Theory on lift

[awblain]

That's the best picture to see the effect on airflow from the passage of the aircraft: the cloud is lowest along the keel, not in two tracks behind the wingtips.

See the 757 wake smoke too - ten lengths back, you have a dipole-field-like flow.

Alongside the posted picture you reference: there is almost no compression of the air. Dynamic pressure, certainly, but no bulk compression.

[HazelNuts39]

Just take a look at Flightpath's pictures - especially the one immediately under "The aircraft wake is compressed, and with the combination of the 2 counter-rotating vortices"I think that picture shows the distribution of downwash velocity at some distance behind the wing (10x wingspan), where the trailing vortex sheet has 'rolled up' into two discrete vortices. Would the distribution closer to the wing be more even (i.e. closer to a constant downwash velocity across the span)?

[Lyman]

Hi awblain...

I may be pedantic, but I think it is important. The aircraft hasn't an "effect" on airflow, the air is still prior to the entry of the airplane. It is the energy of the a/c that develops airflow. The visible effects are the results of energy being imparted to the airmass. Some may think it unimportant whether the air is magically flowing, or is instead disturbed by, the mass driving through it. Strictly speaking, it is the wing that is "flowing" (moving) not the air. The streamlines move downward (wash) long after the wing is gone. It is this transition in angle of streamline that is sensitive to rate of development of pressure, or, velocity of the wing relative to the airmass. How quickly it turns (quantity of lift) is related to velocity and angle of attack.

Dynamic pressure is just compression expressed as a result of motion. To deny there is compression of the airmass as the wing quickly enters is not correct, imho. Mass moves as a result of collision, Third Law? The Air Mass resists motion, First Law, viscosity. Streamline curves, reluctantly, First Law? It retains its shape, Second Law?

There is no wing tip in the standard model drawing, don't know what that's about. Not sure what vortices have to do with lift. Like waterspouts, they are interesting.

Regards

[HazelNuts39]

Strictly speaking, it is the wing that is "flowing" (moving) not the air.Physically it doesn't really matter whether you place yourself in the airplane and describe the airflow moving by, or you place yourself in air at rest and look at the airplane flying past you. If you want to discuss from the perspective of the "air's rest frame":
Dynamic pressure is just compression expressed as a result of motion. To deny there is compression of the airmass as the wing quickly enters is not correct, imho. Mass moves as a result of collision, Third Law? The Air Mass resists motion, First Law, viscosity.Place yourself in the air molecule that meets the approaching wing at the 'stagnation point' close to the leading edge. When the wing has reached you, you will be moving at the same speed as the wing, and the pressure around you will have increased by the 'dynamic pressure'. Your acceleration from rest to the speed of the moving wing is not caused by 'collision' with the wing, but by the increasing pressure as the wing is approaching, because that pressure propagates forward in front of the wing (assuming the airplane is moving at subsonic speed). Sitting at the stagnation point of the moving wing, your air molecule has the choice of moving along either the lower or the upper surface. Whichever way your molecule goes, it will initially be 'pushed' along by the pressure reducing downstream from the stagnation point. Going along the lower surface, the pressure will reduce until at the wing trailing edge it has reached approximately the pressure of the surrounding air at rest, and the molecule will be moving rearward along the wing surface at about the same speed as the wing is moving forward, i.e. it will be approximately at rest relative to the surrounding air. If, OTOH, the molecule chooses to go over the top of a lift-producing wing, it's a different story. The pressure will reduce much more rapidly until it is well below the ambient pressure, by several times the dynamic pressure, and the molecule speed rearwards is much greater than that of the wing moving forward. However, past the point of minimum pressure the molecule decelerates because it is now moving against increasing pressure, until it finally reaches the trailing edge at about the same speed and pressure as the molecules that passed under the wing.

So, in summary, part of the lift is indeed due to the pressure on the lower surface being higher than the ambient pressure, but that contribution is small compared to the pressures acting on the upper surface being lower than the ambient pressure, commonly called 'suction'.

Bernoulli relates local pressure to local airspeed. Other than that, it does not explain why pressures and speeds change as they do. To explain that, you need viscosity and vorticity.

[Lyman]

Hi HN

I really like that explanation. However, I did not say that all the air collides with the wing, that would not be possible, and here is where I like Bernoulli, for as you say, the theory describes focal regions of pressure/gradient

I think you are also differentiating pressure zones as pairs, above, and beneath the wing. For your purpose, I will agree to the use of the word suction, though it is a bit creepy; I was taught to ignore the concept as spurious.

In any case, any differential that works on one streamline works on the other, on a monoplane, they are mechanically connected, No?

I also appreciate the description of velocities as returning to pre entry values and that is why I prefer to see the wing as the donor, the air as recipient.

As to suction. Above the wing, the low is attractive of the open and ambient zone above it, and the closed boundary beneath, as streamline or wing skin? The wing is pressurized beneath, and moves toward the low due to the ambient high beneath the wing? One cannot distinguish which surface is attracted? The Streamline? Or the skin? (which is of course attached to the aircraft?)

Thanks for your kindness, and your patience.....

Cheers

[Owain Glyndwr]

@HN39
Would the distribution closer to the wing be more even (i.e. closer to a constant downwash velocity across the span)? Hi HN,
You are right of course; the picture didn't say how far back and I didn't think it through. Write in haste, repent at leisure. :O
Close up to the wing the downwash will reflect the span loading which may or may not be a constant downwash -but then a constant downwash would not give any shears.

@awblain

As you see, my posted comments were unsound - my apologies.
However I still believe that you cannot generate rotational flow by the action of linear shears

PS - I wonder how many points I lose from that brick :ouch:

[John Farley]

drag, which is a retarding force, the effect of which is proportional to the speed of the airplane

When I was at school it was proportional to speed squared.

Wonder when it all changed.

[Lyman]

Hi Sir.

I think both statements are correct, though they may appear to conflict...

With the greatest respect, I have wanted to ask the following question.

Several times I have been present at a demonstration of the Harrier. I have a friend whose first experience with one is an interesting tale, but for now....

In the hover, one can easily see the application of the Newton Third Law.

Disregarding the dozens of airfoils providing "lift" from Pegasus, the reaction and work is readily apparent.

As you ease into aerodynamic flight, when exactly, does Newton fade away, and Bernoulli replace him? Can Newton yet apply to and through the "transition"?

Most grateful Sir



HazelNuts39

"Physically it doesn't really matter whether you place yourself in the airplane and describe the airflow moving by, or you place yourself in air at rest and look at the airplane flying past you."

Yes, of course. However. This came up as a response to my initial post that posited the wing as the source of energy in the standard diagram seen in the paper under discussion....

The authors purpose in writing the paper was to generate interest in evolving the basic introduction to the Physics of Flight. It is my view that an introduction should be basic, elemental, and exquisitely explicit.

I share that with the authors, imo. So why introduce 'latitude' ("it makes no difference") when restraint is required? To what end? Without a purpose, offering alternate suppositions to frame of reference is gratuitous, and opens the gate to erroneous knowledge.

My drawing has the thrust arrow "within" the wing, denoting motion of the airfoil, not "in front of" the leading edge. The wing is not passive, yet that is the almost universal conclusion students make when first introduced to the "standard" diagram.

Now Camber. The authors show in an elegant way why camber is not necessary in forming a teaching model. They intimate the importance of shape, but dismiss shape as a fundamental necessity, I agree.

The authors show, again simply, why Bernoulli assumes a closed system. As a practical matter, that is deceptive, again for the beginning student.

So off we always go, as the scientists impatiently require a long list of illogical assumptions of their audience, and then recoil in anger as some refuse.

Newton does not 'fail' in any way, throughout the flight envelope, imo....


to awblain..... Thank you indeed for introducing the word 'keel' to the discussion....

[John Farley]

Lyman

As you suggest jet engines are very much Sir Isaac in action.

With the Harrier in the hover the thrust and hence Sir Isaac are supporting 100% of the weight.

As the nozzles are rotated aft the vertical component of thrust reduces as the sine of the angle.

http://img.photobucket.com/albums/v145/johnfarley/Transition_diagram.jpg

As IAS increases the wing provides an ever increasing lift (as with any other wing) helped by the fact that it has an angle of attack of some 8 deg with the aircraft in the hover attitude. The hover pitch attitude is determined by the undercarriage and the need to land mainwheels first to avoid overstressing the nose leg.

The above diag shows how trig works in the pilot's favour when stepping off from the hover. The first 15deg of nozzle rotation produces a .26g accel while only costing some .03g downwards. Something for (nearly) nothing!

The pilot rotates the nozzles at a rate that gives him his chosen accel path - too fast and a sink will occur - too slow and height will be gained. With a level transition the nozzles will be aft after about 13sec.

All extremely easy to do.

[awblain]

Lyman, sir.

`Newton' and `Bernouilli' are both right in their way. `Newton' is couched in terms of conservation of momentum, `Bernouilli' in terms of conservation of energy. Both need to hold.

