Coanda effect Vs Bernoulli

[Maverick2167]

Which is the Correct explanation of Lift on an Aerofoil?? or do they both work together?:ugh:

[awblain]

Please don't rock the already teetering Newton vs Bernouilli boat by adding viscosity too!

They all work together.

If there's a guy who has a compressibility effect named after him then we'll be complete. I guess we can bring Maxwell into the mix there, or just resign ourselves to having to invite both Navier and Stokes to the party, and abandon all intuition.

[kilomikedelta]

Osborne Reynolds?

[awblain]

Fair enough, Prof. Reynolds can be relied upon to select a highlight of all that messy stuff.

[Lightning Mate]

What's his number?

[keith williams]

Surely everyone knows that only money can enable aircraft to fly.........and I mean lots of money!

[Natstrackalpha]

Yes, but to offset the weight of the money, you have to pay engineers to make a curvature in a wing - twice, then stick one on each side of the fuselage, bye bye money hello lift.

NASA, them space people, dispels the idea of the Bernoulli theory and they say that lift is cause another way.

Thinking about it though, a pair of wooden planks could get you airborne if they had angle of incidence and were planed and sanded on the leading and trailing edges to enable air to flow smothly - plus a tailplane thus aligned with adept longitudinal dihedral and voila! You`ve got yourself an A350!

[Maverick2167]

NASA denying Bernoulli, is what make me confused, So are the theory of Lift taught in flight schools all wrong then?? includes the Books Published by Mr Keith Williams, :).. Could someone be kind enough to explain the two theories " Sorry am a rookie :ugh:

[busTRE]

As I understand it, all Newtonian physical theories are only methods to describe the outcome of a given phenomenon. So, by definition they cannot be regarded as 'correct' or 'incorrect'. There value lies purely in how well their results match observed/measured data. So, NASA cannot legitimately say that Bernoulli is wrong. All they can say is that other theories more closely correlate with the experimental data.

[oxenos]

Reynolds is ex-directory.

[TURIN]

At about 35.30 mins in after Nigel has told you how it works, Kingston Uni and their take on Bernoulli.



BBC Airport-Live

[Maverick2167]

But the Pressure difference does not hold true for Aircraft in Inverted flight or does it ??:ugh:

[awblain]

Maverick,

The simple arguments usually presented for lift are:

Newton: the wing deflects air down, leading to net descending air behind the wing, and so the air pushes the wing up. (Momentum is conserved by a net huge-scale rotation to keep the whole world static.) This is broadly conservation of momentum.

Bernouilli: Faster flow above the wing leads to a lower pressure above the wing, pushing the wing up. (Yet of course the air behind the wing is still directed downwards.) This is broadly conservation of energy. Compressibility makes it more complex when the speed rises towards that of sound, but that just means the internal energy of the air needs to be taken into account too.

Both descriptions are correct and required.

Viscosity sets how the flow changes as you move from infinity towards the wing, as neither the "Newton" not "Bernouilli" description provides a way to see where the flow streamlines go. To do that, the flow pattern has to solve the Navier-Stokes equation, which will give pressure and velocity fields that agree with, but encompass these other descriptions. "Coanda" describes the viscous connection that shapes the flow to dip behind the wing, so has more of a link to "Newton" in terms of the bulk flow, shaping the streamlines for "Bernouilli".

If you stick to considering the transfers of momentum and energy in the wing-airflow system, then you'll tell no lies. You might not be able to design a wing that way, but you can explain how one works when it's going fast enough or doesn't when it stalls.

[john_tullamarine]

Following on from awblain ..

As with all explanations, the desirable aim is to match (in a sensible way) the complexity of the explanation to the (educational or other) needs of the listener/user.

A lot depends on how close the listener needs to get to the heart and detail of the problem under consideration ie is a Dalton explanation adequate or does one need to get into Heisenberg, Schrodinger or Fermi ? As for bosons and fermions, spare me the details ...

Thus, there is no point in confusing a young child with an explanation of what might be going on in his digestive tract to cause the pain which is causing him grief and stopping his getting to sleep .. all he needs at the time is to know that it's OK and mummy will cuddle him until it's better.

