Comparing airplanes to women is fine as long as they don't find out we are doing it.

But a question is in order. What accelerates air? How can you move a volume of air? How do you accelerate anything that has mass?

Answer - apply a force to that mass.

How can you apply a force to a volume of air?

Answer - pressure differentials across the volume.

If you can accelerate a volume of air with pressure differentials, then why not accelerate a wing the same way?

Seems pretty simple. Simons in his book Model Aircraft Aerodynamics (a good simple book) talks about heavier airplanes having larger masses of air deflected, the same with higher airspeeds. Is is all consistent with conservation of mass and energy laws. Of course, anywhere in the process those laws must be obeyed. You get upwash, downwash, circulation, etc. All can be determined and analyzed. But it is interesting that he goes on to say on Page 13 while talking about an airfoil,

"On both sides there is still acceleration away from the stagnation point but on the underside the peak velocity is less than on the upper side. Pressure is therefore higher below than that above the wing and lift is produced."
Regardless what you explained to your students, what we are talking about is, hopefully, exact science. What happens when you put an air flow over a control surface? You get pressure differentials caused by some fundamental aero principles. Those pressures put forces on the surfaces. The direction of the forces can be easily evaluated by looking at the direction they are pointing. Like camber effects. Interesting how all of the forces on an airplane are a result of the same process.

The bottom line is - what are the forces on the airframe that actually move the airframe, not the downwash, air, or anything else. What are the forces on the airframe for F=ma to occur for the airpalne? - The answer is pressures on the airframe.

It is amazing how many text books on my shelves talk a lot about pressures on a wing and practically nothing about downwash. But I found page after page of pressure work, Why?, you must ask. Are they all wrong? Why not chaper after chapter about downwash instead of pressures?

Lou you are simply putting the emphasis on the wrong thing.

Ben I think it’s obvious by now that I’m not going to change your opinion and you’re not going to change mine. I’ve enjoyed the discussion and I respect your ideas. Now you can go back to your books, and I’m going flying (and push some air around).

elmerfud, please accept my apology. It must be obvious to you by now that we engineers are characteristically unable to respond to a simple question with a direct answer. We are taught from the beginning to "complexify" stuff. We must maintain the mystery, otherwise how could we justify making the big bucks?

If the only thing creating lift is downwash, which is what you say is necessary for heavier than air craft to fly, then how is that lifting mechanism transferred to the surface of the wing???

It is not a matter of opinion, and never was and it is not just my ideas, it is what I have understood after a lifetime of working as an aero engineer. It is a matter of science and fact. It is also what is taught in every book on the subject that has been written and in every college course. Recently there is a tendency to avoid looking at the wing and look at the downwash as the main emphasis in the process. That has become trendy to teach as a mechanism of lift. It is only a part. It is not necessary to dumb down (probably a bad choice of words but they come to mind) the way an airplane flys.

You and I both know that it is entirely possible to enjoy flying and understand all of the bits of the mechanism too. One does not entirely preclude the other. I know several very good aero professors that enjoy flying a lot.

I have always presented what I hope to be a balanced view with appropriate force transmission mechanisms in place and understanding the resulting motions of the air and airplane. You seem to be locked into the "downwash only" explaniation to the point that you are putting blinders on the fuller more complete description of the mechanism of lift. It is not that difficult to do.

Ben, you continue to misunderstand my comments. The “what” is downwash, the “how” is pressure. I studied the same texts, and through my years in industry probably did a lot of the same stuff as you. Everything you have said about pressures is true. I am the first to agree that when designing wings �n things study of pressures is paramount. I am only puzzled at your reluctance to acknowledge “the rest of the story”.

I think you guys may be missing some facts with the downwash explanation of lift.

1) There is an upwash in front of the wing. (This is subsonic flow after all). I don't think anybody has been accounting for the "negative lift" that this creates.

2) An airfoil has NO net downwash, yet generates lift. Fact: A wing has more downwash than an airfoil because it is inefficient at creating lift.

Downwash does not create lift.... the pressure does... as many of you have pointed out. Any ADDITIONAL downwash created by a wing (this is, downwash created due to the fact that air spills over and around the tips of a real wing) is actually an inefficiency of a wing.... you're getting more drag for the lift that you are creating.