On a hovering Harrier, where does the lifting thrust act? Outwards and upwards equally at the elbow in the nozzles, where Mr Newton's second legal adviser would recommend his engineers to toughen the structure. The fuselage structure is on the rack, with seven tons pulling up, and seven tons pulling the machine apart abeam. As it starts to move forward, the flow over the wings takes some of the upward load, and the nozzles swivel back, rotating those 45 degree forces at the elbows forward, pulling the Harrier along instead of holding it up, while still trying to rip it apart acrossways.

Owain,

I fully accept your point that linear shear is not rotation, but once you account for the descending slab of air behind the aircraft, with the wingtip vortices to connect it to the non-descending air far from the flightpath, it's not a big stretch.

Close to the wing, I agree that the air that moving downwards the fastest is descending round the flap edge in that 757 picture, but that high-speed flow is part of a tight vortex - it spreads and slows behind, making the large scale dipole flow from the C17 wake picture, also effectively seen in the 747 contrail shot.

If the wing was all the way across a smooth windtunnel section, with no tips, the air behind the wing would have a uniform descending velocity component - there would be lift - and yet no rotation. I suspect almost the same is true for a sailplane wing - the downstream airflow is almost all descending slab, and almost no wingtip vortex.

[Lyman]

The natural tendency for any locally derived and accelerated gas path that must re-enter a still mass is to curve. This does not apply to an energetic path that is parallel the direction of entry of the wing. . .

As the upper biased flow spills off the tip, it curves, and contacts the flow off the bottom, which is curving also. The two flows are discrete, and compliant with Newton (#2), wish to maintain their cross section shape. There is no choice for them but to entwine....(Bernoulli, as matchmaker). Unbound, they expand, whilst gradually losing definition, and eventually all memory of their originating shape.

Will that suffice for a description of vortex generation via the Oxford genius?

I thought not. Anything at all useful?

As the wing collects, contains, and deflects (directs) air, can we then call it a 'nozzle'?

[roulishollandais]

Capital Gamma (sorry I can't find a way of inserting the Greek letter)

Owain Glyndwr , OWAIN GLYNDWR

OwainGlyndwr , OWAIN GLYNDWR :D

Method : :( Oh! Sorry ! I was still writing with greek letters ! :}

Method:
Use free Open Office,
select latin letter, write in latin letters, for instance Arial Police
select greek letter, write in greek letters, for instance Symbol Police
Select the mixed lines with as well latin and greek letters
Copy
Paste in PPRuNe Reply !
Add your PPRuNe's smilies as needed .. :)

[Owain Glyndwr]

@roulishollandais

Diolch

[john_tullamarine]

For occasional special characters, if you are using a Microsoft installation, you might try the character applet which gives you access to the complete character set for each font installed .. eg on my installation this is at

Windows\System32\charmap.exe

ie including all the non-keyboard characters ...

I have used the program on each Windows version and the filename has always been charmap so it ought not to be too difficult to find on any installation.

If you know the key combinations, characters can be entered directly but charmap is great for lazier folks like me.

[Mr Optimistic]

We could make it easier....anyone care to explain why water flowing down the plug hole always forms a vortex ? If we can't answer that, not a lot of hope for us......

[Lyman]

Manhole covers are always round.

Nature hates a corner.

Viscosity.

Rate.

"if instead of fluid, the medium was ball bearings, would there be a vortex at the drain?"

Transition.

[HazelNuts39]

Mr Optimistic,

why water flowing down the plug hole always forms a vortex ?

Isn't Coriolis responsible for that? Or is it viscosity?

[Mr Optimistic]

What !?

Manhole covers are always round.

Nature hates a corner.

Did manhole covers evolve from some more primitive closure ?

[Lyman]

They are round because it is the only shape that will not fall into the hole after removal.....

This suggests that Nature is economical, and efficient.

A ninety degree turn in a pipe is wicked drag, hence "elbow" or, camber.

I am trying to evolve an explanation for vortex, and I am not there yet.

Part of it has to do with the containment of energy in "shape", and least drag.

By the way, the vortex forms only within two limiting considerations. Pressure and Volume. I think. So that introduces "locality" (proximity). And 'locality' has to do with the point at which volume and Pressure are interchangeable as to effect, I think.

'Rate' is the bottom line for any Physics problem, imo.....

[mm43]

Vortices and principles of lift - all closely related.

Helmoltz's Theorems (http://en.wikipedia.org/wiki/Helmholtz%27s_theorems) apply to inviscid flows and where the influence of viscous forces are small enough to be ignored.

[roulishollandais]

Thank you John!
Rh:rolleyes:

in Windows
demarrer
executer
charmap.exe
OK

in charmap array
stay in your police for instance arial
for any character of your string for instance : capital gamma,alpha,mu,mu,alpa selectionner
at the end of the string : copier


in PPruNe reply
coller

Result :
Γαμμα :)

[Owain Glyndwr]

@jt

Yes thanks for that John. I was OK with symbols using copy and paste and Word or similar, but I was trying to do it directly using the fonts available inside PPRuNe edit text function.
Could something suitable be added perhaps?

OG

[roulishollandais]

We could make it easier....anyone care to explain why water flowing down the plug hole always forms a vortex ? If we can't answer that, not a lot of hope for us......

Fractals shape is characteristic with his bifurcations.

Feigenbaum unified them. He met Libchaber who has got very famous for his two little vortex in a small box of helium...

Albert J. Libchaber - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Albert_J._Libchaber)

rh

[mm43]

@rh

Worth changing the font to Georgia or Palatino (serif). It will look better!

Γαμμα :ok:

[henra]

Mr Optimistic,



Isn't Coriolis responsible for that? Or is it viscosity?

I also seem to remember it was Coriolis. But in contrast to popular belief not due to rotation of the Earth.

IIRC it is simply that any existing rotation in the water amplifies due to the small size of the outlet compared to the radius of the bath tub thus increasing the rate ot rotaton when approaching the outlet.

[Lyman]

Before there is motion, there are waves. The waves begin at the aperture, they cannot begin in the fluid. At the smallest disturbance in the energy field, there is an angular imprint on the soon to be vortex, an imprint that detemines the poles of the filaments. The angular disturbance is the instigator of the spin of the vortex.

Are you certain this is unrelated to the Gravitational field spinning between two poles?

Water waves react to frequency and amplitude, plus direction?

Only if the Earth was not spinning would gravity be acting ninety degrees to the center, it us always either 90+/-. There is no thing such as 0 rate in our system.
Everything is always moving, and direction is spin dependent, I think......in a passive locale. (tub, drain, water,)

[HazelNuts39]

Google is your friend: The bathtub vortex problem (http://jpsj.ipap.jp/news/jpsj-nc_109.html)

Another one: Which way? (http://math.ucr.edu/home/baez/physics/General/bathtub.html)

[gearpins]

The Equator Water Experiment - YouTube

may beposted earlier....fasinsting nevertheless !

[Lyman]

HazelNuts39

Now we're talkin'! So coriolis can be abandoned since it has "minimal effect".

Conditions must be established to allow it to obtain!

Imagine instead of the standard (typical) airfoil section in the bernoulli model, something different.

Rather than fix the wing at 0 degrees, and provide airflow other than from the motion of the airfoil, let us mount the wing on a spar pivot. The wing can now rotate on command, independent of flow; Angle of Incidence can be controlled.

Let us set 90 degrees, 'flat plate'. We now activate the tunnel turbines, to 200 knots. Without delay, we immediately select and acquire 8 degrees of AoI.

Where did 'Kutta' go?

I envision this experiment as analogous to the lack of coriolis effect in Bermoulli's bathtub.

I think my point is to establish that given a set of "prescribed" conditions, just about anything can make sense, or fail explanation.

How obsessively we unpack the theory of lift requires a man made set of conditions?

A famous aviator has demanded that the shape of an airfoil must be a cambered one, to explain lift. He is not WRONG. Not exactly.

The rotation of a vortex aft of the moving wing tips is predictable, due the architecture of the wing, and has immutable direction. This is to do with velocity and viscosity, plus existing flow.

Once established, prior conditions create a rigid and predictable result.

In reality, and in fantasy, both.

To disprove the bathtub problem, actual conditions must be explained, then accounted for. Does this make the standard bernoulli myth accurate?

Isn't "Suction" yet another mistake?

[roulishollandais]

In France many strange things are told about creationists schools...:E
Some of you perhaps were in such schools ?

A strange ref too, but correct, with bifurcation :
Mitchell Feigenbaum - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Mitchell_Feigenbaum)

Worth changing the font to Georgia or Palatino (serif). It will look better!

Γαμμα http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/thumbs.gif
Agreed !:ok::ok:

[awblain]

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.

[Lyman]

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?

[awblain]

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.

[PJ2]

I thought I would try the "bathtub" experiment as suggested by Arvel Gentry in one of two suggested by Owain Glyndwr (http://www.arvelgentry.com/techs/A%20Review%20of%20Modern%20Sail%20Theory.pdf).

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 (http://www.smugmug.com/gallery/26636897_8nSkqC#!i=2227348875&k=chrsFQN). 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 (http://www.ultraligero.net/Cursos/diseno/La_anatomia_del_aeroplano.pdf), (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!

[FlightPathOBN]

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.

