RE: My head hurts.... LOL
 

Hmmmm... that was worded badly. I should say that forces are forces, the difference is in the sources of the forces. (get that? LOL)

Earlier I posted the rocket equation which has two terms in it... one for a force contribution from pressure (usually small) and one for a force contribution from momentum (large). If you ignore either one of these you will be designing a poorly performing rocket. Both contribute to the thrust, but pressure (it turns out) is a bad way of adding thrust because it is inefficient. (I'll let you look up info on rocket design and find out why... off topic)

Some of the posters that joined today (after you posted) are really starting to get into the difference. I really liked the example posted by Shoe of the Harrier in two different flight conditions. One flight condition using momentum and one using pressure (the "no net momentum" argument).

Regarding the comments on theory versus empirical evidence. Theory allows us to predict things that we do not know to be true, based on what we do know. Black holes are a good example. While we do not know everything about them, they were -predicted- long before they were discovered. This is the process of induction/deduction and experimentation/hypothesizing. We make mathematical models of reality not just to explain what we see, but to gain more -insight- into the physical phenomenon.

RE: Bernoulli's equation
 

LouW.

The sequence of events you described often occurs under specific circumstances, but the answer to the general question of what would happen to an airplane if the thrust is suddenly reversed has to be: it depends.

To answer the question, you would have to clarify what you mean by "the pilot does nothing with the controls". Does he release them? Does he hold them fixed? If he releases the controls, then you need to say whether you talking about an aircraft with a reversible or irreversible control system. You would also have to specify how the airplane's propulsion system affects the trimmed angle of attack. You would need to know how the propulsion system affects the aircraft lift coefficient.

It is well known that some aircraft, such as amphibians with high mounted engines will actually climb initially when you reduce power (stick "free" or "fixed"). Some airplanes won't ever really stabilize in a steep dive after you pull power (the P-3 is a good example, it will descend, but it's "phugoid" mode is so lightly damped that it would likely hit the ground before the speed ever stabilized). These are not mysterious anomalies. The varied behavior resulting from power reduction is well-captured by even linearized stability and control theory.

I think that theory and observations jibe just fine here, just as long as you are careful enough with the theory. To paraphrase Einstein: "Everything should be as simple as possible, and no simpler".

RE: My head hurts.... LOL
 

I believe I know some of how Dr. Frankenstein must have felt. It seems this post is no longer a discussion of technology, but philosophy, and I must have inadvertedly precipitated it.

I meant no offense. All true science is empirical. Experimentation is the touchstone of science. Whether a particular discipline is empirical doesn’t depend on the complexity of its math, but its foundation on data from the real world. Were it not for the mountains of test data accumulated over the past century the present theoretical structure (in aerodynamics) would not have been possible.

I admit that I am no expert on the Navier-Stokes equations. On the other hand I am not totally ignorant either. I know that with enough simplifying assumptions both the incompressible and compressible versions of Bernoulli can be derived from Navier-Stokes. I am also aware that a complete solution to the general Navier-Stokes equations has been elusive so far. As a matter of fact a $1 million dollar prize awaits the first mathematician to provide a complete solution. To quote from:

EXISTENCE & SMOOTHNESS OF THE NAVIER–STOKES EQUATION by Charles L. Fefferman
Princeton University, Department of Mathematics, Princeton, NJ 08544-1000
May 1,2000

Researchers have mapped the human genome within the last several years and I’m sure the solution to Navier-Stokes will likewise be forthcoming in the not too distant future.

Meanwhile back to the discussion at hand, I have not challenged the validity of flow theory. Within its area of application it gives quite satisfactory results. All I have contended is that without considering momentum, the explanation is incomplete.

I am still puzzled as to why some otherwise rational engineers oppose the very idea of the momentum explanation of lift as passionately as any John Bircher ever opposed communism.

