The Mystery Of Lift

[Richard Miller]

There are, as we all know, two schools of thought concerning lift, the one based on Newtonian principles, the other on the Bernoulli equation. Are there any among you who would like, in a friendly way - and I stress that - to discuss the relative merits of the two? Or any others?

-Richard

[Sr71fan]

Hi Richard.

Here's MY theory about lift:

Lift=Deliberate Downward Displacement of Atmospheric Molecules

(DDDAM)

You get lift when you push air down. I guess I'll have to agree with Newton.

Gary

[Richard Miller]

Gary,

Can we ease into this, introducing one concept at a time and building carefully on what we accumulate?

If so, answer me this: Is there a net vertical movement of the air affected by a wing in the process of the production of lift?
Is it up? Down? Up and down? Down and up? Other?

-Richard

[Sr71fan]

Richard,

What part of down, did you not understand?

Gary

[Dave Robelen]

Hello Richard,
In short form, yes there is a net downward displacement of the air from an airplane in flight. As the wing moves throught the air there is first upwash where the air ahead of the wing begins to rise (rather like the bow wave of a boat) , then the air is accellerated to various degrees by the wing (depends on the airfoil section and angle to the airflow) and the the air moves downward behind the moving wing. With all of this going on, the net result is a downward displacement of the air the wing has passed through. This way too short an answer, but maybe it will get someone else going.
Regards, Dave Robelen

[Richard Miller]

Dave,

Proceeding on the one-thing-at-a-time program, what causes the upwash and what magnitude, in force terms, does it have?

Gary,

The part about conservation of momentum, as in what goes down must come up.

-Richard

[SteveH]

The geatest lift I ever experienced? My wife kicking me in the A**
to get me moving.

[SteveH]

What the heck are we dealing with here, rocket scientist's??
Oh!!! We are! Just teasing, I find your conversations interesting.

[Dave Robelen]

Hi Richard,
To preface my remarks, my carrer with NASA was in the stability and control department and not directly involving the research of airfoils and wings specifically. The term upwash was coined because the air can be observed to begin rising as the wing approaches in direct proportion to the amount of lift the wing is producing. This upward flow begins more than a chord length ahead of the wing and the amount of angularity is proportional to the distance from the LE.
The cause of this effect has to do with both the fact that air is compressible at subsonic conditions, and is also "elastic" in it's behaviour. There probably is a better term, but I can't think of it. This phenomenon is the reason that the Bernouli principle can be applied to a flat plate wing. The point where the air splits around the wing is not at the exact center of the LE, but behind and below the LE in proportion to the amount of lift being produced. This point is called the "stagnation point" and is the one location on an airfoil where there is no air pressure due to movement. Because the air must wrap up and around the LE, even the flat plate has some difference in the local air velocity on the upper and lower surface.
A bunch of words, and I doubt if I answered your question, but there you have it.
Regards, Dave Robelen

[paulriseborough]

I'm a guidance and control engineer by speciality (although autopilot designers require some aerodynamics to understand the interaction between the motion and forces acting on a flight vehicle), but when I was studying aerospace engineering, this was one question which we used to enjoy asking lecturers!

One question which was even better related to how Bernoullis theorem applies to lift of a flat plate, ie how does a flat plate generate a higher velocity on the top surface? This is a great question to ask when someone explains lift as due to the greater curvature on the top surface of the wing which forces the air on the top to speed up .............

Both Bernoullis and momentum theories are valid, that is if you measure the flow velocities distribution over a wing (outside the boundary layer), application of Bernoullis theorem will give the pressure distribution which can then be integrated over the surface to provide the lift.

Alternatively, measurement of the three components of air velocity behind the wing enables the rate of change of the airs momentum to be calculated which gives the lift and drag by applying Newtons theorem.

Bernoullis thoerom is more useful for doing calculations, but not always the best way to explain to someone how a wing (particularly a flat plate) generates lift.

Paul

[Richard Miller]

Dave and Paul,

Dave, I am scandalized, and there must be some appropriate smilie for this, by your statement that "air is compressible at subsonic conditions." Would you like to think that over? [I'm new to smilies, but confess I like them.]

A horizontal rectangle, like the screen in front of you, with a line across the middle. If there were cold something in the top and hot something in the bottom, or any other such discontinuity, we could use it to do work.

If we make that an accelerated flow above and a decelerated flow below the horizontal dividing line, the work we can do is lift something. A force perpendicular to the direction of the flow.

Anything place on that line, an inclined plane, a palm stuck out a car window, a membrane between a leading and a trailing edge, a rotating cylinder, an airfoil, inverted if the angle of attack is great enough, a leaf in the wind, that initiates / generates a differential translational flow qualifies as an airfoil and will experience the designate force.

The separation, as you note, begins upstream, a fact, incidentally, that Newton was aware of, but didn't mention in Book II of the Principia. It does so because - the answer I was after from you - pressure propagates upstream in a fluid, from a source of resistance, at the speed of sound. The mean angle of the upwash is always the same; it is always parabolic and simply gets larger as the force increases.