[Lyman]

FlightPathOBN...

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

A "Half Pipe"?

[Lightning Mate]

.....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.

[John Farley]

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.

[Lyman]

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...

[FlightPathOBN]

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.

[PJ2]

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.
Thank you for your response.

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

[Lyman]

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....

[henra]

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 ;)

[FlightPathOBN]

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.

[FlightPathOBN]

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?

Think about that.
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?

[PJ2]

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

[HazelNuts39]

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. ?

[Machinbird]

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 (http://www.aerospaceweb.org/question/aerodynamics/q0130.shtml)

The key word is "effective".

[HazelNuts39]

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.

[Lyman]

Howdy Machinbird....
from the linked essay:
"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.

[roulishollandais]

@Machinbird,
Thank you for this link opening our mind, increasing the quality of the common vocabulary, and pulling us far from fear from new ideas, would they seem iconoclastic at the first glance.
That is Gentry's philosphy too.
Interdisciplinary discussion in that not academic forum is a step forward to fly better. John Tullamarine understands that very well.
Thanks to all.:)

rh

[HazelNuts39]

ground effect works best on a wing of low aspect ratioOnly in absolute terms, because the low aspect ratio wing has a high induced drag. Percentage wise there is no change with aspect ratio, see graph "Percentage of normal induced drag vs percentage of wing span above ground" in the referenced paper.

[Lightning Mate]

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.

No.

Low aspect ratio results in greater downwash, due to increased vortex strength. How on earth does a wing "trap" air????????

I think it's time to avoid further comments here in case some ill-informed moderator bans me!

[Lyman]

'Low aspect ratio results in greater downwash, due to increased vortex strength. How on earth does a wing "trap" air????????'

You confuse 'trap' with your mistaken conclusion re its definition. (in my opinion)

You are likely concluding: "trap and hold". To sequester airflow means to enclose, or to restrict it; there is no rate described. Ram Air is fhe very definition of sequester, or 'restrict'.

I cannot speak for you, but my impression is that you reject any latitude in nomenclature, not helpful, imo.

Perhaps an example will help me. If an instantaneous image of airflow could be shown that demonstrates the zones of pressure the flow inhabits, one could say that for that instant, each "zone" shows 'trapped' air. Of course the flow is transient, or it would not be 'flow'. But without the ability to shepherd airflow, there would be no lift.

Is there a word you can suggest that describes these zones of pressure better than trapped? Screw trap, call it ________ . (contain) ?

Nothing can be compressed without somehow 'trapping' it. imho.

[HazelNuts39]

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. This phenomenon is what we call ground effect ...
The pressure distribution around the wing is not different from that it has out of ground effect at a somewhat greater angle of attack. IOW, no zones of 'trapped air'.

[Lyman]

Hi HazelNuts39...

Yours,

"The pressure distribution around the wing is the same as it would have out of ground effect at a somewhat greater angle of attack."

Are these relative distributions reflective of the arrested trail? Would the clipped vortices not reflect a change in the global distribution?

Doesn't the 'effective increase in AoA' more or less demonstrate an increase in pressure under the wing, forward of center of lift? The RamAir effect alluded to in Machinbird's lonk to the ground effect description?

Thnks

[HazelNuts39]

Doesn't the 'effective increase in AoA' more or less demonstrate an increase in pressure under the wingIMHO the increase in pressure under the wing is not as great as the decrease in pressure above the wing.

[Lyman]

The decrease in pressure causing the "additional lift"?

Is the underwing increase due the "Ram"? The overwing decrease due the foreshortened vortices?

The difference you describe shows the clipped vortices as having a greater effect on the GE than the Ram as described, yes? I agree.....

Are you in substantial agreement that the increased pressure underwing is partially due the effect of the four-sided "box" described by the two tip vortices, the underwing, and the asphalt?

Dare I include the downwash as a "bottom" to the "effective" "barrel"?

From which I would infer a "trap"?

[HazelNuts39]

Lyman,

I would rather say that the changes in pressure are due to the increase of effective AoA, which in turn is the result of the reduced downwash induced by the vortex system, or rather by the superposition of the negative downwash induced by the virtual 'mirror image' of the vortex system.

[mike-wsm]

Half a century ago I made a small surface skimmer and it had a non-aerofoil wing, simply flat under and top surfaces. I can vouch for it producing a lot of lift, the problem was that it produced too much. So no need for it to be an airfoil.

[Lyman]

I like the mirrored silhouette. Alot. I picture it is a little "blurry" in outline, however, since it is composed of streamlines rather than Aluminum.

The incidence of the wing has not changed, hence, "effective". So to garner the benefit of AoA increase, "as if", we need to admit that the volume of airflow has increased beneath rhe wing, as it would had the leading edge raised.

Your preferred description works fine for me, my bent has to do with looking at things from different perspectives, anyway.

The "ram effect" doesn't need any further discussion, imo, except to say it is merely a part of the dynamic system, and should not be excluded in an objective setting?

Thks.

[FlightPathOBN]

I have just looked at the aerospaceweb article on ground effects. Sorry, but I find little, in regards to ground effect, is based on reality as we understand it today.
http://www.aerospaceweb.org/question/aerodynamics/ground-effect/ground-effect.jpg
This diagram is completely incorrect for wake vortex generation, as is the explanation describing an aircraft on final. The wingtips do not generate a vortex. The vortex from an aircaft is a result of rollup, which does not occur at the tips, but inboard of the wing section and reliant on wing configuration and flap settings.
http://blog.aopa.org/asfblog/wp-content/uploads/2008/11/vortices.jpg
The rollup time and distance has many associated factors.
On final, the vortex creation is constantly changing with flap settings, speed and configuration. This is NOT currently explained by any model, especially CFD...why is that? If wing dynamics and principles of flight were understood

There are many issues that may seem simple, but have not been explained by models, or even attempts to model the differences. The effect on crosswinds, headwinds, tailwinds, centerwing, and flap settings.
We have all seen the typical diagram where the aircraft wake drops, then is blown sideways by the crosswind, and there are extensive calculations that try to model that advection rate.
https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcTBbVnQYaW1j7o90PEw7YHyRJGsSi0IXc8EsWv1J1m o4ird18NE
Yet, this has been shown to be completely wrong by field measurements using LIDAR and SODAR.
The models make no attempt to add left/right wing configuration differences, crabbing, or other aircraft/met properties into account.
http://ars.els-cdn.com/content/image/1-s2.0-S0376042110000576-gr4.jpg
Notice how the standard equation is balanced....if not, the foundation of the calculations unwraps very quickly
Does that mean that the calculations are oversimplified, need to be generalized to model, or misunderstood?

[Lyman]

FP....

The center drawing showing "compressed" vortices gives one pause. RE: Ground Effect, a deflection of vortex means exactly what to 'lift'? The vortices may be on "rebound", but how do we conclude they are in "compression"?

As you say, the vortices are not "part" of the wing, but a relic, a "remnant" of lift?

Thank you sir.

[Owain Glyndwr]

Lyman

You may want to make a note of the date or something, but here is a first - I am going to agree with you!

NACA measurements (wind tunnel I'm afraid FlightpathOBN) made back in 1938 show that the build up of lift as you near the ground is NOT equivalent to an effective increase in AoA as suggested by the 'mirror image' modelling, but between say 5 and 10 deg AoA at least is more or less entirely due to a steady build up of pressure underneath the wing, with the upper surface flow changing very little from its free air behaviour. NACA TM 1095 if anyone is interested.

This of course says nothing about how wake vorticity is affected, but I didn't think that was the issue being debated here?

[Lyman]

Hallelujah....

The year of my birth no less. Yes, I think wake is not related here, many strings of discussion at once....

Can you comment on my proposition that the "box" effect of the bound vortices, plus downwash, under wing surface and ground (runway) partially explain the resultant "compression"?

Rgds.

[Owain Glyndwr]

Actually the tests were done in 1938 (editorial correction) but the report was issued in 1946.

Can you comment on my proposition that the "box" effect of the bound vortices, plus downwash, under wing surface and ground (runway) partially explain the resultant "compression"?Sounds reasonable. We know that in free air the greater the vertical distance below the wing TE the lower the downwash [because of the greater 'radius' from the wing bound vortex centre - sorry I couldn't resist saying that ;)] so as the TE gets nearer the ground the more momentum the underwing flow has to shift to get by, so the bigger the force (pressure) it will need.

Different aspect of course, but wings fitted with endplates will need even more pressure build up because the air cannot escape sideways. This is limited though because there is no way the underwing pressure can build up beyond freestream total pressure.

[Lyman]

Hi Owain...

"so as the TE gets nearer the ground the more momentum the underwing flow has to shift to get by, so the bigger the force (pressure) it will need."

Does the streamline contact the ground? It cannot, right? ("no matter can penetrate a streamline")

Does the streamline also "restrict" the passage of the underwing flow?