RE: Bernoulli's equation
 

Ken thanks for your input to the little query about air being pushed or pulled .
My speculation was that airflow being pushed or pulled , is NOT the same .
I assumed a tunnel with a fan at one end - and the other end open to atmosphere.
If it was a steady speed stream (measured before and after the wing)-as you noted -of course it would be the same .
I guess I jumped out of the "wing" science, to looking at wether or not airflow over a fixed resistance changes if the air is pushed ---or pulled over that object.
I have a a number of hobbies and I am not completely versed in this one
.

RE: Bernoulli's equation
 

Hi ya.ll,
Real intereting to see you all have spent pages (and how much time?) with Bernoulli, great?
My 3 cents> this all applies to my model which has a 10% thick 10 inch chord flat nottom foil?
The model wieghs 6 lbs and will probably maintain level flight at 30 mph.
A check of the length of the foil top and bottom edges shows this. Bottom edge is 10" top edge
measures 10 3/16", thus top is 3/16" longer than bottom or about .03%
.03% of 30 mph is about .9 mph. An increase of .9 mph is going to reduce pressure enough
to create 6 lbs of lift?
Another thought> an aircraft moves through static air, the air does not flow past the aircraft
as in a wjnd tunnel. The air being stationary the airfoil splits or displaces it as the airfoil moves.
What force is created by this which would cause the air displaced above the airfoiul to move
faster than the air displaced below it?
Note that Bernoulli considers a confined flow, a restriction reduces the area of the confinement
which causes a compression of the fluid flowing into the restriction, Compression is a force which
adds to the existing flow force. The greater presure causes the flow to increase through the
restriction. Once out the other side of the restriction the flow speed will revert to the origional.
For this therory to be applied to an airfoil there would have to be a resriction. There is none
as when the air is displaced it is not held from moving by anything. Thus it is very hard to
understand how any supposed educated engineer would consider Bernoulli and airfoils.
Another one> displaced air when displacement ends tends to move back to its origional position. Two molecules are seperated at the foil leading edge. What factor says these two
must meet at the same time at the trailing edge? If there is no factor then there is no increase
in speed over the top.
Symmetrical lift> a symmetrical airfoil at zero angle of attack does produce lift, it must as both
sides act as the top does on a flat bottom foil. Lift is created by both top
and bottom and at zero aol the amounts are equal cancel each other.l
When such airfoil is set at a positive angle of attack the top side now creates greater lift and the bottom less.
The differential amount is the lift created,
obviously can be up or down.
Nuff sed? ought to be enough to get your grey cells churning?
Have fun!!

Hal debolt

RE: Bernoulli's equation
 

Hal-- don't stir em up or they may bite ----
I am still tryng to figure out how much wood would a wood chuck chuck if a woodchuck could chuck wood.

Hi Hal
 

Hi Hal, My first RC flight was with a Debolt Trainer with a vibration induced skipping escapement and an unthrottled Cox 15. You would have been impressed if the maneuvers had been done on purpose. Alas they were not and it was the only flight of the machine. I got better service out of the PeaShooter I built some years later. Since I couldn't afford them, I also home built copies of your Demco compound motorized servo. It had one push for right, two pushes for left.

>Real intereting to see you all have spent pages (and how much time?) with Bernoulli, great?
>My 3 cents> this all applies to my model which has a 10% thick 10 inch chord flat nottom foil?
>The model wieghs 6 lbs and will probably maintain level flight at 30 mph.
>A check of the length of the foil top and bottom edges shows this. Bottom edge is 10" top edge
>measures 10 3/16", thus top is 3/16" longer than bottom or about .03%
>.03% of 30 mph is about .9 mph. An increase of .9 mph is going to reduce pressure enough
>to create 6 lbs of lift?

