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Which one to believe; which version of lift is taught for the ATPL theory exams?

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Which one to believe; which version of lift is taught for the ATPL theory exams?

Old 30th Dec 2009, 11:38
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Which one to believe; which version of lift is taught for the ATPL theory exams?

Heya guys I was wondering,
We have the bernoulli model of life with the equal transit times meaning the air has to speed up over the upper surface in order to "catch up" with the air under the wing at the trailing edge. This then creates a low pressure on the upper surface of the wing thus the wing creates lift.

But we also have the stick and rudder version;
The wing is an air deflector and for every action there is an equal reaction, therefore as the air is kicked downwars it kicks itself up.

Is there any other versions?
Which one is most likely to be correct?
WHICH ONE IS TAUGHT FOR THE ATPL GROUND SCHOOL THEORY EXAMS??
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Old 30th Dec 2009, 12:15
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Surely they both are true and should both be taught?
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Old 30th Dec 2009, 12:25
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bernoulli model is the one most likely to be used
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Old 30th Dec 2009, 13:02
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They are both true, of course.
How does the under side of a wing produce lift? By pushing air down.
How does the upper side produce lift? By reducing the pressure.
If you think back to your ppl aerodynamics, you will remember about 2/3 of the lift coming from the upper surface and 1/3 coming from th eunder side.

Some one showed that the spin relied on the lift from the underside and arrived at a figure of very close to this 1/3.
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Old 30th Dec 2009, 15:07
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Bernoulli applies and Newton applies. Can't have one without the other.

However: One thing which is absolutely and provably untrue is the equal transit time theory often taught as the basis of the Bernoulli explanation.

Air parcels separated at the leading edge do not meet up again at the trailing edge. There are plenty of pulsed smoke wind tunnel and CFD imagery available to show this. The air going over the top surface of the wing gets to the trailing edge first.

Regarding the pushing and pulling, they are both the same. The bottom of the wing pushes air down, which goes hand in hand with a relatively higher pressure. The top of the wing sucks air down, which goes hand in hand with a relatively lower pressure. Whatever velocity change you put air through comes with a pressure gradient to match.

To get into a discussion about which surface generates the most lift is opening a can of worms. At low angles of attack, the air pressure at the bottom surface of your typical wing will likely be covered by a pressure lower than ambient, which in fact means it is generating sink rather than lift. How much sink is equal to 1/3 of the lift generated?
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Old 30th Dec 2009, 17:14
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I have to wonder where or how this 'particles separate at the leading edge & must meet again at the trailing edge' nonsense crept into the Bernoulli explanation in our industry.
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Old 30th Dec 2009, 20:50
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The idea of the wing as an air-deflector seems false to me as a physicist. Taken hand in hand with the Bernoulli model air will flow downwards as a consequence of lift, but the wing doesn't water-ski through the air.

Not only would drag be an insurmountable obstacle, but a cambered wing can generate lift with a negative angle of attack, which sort of disproves the whole theory anyway.

I believe (dredging my memory back to ATPL theory) that a very small proportion of lift is from the reaction of the lower surface of the wing from the relative airflow.

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Old 30th Dec 2009, 20:53
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ft.
The explanation put to me was a mathematical comparison of a free falling object with the weight of the aircraft versus the rate of descent of the aircraft in a spin.

Obviously, a fairly rough calculation but it fitted the theory quite well.

I do believe the nearly flat underside of a wing produces lift in the same way as a totally flat shaped wing would.
It would be pointless to curve the underside of the wing so as to create lift in the negative sense.
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Old 31st Dec 2009, 00:54
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Someone sometime gave a rough explanation of how that theory fits fairly well with an aerodynamic state which is far removed from level flight? Well, I guess that settles it then.

Regarding the pointlessness of having convex lower surfaces on airfoils, I think there are people you may want to call and tell them about it.

Note how the airflow curves upward around the forward part of the bottom of the wing in the last pic in the last link. Recall what I said about there never being a velocity change in a fluid without a pressure gradient? Something is clearly making the air turn upwards as it passes approximately quarter chord. What does this tell us about the pressure near the wing as compared with the pressure a distance out from the wing?

To get somewhat back on topic, to the ATPL theory... I for one haven't got the foggiest on what they're teaching this week. I just have a fairly good grip on what really makes aircraft fly. I have long since concluded that this may or may not have a bearing on what is actually being taught at flight schools...

The FAA used to be way off, with half-digested theories about half venturis and what not. In the hope of things having improved I went to check what they currently have to say on the subject and found this gem:

Originally Posted by FAA Pilot's Handbook of Aeronautical Knowledge
The pressure difference between the upper and lower surface of a wing alone does not account for the total lift force produced.
I think I'll just continue to ignore them.
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Old 31st Dec 2009, 10:10
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What's your problem, ft?