[Owain Glyndwr]

Does the streamline contact the ground? It cannot, right? ("no matter can penetrate a streamline")

You can't think of it quite like that. The flow under the wing is properly regarded as a stream tube. The 'lower' limit of the tube is a streamline running just above the ground. The upper boundary of the tube is a streamline that meets the wing where the upper and lower flows divide, then runs along the wing undersurface (outside the boundary layer of course) until it gets to the TE after which it becomes the running mate of the closest streamline that went over the top.
Obviously the lower limit doesn't actually contact the ground.

The upper limiting streamline is the one that gets affected by the direction of flow off the upper surface, and it is this flow direction/velocity I suggested would vary with vertical position relative to the TE (in free air at least), but now I think about it I'm not so sure it works like that if the upper surface flow is unaffected by height changes. It may be that the downwash angle off the TE is unchanged but as height is reduced the relative areas of the stream tube at the LE and TE change so that the relative velocities change and that is all there is to it.

Does the streamline also "restrict" the passage of the underwing flow?

In the sense that the downward pointing streamline(s) off the TE and the ground hugging lower streamline form a converging passageway, then yes, the streamline can be regarded as restricting the passage of the underwing flow.

Being mischieveous, it is fairly easy to explain this increase in lift/higher underwing pressure using the dreaded Bernouilli, but at the moment I don't see how to explain it using Newton :ugh: There must be a way though.

[HazelNuts39]

There must be a way though.Perhaps the 'effective' aspect ratio is increased?

TM1095 compared to Jeff Scott curve:
http://i.imgur.com/HqhRt.gif?1

[Lyman]

I think it is to do with Newton, in this regard. Mass, as unperturbed air in front, is loath to move, "Remain at rest" (#1). Once entered the stream tube, it is loath to rest, 'momentum' (#2). Confronting the wing streamline, it must slow, "damming" the release of air through the aperture: wing, ground streamlines.

Hence the compression, increased pressure, and lift without benefit of incidence increase? (either AoA or incidence).

[Owain Glyndwr]

Hazlenuts

Perhaps the 'effective' aspect ratio is increased?
I think perhaps we are talking at cross purposes, as you are offering a reason why drag is reduced, and I am seeking a Newtonian explanation of why lift is increased.
As a straight response to your suggestion I would only remark that there is nothing in Newton's equations that relates to AR and that infinite AR (2D) wings also show ground effect.
In the case of drag reduction I think it might be very simple, and lift and drag changes are two sides of the same coin:
When we talk of a lift increase in ground effect we usually mean an increase at constant alpha. When we talk of a drag reduction we usually mean a reduction at constant lift coefficient. Invert that first statement and we have 'a given lift coefficient is obtained at a lower alpha when in ground effect' That means that at constant lift the alpha is reduced in GE, so that the resultant force vector is inclined further forward leading to a reduction in drag.

Lyman
Hence the compression, increased pressure, and lift without benefit of incidence increase?Newton's equations as applied to an explanation of lift generation contain mass, velocity, momentum and their derivatives. There is no mention of compression or pressure - indeed such things are foreign to Newton's work.
Consequently I think that you cannot invoke such concepts in a valid Newtonian explanation of how ground effect works.

For myself, I can't get my head around anything further than:

The Newtonian explanation of lift in free air is that starting with a block of air at rest (zero momentum), passage of a wing leaves some of that air with a downward velocity and momentum. to impart that change there must have been some force whose magnitude is given by the rate of change of momentum. There must also have been an equal and opposite force acting on the wing, which we call lift.

Now it seems to me that when the wing is placed near the ground the underwing mass available for deflection is reduced - vanishing to zero if the TE actually touches the ground. In addition I would think that the average downward velocity of the mass of air going under the wing would also be lower than in free air because of the constriction imposed by the ground - in effect being half the downwash actually at the TE.
Both of those seem to me to lead to a reduction in lift rather than an increase. If anyone can point out my mistake or suggest some other Newtonian mechanism that would explain the lift increase one finds in ground effect I will be very interested!

Finally, I found these piccies that might be interesting in this discussion -

http://i1081.photobucket.com/albums/j351/OwainGlyndwr/GEcomposite.jpg

[photofly]

Those are unhelpful, and incorrect diagrams. They give the impression that ground effect is something to do with the height of the wing from the ground being related to the chord, which it isn't. (Just because it's published in a book doesn't make it correct. Doubly so when it comes to explaining aerodynamics to pilots.)

"Ground Effect" is noticeable when the height of the wing from the ground is less than the wingspan.

You can't show ground effect in a two-dimensional diagram, because it's inherently an effect that occurs only in three dimensions, like wingtip vortices.

[Owain Glyndwr]

Those are unhelpful, and incorrect diagrams. They give the impression that ground effect is something to do with the height of the wing from the ground being related to the chord, which it isn't.

Well I'm sorry you find them unhelpful, but incorrect they certainly are not. The pressure distributions are consistent with measurements on a wing of AR 5.0, so what basis have you for saying that is incorrect please? The streamline picture is meant to be consistent with the pressure diagram which, incidentally, relates to a point near midspan. Obviously it relates to a height far less than one would see in actual flight operations, but that does not make it wrong - it is an extreme case to demonstrate effects.

"Ground Effect" is noticeable when the height of the wing from the ground is less than the wingspan.


Sure, when you are flying airplanes that is when it starts to show up. But I certainly didn't say that ground effect depends on how many chords above the ground the wing is - that was your interpretation of the diagrams.

You can't show ground effect in a two-dimensional diagram, because it's inherently an effect that occurs only in three dimensions, like wingtip vortices.

There I'm afraid, you are just plain wrong - there is ample evidence that two dimensional wings exhibit ground effect characteristics.

[HazelNuts39]

Maybe we should consider that the downwash produced by the lifting wing doesn't change in ground proximity, but is reflected by the ground as an upwash. The wing benefits from the upwash in a manner that is somewhat similar to an airplane (or glider) flying in an updraft, or the migrating birds flying in a V-formation. Replace the ground surface by the virtual mirror wing (what Serebrisky&Biachuev call the "method of images") and the upwash it 'induces'.

P.S.
It is perhaps of interest to note that S&B's test setup uses a 'mirror' wing opposite to the test wing to simulate a ground plane.

[Lyman]

Hi Owain

I understand the problem related to compression (pressure) and Newton. That is why I always use mass when discussing the theory.

Any (local) 'modification' of a dynamic system involving a gas involves transient exchanges of energy, and is far more complicated than pushing an object up an inclined plane.

There are seductive traps which require patience. Newton #3 works fine for thrust; there is no reason to eliminate the other two from GE discussion, imho.

Thanks for all your time here, I am learning.

HazelNuts39

I like the upwash imagery. In fact, I like your approach in general, involving as it does visualisation. So far, I am looking at the enhanced products of GE as a result of Newton's Second Law. The 'shape' of the area under the a/c describes a volume of increased pressure, due to the stream tube resisting the descent of the planform onto it. I have used the image 'piston' in the past.

The stream tube is being pinched as it migrates aft toward the convergence of the underwing and ground streamlines?

[photofly]

there is ample evidence that two dimensional wings exhibit ground effect characteristics.Well, of course. Two dimensional wings are always in ground effect, because they have the flow pattern of a three dimensional wing with infinite wingspan. Not co-incidentally, they also generate lift with zero drag. If you want to see what ground effect does on a real wing, looking at two dimensional diagrams isn't helpful.

Those diagrams may be mid-span flow patterns but without an idea of the scale of the wingspan they're not much use. And since they're unrealistically low for aircraft flight, what use are they for indicating ground effect in relation to aircraft?

[awblain]

If you have less "downwash", I suggest you would have less lift, not more, from Newton's second law - less change of downward momentum to the air, less upward force on the wing.

How's this for size: the (above-ground) flow pattern when in ground effect can be described by replacing the ground by a mirror image of the wing and airflow. This flowing air at zero height speeds the wake air backwards, as compared with the flow pattern when far above the ground, thus reducing the slowing parallel to the ground/drag when close to the ground.

For lift: the speeding of the air behind the wing backwards, when close to the ground, as compared with at height, causes an increases in the flow rate over the wing, greater downwards mass flux, so a greater change of momentum and more lift. Also consistent with less slowing in the flow downstream, also consistent with less drag.

[Owain Glyndwr]

Two dimensional wings are always in ground effect, because they have the flow pattern of a three dimensional wing with infinite wingspan. No, they are not always in ground effect. In free air they have the flow pattern of a three dimensional wing with infinite wingspan in free air. Two dimensional wings placed near a ground board have different characteristics than when in free air.

No one is suggesting that studying the behaviour of 2D wings is going to help understanding of a 'real' wing, but looking at the flow on inboard sections of an Aspect Ratio 5 wing doesn't come into that category in my book.

The points to be made are (for a real AR 5 wing)
1. The upper surface flow is not much affected by ground proximity
2. The underwing flow develops steadily into a low velocity/ high pressure zone over most of the lower surface, from which comes the extra lift.

Wing tip vortices don't come into it - bound vorticity might be used to explain things, but I have learned that it doesn't pay to try to describe the flow to pilots in terms of bound vorticity!

Again I say they are meant to illustrate the sort of flow changes that occur near the ground. I assumed (and assume) that viewers would be able to interpolate for greater heights for themselves - it was anyway the only set of illustrations I could find at short notice.

