# Gas flowing quickly over the surface of an aircraft wing

## Main Question or Discussion Point

If part of the upper surface of a jet aircraft wing was replaced by a thin flat plastic sheet containing inert helium gas moving at 2-300 miles an hour (relative to the aircraft),in the direction of motion of the aircraft,the force on the upper wing surface would be reduced significantly and lift increased.Could aviation fuel be used instead of the helium?

## Answers and Replies

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Danger
Gold Member
I am completely baffled by the whole idea. Can you explain that a bit better?

Replace a section of metal wing surface with a layer of gas (or liquid) enclosed in a "flat" plastic bag (the gas would be pumped through the bag from which it cannot escape and can be recycled in a closed circuit).The fast moving gas flowing through the bag would mean there is a greater relative velocity between the replaced part of the wing and the surrounding air the plane is passing through,than between the "normal" part of the wing and the surrounding air.Hence more lift in the replaced section!

berkeman
Mentor
But the surface of the bag is not moving with respect to the outside air, is it?

The surface of the bag doesn't have to move with respect to the outside air because the force gets transferred to the gas inside the bag, the gas decreases its horizontal speed component (in the direction of travel of the aircraft) and there is less force acting vertically down on the wing because the net force vector for a given air molecule (this vector can be considered to be made from horizontal and vertical components added to yield a resultant force vector) turns away from the vertical.There is an increase in drag for the aircraft because the deccelerated gas transfers its
decreased momentum to the rest of the wing structure.

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ouch my brain hurts after read that bit

NO gas vector does not matter inside a skin
and NO ""force gets transferred to the gas inside the bag''''
is pure nonsence skin/bag blocks any tranfer

now you can blow air and get an effect
but there must be direct contact ie NO SKIN

russ_watters
Mentor
The surface of the bag doesn't have to move with respect to the outside air because the force gets transferred to the gas inside the bag...
If that were true, then you'd be able to get this off the ground without the plane moving at all. But Berkeman is right, and let me expand: if the bag is closed nothing going on inside the bag can cause any continuous external forces on the bag, with the exception of buoyancy. In fact, if you combine those two, it is probably simplest if you consider how a balloon works and what you could do to alter it. For example, if you had a big balloon with a fan inside it, would it move?

What you are falling into here is a fairly common perpetual motion type misconception trap.

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Russ is correct. And it should be noted that people can talk themselves into such traps because they do not follow the first "law" of engineering:

Draw A Free-Body Diagram & Label the Forces!

Talk is cheap. Engineering is expensive (but effective!)
Rainman

chroot
Staff Emeritus
Gold Member
Besides, the oft-repeated description of lift caused by Bernoulli's law (i.e. the air flows faster over the top of the wing) is completely wrong anyway.

- Warren

russ_watters
Mentor
Besides, the oft-repeated description of lift caused by Bernoulli's law (i.e. the air flows faster over the top of the wing) is completely wrong anyway.

- Warren
Not wrong at all (air does, in fact, flow faster over the top surface), people just misunderstand it. In fact, the concept the OP is talking about reminds me a little of the effect of spinning an object to increase the speed over one side of an object while decreasing it over the other. Friction changes the velocity, which changes the pressure: http://www.grc.nasa.gov/WWW/K-12/airplane/cyl.html

Where people generally get tripped up is that they think that the two air particles right next to each other that separate at the leading edge must meet again at the trailing edge. That's the only real problem with it.
Arguments arise because people mis-apply Bernoulli and Newton's equations and because they over-simplify the description of the problem of aerodynamic lift. The most popular incorrect theory of lift arises from a mis-application of Bernoulli's equation. The theory is known as the "equal transit time" or "longer path" theory which states that wings are designed with the upper surface longer than the lower surface, to generate higher velocities on the upper surface because the molecules of gas on the upper surface have to reach the trailing edge at the same time as the molecules on the lower surface. The theory then invokes Bernoulli's equation to explain lower pressure on the upper surface and higher pressure on the lower surface resulting in a lift force. The error in this theory involves the specification of the velocity on the upper surface. In reality, the velocity on the upper surface of a lifting wing is much higher than the velocity which produces an equal transit time.
http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html