You have already agreed with the theory in your first post. There was no mention of level flight aerodynmics, just lift.

But you agreed that the lower surface pushes air down and the best way to achieve a figure for that is just beyond the angle at which the flow over the upper surface has broken down.
And the fact fits the theory of just less than a third of the lift. The theory can say nothing about any other angle where the two forces are not separable.

In your effort to put me down you contradict yourself which rather detracts from the put down. Paragraph 4 concurs with the fact that "The pressure difference between the upper and lower surface of a wing alone does not account for the total lift force produced" and yet you choose to ignore that.

Certainly is a can of worms!
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Old 2nd Jan 2010, 10:25
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Some aircraft we know can fly inverted, they do not have symmetrical wings. How can this be wouldn't this exert a down force on the aerofoil?
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Old 2nd Jan 2010, 13:14
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PilotJames,

picture the aircraft remaining in upright level flight and performing a bunt. You'd push the stick forward, first unloading the airframe (zero g). At this point, the angle of attack has been reduced to the where the wing is not generating any lift. The stagnation point, i e the point on the leading edge where the airflow splits, has moved up on the leading edge.

Push the stick further forward and the angle of attack reduces even more, and the lift with it... but wait a minute? You were already at zero lift? Aha! Negative lift indeed!

Now, keep pushing the stick forward and lift continues to decrease, i e the negative lift increases. As you reach an amount of negative lift equal to the weight of the aircraft, you can flip it around 180 degrees and remain in level flight rather than bunt the aircraft.

There's a limit, of course, where you will instead suffer a negative G stall, where the airflow separates from the lower surface of the wing, similar to the separation from the upper surface in positive G stall. If that happens before the the negative lift equals the weight, the aircraft is unable to sustain inverted flight at that airspeed. If increasing the airspeed to a limiting speed does not solve this problem, the aircraft is incapable of inverted flight, at least at the current gross weight.

You can also run into the problem of the elevator authority being insufficient to lower the angle of attack enough for the negative lift to equal weight, in which the aircraft is again unable of inverted flight at the current weight.

Look at the links in my previous post and you will see that most profiles (ancient ones, WWI era, and the Clark Y are the obvious exceptions I can think of) have a convex lower surface, just less so than the upper surface.
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Old 2nd Jan 2010, 13:46
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miserlou,
do not mistake challenging your theories and claims for an attack on your person. Presenting information as absolute facts does however mean investing your personal credibility in your claims and blurring the distinction. Rough fact of life. My problem is that I'm used to a discussion standard where challenges to statements are met by logical discussion or references rather than by reiteration.

Yes, I did state that the lower surface pushes air down. Simplification, overgeneralization, and in hindsight I could have qualified that statement with a "may" to avoid confusion... or just omitted the entire paragraph. That's where you're treading a fine line between writing simplified forum posts and text books, I guess.

You can either write something which brings the discussion forward in relatively few words, or you can write a legal document. It depends on your target audience. The first is for people interested in having an educating discussion. The second is for debating team duels, where the subject at hand is put second to winning debating points. I know which I prefer in my spare time.

Level flight was specified to make clear that it was representative state of flight and I think I did make my point.

I have not agreed to any statement about anything being "just beyond the angle at which the flow over the upper surface has broken down". That's not the case in a spin either, if that's where the confusion stems from.

If a theory presented as a general theory cannot stand the test of the extremes, it is a flawed theory. As stated, for some flight cases the ratio between upper surface and lower surface (positive) lift goes to infinity and then changes sign, making it rather obvious that trying to pin a figure on that ratio is a moot point. Yet you persist to argue a 1:3 ratio, in the face of established science, based on something you heard someone say. That does annoy me. That you try to support your case by saying it would be pointless to build wings the way they are actually built does not improve matters.

I do look forward for an informed discussion on the subject, where it may be possible to figure out where the misunderstanding lies. Please do present the theory you are referring to and we will talk about it and try to sort it out. As long as it remains an unsupported claim I'm afraid it goes in the OWT box, along with the downwind turn stall and the conveyor belt takeoff.

The fourth paragraph was a quip at the FAA. Shear forces through friction will obviously not be a significant factor when talking lift. With that ruled out, if pressure forces from pressure differences across the surfaces of the wing are not responsible for the lift force, then what is? Telekinetics? It would appear that they have some way to go, even though the half venturi is finally binned. Anyway, so much for the attempt at being subtle...