In fairness I should also say that the illustrations were taken from a report discussing wings operating really close to the ground but, and I would emphasise this, the pressure distribution diagram and the general flow behaviour correspond closely to that observed on the AR 5 wing, which was tested at more realistic aircraft heights.

[Lightning Mate]

Let's keep it simple.

When downwash angle decreases due to ground effect, the total aerodynamic force vector is tilted forward.

Upwash only occurs at the leading edge.

but I have learned that it doesn't pay to try to describe the flow to pilots in
terms of bound vorticity!

Some of us understand it though.

[Lyman]

"Upwash only occurs at the leading edge."

Yes, having originated from beneath it. Right?

[Owain Glyndwr]

If you have less "downwash", I suggest you would have less lift, not more, from Newton's second law - less change of downward momentum to the air, less upward force on the wing.


That's where I got to also, but it is less than helpful isn't it :D

For lift: the speeding of the air behind the wing backwards, when close to the ground, as compared with at height, causes an increases in the flow rate over the wing, greater downwards mass flux, so a greater change of momentum and more lift.

I flirted with that idea, but the problem is that the measured data shows that neither the upper nor lower TE static pressures varies with height above ground at a given AoA. That means (using Bernouilli) that the velocities at the TE are not varying with height either, which sort of puts the mockers on it.

[FlightPathOBN]

The reason that I included wake, and wake calculations was relevant. If the resultant cannot be accurately measured, it leads one to doubt the rest of the calculations, methodologies used in determination and validation of wing design, and understanding of principles.

Of the two major manufacturers, ones models the entire aircraft, while the other models wings and fuselage separately. Would one expect the same results?

[Owain Glyndwr]

If the resultant cannot be accurately measured, it leads one to doubt the rest of the calculations, methodologies used in determination and validation of wing design, and understanding of principles.

Of the two major manufacturers, ones models the entire aircraft, while the other models wings and fuselage separately. Would one expect the same results?

With respect, I beg to differ. There is an enormous difference between measuring, let alone calculating, the flow well downstream of the wing with all the attendant uncertainties and random influences and designing a wing. I find it difficult to believe that the major manufacturers should be lacking in understanding of how their wings operate.

So far as the differences in their techniques are concerned, that is really a matter for them. At the end of the day it is their money that will be paid to the customer airlines if they don't meet their guarantees. There is in any case a lot more to the design of a wing than making theoretical calculations of its characteristics.

[roulishollandais]

Did Dubois, Robert and Bonin try to understand theory on lift with all the wrong stuff taught in flight schools when they were deepstalling ??? :{

[awblain]

Owain,

To the first quote: I'd reply that Ms 39's explanation that there's `more upwash' isn't right, as there is in fact more downwash, or else there would not be more lift.

To the second quote: I'd reply that when you flirted with the idea that an increased downwards mass flux, and a greater flow speed over the wing, I think that's absolutely fine: the pressure on the wing is not just static. As ground effect works for very slow gliders, any worries about thermal equilibrium can't be essential either. The boundary layer details must change, since Sir Isaac demands his pound of flesh for the increased rate of change of momentum, and the only way it can be delivered is via changing the integrated pressure normal to the surface over the wing section: the flow changes, the pressure changes. This can only happen if the flow above the wing speeds up by more than the flow below when the underground image wing comes close enough to modify the flow field.

[HazelNuts39]

Who is Ms 39? It wasn't me!

[Lyman]

Upwash increases due the increasing pressure beneath, which is caused by restricted flow due to the 'funnel' created by the proximity of the solid ground.

It is a 'channel', and resists the entry of air that would otherwise flow beneath.

The same thing happens in freestream, but the below wing pressure is less there because the escape route is not bound by a second underwing slipstream, the "ground".

It is all about Rate of flow / velocity of wing. And flow restriction, which is ultimately the creator of all lift.

Altering the rate of flow creates the additional lift. But all these 'causes' are concurrent. At altitude, upwash is produced in the same fashion. Ground Effect demonstrates the complete theory, and Bernoulli does not explain lift, as said prior, he explains conservation of energy. Newton describes the mechanics of moving mass around to create a "plane".

Awblain, how can more upwash result in less downwash?

Owain.

The freebie from Ground Effect is the answer to the dilemma. Without increasing AoA, we get more lift at a constant velocity. Why? Because there is an increase of pressure underneath the wing.

The whole idea is to do just that, by deflecting and compressing it.

[awblain]

Lyman,

Lift is equal to the net rate of change in momentum of the air in the downwards direction as it flows by the wing.

On entering ground effect, `upwash' may increase in some places, presumably ahead of the wing, but `downwash' has to increase BY MORE in others, or else there would not be an increase in lift.

Newton deals with changes in momentum, Bernouilli with changes in energy. They are not redundant. Both must be satisfied.

[Owain Glyndwr]

To the second quote: I'd reply that when you flirted with the idea that an increased downwards mass flux, and a greater flow speed over the wing, I think that's absolutely fine: the pressure on the wing is not just static.
But the static pressure on the wing surface does reflect the local velocity at that station outside the boundary layer does it not?

Sir Isaac demands his pound of flesh for the increased rate of change of momentum, and the only way it can be delivered is via changing the integrated pressure normal to the surface over the wing section: the flow changes, the pressure changes. This can only happen if the flow above the wing speeds up by more than the flow below when the underground image wing comes close enough to modify the flow field.

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. This is just the opposite of what you are suggesting here.

[awblain]

Owain,

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.

I would suggest that enhanced lift and reduced drag in what a glider would recognize as ground effect is a more subtle dynamic process where you slow air less horizontally, which allows you to move more air vertically, thus simultaneously increasing lift and reducing drag.

I agree completely that the surface pressure on the wing is related to the speed just outside the boundary layer, and any compression that's occurred. I'm just not sure the picture in the diagram is right to describe what I'm assuming about the flow around a wing in ground effect.

[Owain Glyndwr]

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 :ugh:

[FlightPathOBN]

This is the established and accepted resultant...

http://ars.els-cdn.com/content/image/1-s2.0-S0376042110000576-gr4.jpg

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)

http://www.nature.com/nature/journal/v445/n7129/images/445721a-f1.2.jpg

Cheers to evolution!

[henra]

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. :O

[HazelNuts39]

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!

[sevenstrokeroll]

Lift=Money

[FlightPathOBN]

HN39,,
Good form!

[roulishollandais]

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

[Lyman]

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?

[Mr Optimistic]

Please do not block my exit; I like to keep my aperture open at all times thank you.

[Bpalmer]

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.

[PJ2]

From the NTSB Report (http://www.ntsb.gov/doclib/reports/2012/AAR1202.pdf) 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.

[FlightPathOBN]

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

http://operationsbasednavigation.com/wordpress/wp-content/uploads/2013/04/FlapWake5-e1364843041888.jpg

http://operationsbasednavigation.com/wordpress/wp-content/uploads/2013/04/FlapWake9-e1364843491854.jpg

[junebug172]

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

Video: How aeroplanes' wings really work - Telegraph (http://www.telegraph.co.uk/science/science-video/9035320/How-aeroplanes-wings-really-work.html)

[FlightPathOBN]

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.....

[A Squared]

Against my better judgement.....

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

[FlightPathOBN]

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

[italia458]

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.[/URL]

[URL]http://youtu.be/BY1k1GcZRww (http://youtu.be/BY1k1GcZRww)

...sorry, I couldn't resist! :)

[henra]

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 :E). 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

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?

[john_tullamarine]

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 (http://arvelgentry.jimdo.com/articles/)

Gentry died last year and his original webpage has disappeared.

[roulishollandais]

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"

[PJ2]

Yes, thank you very much John and as always, Owain!

[john_tullamarine]

Just having a bit of a trawl through old threads and came across this one.

Now that I think upon it, we haven't had a good set to on lift (that I can recall) for some time.

We probably have a great many new chums who wouldn't have seen this history .. so, I shall play "moderator troublemaker" and bring it back to the top to see what may transpire.

If there be little interest then it will sink rapidly back into the bog whence it came ..

[VinRouge]

Just having a bit of a trawl through old threads and came across this one.

Now that I think upon it, we haven't had a good set to on lift (that I can recall) for some time.

We probably have a great many new chums who wouldn't have seen this history .. so, I shall play "moderator troublemaker" and bring it back to the top to see what may transpire.

If there be little interest then it will sink rapidly back into the bog whence it came ..

Ok John! (you asked for it)

How does the traditional theory of lift as taught on most groundschools and even at GCSE, explaiin how a paper aeroplane flies? Or a symmetric foil section?

Ive also done the bernoulli sums on a standard NACA section and considering 2D flow only, increased velocity due to "further to go"...it explains at maximum about 5% of the lift actually generated in accordance with the standard lift equation and experimental Cl figures.

Still check whether the starting vortex forms in the bath with my kids and their toy aeroplane!

[Meikleour]

Add to the list - lift produced by a sail on a yacht versus the "equal tranit time" explaination often given when describing Bernoulli?

[pattern_is_full]

No problem with a little revival.