Other misconceptions...
If we know the correct velocity distribution, we can use Bernoulli's equation to get the pressure, then use the pressure to determine the force. But the equal transit velocity is not the correct velocity. Another incorrect theory uses a Venturi flow to try to determine the velocity. But this also gives the wrong answer since a wing section isn't really half a Venturi nozzle. There is also an incorrect theory which uses Newton's third law applied to the bottom surface of a wing. This theory equates aerodynamic lift to a stone skipping across the water. It neglects the physical reality that both the lower and upper surface of a wing contribute to the turning of a flow of gas.

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chroot
Staff Emeritus
Gold Member
Sounds like it's pretty wrong to me, Russ. :rofl: Maybe my definition of "wrong" is different from yours, however. Lift is essentially entirely created by the reaction force caused by the wing's angle of attack pushing air down. The Bernoulli effect has very little to do with the lift created by normal airplane wings, and absolutely nothing to do with symmetric aerobatic airplane wings, which still fly just fine...

- Warren

I still disagree with people who say that the gas bag idea wouldn't work.A moving aircraft wing is covered by thin layers of air which are,like the walls of the bag,a barrier to air molecules attacking the wing structure from further afield.The thin layers of air transfer forces generated by air molecules above the layers to the metal wing structure.And yes,there would be some lift generated even if the aircraft was stationary.Perpetual motion doesn't come into this idea because a pump that expends energy would be required to keep the gas flowing.

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russ_watters
Mentor
Sounds like it's pretty wrong to me, Russ. :rofl: Maybe my definition of "wrong" is different from yours, however. Lift is essentially entirely created by the reaction force caused by the wing's angle of attack pushing air down. The Bernoulli effect has very little to do with the lift created by normal airplane wings, and absolutely nothing to do with symmetric aerobatic airplane wings, which still fly just fine...

- Warren
I'm really not following you. You can use the Bernoulli effect just fine on a symetric wing. As you'd expect, at zero angle of attack, the velocity profile is symmetric and thus the pressure profile is symmetric and there is no lift. But a positive angle of attack essentially makes the airfoil asymetric and the velocity and resulting pressure profiles are asymetric. And "pushing air down" implies that it is the bottom surface of the wing that produces most of the lift - but it isn't(that's one of the misconceptions in the NASA link). If that were true, a flat-bottom wing (with a sharp leading edge) would produce no lift at zero aoa. For such a situation, all of the lift comes from air being accelerated over the top surface of the wing - air flowing over the bottom surface isn't affected at all.

The difference between Newton and Bernoulli is that to use Bernoulli, you integrate the pressure profile over the area to get lift (very common in wind tunnels) and for Newton, you measure the downdraft behind it.

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russ_watters
Mentor
A moving aircraft wing is covered by thin layers of air which are,like the walls of the bag,a barrier to air molecules attacking the wing structure from further afield.The thin layers of air transfer forces generated by air molecules above the layers to the metal wing structure.
Those thin layers of air are not separated from each other by an impenetrable barrier.
And yes,there would be some lift generated even if the aircraft was stationary. Perpetual motion doesn't come into this idea because a pump that expends energy would be required to keep the gas flowing.
Actually, perpetual motion does apply here because you aren't doing any mechanical work outside your control volume.

Can you explain what you think would be going on in my balloon example to cause it to rise?

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Warren,
The Bernoulli effect has very little to do with the lift created by normal airplane wings, and absolutely nothing to do with symmetric aerobatic airplane wings, which still fly just fine...
In my opinion you should be careful about statements of absolutes in your pronouncements. There is one aspect of lift production by a wing that you cannot escape as truth, and it is derived from Bernoulli's principle:

The lifting force of a wing scales directly with dynamic pressure.