You have made the assumption that the only thing at work here is the difference in length of the top and bottom. That airfoil at the angle of attack the model has in level flight and at 30 mph will have a measureable velocity and pressure field about it. The integral of the pressures is it's total lift. As a result of having the pressure field set up around the wing it there is a resulting downwash behind the wing. Looking at two molecules at the leading edge will show that the one taking the trip on the top of the airfoil makes no effort to meet the one on the bottom again exactly at the trailing edge. That really isn't in question. The same effect occurs with a semi-symmetrical airfoil wing at zero angle of attack or a symmetrical airfoil or flat plate wing at an angle of attack with respect to the air. The top molecule will reach the trailing edge at what ever the angle of attack, wing thickness, camber, etc. allows it to do.

>Another thought> an aircraft moves through static air, the air does not flow past the aircraft
>as in a wjnd tunnel. The air being stationary the airfoil splits or displaces it as the airfoil moves.
>What force is created by this which would cause the air displaced above the airfoiul to move
>faster than the air displaced below it?

It doesn't make a whole lot of difference whether or not the air is moving or the airplane is moving. Put a model in the wind tunnel with a camera in the fuselage. Throw some smoke in the tunnel and video the flow. Put the model in flight with smoke and video the flow. At the same flight conditions the flow field as seen from the fuselage will be the same (assuming that the model is within the sizes that will not choke the wind tunnel).

Without a lot of math, the thing about a shape moving through air is that as the air moves out of its way the air tends to speed up. If the shape is a flat plate or airfoil-wing shape then the side defined as the top (due to angle of attack) pretty well always has the air moving faster over it resulting in lower pressures. This is the same whether in flight or in a wind tunnel. It is measureable and with lots of math is uniquely and precisely described (but not by me, I barely made it through the aero courses at Purdue).

>Note that Bernoulli considers a confined flow, a restriction reduces the area of the confinement
>which causes a compression of the fluid flowing into the restriction, Compression is a force which
>adds to the existing flow force. The greater presure causes the flow to increase through the
>restriction. Once out the other side of the restriction the flow speed will revert to the origional.
>For this therory to be applied to an airfoil there would have to be a resriction. There is none
>as when the air is displaced it is not held from moving by anything. Thus it is very hard to
>understand how any supposed educated engineer would consider Bernoulli and airfoils.

To an extent the inertia/pressure/whatever of the air in the near vicinity of the airfoil is acting sorta like the top wall of a restriction, the wing being the other. Bernoulli and airfoils are considered similar because in each case when the velocity of the flow is increased the pressure in that region decreases. Bernoulli is a very precise pipe thing, yes, but the same concept with appropriate modifications is taking place with a wing. A measurement of flow field velocities and pressures will indicate that this is happening. We tended to measure them all the time when I worked at McDonnell Douglas as it is the only way to be able to precisely taylor an airfoil to the job at hand. Computer modeling will take you to a point and then the interaction of the fuselage/wing/external stores,control surface deflections/etc. causes local flow changes to occur and you try to vary the flow as needed for control and drag, etc.. Knowing the velocity-pressure effect will allow this to be done with some degree of feeling like you know what is happening.

>Another one> displaced air when displacement ends tends to move back to its origional position.
>Two molecules are seperated at the foil leading edge. What factor says these two
>must meet at the same time at the trailing edge? If there is no factor then there is no increase
>in speed over the top.

They will meet if the airfoil is symmetrical and at zero angle of attack. The velocity of the molecules at zero angle of attack will both increase in speed as they go around the leading edge of the wing and meet exactly at the trailing edge where they will return to the original state. But - put the airfoil at an angle of attack and the molecules no longer will meet at the same time. When the wing is at angle of attack the speed is greater along the top. This is easily measured, also the pressures will decrease.


>Symmetrical lift> a symmetrical airfoil at zero angle of attack does produce lift, it must as both
>sides act as the top does on a flat bottom foil. Lift is created by both top
>and bottom and at zero aol the amounts are equal cancel each other.l

Not the way I look at it I guess. Mentally I never did the divide it down the center exercise. I tend it look at it as a total airfoil and that the summation of pressure forces is zero.