So, having answered my own question, I leave you with a secondary part of it. What causes the upwash? Why is it parabolic? And Oh yes: Why call that locus of maximum dynamic pressure on the leading edge the stagnation point?

-Richard

[marcus]

Originally posted by Richard Miller
Dave and Paul,
The separation, as you note, begins upstream, a fact, incidentally, that Newton was aware of, but didn't mention in Book II of the Principia. It does so because - the answer I was after from you - pressure propagates upstream in a fluid, from a source of resistance, at the speed of sound. The mean angle of the upwash is always the same; it is always parabolic and simply gets larger as the force increases.

So, having answered my own question, I leave you with a secondary part of it. What causes the upwash? Why is it parabolic? And Oh yes: Why call that locus of maximum dynamic pressure on the leading edge the stagnation point?
-Richard

If you're going to pick on Dave's "compressible" then you'll have to do better than "the mean angle of the upwash is always the same; it is always parabolic", reference frames please.

The locus of of MDP is the stagnation point because ideally a particle will not move from there. The forces upward and downward and forward and backward are balanced there. The uplift is caused by the pressure differential between the top and bottom of the wing. IOW, the center of drag "a source of resistance" is not the stagnation point, nor is it on a line extended from the SP parallel to the flight path.

[paulriseborough]

Richard,

All wings generate what is called a 'bound vortex' which in simple terms is a rotating flow superimposed over the freestream velocity. This vortex actually forms a horse-shoe shape, because the vortex bound to the wing is shed as the wing loses lift towards the tip to eventually roll up into the trailing vortices which form behind the wing.

The velocity of a vortex is inversely proportional to the distance from its centre, so if we take a section through a wing and ignore the trailing vortices, you will have an upwash component in front of the wing and a downwash component behind the wing that vary proportional to the distance from the 1/4 chord.

Therefore the variation in upwash and downwash with distance from a wing is NOT parabolic, it follows a 1/R relationship.

In reality, the bound vorticity needs to be modelled as a number of smaller vortices which run spanwise at different chord locations.

This is the basis of the so called 'vortex panel' method of calculating flows over a three dimensional wing. This method was quite popular until the development of modern Computational Fluid Dynamics codes and powerful computers.

Regards,

Paul

[Richard Miller]

I am going to attempt, for the time being, to restrict discussion, if there is any more, to what happens ahead of the leading edge.
And I am going to confess to misspeaking myself: I affirm that the upwash has a parabolic profile; what I meant was that it begins little and gets bigger as the airspeed increases, that a line from its point of origin, however far ahead of the leading edge, to the stagnation point, remains the same.

Marcus,

Air is either compressible at Cub-flying speed, for example, or it isn't. "Pick on Dave..." What does that mean? Is this a friendly discussion of physical phenomena or are your shorts pulling up on you?

The greatest manifestation of dynamic pressure on an airfoil is, or may well be, at the stagnation point? I'll allow an ideal non-moving molecule at that point, but why the pressure, and if dynamic pressure is the business of the airfoil should its creation be referred to as stagnant in this instance?

Paul,

I am intimately familiar with the Lancaster-Prandtl theory as well as with its shortcomings. But I want to stay up front for the time being.

-Richard

[EUT]

I can't believe that you guys still belive Bernoulli, I bet you think the world is flat and revolves around the sun! Have you not heard of Coanda? He , and Newton have it all.

Eut

[paulriseborough]

Richard,

A scientist would argue that a gas can never be regarded as incompressible. However as an engineer I'm quite happy to ignore the change in density that occurs for flight speeds that our models fly at as there are many other approximations in out calculations that are of more significance.

In other words 'incompressible' means we ignore the density change because its effect is insignificant , whereas 'compressible' means that we can no longer do so.

Anyway, back to what happens in front of the leading edge, are you still maintaining that the streamlines are parabolic in a wing fixed coordinate frame?

EUT,

A flat earth model can work quite well as an approximation in a flight dynamics simulator, so don't go knocking flat earthers OK!!!!!!!!!

Oh and by the way, Bernoullis theorem does work.

Cheers,

Paul

[Blackhawk]

The rest of your readers ever wonder what happens when the worker bees takeover? This discussion is really obtuse for balsa benders.

Pat

[Dave Robelen]

Hi Pat,
I got lost in the dust quite a ways back, but I still am enjoying this exchange. Knowing that airplanes fly is easy, but really knowing why they fly is quite another matter! Thank goodness there is room for applied aerodynamics in model airplanes at various levels.
Why the next thing you know, these fine folk will start talking about the laminar bubble at low reynolds numbers and the merits of turbulators (I hope). Lets hang loose and see where the discussion goes from here.
Cheers, Dave Robelen

[SteveH]

It is definitely for a select crowd. I'm not one of them.
My theory of lift? Throw it, if it stays in the air....eureka! LIFT!!