This is a basic fact of flight dynamics that cannot be refuted, and which is directly derived from the Bernoulli relation which defines the distinction between static, and dynamic pressure. So I am totally in Russ' court here: There are slight misunderstandings and misinterpretations with respect to the mechanism of lift production viz-a-viz air transit over the upper and lower surface and a "requirement" that molecules that split at the leading edge must meet at the trailing edge. But to claim that the Bernoulli effect is "completely wrong" with respect to production of lift is throwing away a great deal of verified science, and that can lead many unscientific "cranks" to make a lot of other cranky claims. :uhh:

Rainman

The gas bag idea is wrong in its current form because in a closed bag the forces will be transferred to the rest of the wing.But if the bag had holes in it
then gas could be squeezed out and in a downward direction and would transfer the force on top of the wing to the air beyond the trailing edge of the wing.In terms of Newton's law - for every action there is an equal and opposite reaction - this would mean that the aircraft wing has increased lift as a reaction to the gas coming out of the holes.Helium being so light could be a viable gas for the bag.But it should also be noted that,in place of the bag, a flat metal strip rotating on rollers would increase the lift of a wing by increasing the relative velocity between the strip and the air.The balloon with the fan inside it wouldn't move.And one more point:as a wing sheds a vortex rotating anticlockwise,air travels under the wing from the trailing edge to the leading edge and slows down the air that comes under the wing as a result of the aircraft moving forwards,increasing lift.

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Warren,
Lift is essentially entirely created by the reaction force caused by the wing's angle of attack pushing air down.
I've highlighted the absolute in your statement to show you where it can lead to trouble. If what you are saying is true, then a cambered airfoil flying at zero angle of attack should produce no lift. But clearly we know that this is not true.

The Bernoulli effect has very little to do with the lift created by normal airplane wings, and absolutely nothing to do with symmetric aerobatic airplane wings, which still fly just fine...
Again the penchant for absolutes. If (as you say) it has "absolutely nothing" to do with lift production on a symmetric wing (which produces NO lift at zero angle of attack), then I wonder why it is that we can measure static pressure along the curvature of such a wing and relate it to the velocity of air flowing over the wing assuming constant total pressure in the flowfield (at subsonic/incompressible conditions)? In other words why do measurements of the flowfield (even around a symmetrical wing) consistently show that Bernoulli's effect DOES have something to do with ALL wings?

It seems to me that you are ignoring that lift has two aspects: Lift due to geometric shape (camber) AND lift due to angle of attack. Even a flat plate will produce lift when canted at some angle of attack with respect to the airmass flow! :tongue2:

Rainman

verdigris,
The gas bag idea is wrong in its current form because in a closed bag the forces will be transferred to the rest of the wing.But if the bag had holes in it
then gas could be squeezed out and in a downward direction and would transfer the force on top of the wing to the air beyond the trailing edge of the wing.
Methinks your verbal analysis is quickly turning your idea into a Rube Goldberg special! :surprised Once you put holes in your bag you are now faced with another aspect of nature and that is pressure equalization. While you may think you can control the rate at which the gas is "squeezed out", Mother Nature will take care of it for you... and pressure will be rapidly equalized and your gas would be gone.

But it should also be noted that,in place of the bag, a flat metal strip rotating on rollers would increase the lift of a wing by increasing the relative velocity between the strip and the air.The balloon with the fan inside it wouldn't move.
So now you are adding a great deal more weight to the wing, in addition to a requirement for power to spin a fan? Efficiency of lift production is quickly falling, I assure you... and it is never good when an airplane's Operating Empty Weight (OEW) goes up!!

My suggestion: If you wish to continue to convince yourself your idea will work with only words, without doing the appropriate engineering analysis (diagrams and equations), then I suggest the best way to go would be build your idea and test it. That will rapidly show you just how incorrect (and unworkable) your idea is. And beyond that, if you can prove that we are wrong and your idea is right, you are well on your way to a patent that you could sell.