>When such airfoil is set at a positive angle of attack the top side now creates greater lift and the bottom less.
>The differential amount is the lift created,
>obviously can be up or down.
>Nuff sed? ought to be enough to get your grey cells churning?

But at angle of attack the side creating the lowest pressure has the greater velocity along the surface of the airfoil and the sum of pressures around the airfoil is creating the lift. And as some others would insist, a measureable downwash close to the magnitude of the lift being produced is created and is evident aft of the wing trailing edge.




Heck Dick, come down anyway and visit. Howabout the Scale Nats or Scale Masters? I will be photographing both.

The thing about pushing or pulling the air, etc. is a good question. In Saint Louis we have a Polysonic Wind Tunnel with a 4 foot cross section. It is powered by compressed air from a really big tank up wind. For a run the valves are opened letting air through a variable throat nozzle from where it enters the test section. The flow is pretty straight in the test section and then the air is exhausted to outside air. It is noisy and works well covering a Mach number range from .5 to 2.5 or so, but it has a short run time and a long pump up time.

We have a Slow Speed Wind Tunnel that is a continuous flow tunnel. The motors and fans are basically across from the test section. The overall shape looking down is a big rectangle of square cross section pipe. There are lots of flow straighteners, cross section area changes and diverters such that the flow is very smooth in the test section.

In the Arnold Engineering Development Center in Tullahoma, TN they have continuous flow transonic and supersonic wind tunnels with a 16 foot cross section in the model test section. These are truly huge pieces of machinery and cover a block or so of area, I forget exactly how much. Again it is a matter of smoothing out the flow in the test section and it then doesn't matter if pushed or pulled. Any difference between flight and tunnel is totally insignificant as far as flow is concerned. Of course they do allow for a large model which makes measurements more accurate.

It pretty well doesn't matter if the air is pushed, pulled, or the airplane moved through the air in the end as long as the turbulence is removed from the flow before it hits the model and that the model is sized to the test section size to keep tunnel wall effects to a minimum. These things are determined in the initial tunnel setup.

A side note - in many years of testing many models in a lot of tunnels we didn't measure downwash once. Nor did we in flight test.

Some forum members (Lou :-) have maintained that the wing acts as a flow diverter and that we should look at this as the mechanism that causes the airplane to create lift to fly. Saying the wing action as a flow diverter is causing flight is truly a case of the emphasis being put on the wrong syllable. It is like saying, I get the energy and nutrients for my body to live - by pooping and not by the action of the stomach and intestines. Poop being like downwash, after the fact - get it? Come on, you have to admit it is pretty close to what we are talking about.

RE: Bernoulli's equation
 

How do you measure hogwash?
I have never paid any attention to the downwash theories as they simply did not seem to be of any value to my needs.
On the tunnels - push or pull - I just don't see how there would not be some difference - if there were no correcting devices after the wing - on the pushed air example.
Air pulled - is self straightening -
I will cogitate.

RE: Bernoulli's equation
 

Hal,

Yes, this applies to your 10 inch chord wing.

The Bernoulli equation is not applicable only to flow through a restriction.

Air molecules parted at the leading edge do not need to reach the trailing edge at the same time for the velocity on the upper surface to be higher than the velocity on the lower surface.

It is not valid to analyze a symmetric airfoil as if it were two independent flatbottom airfoils stuck together.

banktoturn

RE: Bernoulli's equation
 

Dick,

In order for the flow in a wind tunnel to represent the flow in "free flight", tunnel designers have to work hard in a few areas.