Conversely, the RMO has no use for the child's explanation and seeks the detailed ins and outs of the relevant internal medicine considerations.

The pilot needs an idea of what is going on but, as he is not involved in estimations and the like, an overview is fine and Bernoulli fits the bill to an acceptable level .. warts and all. As to whether it is the best simple explanation is a moot point for which the answer is up for grabs. As for me, my preferred simple explanation is a Newtonian one which looks at the wing's magic bag of pixie dust which throws a bunch of air downwards .. giving the wing a lift (force) upwards ...

At the other end of the spectrum, the aerodynamicist generally gets revolved in detailed consideration of circulation theory from which he can estimate numbers (an outcome beloved of all engineers).

Point is it comes down to horses for courses .. pick the range of explanations which fit the bill and don't fuss too much about the others ...

As for Navier-Stokes, I could never figure out the answer ..

[16024]

pick the range of explanations which fit the bill and don't fuss too much about the others ...

...and then Google the definition of "superstition"!

And if Coanda seems weird there's plenty witchcraft went on in old Eastern Europe. Check out Tesla and Schauberger.

[TURIN]

....and if anyone mentions conveyor belts.

Shoot them. :ok:

[FlightPathOBN]

Newton explains lift. :ok:

Bernoulli explains stall. :eek:

Coanda explains cavitation :hmm:

Reynolds doesnt have anything to do with flight :mad:

[YRP]

I'm probably less knowledgeable on this than others on this thread [1].

The best answer is that all of them are true. Newton & conservation of momentum is a basic law. The wing does push the air down, hence the air pushes the wing up. A planar wing would deflect air downward and provide lift.

The way I've always thought of Bernoulli is that it enhances / increases the "Newtonian" air deflecton. The curvature changes the pressure on the wing, gives an even lower pressure on the top and higher on the bottom than the planar wing would get. So the wing pushes the air down harder than a flat plane would, so gets a bigger Newtonian push up.

This gives a way to think about flying inverted... the same basic air deflection is happening, just less efficiently because the Bernoulli effect is worked against the lift rather than for it.

I think the confusion is that some textbooks describe some percent of lift as from Bernoulli and some from deflecting air. It is really all from deflecting air, just that some is deflected because of the basic planar shape, some increased amount because of the curve.

[1] But sadly, to everyone's disappointment, that won't keep me from posting. :)

[Typhoon650]

I really don't care for the specifics of how wings work, just that they do if used correctly.:ok:

[Maverick2167]

So, let me put down what I,understand reading the above comments Please correct me If I,am wrong.

Lift is a Combination of both Newtons Law and Bernoulli's Principle, Newtons law defines the reaction of an Aerofoil at an angle to a free stream flow of Air/fluid. An aerofoil pushes the air down which results in the air pushing the Aerofoil up, but an Asymmetrical Aerofoil would push the air downwards at greater velocity due Bernoulli's principle and thus create more Up force (lift)

Coanda effect describes the movement of this Air/fluid Particles around the airfoil and reason the air on upper surface dips below and Vice-Versa creating what we know as Wingtip Vortices.??

Inverted flight:-, a conventional Aerofoil can sustain such flight because of Newton's law but inefficient and would require higher AOA and for Symmetrical aerofoil like that of a fighter jet, it is easier to sustain such maneuvers because there is less pressure differential (Bernoulli) due to the aerofoil design.....:confused:

[john_tullamarine]

I think you try to pigeon hole things too neatly.

Consider that, however you might want to explain things, an aeroplane doesn't fall down because, quite magically, it throws air downwards and, as a result, develops a force (lift) upwards.

Different explanations are useful for different purposes but all arise from the above observation.

If you want to get a handle on what really is going on, you need to read up on circulation theory. The wing generates a circulation and that is an observable thing experimentally. The easiest way to see what that does is to look at spinning cylinders in a tunnel or tank. A search on the Magnus Effect will bring up interesting sites for you to review .. you don't really want to play with the mathematics, I suggest, unless you have nothing better to do this evening ...

The pilot doesn't need to know anything much about this level of detail .. but, if you are interested, then go for it.