Rainman

Rainmanaero said: "a cambered airfoil flying at zero angle of attack should produce no lift."
But if the vortexes shed have a net downward momentum then the lift of the wing can be considered to be due to a reaction force as momentum is conserved.Also the pressure of the gas in my bag idea will not be equalized.I'm going for that patent!

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verdigris,
Rainmanaero said: "If what you are saying is true,a cambered airfoil flying at zero angle of attack should produce no lift."
But if the vortexes shed have a net downward momentum then the lift of the wing can be considered to be due to a reaction force as momentum is conserved.
(You took my quote out of context, so I had to correct it above so folks would not get the wrong idea of what I was saying).You are now obfuscating the issue by mixing apples and oranges. An airfoil (by definition) is a 2-dimensional object and the flow analysis is likewise only two dimensional. Spanwise flow considerations, which give rise to vortex shedding, are only relevant when we are talking about a 3-D wing. But if you are going to bring in 3-D wing effects, I assure you that you are going to have bigger problems with your idea, and the idea that lift is only due to angle of attack (Warren's push) is also going to have problems. This is because downwash (which is the result of vortex shedding) actually causes a REDUCTION in the effective angle of attack. This is why a 3-D wing ALWAYS produces less lift than its 2-D airfoil counterpart. I can also assure you that momentum conservation (and pressure equalization) are the exact reasons that vortices exist for 3-D wings. Regardless, vortex shedding is NOT the reason why cambered airfoils produce lift. It is the pressure differential between the upper and lower surfaces. This has been shown experimentally time and time again.

Also the pressure of the gas in my bag idea will not be equalized.
I certainly would like to see an engineering analysis (not just verbal diatribe) that bears this conclusion out.

I'm going for that patent!
Good luck! But you won't find me holding my breath!

Rainman

russ_watters
Mentor
The gas bag idea is wrong in its current form because in a closed bag the forces will be transferred to the rest of the wing.But if the bag had holes in it
then gas could be squeezed out and in a downward direction and would transfer the force on top of the wing to the air beyond the trailing edge of the wing.In terms of Newton's law - for every action there is an equal and opposite reaction - this would mean that the aircraft wing has increased lift as a reaction to the gas coming out of the holes.
Ahh, now you're getting somewhere, but the effect is much, much better than even that. Have a look at the concept of a "blown flap" (specifically, the last sentence):
In a conventional blown flap a small amount of the compressed air produced by the jet engine is "bled" off of the compressor stage and piped to channels running along the rear of the wing. There it is forced through slots in the wing flaps of the aircraft when the flaps reach certain angles. This air follows the flap profile, aimed downward to provide more lift. The bleed air prevents the boundary layer (slow-moving air that accumulates on the airframe surface) on the upper surface of the flap from stagnating, further improving lift.
http://en.wikipedia.org/wiki/Blown_flaps

Here are a handful more variations on the idea: http://www.aerodyn.org/Drag/blc.html [Broken]

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russ_watters
Mentor
If what you are saying is true, then a cambered airfoil flying at zero angle of attack should produce no lift. But clearly we know that this is not true.
Just to stave-off a possible nitpick, that would be geometric angle of attack (defined by the line between leading and trailing edges) as opposed to effective angle of attack (defined at zero lift). We've had that argument before...

Rainmanaero said: "
vortex shedding is NOT the reason why cambered airfoils produce lift. It is the pressure differential between the upper and lower surfaces"

As a trailing edge sheds a vortex air flows under a wing from the trailing edge
to the leading edge.This air collides with and increases the density of the
air flowing because of the aircraft's forward motion.This produces a lifting force.If the vortex is not shed then there would be no flow under the wing from the trailing edge to the leading edge.The lift on an airfoil depends on the totality of what is happening.