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Feb 19, 2020, 02:05 AM
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ShoeDLG's Avatar
Quote:
Originally Posted by Designer2010
Is there an experiement which will settle it one way or the other?
If the question to be settled is: “Does a wing generate lift by imparting downward momentum to the surrounding air?”, then yes, there is an experiment that will settle it.

If a wing generates lift by imparting downward momentum to the surrounding air, then the downward momentum of the surrounding air must always be increasing at a rate equal to lift. In theory you could measure the rate at which the downward momentum of the surrounding air is increasing. The challenge is that you would need to measure the flow field around the wing at many (likely tens of thousands of) points... this requires significant resources and doesn’t offer much practical benefit in return.

You could instead use analytical methods or computational fluid dynamics to solve for the flow around a steadily lifting wing. With such a solution in hand, it is straightforward to calculate the rate of increase of downward momentum in the air surrounding the wing. This has been done... by Drela, McLean, Lissaman, Kroo, Montag DP, and others. In all cases, it has been shown that the downward momentum in the air surrounding a steadily lifting wing is not generally increasing at a rate equal to the lift. This is extremely problematic for the idea that downward momentum transfer is THE mechanism responsible for lift.
Last edited by ShoeDLG; Feb 20, 2020 at 12:10 AM.
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Feb 19, 2020, 09:11 AM
2 infect U it 1st has 2 find U
Miami Mike's Avatar
It seems to me that the Newton's Third Law discussion that we had above, which seemed a bit off-topic at the time, actually answers this question of “Does a wing generate lift by imparting downward momentum (movement) to the surrounding air?”

The answer is yes, it has to. Lift is a force between a wing and the air, so the action of upward lift must be accompanied by a downward acceleration of air.

And as we established above, neither causes the other.
Latest blog entry: A short message...
Feb 19, 2020, 09:27 AM
Sink stinks
Montag DP's Avatar
Quote:
Originally Posted by Miami Mike
It seems to me that the Newton's Third Law discussion that we had above, which seemed a bit off-topic at the time, actually answers this question of “Does a wing generate lift by imparting downward momentum (movement) to the surrounding air?”

The answer is yes, it has to. Lift is a force between a wing and the air, so the action of upward lift must be accompanied by a downward acceleration of air.

And as we established above, neither causes the other.
You've skipped a step in the logic there. It is true that a lifting wing applies a downward force to the surrounding air as required by Newton's third law. Whether that force actually imparts downward momentum to the air depends on a couple things:

1) Which air you are talking about (what are the actual extents of air in consideration, and are the extents fixed or moving with the air)
2) What other forces are acting on the air in consideration; for example, pressure fields from wind tunnel walls, the ground, etc.
Feb 19, 2020, 05:21 PM
Registered User
So why the upward lift should not also be accompanied by an upward acceleration of the wing?
Feb 20, 2020, 04:48 AM
An itch?. Scratch build.
eflightray's Avatar
"To get the right answer, you have to ask the right question."

But then, people will interpret the question in their own way to suit their own answer.

Then people will interpret that answer to agree or disagree with their own thinking.

I went back to the start of this thread and re-read the first couple of pages. (worth doing ?)

Right from the start of the thread, it would appear, "lift is like a box of chocolates," (sorry), everyone has their own favorite flavors and dislikes.

My favorite flavor is, it's about pressure differences. (Do I prefer the hard ones or the soft ones ).


.
Feb 20, 2020, 05:29 AM
Registered User
one can enhance the process of generating lift, by reducing the deflection, by flying close to the ground surface, groundeffect. since the wing stays the same, and lift stays the same, and (i guess) pressure difference stays the same, and drag reduces, one can say that it is most efficient to reduce the deflection, by inhibiting the deflection, or by involving as much air as possible in order to move it as little as possible. what glider builders do. there are ofcourse practical and other limits to this.
Feb 20, 2020, 07:25 AM
Registered User
That fits with the lizard shown above who not only makes waves but has underwater "cavitation" bubbles maybe acting like a bypass jet engine, moving more water less distance or slower: swim fins vs faster moving natural human feet. Anyway the little lizard does have to work hard, so we know that running on water is easier for us when the water surface is frozen, spreading each impact with the actual surface moving less max vertical displacement with each step ...

.... while wearing cleats, of course!
Feb 20, 2020, 10:33 AM
Design is everything.
Quote:
Originally Posted by ShoeDLG

.....

You could instead use analytical methods or computational fluid dynamics to solve for the flow around a steadily lifting wing. With such a solution in hand, it is straightforward to calculate the rate of increase of downward momentum in the air surrounding the wing. This has been done... by Drela, McLean, Lissaman, Kroo, Montag DP, and others. In all cases, it has been shown that the downward momentum in the air surrounding a steadily lifting wing is not generally increasing at a rate equal to the lift. This is extremely problematic for the idea that downward momentum transfer is THE mechanism responsible for lift.
I am not surprised. Has anyone analyzed this using energy transfers: energy is transferred into thermal energy by heating the air, also, there is a change in entropy as well?