Now, can we get back to the subject at hand and try the best we can to understand each other, or are we to continue disassembling posts into molecules in order to find minor points to squabble about and win debating points?
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Old 3rd Jan 2010, 16:30
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Given the the two facts that (a) a flat plate will produce lift at positive angle of attack and (b) a cambered airfoil will produce lift at zero angle of attack, both theories are equally valid. However, being that a cambered wing is more efficient in most flight regimes, and is actively used, it is reasonable to assume that the Bernoulli part is more important than the Newton part. But one does not exclude the other.
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Old 3rd Jan 2010, 19:13
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bfisk,
could you explain your reasoning for flat plates and cambered airfoils existing in different worlds from an aerodynamical point of view?

They don't, just FYI.

Without Newton, no lift. Newtons laws are laws of nature and dictate that air goes down, lift is generated. Equal and opposite reaction. 100%, always, no getting around it.

Effectively, Bernoulli's theorem tells us that there's no acceleration in a fluid without a pressure gradient, and that where there's a pressure gradient there's also acceleration. Now, that does sound familiar, doesn't it? Bet there's some annoying bloke in these forums going on and on about that relationship...

Without a pressure gradient and acceleration in a fluid (i e air, in this case), no change of direction of the airflow. Without a change of direction, no air goes down and no lift is generated.

Bernoulli and Newton co-exist, and neither is more important than the other. It is not a one versus the other situation at all. Both are 100% true, always. They both apply just the same whether the wing be a flat plate or a cambered airfoil. The latter just happens to be significantly better at generating useful lift.

Cheers,
/Fred
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Old 3rd Jan 2010, 22:33
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bfisk,
could you explain your reasoning for flat plates and cambered airfoils existing in different worlds from an aerodynamical point of view?
I don't quite see how my previous post would lead to such a conclusion, perhaps my post was ambiguous.

My reason for bringing up the two as examples, is that the flat plate disproves the common misbelief that "air has to meet at the trailing edge"/"longer way" way of thinking, and the fact that a cambered wing will produce lift at 0 angle of attack disproves the other common misbelief that the underside of the wing has to deflect air downwards in order to produce lift. Thus both theories are equally valid.

The question remains, to which extent are these phenomena responsible for the creation of lift on a modern airfoil; I think the general consensus is that the lower pressure above the wing/pressure gradient (ref Bernoulli) is stronger than the reaction from the downward displacement of mass (ref Newton).

As I wrote:
But one does not exclude the other.
I think we agree in general.
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Old 4th Jan 2010, 20:09
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Among professionals, the general consensus is that they are both involved in 100% of the generation of lift, and if one was lacking nothing would happen. Even initiating the discussion about which one is "the strongest" implies having taken a wrong turn somewhere in the maze of theories, I'm afraid. It's a chicken or egg discussion, in a setting where it is known that both the chicken and the egg have existed since the start of time.

The reaction from the downward displacement of mass is exactly as much as the pressure gradient will cause, and the pressure gradient is exactly that which corresponds to the downward displacement of mass.

The one caveat would be that Bernoulli's theorem is invalidated as compressibility builds up to be a significant factor.
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Old 6th Jan 2010, 00:08
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Among professionals, the general consensus is that they are both involved in 100% of the generation of lift, and if one was lacking nothing would happen.
May well be so, but you must have sounded out a very selective group of professionals to arrive at your 'general consensus'!

Try instead Barnard and Philpott (aerodynamics at ... ah, de Havilland Hatfield!), for example:
"It is often thought that the downwash is entirely responsible for the lift, by the principal of momentum change. This is not so. What is invariably forgotten is that the trailing vortices also produce a large upwash outboard of the wing tips. The upward momentum change thus produced cancels out the downward momentum change of the downwash. If we sandwich a wing between the walls of a wind-tunnel, so that there are no trailing vortices, air particles behind the wing will return roughly to their original height, and yet the lift is greater than when downwash is present."

Or Gibson (aerodynamicist at BAe nee English Electric Warton):
"It cannot be said that the current enthusiasts for the sole application of Newton's laws to explain lift have succeeded in rubbishing circulation theory, because they appear not to know it exists. Their ideas are assertions that have utterly failed to provide any quantitative measures of lift, or to explain the physical basis of lift generation, or to suggest how an aerofoil might be designed. It is these ideas that I referred to as nonsense."