[Blackhawk]

Steve

I agree-- it is a lotta theory and it can get overbearing at times..., Meanwhile, I build lotsa planes and win lotsa contests with French curve airfoils. But then, I guess arguing over what makes a plane fly is almost a much fun as building them and a whole lot easier! Just don't put them in charge of anything.

Pat

[EUT]

Originally posted by Richard Miller
There are, as we all know, two schools of thought concerning lift, the one based on Newtonian principles, the other on the Bernoulli equation. Are there any among you who would like, in a friendly way - and I stress that - to discuss the relative merits of the two? Or any others?

-Richard

At the time there were two schools of thought on whether the world was flat or round too!

[Richard Miller]

Paul, mainly,

Thanks for differentiating between an ideal and the engineer's incompressibility. There's a nit's difference there, and it's easy to get side tracked.

It was Huygens who first, to my knowledge, determined that resistance in fluid flow varied with the square of the velocity. Newton came to it later, and as he was wont, claimed priority.

Every aerodynamicist and aeronautical engineer uses that V\2 when he's calculating lift, or had better, but it rarely comes up in discussions of theory. If - when - you think it through you find the inverse-square law ruling at the source of resistance and the concept of the field ruling everything. [You following this Eut?]

Another thing you find is X plotted against Y\2 in any fluid dynamic situation, n'est-ce pas? What kind of curve does that give you?

And the rest of you guys. If this doesn't interest you, if its too abstruse or arcane, why do you bother? Not said in any mean way. I still in a mood.

-Richard

[paulriseborough]

Richard,

Could you please explain your statement "inverse-square law ruling at the source of resistance and the concept of the field ruling everything" It reads like gobbledygook, but then again I speak Australian english (as opposed to the Queens English).

"Another thing you find is X plotted against Y\2 in any fluid dynamic situation, n'est-ce pas? What kind of curve does that give you?"

I can't read French either, and you haven't defined what your X and Y are, but so far your haven't challenged the validity of the vortex panel method of predicting velocity distribution around a wing or the fact that a vortex induces a velocity that varies inversely with distance from the vortex core, both of which cause upwash to vary inversely with distance from the wing.

Keep on 'stirring the possum' as we say on Oz.

Regards,

Paul

[Richard Miller]

Paul,

Imagine a fan blowing a stream of air against a paddle. If the resistance to that flow at the face of the paddle is proportional to the square of the velocity [of the flow], it reads out as the inverse square. This much, incidentally, Newton recognized, although he didn't know where to go with it. In other words, these are the reciprocals: the structure of the resistance is the reciprocal of the flow.

Look at the accumulation of momentum on the face of the paddle. Imagine it as a hemisphere with the force, dynamic pressure substituted for gravity, manifest.

One is obviously dealing with a field, an area in which the designate force is dependent on the inverse square of the distance from the source. Don't look in the text books.

The X and Y are the first two dimensions of the Cartesian coordinates. The square is that derived from the law of resistance, aforementioned. Maintaining any such curve, or shear line, is dependent on dynamic pressures and it would have to be ruled by an exponent. A straight line wouldn't hold.

Is that more answers or more questions ?

-Richard

[Richard Miller]

One More Time

Let's define a field as the continuous distribution of some quantity or quality. It is obvious how readily gravity and the EM forces fall into this category. It is less obvious how aerodynamic forces do.

Whitehead, writing about fields, noted the confusion related to contiguous points, of which there are none in any field characterized as the continuous distribution of a quantity or a quality, and coincident bodies. These, with just about everybody else, Descartes and Leibnitz and Huygens and Newton, assumed were necessary for the transmission of momentum, and the alternative was action at a distance, which nobody wanted to champion, until Newton was forced to in The Principia.

There were two ways to address the problem. One addressed gravity and EM with the concept of a universal luminiferous aether, about which, as you know, there's been a lot of contention over the centuries.

In fluid dynamics appearances were saved by treating fluid as if it were a solid, as if there were a direct-contact transmission of momentum between air, or water, and the surface of the body comprising the source of resistance. Call this the justification of a facile intuition. One thing must impinge on another if momentum is to be transmitted. The Newtonian explanation of lift.

All fluid dynamics is field dynamics. [That's my mantra, one I broadcast repeatedly to a universe that never sends a signal back.]

The gravitational field strength at, say, 4,000,000 miles from the Earth would induce, in a given body, a [potential] velocity of X. At 2,000,000 miles the figure would be 4X, and at 1,000,000 miles, 16X. In this we see that [potential] velocity is the reciprocal of the field strength; the reciprocity of the square of the velocity and the inverse-square of the field strength.

Same thing in aerodynamics. The pressure on any point on the surface of a body in a flow = X, as up from the top surface of an airfoil, down from the under surface, and falls away in accord with the inverse-square law. That's obvious.

We draw picture of the presssure distribution, top and bottom, but never specify the immediate cause of that pressure, that it's dynamic presssure, and dynamic pressure is the disposition of momentum within a fluid. Despite all the obvious indications that we are dealing with field dynamics, no one acknowledges it. That's one of the things I keep saying, and keep puzzling over.

-Richard