Momentum transfers as I recall, apply to rigid bodies in an elastic collision, not a stirring up of a fluid.

Maybe there is a way to relate energy transfer to change of momentum and compare the results.
Feb 20, 2020, 01:40 PM
B for Bruce
BMatthews's Avatar
Quote:
Originally Posted by Miami Mike
It seems to me that the Newton's Third Law discussion that we had above, which seemed a bit off-topic at the time, actually answers this question of “Does a wing generate lift by imparting downward momentum (movement) to the surrounding air?”

The answer is yes, it has to. Lift is a force between a wing and the air, so the action of upward lift must be accompanied by a downward acceleration of air.

And as we established above, neither causes the other.
There's different ways to get that downward acceleration though. Look on You Tube for "wind tunnel airfoil" and watch a couple of the smoke stream tests which do not have a lot of "stalled" flow.

The air in this case is pulled first pulled upwards from ahead of the airfoil and meets the airfoil at the stagnation point somewhere on the lower side of the nose. So it's not just a straight on flow that gets diverted like billiard balls off an angled surface. Instead it's got some initial upward direction that is then pulled back down. So perhaps a little like whipping the last guy on the Conga line around? OK, perhaps not a great analogy.

But I've long wondered about the effect of the air lifted at the leading edge. If this upwards and around the nose flow is rapidly and forcefully blocked and turned back down could be where the momentum transfer takes place? And could this be why there does not seem to be a noticeable amount of downward flow in the air off the trailing edge?
Feb 20, 2020, 03:43 PM
An itch?. Scratch build.
eflightray's Avatar
What about a flying wing with a reflex section, isn't the air flow turned up at the trailing edge ?

.
Feb 20, 2020, 05:40 PM
Registered User
richard hanson's Avatar
How many ways can lift occur?
Let me count the ways——————
As long as a difference in pressure is involved, lift may occur
L eave out pressure difference , no lift
Ever
If you want to teach kids to memorize the various arrangements which may produce lift,
Thats a different question
Last edited by richard hanson; Feb 20, 2020 at 05:45 PM.
Feb 20, 2020, 07:24 PM
Registered User
I'd love to see a smoke test for a reflex wing. If my instinct is right the air will leave the trailing edge travelling horizontally or slightly upwards, and then drop down a short distance behind the wing. The normal purpose of the reflex is to create an inverted pressure difference on the trailing edge, to compensate for the nose-down pitching moment of a simple cambered wing. Though I guess it can also be used to "smooth out" the re-balancing of the pressure difference between the upper and lower wing to reduce slightly induced drag.
Feb 20, 2020, 08:41 PM
Sink stinks
Montag DP's Avatar
Quote:
Originally Posted by Brandano
I'd love to see a smoke test for a reflex wing. If my instinct is right the air will leave the trailing edge travelling horizontally or slightly upwards, and then drop down a short distance behind the wing. The normal purpose of the reflex is to create an inverted pressure difference on the trailing edge, to compensate for the nose-down pitching moment of a simple cambered wing. Though I guess it can also be used to "smooth out" the re-balancing of the pressure difference between the upper and lower wing to reduce slightly induced drag.
If you can believe Xfoil...

(I rotated the image 5 degrees because the AoA was 5 degrees, so that makes the flow from left to right and lift pointed straight up.)
Feb 21, 2020, 08:11 AM
Registered User
Quote:
Originally Posted by BMatthews

But I've long wondered about the effect of the air lifted at the leading edge. If this upwards and around the nose flow is rapidly and forcefully blocked and turned back down could be where the momentum transfer takes place? And could this be why there does not seem to be a noticeable amount of downward flow in the air off the trailing edge?
yes, the process of aerodynamic lift is not unlike a small wave in a rope, laid out over ground and given a small special jerk. the small bulge experiences a kind of "lift" as it moves along. the top part is carried by the lifting, curving, and laying down again. i suppose, a wave in a rope, on a inclined plane, actually does not dissipate but maintains while "gliding" down the inclined plane.

another phenomenon which resembles this, is the solitary wave. large ones form regularly on certain land and sea features, "Morning Glory" in Australia.
Feb 21, 2020, 10:07 AM
Registered User
Quote:
Originally Posted by Montag DP
If you can believe Xfoil...

(I rotated the image 5 degrees because the AoA was 5 degrees, so that makes the flow from left to right and lift pointed straight up.)
I believe Xfoil. Thanks Montag DP.


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