I'm giving this explanation because someone compared the lift on a wing to the pressure in a rocket. I did a paper on this a while back. Back to the explanation....

Lift can be created in two ways.... pressure lift and mass lift. This is similar to the way that a rocket/jet creates thrust... there are two kinds... pressure thrust and mass thrust. In a jet or rocket, the primary mechanism for creating thrust is moving mass at high speed out the back of the engine. You do not want a pressure difference between the exit plane and the free stream. If you do have a pressure difference, you CAN get more thrust, but at the expense of efficiency (not good if you are trying to conserve fuel).

Now... think the exact opposite for wings. As Ben has pointed out... the only thing that the wings "feels" is the pressure. All it needs is a pressure differential to create lift. Upwash.... wing... downwash. The upwash and downwash is an effect of the wing moving through the air. More net downwash (as in the case of a real wing) means that there is mass moving downwards. Does this create lift... yes... but only a small portion of it... it DOES NOT account for the majority of lift.

Here's the kicker...

YOU DO NOT NEED A MASS OF AIR MOVING DOWNWARD TO CREATE LIFT... END OF DISCUSSION. All you need is energy to create that pressure differential... where does it come from.... the thrust from the engine.

Please please... no more explanations of lift as a mass of air forced downward by the wing... it just doesn't work that way.

Maybe in some other universe. On this planet , this is absolutely wrong.

Why is it called a foil?

An efficient wing (high AR) pulls air upward as much as it pushes it downward... with no -net- downwash. It bends the air. This is how lift is created. Look at circulation theory.... it's in the equations.

The airfoil explanation is relatively simple... an airfoil is an infinite wing. While we cannot build an infinite wing, we can simulate one. When we test this airfoil, if there is any net change in momentum around an airfoil... upwards or downwards... it logically follows that because an airfoil is of infinite length... we have just created an infinite force (change in momentum). There is no infinite force, yet we still get lift.

Are you going to argue that there is no upwash in front of the wing? Or that that same upwash creates a negative lifting effect? I don't think i've seen anything in this thread to account for what happens in front of the wing... only behind. It's like trying to explain road apples without seeing a horse eat... sorry that was the best analogy I could come up with! LOL

Remember a wing does not simply push on the air... it itself is pushed -through- the air... if it isn't, it falls out of the sky.

OK.. make a home-brew wind-tunnel... and we can settle if a wing HAS to produce a downwash to create lift. The crudest of wind tunnels will work well enough.

Here's the wind tunnel plan: get a refrigerator box and a box fan. Cut the side of the rifrigerator box to insert a window of some sort. (you have to be able to see whats happening..) The box fan is set to PULL air out of the refrigerator box.
(The crude wind tunnel will only give about 5 to 7 mph equiv... so you aren't going to do a lot of tests with it...)

Build a rig to hold a flat bottom wing at ZERO AOA.

Build another little stand to hold a cigarette appx 4 inches in front of the LE. (need a smoke trail to observe...)

Light the cigarette and turn on the fan. If you have it all set up correctly... the airflow will pull the smoke right along the upper srface of the wing... and you wll see it droop below the bottom of the wing as it goes off the TE.

Now how the heck did that happen if there is no downwash?

Try again at -2 deg AOA. You STILL get the downwash. (+2 to -2 deg AOA is about all this setup is good to test. You'll stall the wing with much more than that. Airflow is too slow.)

I will explain one more time (maybe if I speak slowly and distinctly……).

First let me clarify a definition. The term “downwash” is technically defined as the angle through which air is deflected behind a wing and has units of radians. The only time I used the term earlier in this discussion I was referring to the downward acceleration of a mass of air by the passage of a wing, which units would be slugs/sec/sec. Even though they refer to the same phenomenon, they are not interchangeable. I will try to be more precise.

There is a tendency of technical folk to sometimes confuse the mathematical model representing reality with reality itself. There are two concepts that make higher math possible. Without them we could only count and measure. They are the abstract concepts of zero and infinity. As absolutely indispensable as they are, if they are not carefully considered, results can be obtained that are at best misleading.