My take is - one can fly a barn door (or a paper airplane) with no camber and no significant "Bernoulli" lift, simply with enough AoA and power (or gliding force from gravity).

But the process is much more efficient using every possible physical effect one can. Whether one is the Wright Bros. with a pitiful little 12-hp engine, or today's commercial jets trying to save evey penny on fuel while going fast.

Lift is a lot of little pieces, which in the final analysis, together push down (or in the case of Bernoulli, pull down) a mass of air equal to the mass of the aircraft, holding it up against the force of gravity (Newton's 3rd). Actually, probably slightly more than the weight of the aircraft, due to losses to tip vortices and such, but with a net vector "down" that is equal to aircraft weight. Newton, Bernoulli, Coanda, Kutta-Joukowski - they all identified things that contribute something to lift, or rearranged the pieces to look at them from a different angle.

But unless there is room and time to write a thick textbook covering all those pieces, any "popular" explanation of lift has to be abbreviated, and thus only partly correct. It is a bit like "Is light waves or particles?" - the correct answer is "both" or "all of the above."

In theory, one could just put a tube full of people on top of a 12,000m girder, and move it around on wheels. The weight of the craft (mass x G) is supported by the girder, and eventually transferred to the wheels and thus the ground. Lift does the same thing, and someone has even detected the "weight" of an aircraft flying in the flight levels, as a tiny increase in local baro pressure (tiny because the weight is distributed over of billions of square inches, in a wide, spreading "footprint" below the aircraft).

[VinRouge]

Interesting. So, for me, I wish we didnt delude people with fudges to explain WHY aircraft fly. Instead, It should be explain the theory of lift is a highly complex topic, with the overall production of lift explained through a number of effects which by themselves do not explain fully the theory of lift, however, assist with the understanding of what is going on in terms of flow fields and on the 2D level.

In ,my mind, there are a few truths that can be stated which at a deeper level, assist understanding of lift. They are:

There can be no lift without Viscosity. Without viscosity, the flow will not remain adhered to the wing and work can therefore not be done on the flow. The wing slips through the flowfield and does not impact it. As there is no change in flow direction, there is no change in inertia and thus no downwash.
There can be no lift without considering the 3D flowfield.
There can be no lift without a circulatory flow, which by thought, can be tied to the requirement of viscosity in the production of lift. Circulatory flow is not a fudge to explain lift. It actually occurs (dont believe me, move your hand through water at an angle of attack. you can observe the reverse circulation depart the trailing edge as your hand starts to move).
The net effect of the complex flow around a wing is the generation of downwash, the effect of which due to Newtons third law exerts a force upon the wing and thus doing work. Without viscosity, we cannot have downwash as work cannot be done on the flow by the wing.
An ideal wing that is 100% efficient would convert 100% of downwash into lift with no drag, other than that induced by the rearwards deflected lift vector (what we call induced drag).
We dont live in the ideal world. Ignoring compressibility,, the effect of viscosity upon which lift depends is responsible for the generation of non-lift dependent drag.
In the compressible world (Re dependent, big planes, assume M >0.4), the impact of compressibility on the flow field (particularly pressure field) can both positively and negatively impact both lift and drag.
The 3D flowfield significantly impacts the properties of the wing, particularly the impact upon downwash across the span.


Just my musings. Please feel free to rip apart my mental model!

[DaveReidUK]

No problem with a little revival.

True. After all, it's not as if we're going to get posters who haven't bothered to read the thread so far, or have but can't remember what was said ...

[VinRouge]

Well that killed that conversation.... :rolleyes:

[Big Pistons Forever]

Fortunately President Trump has directed the FAA to stop teaching the fake news theories and instead teach his approved description of lift and thrust.


LIFT DEMONS AND THRUST PIXIES

Title of Paper: The Role of Lift Demons and Thrust Pixies in Heavier Than Air Flight

Publication Date: 2004

Abstract: The role of Lift Demons in aeronautics was first explained in 1994 by Mary Shafer (NASA). Since then, Shafer's work has been explored and revised. This paper summarises advances in Lift Demon technology over the last decade.

Keywords: Lift Demons, Thrust Pixies, lemon fondant

Authors: Sarah Hartwell, DEF Smith, Peter Rieden, Gavin Bull

Question: How did the secret information on Lift Demons make it into the public domain? I've been a practising Aircraft Performance Engineer for the past 26 years and have always tried to explain how aeroplanes fly by using the official public explanations regarding Bernoulli, airfoils and other such rot. Civilians just weren't ready for the truth. In fact, we generally don't speak about the magic directly. Most of our plans and estimates usually end with the phrase "and then a miracle happens".

Answer: The science of Lift Demons was declassified in 1994, throwing this topic wide open for discussion and research.

According to Shafer "Lift is caused by Lift Demons. These little, invisible demons hold on to the leading and trailing edges of the aircraft and lift it into the air by flapping their wings (so, in a reductionist sense, lift is actually caused by feathers). Some of the demons are a little confused and they hold on backwards, causing drag. The reason that planes stall at high alpha is that the leading edge demons get scared and let go when they can't see the ground anymore. Lift demons have good taste and don't like to look at ugly aircraft, so they hold on backwards on ugly planes. That's why gliders have so much lift and so little drag and why F-4s have lots of drag." This, however, did not address what gives lift to the Lift Demons' wings. Again, according to Shafer "Feathers. The multiple filaments on feathers trap the air molecules and they struggle to escape, which causes the action-reaction that we call lift. Bat wings don't have feathers but they're hairy and that works just about as well (air molecules are a little claustrophobic)."

Because Lift demons hold on backwards on ugly planes to avoid looking at the planes, drag can be calculated using CDU (Coefficient of Drag due to Ugliness) used in correlating aircraft ugliness against Lift Demons' unwillingness to hold on forwards and provide lift.

It was originally believed that Lift Demons got their lift from smaller Lift Demons whose lift was, in turn, produced by even smaller Lift Demons leading to the "Infinite Demons Theory" as proposed by the great Greek philosopher-scientist Miasma. However, with the revival of scientific knowledge that eventually ended the Dark Ages, it was realised that this situation unresolvable according to Zeno's paradox. The "Infinite Demons Theory" works in many problems of engineering significance, however, real understanding requires that the ether be introduced into the analysis at some point. The "Ether Concept" explains why planes fly more efficiently at higher altitudes. This is an absolute necessity when studying orbital and interplanetary travel where, it is believed, many of the Lift Demons are unable to breathe.

As always the Chinese seem to have been there first. "The Genius of China", tells of one Ko Hung. "Someone asked the Master (Ko Hung) about the principles of mounting to dangerous heights and travelling into the vast inane. The Master said "[...] some have made flying cars with wood from the inner part of the jujube tree, using ox leather straps fastened to returning blades so as to set the machine in motion. Others have the idea of making five snakes, six dragons and three oxen to meet the 'hard wind' and ride on it, not stopping until they have risen to a height of 40 li (about 65,000 feet). The the ch'i [emanation of the sky or perhaps wind] is extremely hard, so much so that it can overcome the strength of human beings. [...] Take dragons, for example; when they first rise they go up using the clouds as steps, and after they attained a height of forty li then they rush forward effortlessly gliding." The jujube tree device sounds like the Osprey. Scholars believe that the term "dragon" referred to the Lift Demon and that the Chinese had harnessed their power before the Western world had even discovered the Lift Demons' existence.

Lift Demons operate differently on different aircraft types. They get dizzy holding onto a spinning prop and become disorientated, thus more of them face the wrong direction which increases the drag. Lift Demons have to be magically bound to jet engines and an Airbus propulsion engineer explained "It takes an average of three day's solid chanting and ritual to get one of those 'engine' things to work. If us witches stopped our hard work, there'd be planes dropping out of the sky all over the place. We should never have let you lot find out about being able to fly, it's been nothing but trouble ever since."

Recognition of Lift Demons has also solved one of aviation's oldest puzzles: Gremlins. There appears to be a connection between Gremlins and Lift Demons. Gremlins may be Lift Demons that have, in some way, turned against the pilot. Instead of assisting him in his task of keeping the machine in the air, they do the opposite. The reason may be that, after reading about the Bernoulli/Newton dichotomy, they have become confused. Much has been written about Gremlins, dating back to WWII. Gremlins have been known to run towards the nose of an aircraft, causing it to dive into the sea, showing that they have at least some knowledge of the principles of flight. These days, most planes carry a full complement of In-Flight Gremlins; although they must be carefully managed, their presence generally inhibits the transformation of rogue Lift Demons into Gremlins.

Further study of Lift Demons was carried out by an aero-industry weights engineer who submitted the following scientific paper in 2004 to celebrate the anniversary of Shafer's ground-breaking article:

"There are still people in this company who think we weigh aircraft to find out how much they weigh, not to calculate stresses. Of course we need to know how much the thing weighs. How are we ever going to know how many Thrust Pixies we need to get the thing off the ground if we don't know the weight? Or should that be "Lift Demons"? Pixies have largely fallen into disrepute - something about Bernoulli not being representative in unbounded conditions and cause and effect being transposed in the Newtonian model.