First, the flow should be as uniform as possible across the cross section of the tunnel. If the fan is upstream of the test section (or it is a closed-ciruit tunnel), getting the flow back to uniform often requires a set of straightening vanes. In order to keep the flow from being faster in the center than near the walls, the flow is often sent through one or a set of screens (this is the case whether the fan is upstream or downstream). Other devices such as honeycombs are often used to straighten and condition the flow before it enters the test section. Tunnel designers will often accept higher "freestream turbulence" in order to get more uniform flow (note that freestream turbulence is virtually non-existent in the real world)

Secondly the flow shouldn't accelerate or decelerate in the test section (otherwise your model will experience "buoyancy" forces). It would seem that you could do this by making the tunnel walls parallel, but this doesn't account for the boundary layer that develops along the wall. The walls actually have to diverge a little in order to keep the flow outside the boundary layer at constant speed.

You are right, there are subtle differences between "suck down" and "blow down" tunnels, but if you are careful enough that the air entering the test section is the same in both cases, you should get the same results.

RE: Bernoulli's equation
 

I figured there had to be some differences -tho small
Why?
the base line is different-
push is different than pull.
I only poke at the established guidelines to see where the real limits exist.
In finding thru hands on experience how well flat plates fly ( or don't fly), I see that most developed data is for entirely different applications .
Why?
first off - flat plates tho provide terrific lift - they are super critical to AOA.
also they are structual nightmares.
but reduce the weight to almost nothing and they work -very well.
On our models - we can easily have a thrust to weight ratio of 2-1 --- instantly applied
On even the best full scale acrobatic planes - there are non which can do this.
Tho no such airframe and power combo exists at the moment, it willat somepoint in time.
It is apparant to me that all of the hard work on airfoils has been a long term evolution which was a concession to power available .
It was and is, necessary work.
As a rabid modeler - I get to enjoy the low risks --in time-money to see "what happens if"--
I really enjoy reading the inputs here-
I really don't find it necessary to agree on all viewpoints .
That is really flies beyond all the rules of logic.

RE: Bernoulli's equation
 

Dick,

In a wind tunnel, the difference between "push and pull" is entirely due to the potential difficulty of smoothing the flow downstream of the fan. The bottom line is that for air, there is no difference between push and pull. You can't pull air, because it has zero tensile strength. We naturally think of pulling air if we place a fan downstream, but if you look at the pressure difference before and after the test section, it will be the same, regardless where in the duct you place the fan. Think about a closed-loop wind tunnel. I could place the fan anywhere from just behind the test section to just in front of it. At what point does it go from "pull" to "push"? It doesn't. The distinction is fictional.

Flat plates are not structural nightmares, thin plates are. A thick flat plate is just as easy to make strong as a thick traditional section.

All the established guidelines remain valid for the wing sections, wing loadings, and power-to-weight ratios you favor. You simply work with a loose set of constraints, so that a wing with very poor performance is quite good enough. You aren't working with a different set of rules, you are simply working in a remote corner of the design space. As you point out, full scale aircraft, and most models, don't operate there. That doesn't mean the rules aren't valid, even though you find that you need not be concerned about them.

banktoturn

RE: Hi Hal
 

Now that you’ve gotten that little toilet humor out of your system, maybe we can have an adult discussion. I have a question.

Solids can exert or react to a force by deformation (elastic or plastic) and can thus produce a force without motion. On the other hand an inviscid fluid can only exert or react to a force with inertia, which requires acceleration. Since you claim that there is no vertical acceleration in the vicinity of a lifting wing, please explain how horizontal accelerations alone can produce a vertical force (lift).

(Please spare me a rehash of Bernoulli. I am well versed in flow theory and I have never questioned its validity. I merely contend that it is only part of the story.)

RE: Bernoulli's equation
 

Can that explain why a phart runs up the back of your shorts?
(sorry)

RE: Hi Hal
 

LouW,

Sorry to always be nitpicking, but inertia is not required for an inviscid fluid to exert a force. A(heavier-than-water) boat can float just fine on an inviscid fluid that is at rest.

