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Theory on lift

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Theory on lift

3rd Dec 2012, 19:31

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I think your baulked flow beneath the wing picture, where the wing is within a fraction of a chord distance of the ground (Fig 4 of your diagram) is effectively describing a sort of `ram-air hovercraft', where there is an enhanced static pressure beneath the vehicle - plus a lot of extra drag, as you're slowing more mass down in the horizontal direction than you would a long way above the ground.
Yes, I would agree with that for that particular picture. As I said, my hope was that people would interpolate for greater heights, although that of course depends to some extent on there being some linearity.

My comments really were based on the data of that NACA TN, but now I come to look at that more closely I see that also was biased towards quite low ground heights. The trend to increased pressures under the wing is still there though even at heights of around 30% span which is reasonable for a modern design close to touchdown. It is MUCH less obvious however and, again looking more closely, I see there is some increase in LE suction peak in this condition.

That may be enough to suggest that for what most would regard as 'normal' the ground effect can be simply regarded as due to increased circulation (arising from the 'mirror image' wing) which adds to the lower surface flow slowing down from the main wing circulation, and must also result in increased circulation around the main wing.

That would satisfy me - now back to Newton
3rd Dec 2012, 22:13

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This is the established and accepted resultant...

does that fit within that diagrams parameters?

In CFD, would this look the same as a flat bottomed boat with the same side angles as an aircraft wing bottom profile?

Would one assume flap setting affect this calculation?

On final, when wheels are down, would the couple between the cores be influenced?

If the aircraft on final encounters a crosswind, the unbalanced diagram would produce what sort of result?

All questions that need to be answered, but how?

( I would note that images of ac on final can be misleading as the rate of descent of the ac, are typically close to the rollup rate of descent)

Cheers to evolution!

Last edited by FlightPathOBN; 3rd Dec 2012 at 22:36.
4th Dec 2012, 20:53

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Originally Posted by Owain Glyndwr
No problem with Sir Isaac and changes to the integrated pressures normal to the surface, but the measurements (NACA TN 1095 again) show that the pressure changes are such that the pressures on the upper surface are barely affected by the presence of ground whereas the pressures on the lower surface increase substantially in a manner that indicates that the flow over the lower surface is retarded.
That is why I'm still convinced that the downwash behind the wing leads to a 'choking' of the mass stream underneath the wing as it displaces the streamlines normally exiting along the lower surface downward toward the ground which themselves are displacing other streamlines.

As the ground puts a fixed barrier to this mutual displacement, the effective freestream area behind the TE is reduced.
Assuming mass flow = free stream speed x effective free stream area this will reduce mass flow and thus velocity underneath the wing, leading to increased static pressure on the lower side.
Sorry for defending Mr. Bernoulli again.

Last edited by henra; 4th Dec 2012 at 20:54.
4th Dec 2012, 21:19

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Sorry for defending Mr. Bernoulli again.
Does he need defending? After all, he's not responsible for the misconceptions based on wrong applications of his equation!
4th Dec 2012, 23:03

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Lift=Money
4th Dec 2012, 23:35

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HN39,,
Good form!
5th Dec 2012, 17:02

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Thank you for that drawing from Leonardo da Vinci !
He did not chose to draw streamlines, nor streamtubes, only vortexes

Last edited by Jetdriver; 26th Feb 2013 at 17:29.
5th Dec 2012, 18:20

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The airplane in level steady flight knocks the air into many "pieces", and it is not willingly the means of separation; here is drag. And Friction, and 'resisted', and viscosity, and compression. These nouns describe what is intuitive into the manufacture of what they call 'chaos'. With some forgiveness for the artist, who owns no wind tunnel......

But wait, he is watching the natural tunnel. The boundaries of the tunnel, the visible limits, are described by the shapes so important to the one who is 'visual' in his genius, the Italian.

The artist who cannot see boundaries cannot see shape. Some of us demand precision that is not available, and request of the intuitive that assumptions be made. It is then organized to further 'knowledge'.

The artist is satisfied with space and its inhabitants. The observer demands more, and makes up a personal 'language', a 'jargon'.

In jargon are rules, and code. From chaos, the artist sees order. The inventor makes the order perform, and the circle is complete.

The wing collects the workers, and pushes them away. In pushing, he is raised, and supported.

If I block your exit, will you take another?
5th Dec 2012, 20:32

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Please do not block my exit; I like to keep my aperture open at all times thank you.
7th Dec 2012, 04:42

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ABSOLUTELY!