In fact the use of Lift Demons on civil aircraft programmes is generally not that good an idea. The Demon binding contract tends to specify payment in blood or souls. This is readily achievable with aircraft of military function, but frowned upon in civilian circles as they may attempt to acquire payment outside of the terms of their binding contract. Lift Demons are not used on Elf bombers. We don't talk about Lift [Thrust] Pixies too often as it seems to upset the self-loading cargo [passengers]. Pixies require payment in cakes, flowers or nice thoughts. These are readily sourced either from the in-flight catering, or provided cost-free by the passengers. Clearly this would not work well within an operational military environment. Air force cooking is not renowned for the "light and fluffy texture" that Thrust Pixies demand, the availability of flowers might be problematic in desert operations, and nice thoughts may also be hard to find during times of active operations.

There is also a scalability issue. While one rampant Lift Demon would have few problems supporting a fighter aircraft (particularly if there is an immediate prospect of blood), it'll struggle to achieve level controlled flight of a 560 tonne Airbus A380. Use of more than one Lift Demon on the same flight vehicle is contra-indicated (they squabble and eat each other). Communities of Thrust Pixies can be encouraged to work together on the same aircraft by the provision of advanced technologies such as Lemon fondant icing, variegated tulips or in-flight romantic comedies. Ryanair once requested Leprachauns be installed in place of Lift [Thrust] Pixies, but Leprechauns have a mission statement which indicates their desire for monetary gain, and their willingness to search all over the world for it. This makes Lift Leprechauns expensive to keep (gold vs. lemon fondant icing), and makes it difficult to establish a regular route network as the Lift Leprechauns don't like to continuously visit the same locations. By law, aircraft also have to have a full complement of In-Flight Gremlins, but these are generally not a problem unless you feed the Wingtip Vortex Faeries after midnight."

His article shows that we've moved on a great deal since 1994. It has been known for some time that Lift Demons are best suited to military uses. In spite of suggestions that Pixies have fallen into disrepute, outside of military aviation it is Lift Demons that are considered passé - they have notoriously short attention spans and the ones used on Harriers have problems discriminating between air and water. Inbreeding in pursuit of the ultimate high performance Lift Demon may have been the cause. In civil aviation, the way ahead still lies with Thrust Pixies, large amounts of lemon fondant icing and in-flight Hugh Grant movies. Many instances of civil planes mysteriously falling out of the sky can be traced to (a) cessation of happy thoughts/lack of lemon fondant for the pixies (resulting in "Pixie Fatigue" or even "catastrophic pixie failure") or to (b) a large amount of happy thoughts/gateau mountain at ground level distracting the pixies from their task. Thrust Pixies dislike Marmite/Vegemite (yeast extract) and since such spreads can cause unhappy thoughts in some passengers, airline caterers are careful to avoid these.

A note on Catastrophic Pixie Failure: Temporary pixie problems resulting in turbulence or sudden loss of altitude causes unhappy thoughts in the passengers (which is why cabin staff and pilots always play down such occurrences). The resulting loss of happy thoughts causes further Pixie failure, worsening the situation. This makes passengers have even more unhappy thoughts and the Pixies become more fatigued; some may even leave the aircraft. Unless counter-acted by large lemon fondant reserves, Pixie Failure reaches catastrophic proportions and the aircraft is doomed.

Modern aircraft designers use Lift Fairies and avoid the whole controversy regarding the used of Lift Demons on civil aircraft programmes. Coming from a less benevolent cast, Lift Fairies are less dependent on nice thoughts or cakes. Fairies tend to less concerned with good and evil and hence make better dual use aircraft. Another major headache today is how to get 707 Lift Fairies at a reasonable price. Many of the older aircraft-specific fairies have become rare, if not unavailable. Aircraft numbering relates to the type and quantity of Lift Demons or Lift Fairies needed. For example a C-130 requires 130 "C class" Lift Demons while an Airbus A380 uses 380 "A" class Thrust Pixies. Aero engineers have a scale of values (proprietary information, not available in the public domain) allowing them to substitute different numbers of Pixies and Demons with different lifting abilities, thus making best use of the available surfaces.

There was short-lived interest in breeding a generic, or at least dual-use, "Thrust Demon" but the blood-loving Lift Demons ate the gateau-loving Thrust Pixies. In vitro techniques failed because their genietic [sic] material is incompatible so there are no hybrid Thrust Demons. DNA sequencing has allowed us to distinguish between many castes of Lift Demon, Thrust Fairy etc. For example due credit should be given to the inelegantly named Fart Fairies without whose bean eating and gas production, no machine could power itself from the earth's surface. The shy Turbine Winder-Rounder Gnomes hide inside those so-called engine nacelles from the public and indulge their serious kerosene drinking problems. Why else would the engines be called Gnome engines? Leading-Edge Leeries give the extra little push that keeps the nose up. The unfortunate tendency for RAF Harriers to crash into the sea has led to speculation that Harriers are equipped with Sirens or that the crashes resulted from an ill-advised experiment in using Water Nymphs (these are better suited to submarine propulsion). In any case, only export versions of the Harrier are likely to use Sirens, while those for the British domestic market use Banshees leased from the Irish.

Thrust Pixies are adequate for civil aircraft, while Lift Demons were good in military applications in years past, but modern Naval Aviation is currently all Angel-powered. The very best ones are, of course, Blue Angels. This is why they report their altitude as "angels twenty" or such. In addition, many Navy pilots claim that black air has no lift, which means they can get all night in to supplement their daytime naps between meals. This shows that Angels and Naval Lift Demons are strictly diurnal. Genetically engineered, military-tolerant Thrust Pixies may be what is needed. Thrust Demons might also have applications in getting Air Force maritime patrol aircraft to stop reaching "prudent limit of endurance" by 2 p.m. (local time) every Friday just after reporting a "possible intruder" submarine in the exercise area, and not be restored to flying status until 9 a.m. Monday, thus leaving the ships to stay out over the weekend looking for the "intruder".

There are no Anti-Gravity Demons so a different approach has to be taken in this field of research. Current research into inertia-free propulsion has shown great promise through the use of properly modified felines. Butter is spread evenly over the felines back. When the creature is tossed lightly into the air, the third law of universal fate dictates that it must land butter side down. However this does not occur due to the intervention of the feline landing axiom (feet first). The above conflicting forces result in a stable hover. The subject felines have demonstrated the ability to control their own velocity at will. The only loose ends delaying the full commercialisation of this process is the matter of persuading the felines to (a) work in teams; (b) not lick off the butter and (c) follow a flight plan. As there seems to be a deficiency in feline herding instincts, any suggestions would be appreciated by the researchers involved.

[dook]

…..explaiin how a paper aeroplane flies? Or a symmetric foil section?

Circulation and vortices.

[A Squared]

Ok John! (you asked for it)

How does the traditional theory of lift as taught on most groundschools and even at GCSE, explaiin how a paper aeroplane flies? Or a symmetric foil section?

The tired old "symmetrical airfoil" argument is only appealing to those who believe that the stagnation point (point where airflow divides between under the wing and over the wing) on a symmetrical airfoil operating at a non-zero angle of attack is located on the airfoil's axis of symmetry. It's not. There are numerous wind tunnel photographs of symmetrical airfoils with smoke generator streamlines switch demonstrate the fallacy of this.

Ive also done the bernoulli sums on a standard NACA section and considering 2D flow only, increased velocity due to "further to go"...it explains at maximum about 5% of the lift actually generated in accordance with the standard lift equation and experimental Cl figures.

*sigh* the "equal transit time" strawman. That would only prove something if the equal transit time was some fundamental element of bernoulii's principle. In fact it is not. In fact, the air flowing over the "top" of the airfoil arrives at the trailing edge of the airfoil well before the air flowing beneath. This can be seen in wind tunnel photos and footage of pulsed smoke steamers flowing over air foils.

[VinRouge]

The tired old "symmetrical airfoil" argument is only appealing to those who believe that the stagnation point (point where airflow divides between under the wing and over the wing) on a symmetrical airfoil operating at a non-zero angle of attack is located on the airfoil's axis of symmetry. It's not. There are numerous wind tunnel photographs of symmetrical airfoils with smoke generator streamlines switch demonstrate the fallacy of this.


*sigh* the "equal transit time" strawman. That would only prove something if the equal transit time was some fundamental element of bernoulii's principle. In fact it is not. In fact, the air flowing over the "top" of the airfoil arrives at the trailing edge of the airfoil well before the air flowing beneath. This can be seen in wind tunnel photos and footage of pulsed smoke steamers flowing over air foils.

Sigh. You completely missed my tongue in cheek jibe in that post. My original point if you read my later post, is that we shouldn't be teaching incorrect science as fact to those at GCSE and even A Level Physics.

Having 30+ hours in labs in sub, trans and supersonic tunnels, I can testify to the above and stagnation point/ Kutta condition.

Unless your repose was as equally tongue in cheek of course.

[KenV]

Put propellers in a tube, and spin them, will there be thrust? Yes.While this statement is technically true, in reality no jet engine puts "propellers in a tube." Unlike propellers which accelerate air, those "blades inside the tube" of a jet engine compress air. The compressed air is then accelerated in the nozzle of the engine. Similar principles, yet very different.