RE: Hi Hal
 

LouW,

Quite right about solids. You are defining the distinction between solids and fluid. However, we are not considering an inviscid fluid here. What is required, for a fluid with non zero viscosity, to support a load (apart from the case of bouyancy, mentioned by Shoe, and the case of a confined space, like a piston in a cylinder) is a non-zero rate of shear strain. I haven't yet convinced myself whether a sufficient downward massflow rate is required to support the weight in the manner of a reaction motor ( like a rocket ), but it is definitely the case that a sufficient shear rate must be sustained. In an inviscid fluid, a wing would generate no lift in any case.

banktoturn

RE: Hi Hal
 

BankToTurn,

I think it is accurate to say that there are inviscid solutions for the flow around a wing that do result in lift. There are many inviscid solutions that can satisfy the boundary conditions imposed by a wing in flight. The zero vorticity/zero lift solution is just one of them. A solution that satisfies the "Kutta condition" at the wing trailing edge is an equally valid solution, and is one that has lift and drag associated with it. You could argue that the zero vorticity solution is the one that nature would choose, but I'm not sure that that has been experimentally verified (and I'm not sure that th answer is all that relevant).

Comments
 

>Now that you’ve gotten that little toilet humor out of your system, maybe we can have an adult discussion. I have a question.

Lou, it was funny and was appropriate.

>Since you claim that there is no vertical acceleration in the vicinity of a lifting wing,
>please explain how horizontal accelerations alone can produce a vertical force (lift).

I don't think I ever said that. We all know that air hitting an airfoil goes around it with vertical and horizontal accelerations and changing velocities producing pressures which futher effect the process. We both have done the smoke trails in wind tunnels, ink flow studies, tufting studies in flight and so on. We have measured the effects with pressure rakes and probes. It all matches within experimental accuracy to theory - as it should.

The vertical and horizontal flows happen during the entire length of the airfoil. At the trailing edge what has happened to the air on both sides of the airfoil combines as the individual flows come together and the resultant flow is downward. We have measured, evaluated, tufted and whatever the downwash for years.

It is incidently nice to know downwash values to compute horizontal tail effectiveness. If we could measure the strength of the downwash and take care of the bookkeeping of all of the energy inputs to the system we would find that the downwash is probably equal to the lifting process. I don't think that is surprising, the downwash is a result of the lifting process ocurring in over around, etc. of the wing.

Do a wind tunnel test. Put a horizontal flat plate aligned with the tunnel floor and located at the trailing edge of a wing's airfoil (the airfoil being at angle of attack) and eliminated the downwash as a free flowing thing and caused the velocities and pressures involved in making the downwash to impact on the plate - the wing would still produce lift.

>(Please spare me a rehash of Bernoulli. I am well versed in flow theory
>and I have never questioned its validity. I merely contend that it is only part of the story.)

I will spare you even though you don't like my humor. Way back on page 2 of this thread you had originally said, "The pressure distribution around the wing is the proximate cause of lift (the Bernoulli side), but the ultimate source of lift is the acceleration of a mass of air (the Newton side)."

I think that when you say the ultimate source of lift is the acceleration of a mass of air (downwash) it is doing the listener a disservice. The pressures impacting the wing are doing the lifting (pressures developed as a result of air flow around the wing). The downwash is a major result of creating lift, yes, but downwash is not creating the lift.

RE: Bernoulli's equation
 

Ben - I was really lost in the waves of downwash -
I also could not see how it created lift -
So far, I have been able to decipher most of the arguments and have not seen much to change my thoughts on what really makes things go up.
Kinda like threshing wheat------

RE: Hi Hal
 

Shoe, you are falling for a trap that almost got me when I was thinking this through. Water only has buoyancy because of its inertia reacting to the acceleration of gravity. This is the same condition as a gas within a vessel that is being swung in a centrifuge. Anything less dense than the gas will be “buoyed upward” toward the axis of rotation. The only way to consider water without inertia forces acting is in free fall, like in orbit around the earth. In such a case, a something within the boundary of the fluid will remain stationary and will not be “buoyed” in any direction unless there is some acceleration of the fluid, regardless of its density.