The lift of an infinite span wing is one such example. From Newtons second law,

F=ma

Or transposing,

a= F/m

For a hypothetical wing of infinite span, the mass of air affected by its’ passage becomes infinite. Dividing F by infinity results in acceleration being zero (for any finite value of F). This is all very interesting, however it is also obvious, that for a real wing with a finite span affecting a finite mass of air, there must be a downward acceleration for any real value of F (lift). A high AR wing effects a large mass of air so the acceleration is small. A low AR wing affects a lessor quantity so the acceleration is more.

I could go on with the development of circulation theory which is again the combination of several mathematical constructs to make the mathematical model represent more closely actual observed phenomenon. But I move on.

All that has been said about the pressure being the only thing the wing “sees” is correct. The pressure distribution around the wing is the only direct force acting on the wing and is the proximate cause of lift. The argument comes with the analysis of a wing as a “free” body. Now this is a perfectly valid engineering technique for force analysis. It is used to isolate joints in trusses, beams, and other structural elements, as well as all manner of static and dynamic problems where things can be represented by vectors to simplify analysis. Lets look at an example of an airplane not moving but suspended from a rope. If the rope is represented by a vector acting on the airplane as a free body, it is easy to see that the weight is balanced by the tension in the rope, and the rope may be said to be the source of “lift”. However, a rope pulls two ways. (See Newton Law 3.) It is entirely appropriate to ask, what is supporting the other end of the rope? It could be a crane, a parachute, or a rafter in the Smithsonian museum. To fully understand what is supporting the airplane it is important to know what is happening on the other end of the rope.

Back to the airplane flying through the air, there is an area of low pressure just above the wing, and its net effect on the wing is the lifting force supporting the wing. The net effect of that low pressure on the air is to cause the air above to flow downward toward the wing. The air thus set in motion continues to move downward after the wing (and its area of low pressure) has past (Newton 1.). The resulting downward flow is not some arbitrary side effect, but the direct result of the pressure (force) supporting the wing. In fact one simply cannot exist without the other. (Newton law 3.) It is the other end of the rope.

In the above example it is OK to say that the rope is supporting the airplane but it is a more complete understanding to say that it is supported by the crane, or rafter, or whatever is on the other end of the rope. I agree completely that the net pressure distribution is the proximate cause of lift, but in a fluid there is simply no way to create a force on a body other than by accelerating a mass of the fluid. In the final analysis, when you cut through all the details, a wing is simply an air deflector.

Sure there is a way to create a force without moving mass.... F=ma can be re-written as F=PA... Force is equal to pressure times an area.... no mass movement necessary. At least not with that equation.

The difficult thing to understand is how you get a pressure field set up in a fluid without moving it. After all... a fluid cannot sustain a shear force... right? Any pressure field in a free fluid will quickly dissipate because there is no way to confine it by using the fluid itself. Think of an explosion.... a quick, intense, pressure field, then nothing.... because it dissapates.

Like i mentioned before, a wing does not simply move through the air... it is pushed through the air. This "pushing" on the air creates this pressure field.

Don't be so quick to dismiss the theory as just a bunch of equations. That bunch of equations is pretty good at predicting the performance of a real wing.

BTW... you still haven't argued anything about the upwash. In order for this "mass of air moving downward", as you say, to create lift... it has to overcome the "mass of air moving upward" in front of the wing.

Another BTW... i goofed that explanation of an infinite force created by a airfoil... it's not infinite because it occurs along the length of the airfoil... it's infinite because each molecule of air moving downwards behind an airfoil would need to be replaced by a molecule of air above it... in an infinite stack... this infinite stack of molecules accelerated downward by an airfoil is what would cause an infinite lifting force to be developed. This is simple continuity.

This web page has a surprisingly good explanation of the concepts i've described:
http://regenpress.com/circlatn.htm

F=mass * acceleration
and..
F=pressure * area

... and if we're talkign about the same lifting force, F, then

mass*accel = pressure*area

So you don't get one without the other.

Sorry if I'm stating the obvious.

It is pretty easy to measure the pressure and the area, and pretty darn hard to measure the mass and acceleration. So pressure is preferred in the analysis. That's all it comes down to IMO.