[megan]

in reality no jet engine puts "propellers in a tube." It does if you consider the loads imposed by the various components. Here the compressor is providing some 19,000 lbs of thrust. You can even see the principal on which the SR-71 derives the majority of its thrust from the inlet, where the diffuser here is providing some 2,000 lbs thrust. You'll note that aft of the burner cans very little thrust is developed, the vast majority of the loads being drag, 46,678 lbs drag against 2,419 lbs thrust. The compressor is providing 33% of the total thrust load, a prop in a tube.

https://cimg0.ibsrv.net/gimg/www.gmforum.com-vbulletin/1152x648/thr_82c6c2da5528d461c8ecf54209e244edcffe2183.png

[fizz57]

the SR-71 derives the majority of its thrust from the inlet, where the diffuser here is providing some 2,000 lbs thrust.


I've always disliked statements like these (true though they may be) as it's no different to saying that the forward force in a conventional-drive car is generated by the rear axle. Airflows and pressures are a bit less visible than shafts and gears, but the principle is the same.

I wouldn't be surprised if someone comes up with an inlet design that actually produces a net backward force on the engine mounts....

[LeadSled]

Big Pistons Forever,
Love it!!
Rather overshadows the "Acoustic Lift Theory" --- usually applied to the Brittain Norman Islander ---- when the noise stops, the aeroplane quits flying.

Now back to serious explanations --- in its simplest terms, it is the basic physics, laws of motion, action and reaction, that creates lift.
All the shapes and sizes of aerofoils are just refinements to maximise "lift" for a given power.
The papers of Lawrence Hargrave from Sydney, AU, ( the Wright Flyer was fundamentally a Hargrave Box Kite with an engine --- something acknowledged by the Wright Bros themselves, it's all in the Smithsonian) detail the whirling rig he built, starting with flat plates, moved on from there. with tabulated results for "lift" from various shapes.

As for the "friendly little molecules" that separate at the leading edge, and are miraculously re-united at the training edge, having had many adventure in-between, how many students have been taught that over the years.

As an aside, I have always been fascinated by the history of the development of the Navier-Stokes equations for analysis of fluid flow, to me that brain ranks with Newton and not far behind Einstein.

Tootle pip!!

[john_tullamarine]

+1 for post #369 .. should be compulsory reading for all engineering undergraduates ...

As for the "friendly little molecules" that separate at the leading edge, and are miraculously re-united at the training edge, having had many adventure in-between, how many students have been taught that over the years.

I'm presuming that Leadie is being a tad tongue-in-cheek.

However, one hopes that any newchums reading this thread are comfortable with the notion that the truth of the matter includes some extra words -

that don't separate at the leading edge

and are miraculously not re-united at the training edge

how many students, incorrectly, have been taught that over the years.

As to being re-united at the training edge that just doesn't happen and the fact that it doesn't happen is fundamental to lift production.

While I'm on the soap-box, the old nonsense about parcels of air separating up at the front and then joining up at the back ... is, likewise, nonsense. Just doesn't happen .. the bits going over the top win the race handsomely.

[DaveReidUK]

Isn't it wonderful that, more than a century after the Wright Brothers and half as much again since Cayley, we still can't agree why aircraft fly ? :O

[KenV]

Ken: in reality no jet engine puts "propellers in a tube."
It does if you consider the loads imposed by the various components..... The compressor is providing 33% of the total thrust load, a prop in a tube. You missed the entire point of my post. It has nothing to do with "the loads imposed" or even "thrust generated". Yes, the intake system can provide a considerable amount of thrust. But it does so NOT because the blades in the intake system are accelerating the air, but because the blades are compressing the air and then the constriction of the casing acts as a nozzle to accelerate the air. Propeller blades directly accelerate air. The blades in the jet engine are compressors, not accelerators. Propeller blades and jet engine blades behave similarly from a pilot perspective (they provide thrust), but employ fundamentally different principles.

You noted that the SR-71 inlet/diffuser provides a lot of thrust. That is NOT because the inlet/diffuser has a bunch of little propellers in the tube accelerating the air. A fundamentally different principle is being employed.

[john_tullamarine]

Isn't it wonderful that, more than a century after the Wright Brothers and half as much again since Cayley, we still can't agree why aircraft fly ?

The techo folk have been in agreement for most of that period. I might refer the remainder to readings on such folk as Lanchester (no slouch, an FRS, no less), Kutta. Zhukovsky, Prandtl, and others all of whom were involved in getting circulation theory sorted out. What amazes me is that the pilot theory examination stuff more or less stagnated a long time ago ....

... whilst still adhering to the lift pixies and thrust demons view of life, death and the universe ... which, quite obviously, summarises everything in a most elegant manner ...

[HOCHWALDSPRUDEL]

By curiosity I have just found this threat. For a long time I have been looking for reasonable explanation of lift, and guess what I have found one. Not me, but Dr Wolfgang Send. He has a webpage with some good explanations. Www.aniprop.de
The origin of all lift is because more more air is flowing over the top of the wing than underneath. As the separation point of the airflow moves forward and down (that is the interesting fact) from the leading edge at a positive angle of attack, more mass of air is now routed above the wing (might it be a symmetrical wing or whatever). Thus, the air is being accelerated and you get this tube-effect described by Bernoulli resulting in low pressure above the wing and high pressure below. The wing gets sucked up.https://cimg1.ibsrv.net/gimg/www.gmforum.com-vbulletin/1120x2000/bc0555be_e3c2_4eb6_9ef4_0011d556b0ef_c4660770e4fa48cda695211 dff4fa9c685b2eceb.pnghttps://cimg2.ibsrv.net/gimg/www.gmforum.com-vbulletin/1120x2000/3359058a_eb0a_4a6a_8001_41e1ee6d8f88_342e95287cd84309e298988 26c27dd2c8b338e92.png
https://cimg3.ibsrv.net/gimg/www.gmforum.com-vbulletin/1120x2000/69501fa0_0e3c_40e5_9f6d_910161b41b59_86cd2f72bf46a7a1cff7d55 697a0f3d6b1a599b5.png

[dook]

Mr. Tullamarine,

I couldn't agree more with your comments / sentiments.

I also think that our German friend above is confusing stagnation point with separation point.

dook
Central Flying School (retd.)

[HOCHWALDSPRUDEL]

Yes sorry, I mean the stagnation point when I talk of seperation point. Sorry. Seperation happens later. As the streamlines separate at the stagnation point, a non native English speaker like me would simply call it seperation point, not taking into account that something else is meant with seperation by definition when the laminar/turbulent flow separates from the wing....

[john_tullamarine]

My German is worse than non-existent so I have no idea what the text in the graphics is saying. However, the graphics appear to suggest that the writer does not acknowledge downwash ? Looking at the cited url, I gather that the researcher is a bird flight man. I have read a bit in that area and, interestingly, the natural world flight researchers appear to have a generally better understanding of matters relating to aerodynamics than many of our aeroplane colleagues.

As to why the pilot mantra reflect thinking from a hundred years or so, ago ? Who knows ....

The following youtube video might be of interest to those who haven't seen it or something similar ..

https://www.youtube.com/watch?v=6UlsArvbTeo

In addition to Arvel Gentry's papers which were referred to earlier in this thread, a related (to the above video link) AeroSoc paper is useful ... no, make that essential, parallel reading (Ackroyd was a very well regarded aeronautical academic) ... https://www.aerosociety.com/media/4841/babinsky-s-demonstration-the-theory-of-flight-and-its-historical.pdf

[HOCHWALDSPRUDEL]

Hi!
The author also researches the downwash and the vortices....in the 3D case. Pictures above show the 2D case, to illustrate how the phenomenon of lift is initially created by forward movement and down going movement of the stagnation point and showing, that then more air mass is flowing over the wing than underneath with the positive angle of attack. The solution for an infinite wing span (2D case) is without power and only delivers a force component crosswise to the flow. This also shows the video above by the way!
In the 3D case the vortices form as the result of pressure equalization at the edges of the wing. The vortices stay behind streamwise as the wing travels further in the direction of flight. Honestly, it is very complicated. But for me it really helped to understand, and this is a fact, that more air is flowing above the wing.....it is obvious and I have never seen it anywhere before in a book until I have found the explanations of Dr Send. The rest...is very mathematical.

[Pugilistic Animus]

Just to add: one may write Bernoulli's equation for example,as aTaylor Series. One may get better results with the lift equation

edited to adjust punctuation and add a little bit more

[A Squared]

As for the "friendly little molecules" that separate at the leading edge, and are miraculously re-united at the training edge, having had many adventure in-between, how many students have been taught that over the years.


A previous iteration of the FAA's flight training handbook contained the "rejoined at the trailing edge or "equal transit time" fallacy, so I would venture to say that a lot of students in the US were taught that.

[HOCHWALDSPRUDEL]

Yes, this obviously wrong, as they never meet again behind the trailing edge....as one can see on the video as well...

[Pugilistic Animus]

Theory of lift actually involves 40, 5000 page volumes and you will never find a publisher because of that :}

just joking around