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# Theory on lift

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# Theory on lift

19th Nov 2012, 17:03

Join Date: Dec 2007
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Down the plughole

Why a whirlpool? - conservation of angular momentum, as speed increases into the drain.

Why a tube of air in the centre into the plughole? Conservation of energy - a dense whirling water core would be moving too fast to sustain from the potential energy available from the depth of your bath.
19th Nov 2012, 17:16

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Why such definition? A Shear face, the airtube does not exist until the water becomes shallow enough to allow the shear. No maelstrom can exist at depth.
No filament may originate in the fluid.....

Some one must open Bernoulli's pipe to explain the airtube. Unless we want to introduce cavitation?
19th Nov 2012, 22:29

Join Date: Dec 2007
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Why discuss energy and momentum?

Without discussing energy and momentum, I always think that the connection to the physics at work can be lost. Newton's laws deal with momentum conservation and input, Bernouilli's theorem deals with energy conservation and input. Both are necessary. Viscosity moves momentum around, and turbulence dumps energy, aided by viscosity. These processes add extra complexity to dealing with fluids, but don't take away from the need to conserve the normal stuff that needs conserving. If you jump to the Navier-Stokes equation, it includes everything you need, but doesn't immediately lend itself to intuitive explanation.

For whirlpools: try spinning a tall perspex tube filled with water along its axis: as it speeds up, the surface dips in the centre, and a thread of air can go right to the bottom. The tube can be huge. There's one that's at least 12 feet high at Techniquest on the Cardiff waterfront. You can see it on their virtual tour.
20th Nov 2012, 22:30

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I thought I would try the "bathtub" experiment as suggested by Arvel Gentry in one of two suggested by Owain Glyndwr.

The setup is in my darkroom, using a 23" x 27" processing tray. I discovered that the best overall material to make streamlines visible on the surface of the water was pepper. I cut and bent a 4" x 6" piece of aluminum and mounted it on a make-shift handle. I filmed with a small video camera mounted on a tripod directly above the tray. The lighting was indirect flourescent.

As per Gentry's comments in the paper referenced I waited until the water was still and placed the airfoil in the water such that the leading edge would meet the oncoming fluid with a positive AoA. I drew the airfoil through the water and watched how the streamlines developed.

After a day and dozens of trials a lot of pepper had sunk and I discovered that this was even better for showing up streamlines as it wasn't just on the water surface but "3D" - about 2 inches worth. I ensured that the airfoil was as close to the tray as possible, (attempting to simulate an "infinite" airfoil) and drew it, left to right.

The experiment can be viewed here. I don't know how, yet, to link videos which are hosted on personal photo-video hosting sites like "SmugMug" such that they are visible and clickable here. Tried everything...the embedded links, the blog links, the forum links and lo, nothing works.

Microburst2002, could this, (The Anatomy of the Airplane), be the link to Darrol Stinton's book that you were looking for? This document is in pdf format - Sections 5 & 6 are relevant to the discussion.

Again, I'm not an engineer but am an interested bystander in this particular conversation which is really fascinating to a pilot. Thank you Owain Glyndwr, roulishollandais and others for a very stimulating discussion on lift, and some interesting reasons why Bernoulli, and Newton, don't make it into too many works on aerodynamics when it comes to discussing lift - not Stinton's, Gentry's or Shevell's, anyway!

Last edited by PJ2; 21st Nov 2012 at 00:50.
30th Nov 2012, 16:05

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If Bernoulli's principles explain lift, how does that explain ground effects on final?

PJ2...I really dont think this experiment explains lift, as there is no load on the airfoil. It may explain airflow, but not lift.
30th Nov 2012, 16:33

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FlightPathOBN...

Some one must open Bernoulli's pipe to explain it? At the ceiling?

A "Half Pipe"?

Last edited by Lyman; 30th Nov 2012 at 16:36.
30th Nov 2012, 17:15

Join Date: Apr 2009
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.....how does that explain ground effects on final?
Bernoullis' theorem does not explain lift, and never has. He simply stated that the sum of the energies in a gas remains constant.

We therefore derive that the sum of kinetic energy and static pressure energy remains constant.

When we add the equation of mass flow being constant in a tube (area, velocity and density) assuming incompressible flow , we are getting somewhere.

Ground effect simply arises because of a reduction in downwash, which is why low aspect ratio works well in ground effect.
30th Nov 2012, 17:17

Do a Hover - it avoids G

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I'm sure these ground effect comments are a wind up.

After all what has the presence of the ground plane got to do with how lift is generated? - as opposed to what happens to the flow round an aeroplane when a ground plane is introduced into its vicinity.
30th Nov 2012, 17:58

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I don't think it is a wind up. There is a fundamental communication problem in this discussion that may never go away.