[Lightning Mate]

Ahh - circulation theory at last, not to mention dimensional analysis.

Reynolds doesnt have anything to do with flight http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/censored.gif

So density and viscosity has nothing to do with it then!

[TURIN]

I couldn't agree more L.M. :ok:

Reynolds Number (http://www.desktop.aero/appliedaero/fundamentals/reno.html)


Otherwise I wasted several years at college.
(Actually, now I think about it, I did).:\

[awblain]

Lift is due to the change of momentum of the airflow over the wing. The wing can be on an A350, or a tea tray with an angle of attack, or a piece of smooth or rough balsa wood.

The effect of the wing is manifested in changes in the air flow's speed and direction that extend for at least about a chord's length above and below the wing, and can be approximated by the circulation theory description.

The total force on the wing is consistent with Newton's rate of change of momentum derived from the velocity field of the wake, and can be resolved into components both up and back, giving lift and drag respectively.

Along any streamline, Bernouilli's equation holds too, if you take care at shocks.

The ratio of inertial to shear forces, diagnosed by the Reynolds number, controls whether/how-quickly turbulence builds up in the flow near the wing, and whether you can model it in a wind tunnel of a specific size, operating at a certain speed and density.

The compressibility of the air affects how shocks (sharp changes in the air's speed, density, pressure and direction) form in the supersonic air above the wing at high speeds.

Individually, none of these elements gives a complete picture of how the air moves as the wing passes and the effect it has. Their interplay is required to describe and understand what happens.

Mr Tullamarine is absolutely right though - you probably need to know how it works, rather more than why it works, and to tell whatever suitable story keeps in your mind how it works.

[Volume]

Except for one statement I fully agree.
The ratio of inertial to shear forces, diagnosed by the Reynolds number, controls whether/how-quickly turbulence builds up in the flow near the wing, and whether you can model it in a wind tunnel of a specific size, operating at a certain speed and density.That statement holds true for Reynolds numbers of large enough magnitude, let´s say from 1000 up. That is where Airplanes do operate (far up from 1000...). Thats where wind tunnels do operate (still a fair bit above 1000) If we scale down to insects, the effect of friction is no longer limited to the boundary layer ("near the wing"). And the change of momentum of the surrounding air is no longer restricted to the effect of pressure (normal forces), but may also be an effect of friction (shear forces).

The pilot doesn't need to know anything much about this level of detailThe Pilot just needs to fully understand what an aircraft does, not how and why. Unfortunately many do not even know what their aircraft may do in certain conditions... And don´t believe those who do :ugh:

[balsa model]

Aaaah... This topic never fails to stir some back-n-forth, around here.
On previous occasions, my confidence in understanding lift has been stripped to pretty much nothing. By "understanding" I mean (for my interests) "being able to predict how changing shape will affect the lift (and L/D)". Without CFD modelling. Preferably without contour integrals either.

Yet today, I would like to ask a simple question:
Is there an airfoil shape that does produce (non-zero) lift and yet has the aiflow above its upper surface move at the same speed as the airflow below its bottom surface? A kind of "non-Bernoulli" airfoil.
I'm guessing there is not such thing, but I've never seen it stated categorically.

Thank you for the enjoyable reading, as always. Balsa

[FlightPathOBN]

CFD and Reynolds, while may be fine for subsurface vessels, are just a mere estimate of aerodynamics.
Fooling with Reynolds numbers to 'scale' aerodynamic flow is simply that, a model, with only relative meaning in actual aerodynamics. While fluids have a nominal ability to compress, this is not well understood, nor defined in any but the theoretical complex dynamic analysis models.
Air can be compressed, and has an infinite number of compression ratios due to temperature, altitude, and humidity, far too many variables for a CFD model.

If the mechanics of aerodynamic wing design were understood, the wing designs would generally tend to gravitate to a similar solution over time. This has not happened, as each manufacturer claims to have the most efficient design, yet the wings look completely different.

Look at wake turbulence, the rollup of the lift component of the aircraft, the entire aircraft including the center wing, and influenced by the flap configuration, landing gear, and engine location.
AND completely misunderstood in mechanics of creation, rollup, velocity, and advection.