Bank To Turn, I mentioned an inviscid property because for speeds well below sonic, air is usually considered to be inviscid and incompressible. Introducing viscosity does complicate the picture a bit, even if the fluid is considered to be Newtonian. (There are any number of viscosities and there will be different relationships depending upon what the particular viscosity characteristics are.) However, viscosity only presents a force in reaction to relative motion between parts of the fluid. Something else has to make that motion happen and that something else is inertia. (I can’t imagine a condition where viscosity alone initiates motion.)

Shoe is correct. There are inviscid solutions for flow around a wing that do result in lift.

RE: Comments
 

What I like most about these discussions, is the quick bypassing of an essential issue. For instance, let's take two statements out of the above quote "as the air moves out of its way the air tends to speed up" and "resulting in lower pressures".

Hmmm, the air speeds up. Yes, that's true, but the author conveniently bypasses the question "why did the air speed up?", and quickly moves to the assertion that the accelerated air caused the lower pressure.

Well hey, if the pressure goes lower first, then the air will speed up second. So - which comes first? Faster air, or lower pressure?

To me, it's obvious. It's the low pressure that comes first, because that then explains the air speeding up.

Otherwise, there's no reason for the air to speed up. Then, the question begs to be answered - "what causes the low pressure?" But that one's easy - when you move anything through the air, it creates a temporary vacuum behind it - which the air tries mightily to fill.

And fill the vacuum it will.

If the object moving throught the air is your fist, the air filling the vacuum behind your hand will be quite turbulent. Doesn't do much. But if the object moving through the air is an airfoil with a normal AOA, then the air moving behind it does wonderful things for us - as it attempts to fill the vacuum created by the positive AOA. Downwash, and a pressure differential. The pressure differential can be measured, and used to predict lift (downwash).

RE: Hi Hal
 

LouW,

I suppose general relativity might suggest that you consider gravitational forces and accelerations as equivalent. Most fluid mechanics I know bookkeep gravity as a body force - distinct from inertial terms in the field equations. If you choose not to bookkeep gravitational forces this way, you can still have "buoyancy forces" in an inviscid fluid without gravity or inertia. Suppose you had a container of charged gas outside of any gravitational field. If you applied an electric field across the container, you would develop a pressure gradient in the gas. This would cause a bouyancy force on any immersed body. No inertia involved.

RE: Hi Hal
 

Shoe & LouW,

Yes, there are inviscid solutions which result in lift, only by the imposition of the Kutta condition. Imposing the Kutta condition is introducing the influence of viscosity, because the presence of viscosity is what makes that solution the 'right' one. Having said all that, I was not talking about inviscid or viscous solutions, I was talking about inviscid or viscous fluids. Viscosity is required for a wing to generate lift by virtue of its motion relative to the atmosphere in which it finds itself, and ongoing shear is required for that lift to be generated. The ability to find solutions to the inviscid approximations to the flow equations which result in realistic values of lift does not change that.

banktoturn

RE: Hi Hal
 

BankToTurn,

I don't understand. What do you mean by ongoing shear? Do you mean that shear stresses are required for lift?

RE: Hi Hal
 

Another way to witness an effect identical to that of downwash is the Coanda effect of a gentle stream of water falling on the back of a spoon. You hold the handle of the spoon, letting the spoon part hand down. Then back the spoon into a gentle stream of tap water. The Coanda effect is that the water hits the back of the spoon, but then follows the curve of the spoon.

But the water sticking to the spoon and following it's surface is only half the story.

The other half is that the spoon moves in an opposite direction from that of the stream of water leaving it's surface. Yes - when you back the spoon into the stream of water, once contact is made, the spoon moves about a 1/4" or so more into the vertical stream of water, all on it's own accord - due to the fact that the stream of water being ejected from the spoon now has a slight horizontal force vector. And that's the same effect as on a wing, when the stream of air leaves the surface of the wing.