Mass is perturbed, redirected, and the perturbing mass changes direction and velocity. The Mass also loses energy. Having a constant source of 'new' energy (thrust) it creates a dynamic system that becomes mysterious only in the perception of those who then discuss it.

I started fifty years ago, and the frustration was mine, due to an intuitive rejection of the proposition, specifically equal time transit, and camber.

As a dynamic system, an aircraft in unaccelerated flight is Newtonic. I do not disdain the effort to describe it however one chooses to. My beef is with the (apparent) effort to make the endeavour overly dramatic, overparsed, and obtuse. I think the problem arises because 'air' resists a simple 1,2,3 explanation.

What is 'wrong' with an attempt to describe ground effect as a result of mass being partially supported by a 'compressed' cushion?

All respect...
30th Nov 2012, 18:13

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What is 'wrong' with an attempt to describe ground effect as a result of mass being partially supported by a 'compressed' cushion?
exactly...this 'unseen cushion' in flight, becomes apparent next to the unmovable surface, and is how ground effect vehicles maintain altitude.

Ground effect is relevant, by illustrating the issues associated with using Bernoulli's principles to explain that the airflow over the top vs the bottom of the wing creates lift.
30th Nov 2012, 18:19

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FlightpathOBN;
PJ2...I really dont think this experiment explains lift, as there is no load on the airfoil. It may explain airflow, but not lift.

After reading, (and finding it a distinct challenge!), a bit in Gentry, Shevell and Stinton as kindly pointed out by some posters and finding this discussion so interesting, I decided to do the experiment, and set it up more or less as advised by the paper.

The lift and trailing vortices generated, even at lower AoA's, were as described in Gentry's work (I have to return to Stinton though!), and the downwash visible here in the tray of standing (static) water until "disturbed" by the movement of the airfoil from which I definitely felt "lift", and in the descending vortices that I've seen (and been hit by!) in cruise by opposite direction aircraft 1000ft above, to me demonstrates downwash and the fact that lift was created.

I would disagree that there is no "load" on the airfoil. Wouldn't load be a resistance against a force whether something is held in place or whether it is dead weight plus Nz?

The airfoil is held "against its will", (if you will!) by the metal handle and my arm. That is as valid a "load" as is the sophisticated fastenings to models one sees in wind-tunnel work. If it works and is valid there, it works and is valid here, I believe.

PJ2

Last edited by PJ2; 30th Nov 2012 at 18:27.
30th Nov 2012, 18:46

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Hi PJ2

This is as close to perfection as i can imagine.

"I would disagree that there is no "load" on the airfoil. Wouldn't load be a resistance against a force whether something is held in place or whether it is dead weight plus Nz?"

******************************************************

When viewing a film of an aircraft that tragically lost its airfoil at high speed, I noticed its "vector" as it flew through the air behind the doomed a/c. It was not rising, it was not falling, so much as it was horizontal (and of course tumbling). This is instructive of your comment. The "load" on an airfoil is aft (predominantly), not up, not down, but opposite the direction of flight. So discussions about the relative static loads are moot related to the vertical. The 'lift' is forward with an upwards component.

The wing, in my opinion, is aboard to produce drag as a product of its incidence, the lift comes from the momentum it imparts to the air, due to this drag.

The arrows in the primitive diagram are completely misleading, because there is Rate which serves to rotate all four arrows clockwise, sustantially.

Imo....
30th Nov 2012, 18:53

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Originally Posted by FlightPathOBN
If Bernoulli's principles explain lift, how does that explain ground effects on final?
I don't see a contradiction there necessarily.
The downwash behind the TE will displace the air beneath/behind the airfoil thereby reducing the area where the mass flow underneath the wing from the LE can 'escape'.
As the area between TE and the ground is (solidly) limited the mass flow and thus speed below the wing will reduce. Reduced speed will increase pressure.

At the same time center of pressure starts to shift backwards. If you go to the extreme i.e. when the TE touches the ground (in case of a WiG) cp from the lower side lift component will be at 50% MAC and static pressure component below the wing will be Rho/2 *v^2 *A.
Normally the static pressure component of the underside of the wing is only Rho/2 (V2^2 -V1^2)*A. (V2 being free stream speed at TE and V1 being free stream speed at LE of the lower side streamlines).

I hope the point I try to make is sufficiiently clear and I didn't explain it in a too fuzzy way
So for me Bernoulli's general principle doesn't even conflict with this effect. At least I have an explanation for my peace of mind
30th Nov 2012, 19:04

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Unfortunately, I am not a big fan of wind tunnels.
While one can scale the model, you cannot scale the air. There are a few full scale wing only tunnels I believe, but again, I have not seen results that match what has been measured in-flight.