Yes, with advances in technology, unfortunately, many, many people have wasted their time in school..then again, history is fraught with examples of this..unless you still think that you can drive off the edge, continents dont drift, and of course the heresy of evolution.

[henra]

Is there an airfoil shape that does produce (non-zero) lift and yet has the aiflow above its upper surface move at the same speed as the airflow below its bottom surface? A kind of "non-Bernoulli" airfoil.


I'm quite confident in saying: No, that doesn't exist.

Because at the same time the other theories will provide zero as well:
Same Speed above and beklow means => no circulation (obviously) and also no net vertical component behind the wing, because that would require more mass flow and thus flow velocity above the wing than below.
So, I give you a 'No'.

[TURIN]

Upper Surface Blowing?
I know its cheating but it fits the criteria.

[awblain]

A supercritical section looks rather like it fits your bill - although the airflow over the top travels relatively less far than the air under the bottom as compared with a classical airfoil section, it still gets to the back first, and leads to a net downwards flow in the wake.

[FlightPathOBN]

Many fighter aircraft have a symmetrical wing section...

Then there is this...

http://www.grc.nasa.gov/WWW/k-12/airplane/Images/wrong1.gif

[kilomikedelta]

Balsa model;

The P-51 Mustang airfoil is almost symmetrical.

[balsa model]

Thank you all for the answers. (I hope the OP finds it all "on topic", as I do.)
No, that doesn't exist.

Because at the same time the other theories will provide zero as well:
Same Speed above and beklow means => no circulation (obviously) and also no net vertical component behind the wing, because that would require more mass flow and thus flow velocity above the wing than below.
Note that I am asking about existance of a shape with certain properties; not whether it is consistent with the theories. Not that I insist that the two are necessarily different; it's just that I'm looking here for some factual evidence.
You obviously saw that I was trying to do away with the circulation theory, for at least one airfoil case. But I don't get the reasoning about equal speeds leading to "no net vertical component".
In the simplified Newtonian theory, extremely easy to catch intuitively, the stagnation line (SL) is approximated to be level and passing close to the leading edge. Then the air below SL has to accelerate down (because of the approaching non-porous barrier = wing) and the upper air fills the vacuum behind the wing (which is its top side), also accelerating down. Simple and no contradictions. Except that I found out that the SL is not where it was postulated.:hmm:
supercritical section...
the airflow over the top...
gets to the back first...
How about a flat plate, to keep it simple. Or your typical rudder section, to keep it realistic. Symmetrical by design, since Wright brothers.
What is this obsession of the air "above" with getting to the back first, at positive angles of attack? (Besides "so that it satisfies Mr. Such-n-Such equations".)

[HazelNuts39]

What is this obsession of the air "above" with getting to the back first, at positive angles of attack? (Besides "so that it satisfies Mr. Such-n-Such equations".)If you have no confidence in any theory, that leaves only the experiment.

[westhawk]

Keeping lift theory simple seems to be at the root of the misunderstanding.

Reading further on the NASA website, a fairly good synopses of the sources of misunderstanding regarding aerodynamic lift can be found HERE. (http://www.grc.nasa.gov/WWW/k-12/airplane/bernnew.html)

[A Squared]

NASA denying Bernoulli, is what make me confused, So are the theory of Lift taught in flight schools all wrong then?? includes the Books Published by Mr Keith Williams,

The thing is, NASA *hasn't* denied Bernoulli. What they have done is pointed out that the "equal transit time" explanation is complete hogwash. And it is, there is nothing that compels air flowing over the "top" of an air foil to reach the trailing edge at the same time as the air flowing under the bottom of the same airfoil. And it doesn't. Actually, it can be shown that the air over the top of a typical asymmetrical airfoil reaches the trialing edge ahead of the air underneath. The fact that NASA has rejected the "equal transit time" notion, doesn't mean that they have rejected any relevance of Bernoulli's principle to lift.

[fizz57]

From a physicist's point of view, a wing is just a machine for producing downwash - a glorified wedge, if you like. The momentum change in deflecting the air in a given direction is what provides the "lift" force in the opposite direction - whether up, down, sideways or forwards.