Jim,

Quite true.... but most people believe that the air is deflected downward. It's not.... it's circulated around the wing. Another way of saying it is that the air is accelerated around the wing.

The key is that it's around the wing... not down and away from it. Maybe a better way to write it is as angular momentum...

F = ma = mv^2/r = PA

The air is accelerated over the top and decelerated underneath. This causes the pressure field. This causes lift.... not the wing pushing air down and causing a newtonian "push" upwards.

This does not say that newton and bernoulli are wrong!!! The equations of lift are all derived from newtonian concepts, just like bernoulli is.

I agree, Both are true... when designing a wing use pressure, as a pilot figuring out how airplanes work, how different aerodynamic surfaces interact, and so on, physical lift is better. Upwash is the enemy, it is moving air the wrong way to create lift... the only reason the increase in downwash created by the wingtip vortex doesn't increase lift is because an equal amount of upwash cancels it out. In fact that effectively increases the angle of attack and increases the drag. But the effects from tip vortex are superimposed over the downwash that exists in 2D flow... in other words there is always more downwash then upwash... Here is a good website on the subject http://airsports.fai.org/feb99/feb9904.html

Ty

I think you are confusing the mathematical model with reality. Circulation theory was developed to make the math conform more closely to the observed flow, not visa versa. Perfect fluid theory doesn’t predict lift (or drag) but if you add circulation to the rectilinear flow it quite closely conforms to experimental data. As far as the upwash ahead of the wing, it obviously exists, but when the downwash aft of the wing is considered, for a finite wing, the net motion of the air is downward.

If you have discovered a new physical principle so that a finite wing can create lift without producing a commensurate downwash, by all means share it with the industry. It could be of enormous benefit. ATC could reduce the spacing between aircraft, especially the jumbos, and it could double the capacity of the worlds busiest airports overnight at no cost. It would save millions of dollars and revolutionize the whole aircraft industry. Maybe you could patent it (I don’t think just building all aircraft with an infinite wing would work).

I would never dismiss theory as just a bunch of equations. The years I spent in industry as a working engineer after earning my degree more than fifty years ago has given me a respect for their usefulness in design and research. On the other hand, to deny the observed acceleration of the air as an integral part of understanding lift is to lose touch with reality. As acropilot_ty said, “both are true”. When designing wings �n things use the pressure stuff, but when flying, the momentum explanation makes the most sense. It’s not a matter of one being wrong and the other right. They are both valid. They are just two ways at looking at the same thing. They are two ends of the same rope.

We're not talking about perfect fluid theory. We're talking about a newtonian fluid... a fluid with viscosity. You're kinda referring to d'Alembert's Paradox, but not quite. That's not what we are talking about.

There is a small net motion of the air downward, but it is because real wings are finite and inefficient. The small downward component is not the lift.... it is due to induced drag and the lift vector being tilted back. It is the inefficiency of the wing... not the cause of lift. The pressure field around the wing causes the lift... the pressure field is caused by the thrust from the engine. (in the case of a glider.... the thrust is replaced by an exchange of potential energy and kinetic energy as the glider flies through the air)

I don't deny that the air is accelerated, i'm just saying that it is not in the direction you think it is. It is accelerated around it... not down and behind it. Accelerated over the top... decelerated over the bottom.... both of which contribute to upward force by causing lower pressure on the top and higher pressure on the bottom. This is circulation.

It doesn't make the most sense... it's just the easiest to visualize.... "air deflected in this direction moves me in this direction". I don't deny that... it's a great explanation for a lay person.

But try this... ask someone unfamiliar with engineering to explain why, when you blow over the top of a tissue held in your hand... it moves up toward the flow, not away from all the air your'e blowing on it. When the angle between the "jet" coming out of your mouth and the surface of the tissue changes from blowing over it... to blowing on it... they get the expected effect.. the tissue moves away.

That difference is exactly what i am trying to explain.... pressure lift verses momentum lift.... they are two different things. Pressure lift is good and efficient.... momentum lift is bad and inefficient... as least as far as wing design is concerned. Downward motion of air does not explain lift... pressure does.... at least to this engineer. There are much easier ways of pushing air downward to cause a vertical force... such as standing a jet on end... but airplanes are not designed that way.