I would also point to wake turbulence modelling. After over 40 years of trying, the model, transport, and generation has yet to be understood, nor able to be recreated in any form of modelling medium (wind tunnel, computer, etc)....

There is the never ending winglet/sharklet/xwing configuration issue. If the wing dynamics were understood, there would not be an issue.
30th Nov 2012, 22:32

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I would disagree that there is no "load" on the airfoil. Wouldn't load be a resistance against a force whether something is held in place or whether it is dead weight plus Nz?
Hold a wing section in place. Now consider a 'scale weight' for your section. Your assumption of simply holding it in place, accounts for the weight of the aircraft, and the weight/thrust has no bearing on the airflow characteristics?
Would you assume that the same aircraft. lightly loaded, would produce the same reactive results moving through the air, as the very same aircraft at max weight?
It IS the same wing section after all....

Look at stages of flight, at the beginning, when the ac is loaded with fuel, the angle of attack is much different than the angle of attack when the aircraft is near destination with much less of a fuel load.
Using the principles that you are stating, the wing section would simply have a fixed angle, and require more thrust at the heavier weight, not a different angle of attack?
30th Nov 2012, 23:04

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FlightpathOBN, First, this discussion is way above my pay grade and so my stance is, "I'm learning". I did the experiment out of interest, not to "prove" anything. That said, I wonder if you haven't misunderstood just a bit what I'm thinking? By "load" I simply meant resistance to a force, in this case the lift axis. It doesn't matter that such load increases/decreases with weight etc, which encompasses your point. I was responding to your "no load on the airfoil" point - there is "load" in the form of resistance to the lift generated, and of course it will be different for different cases. Is "load" not indistinguishable from weight & Nz?

Cheers!

PJ2

Last edited by PJ2; 30th Nov 2012 at 23:05.
1st Dec 2012, 09:01

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Originally Posted by FPOBN
While one can scale the model, you cannot scale the air.
The element of 'scale' is represented by the Reynolds number. For some tests it is important, for many it is not. NASA has or had a 'variable density' tunnel that permitted variation of Re independent of tunnel speed, but its draw-back is a relatively high turbulence level.
I have not seen results that match what has been measured in-flight.
What an amazing statement. You can't have looked very far.
I would also point to wake turbulence modelling.
Wind tunnels are not suitable for that. They are however quite suitable and accurate for studying/measuring many other airflow phenomena, in particular for measuring the lift force generated by an airfoil or wing.

P.S.
Did you know that wind tunnel tests are not limited to airplanes but include items like cars, trucks, buildings, bridges, cyclists, skiers, golf balls, jet engines etc. ?

Last edited by HazelNuts39; 1st Dec 2012 at 11:23. Reason: P.S.
1st Dec 2012, 16:11

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Ground Effect

However, there is no "cushion of air" holding the plane up and making it "float." What happens in reality is that the ground partially blocks the trailing vortices and decreases the amount of downwash generated by the wing. This reduction in downwash increases the effective angle of attack of the wing so that it creates more lift and less drag than it would otherwise.
Taken from this link Aerospaceweb.org | Ask Us - Ground Effect and WIG Vehicles

The key word is "effective".
1st Dec 2012, 17:08

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Machinbird,

Good article. But why only the trailing vortices and not the 'bound' vortex from wing tip to wing tip?

Reminds me of how 'ground effect' is modeled in CFD. Imagine an identical airplane upside down below the first one, so that the ground surface is the plane of symmetry between the two (don't ask how the mirror airplane deals with gravity, just consider the flow disturbance it creates for the first airplane). Since there can be no flow across a plane of symmetry (it wouldn't be symmetric otherwise), the airflow encountered by the first airplane is identical to that above ground surface.
1st Dec 2012, 19:47

Join Date: Aug 2011
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Howdy Machinbird....
"An additional bonus of ground effect that becomes more significant as speed increases is called ram pressure. As the distance between the wing and ground decreases, the incoming air is "rammed" in between the two surfaces and becomes more compressed. This effect increases the pressure on the lower surface of the wing to create additional lift."

Bolding mine.....

Such heresy must not go unpunished. I do like the description, however, since it implies a 'cushion'.

Unfortunately, the word conjures up a visual of a "Whoopee" cushion.

We are not to use the word compressed, nor viscous, so I am exonerated for earlier offenses?

I will eschew the word cushion only, since 'air' is so effective in describing, well, AIR.

I will call my personal visual of ground effect: "Transient Atmospheric Gasket".

Hope no one beats me to GoDaddy.... TAG

************************************************************ ****

Lightning Mate has offered a very good point, that ground effect works best on a wing of low aspect ratio. My takeaway is that it is because the extended chord line makes a more concentrated area of "capture"; there is less loss of compressive force, since the wing is not skinny, but traps more air, for a longer time.