Now the only way a gas can exert a force is through pressure - remember force over area? So, if there is a lift force there must be a pressure difference between the two sides of the wing.

If you then assume that the airflow is laminar and incompressible, and apply conservation of energy, you conclude that the air on the low-pressure side is travelling faster than the air on the other side - hey, you've derived Bernoulli's principle!

So much for the basic idea. In practice, most of the pressure change,and hence lift, occurs on the low-pressure side of the wing, and not, as you may expect, on the underside. This is where all the whizz-bang of streamlines, stagnation points and circulation comes in. It's not possible to explain this in simple terms, yet it is vitally important in keeping an aircraft in the air (a fully stalled wing, where virtually all the lift comes from the lower surface, can provide as much lift as an unstalled wing at a great enough angle of attack. But of course the drag is horrendous, the engines can't keep up and so gravity chips in to maintain equilibrium - AF447 in a nutshell). Hence the prevalence of Bernoulli in the teaching texts, and the resulting misconceptions from the pretty pictures.

You'll notice I haven't answered the question as to which shape provides the most lift. But then, that's just a detail :)

[awblain]

The air flows as the air flows, whether individuals have written down partial or complete explanations or not.

I don't know what the "obsession of the air with getting to the back first" is. That's just what the air does. Often this idea of "equal transit time" is mentioned, perhaps to try to "explain" why the top-surface air is faster than the bottom-surface air; however, it doesn't explain anything, and it's not true:
not for a balsa stick, a rudder, a tea tray or an A350.

Only Monsieur Navier and Mister Stokes' famous equation includes all the terms that are necessary to describe the flow. However, that doesn't lend itself to obvious intuitive understanding, has no analytic solution, and if there is turbulence present becomes very unwieldy.

[md80fanatic]

"As I understand it, all Newtonian physical theories are only methods to describe the outcome of a given phenomenon. So, by definition they cannot be regarded as 'correct' or 'incorrect'. There value lies purely in how well their results match observed/measured data. So, NASA cannot legitimately say that Bernoulli is wrong. All they can say is that other theories more closely correlate with the experimental data."

This reply most closely matches reality IMHO. Absolute 100% accuracy cannot be had by utilizing imaginary numbers in the calculations. :8

[henra]

You obviously saw that I was trying to do away with the circulation theory, for at least one airfoil case. But I don't get the reasoning about equal speeds leading to "no net vertical component".


Considering air as incompressible means if the speeds are constant so is volume flow.
Where do you want to get the (additional) vertical volume flow (the vertical component that obiously didn't exist before) from if we assume the horizontal component to remain constant? (Free stream air flow/speed behind the wing, no circulation). Air doesn't decellerate below free stream velocity without a good reason to do so.
If you want to add a vertical volume flow (Newton), you need more volume flow upstream over the wing. Considering air incompressible this requires higher velocity.
Bernoulli, here we come...

[Mr Optimistic]

Where does Coanda come in to this or have I not been paying attention ? Isn't that what makes my teapot drip ?

Equal transit time is clear and present :mad:: only used to fob off 10 year olds in school.

Circulation is bernoulli isn't it; speed of flow difference generator.

From memory, no viscosity (inviscid flow) denies lift and drag, but I am with the accelerate the air downwards school and tire of the details of 2D potential flow which again is only a model................

[henra]

Where does Coanda come in to this or have I not been paying attention ? Isn't that what makes my teapot drip ?


Coanda makes sure the air follows closely the curved surface of the airfoil and leaves the TE in the desired direction....
No Coanda => No Newton (and no Bernoulli/Circulation either)

[Mr Optimistic]

Ah, best get a new teapot then I suppose.

[HazelNuts39]

Or fit an 'exhaust mixer' that mixes the spout boundary layer with the main stream.

[henra]

Ah, best get a new teapot then I suppose.

Alternatively you might want to try and sharpen the TE of your teapot... :E
In contrast to an airfoil this will also help reducing the starting vortex :ok:

[FlightPathOBN]

seems that the leading edge of the spout would influence the rollup of the vortex.

we have done some very, very interesting things messing with vortex creation in water systems, especially dams.