept thinking and realized this can’t be right.

 

 

If you have different airfoils all with the same mass and same volume, but with different shapes, then there must be the same amount of additional drag as each one rises since there is the same increase in gravitational potential energy as each one rises.

 

Is there?

 

When each differently shaped airfoil is held in place and the wind passes over them, given that they are each shaped differently, the odds are there will be different amounts of drag with each one.

 

Now, when each of them rises, one, there needs to be an additional amount of drag to slow the wind down more and to offset the increase in gravitational potential energy and, two, this additional amount of drag must be the same for every airfoil of the same mass and volume regardless of its shape.

 

But it stands to reason that if there is an additional drag when an airfoil rises that that amount of additional drag (and so the amount of slowing of the wind) will vary and depend on the shape of the airfoil.

 

But this means (if there are different amounts of additional drag when the differently shaped airfoils of the same mass and volume rise) that the decrease in kinetic energy of the moving wind cannot be precisely equal to increase in gravitational potential energy for every differently shaped airfoil.

 

And, so, energy could not be conserved in every differently shaped case due to this proposed additional, but varying, drag.

 

No?

 

If this thinking is right, then the conservation of energy analysis is still not yet resolved.

 

?

[J.C.MacSwell]

1. Generally speaking, if you restrain an airfoil both horizontally and vertically you will have higher drag than the same set up where the airfoil rises.

 

2. When the airfoil rises there will be slightly less angle of attack from the reference frame of the wing, so induced drag should be reduced. Less energy will be removed from the free stream, so the overall velocity will remain higher than in the first case

 

Energy is of course conserved in each case. The gravitational/potential energy gain in the second case is accounted for as a loss of the free stream energy, along with other losses associated with drag, which together will be less than the drag associated losses in the first case.

[studiot]

 

JCMacswell

Generally speaking, if you restrain an airfoil both horizontally and vertically

 

Zet, please note that you have to restrain the object both against the drag and the lift in the restrained case.

JC has the right of it +1

 

However please also note we are talking about the lift and drag of aircraft in the atmosphere.

 

The aircraft has kinetic energy, which is replaced by the engine when under power.

When gliding the aircraft looses both gravitational and kinetic energy, eventually coming to rest on the ground.

No glider flies forever.

 

You are describing the action in a wind tunnel, where the energy comes from the fan that drives the airflow.

In the free atmosphere the energy comes from the natural forces that created the wind, which (I think) are usually temperature differentials.

 

As regards you question comparing the energy in lifting a heavy object v a lighter one.

 

The lift force is determined by geometry so will not be able to lift a heavy object so far or so fast ( as you have shown in your diagrams) so energy conservation is preserved.

Edited February 2, 2015 by studiot