If you take a liquid and spill it over a surface the liquid will take the paths of least resistance. The liquid will flow until it builds up then it will spill in other areas. Air will try to spill around all the edges of a wing if held static, as air is pushed over the wing less air is spilled by the leading edge. Flowing air now spills off the trailing and wingtips, the interesting thing is air can be compressed so the amount of down draft you are discussing would not be created by the main airfoil. The wake would have to be created by air spilled by the wingtips of flying structures and by the fuselages structure. I think your down drafts would cause the air platform to move foward like a watermelon seed that has been squirted out from the pressure of your thumb and forefinger coming together. Heck, gentlemen lift is a dying factor in this jet age, strap a jet engine to a brick and the directional noozles with make it fly no matter what shape of the airfoil. Some sarcasm, to think upon!!!!

will

That is a real stretch of the imagination and missing some engineering too :-) But you get an A for imagination.

I think Ken has added a lot and supports what I have tried to say.

One thing Lou you said, " Back to the airplane flying through the air, there is an area of low pressure just above the wing, and its net effect on the wing is the lifting force supporting the wing. The net effect of that low pressure on the air is to cause the air above to flow downward toward the wing. "

Haven't you just said what I have been saying all along. It is a little out of context, granted.

Backing up a little, when we talk about the accelerations that can occur on an airplane, those accelerations, both angular and linear, are about the CG depending on mass and inertias.

In the equation F=ma we have the mass of the aircraft and a desired acceleration we want. What do we do then, we apply a force F to the airplane.

The units of pressures over an area give a force directly.

The units of downwash do not give a force directly, you have to know the mass flow of the air and some angular velocity changes, etc..

The only thing I am trying to get across is to call a force a force. When you are lifting the airplane talk about force.

When you are measuring downwash/upwash/flow angularity, characterics of air flow, etc., you then talk about flow direction/velocity/acceleration/mass flow rates/etc.

When you tell a beginner that the airplane flies because of downwash you are doing them and you a major disservice.

Exactly! I never said that the pressure approach was wrong. All I've said is that it only tells part of the story. Thats still the case.

I should probably stay out of this, but I'd like to add a few points.

I don't think anybody will disagree that there is downwash directly behind a lifting wing of finite span. But to state that the momentum of this downwash is in some way equal to the lift glosses over one inconvenient fact: There is also UPwash outside of the tip vortices, and the net momentum of this upwash is equal and opposite to the downwash between the tip vortices. The net vertical momentum in the entire "Trefftz Plane" behind the wing is zero.

It's easy to see this if we examine the wing's wake one trailing vortex at a time. Each vortex has equal and opposite vertical velocities on the left and right, so the net vertical velocity integrated over the vertical plane is zero. Adding another vortex, and then another, and another, doesn't change this, since the velocities just linearly superimpose.

So we have to be more precise when stating something like: "The wing lift is equal to the downward momentum in the wake". Well.... sorta. Depends on how one defines "momentum in the wake". How much of the wake? Just behind the wing between the tip vortces, all the way up and down? Or the whole vertical plane behind the wing? You get a different answer for each choice, anything from the full wing lift to zero.

Here's the precise way to apply Newton's law for the airflow about a lifting wing:

First you must define a "control volume" of air which contains the wing. The size and shape of the volume doesn't matter, but you have to define a volume if you want to invoke Newton correctly.

One can define two vertical pressure forces acting on the air inside this volume:

1) The force F1 imparted by the wing from inside the volume, equal and opposite to the lift: F1 = -Lift (the lifting wing pushes down on the air).
2) The force F2 imparted by the air outside of the volume, via the unbalanced pressures on the volume's surface.

There is also a net vertical momentum change of the flow through the volume, such as the difference between inflow upwash and outflow downwash. Call this vertical momenum change delta(M).