F=Ma

Force = Mass x acelleration

That's the forumla. The speed changes, the pressure changes (all over the wing)-
some higher (leading edge) some lower( aft of that). Cause or effect.
The basic principle is that you must acellerate a mass of air (change its direction and/or velocity. The prop, the wing, the rotor, the jet they all do the same thing. Some do a small mass of air with a big acelleration, some to a big mass of air with a smaller acelleration. Diagrams like the rotating cylinder do nobody any good. That was apparently drawn by somebody who didn't get it himself, and had a hard time explaining it to anyone else.
26th Feb 2013, 15:55

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From the NTSB Report on the G650 flight test takeoff accident at Roswell, NM on April 2, 2011:
The National Transportation Safety Board‟s (NTSB) investigation of this accident found that the airplane stalled while lifting off the ground. As a result, the NTSB examined the role of “ground effect” on the airplane‟s performance. Ground effect refers to changes in the airflow over the airplane resulting from the proximity of the airplane to the ground. Ground effect results in increased lift and reduced drag at a given angle of attack (AOA) as well as a reduction in the stall AOA. In preparing for the G650 field performance flight tests, Gulfstream considered ground effect when predicting the airplane‟s takeoff performance capability but overestimated the in - ground - effect stall AOA. Consequently, the airplane‟s AOA threshold for stick shaker (stall warning) activation and the corresponding pitch limit indicator (on the primary flight display) were set too high, and the flight crew received no tactile or visual warning before the actual stall occurred.
1st Apr 2013, 19:05

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Couple of aircraft here illustrating Flaps and Vortex generation...

Last edited by FlightPathOBN; 1st Apr 2013 at 19:12.
2nd Apr 2013, 06:18

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I didn't see this posted but its very good:

Video: How aeroplanes' wings really work - Telegraph
2nd Apr 2013, 15:16

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junebug...interesting video, although the airfoil they used is in stall.

A wind tunnel does not model the compression of the air below the wing due to the weight/force of the aircraft 'planing' through the media.....

Last edited by FlightPathOBN; 2nd Apr 2013 at 15:17.
2nd Apr 2013, 19:56

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Against my better judgement.....

If the air is moving less than .3 mach relative to the airfoil there is no compression.

Last edited by A Squared; 2nd Apr 2013 at 19:56.
2nd Apr 2013, 20:54

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If there is anything more than an AoA of 0, then there is compression.
3rd Apr 2013, 04:23

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If the air is moving less than .3 mach relative to the airfoil there is no compression.
versus

If there is anything more than an AoA of 0, then there is compression.

http://youtu.be/BY1k1GcZRww

...sorry, I couldn't resist!

Last edited by italia458; 3rd Apr 2013 at 04:24.
3rd Apr 2013, 08:32

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Originally Posted by FlightPathOBN
If there is anything more than an AoA of 0, then there is compression.
Technically speaking you are obviously correct. As soon as the static pressure on a medium increases there will be compression (That's true not only for a gaseous medium, even your wood furniture will be compressed if a Battle Tank accidentally runs over it ). The only question is: Is it big enough that it's relevant. For air this effect is usually neglected up to M0,8 - 0,9.

But
Originally Posted by FlightPathOBN
A wind tunnel does not model the compression of the air below the wing due to the weight/force of the aircraft 'planing' through the media.....
this I don't quite understand.
Why wouldn't the wind tunnel reproduce this effect?

Last edited by henra; 3rd Apr 2013 at 08:38.
22nd Nov 2016, 02:11
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It may have been posted previously (and I missed it) but, just in case it hasn't been ... the article by Arvel Gentry (referred to on p2 - post 33 by Owain Glyndwr) can be found as the final link in Articles - Página web de arvelgentry

Gentry died last year and his original webpage has disappeared.
22nd Nov 2016, 09:00

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Thank you John and Owain.
An interesting reference about creation of vortices is the 2016 Nobel Prize of Physic too, despite it matters about topological phase transition and quantic theory.
And don't forget Libchaber vortices .…
What a beautiful science, that theory on lift !
roulishollandais

"Scientific Background on the Nobel Prize in Physics 2016
TOPOLOGICAL PHASE TRANSITIONS AND TOPOLOGICAL PHASES OF MATTER
compiled by the Class for Physics of the Royal Swedish Academy of Sciences"