Now... Newton sez (drumroll): F1 + F2 = delta(M)

All the squabbles here arise from the fact that F2 can be anything, since it depends on the shape and size of the chosen control volume. Some possible choices:

a) Volume is a huge cube with the wing in the center. In this case, F2 = Lift, so F1+F2 = 0, and delta(M) = 0.
The net momentum change of flow though the volume is zero. So the "no downwash" people are correct in this sense.

b) Volume is huge slab up and down, and fore and aft, but its width is equal to the span. In this case F2 = 0, so F1+F2 = -Lift, and so delta(M) = -Lift. The momentum change set up by the downwash of the vortices is indeed exactly equal to the lift. So the "lift=downwash momentum" people are correct in this sense.

So everyone is right some of the time, OK?

drela, well said. Thanks for jumping in. Like I said in the beginning, both explanations are correct, they are just looking at it from a different viewpoint.

I know, I know.... beaten to death... dragged out.... pummeled....

Anyway... i found this old thread on this subject from a listserver discussion and thought you guys would be interested:

http://www.amasci.com/wing/wphysl2.txt

Drela,

I would expect to be able to say whether an airplane actually does or does not impart "vertical momentum" to the flow. The fact that the answer depends upon the choice of control volume aspect ratio isn't very satisfying. Suppose I take a REALLY big control volume that is still far from the ground, and I make it a tall vertical slab. The width of the control volume might be many spans, but the height is enough that the aspect ratio is still very large (effectively infinite). As I make this control volume bigger, I might expect to capture all of the momentum imparted by the airplane. Once I'm satisfied that I have captured all of the momentum, I sum it up and find that the rate of momentum addition is equal in magnitude to the airplane lift.

Then, while keeping the height of my control volume fixed, I imagine increasing the width until the control volume becomes very short and wide. If I captured all of the momentum with my original (REALLY big) control volume then I should get the same answer when I sum all of the momentum. But I don't, I get zero. The conclusion has to be that you can never make the Trefftz plane big enough to capture all of the momentum imparted by the airplane. (Not too surprising given the nature of a 2D vortex).

One difference between the real world and one in which you can imagine arbitrarily large control volumes is that real world flows have viscosity. With viscosity included, I would expect to be able to truly capture all of the momentum imparted by an airplane inside a finite control volume. What is the rate of momentum transfer in this case?

One other question about your reply. Your case a) with a cubic control volume would have a square Trefftz plane. You suggested that delta(M) would be zero in this case. I get something like half the lift. I only get zero when I stretch the Trefftz plane to be infinitely short and wide. Am I doing something wrong?

Thanks for your reply!

This is not true in general. The wing is not the only force which imparts momentum to the air in the volume --- there are also the pressure distributions on the top and bottom planes of the volume. These can be almost anything, depending on the planes' fore/aft extent, left/right extent, and distance below/above the wing or 2-D airfoil. Below are some particular cases. The top and bottom planes are not quite flat, but are chosen to lie along streamlines, so that the inflow and outflow momentum change delta(M) occurs only through the vertical planes.

2-D airfoil, volume is infinitely long fore/aft, finite height.
Vertical velocities at inflow and outflow are zero, so we expect no vertical momentum change.
F2 = Lift -> delta(M) = 0 (verified)
---------------------------------
=
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2-D airfoil, volume is finite fore/aft, height is infinite.
Pressures forces at top and bottom are equal and opposite.
F2 = 0 -> delta(M) = -Lift
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2-D airfoil, volume is a square, very large in extent. A detailed analysis reveals....
Half of the wing's downward lift force is canceled by forces on top and bottom planes.
The remaining force imbalance of -Lift/2 produces a nonzero delta(M)
F2 = Lift/2 -> delta(M) = -Lift/2
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No. Viscosity doesn't change the picture at all. The only effect it has is that F1 and F2 might also include viscous shear forces in addition to the pressure forces.

Correct for the 2-D case (see 3rd case above). In 3-D, the momentum imabalance goes to zero.

An airplane cruising at 30000 ft has an overpressure "footprint" on the ground covering many square miles. This overpressure integrated over the whole footprint is equal to the lift on the airplane. So the ground pushes up on the atmosphere with a force equal and opposite to the airplane's downward force. The net force iz zero, and so the net momentum change in the atmosphere is also zero.
F1 + F2 = -Lift+Lift = 0 -> delta(M) = 0