Or Anderson (Curator for the National Air and Space Museum, Professor of Aerospace Engineering and quite possibly the most authoritative researcher into the history of aerodynamics):
"First, let us consider what this author advocates as the most fundamental explanation of lift. It is clear [........] that the two hands of nature that reach out and grab hold of a body moving through a fluid (liquid or gas) are the pressure and shear stress distributions exerted all over the exposed surfaces of the body. The resultant aerodynamic force on the body is the net, integrated effect of the pressure and shear stress distributions on the surface. Because the lift is the component of this resultant force perpendicular to the relative wind and because the pressure on the surface of an aerofoil at reasonable angles of attack acts mainly in the lift direction whereas the shear stress acts mainly in the drag direction, we are comfortable in saying that, for lift, the effect of shear stress is secondary and that lift is mainly due to the imbalance of the pressure distributions over the top and bottom surfaces of the aerofoil."

Last edited by Islander2; 6th Jan 2010 at 00:20.
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Old 13th Jan 2010, 18:31
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A very select group, indeed! Those I trust to know what they're talking about. I strongly recommend everyone to be no less selective. There is no shortage of bad references out there, and you are certain to find them if you e g google for references to support your opinion without being very critical and making sure that you fully grasp what they are saying.

Always apply a basic sanity check. One example would be to check if the author has received a Nobel prize if he or she is claiming to have found a way to circumvent the basic laws of physics, such as Newton's laws.

In actual practise, this means that if someone claims to have a force without an equal and opposite reaction, or to be able to apply a force to a mass without accelerating it, you should check for a Nobel prize in physics. If their work is more than a year old and there's no Nobel prize in their CV, their work likely did not meet the scientific standards.

Also be very vary of any reference selling by being devoid (or almost devoid) of mathematical formulae, such as your first reference. Many very good books are without formulae, but to have it as a selling point is a warning sign of simplifications. While the book in question looks to be well written and interesting, to the point where I'm contemplating getting hold of it and reading it (thank you for bringing it up), the quote chosen is peculiar.

As the air forms a vortex after being pushed down by the wing, there's no momentum change? No momentum imparted to the air since it rotates? Now, that's an interesting take on certain other things, such as propellers and flywheels.

Also check out the way smoke rings move the next time you are attending a party. A smoke ring is a vortex. There's a similarity to be found - and I'm fairly certain it's even in the PPL syllabus.

As for your second quote, it is first off obviously out of context as it is refers to a previous statement. I'd like to read it in context. Secondly, no, as he rightly points out momentum change does not succeed in rubbishing circulation theory as they go very well together. For the rest, I again refer back to Newton, sanity check and out of context quoting.

Your third quote is from an author I respect immensely. Thus it is with great relief, if little surprise, that I see that he supports my case against the FAA. I must admit to being bewildered about your choice to include the quote though, as your reference to the honourable John D does not contradict anything I have said. BTW, read his books if given a chance.
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Old 17th Jan 2010, 18:45
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ft, thank you for your response. Might I ask, do you have to work hard at being so patronising, or does it come naturally? It hardly flatters you.

None of the authors I quoted are claiming to have a force without an equal and opposite reaction, so your condescending 2nd and 3rd paragraphs are wasted words. The authors are all well-regarded industry professionals ... and I was merely taking you to task over your 'general consensus' comment.

Also, thank you, I'm not given to quoting out of context, and I didn't. And, since it was me that was providing the quotations, I should have thought it self evident that I had already read the books!

Also be very vary of any reference selling by being devoid (or almost devoid) of mathematical formulae, such as your first reference.
Which is utter nonsense!

There is a small number of truly excellent, substantially non-mathematical texts that do a fine job of explaining the mechanics of flight, and there numerous theory texts where even the mathematically-literate will be left wrestling with the Navier-Stokes partial differential equations without having gleaned any basic insight into the relevant principles.

But if you want to base your consensus argument on mathematical texts, I've just picked two of the all-time classics off my bookshelf: von Mises and Glauert. A quick review reveals that, in both cases, their entire analysis of the lifting force is in terms of circulation and the pressure differentials around an aerofoil. Both texts are devoid of any explanation or analysis based on changes of momentum or equal and opposite reactions.

Which, of course, is not to say that Newton's theories do not apply to the generation of lift. Just that, for all the professionals that I interfaced with during my aeronautics career, those theories weren't a particularly useful way of either thinking about the mechanism for generating lift or for quantifying the size of the lifting force. Since you say you have immense respect for John Anderson, I'll leave my last word on the subject to a quote from him: "the net rate of change of downward momentum created in the airflow because of the presence of the wing can be thought of as an effect due to the surface pressure distribution; the pressure distribution by itself is the fundamental cause of lift."

So given your view that:
Even initiating the discussion about which one is "the strongest" implies having taken a wrong turn somewhere in the maze of theories, I'm afraid. It's a chicken or egg discussion ...
perhaps you'd better let Mr Anderson know that he's lost his way! Oh, while I'm at it, let me add Chris Carpenter (one time head of aerodynamics at the RAF College, Cranwell) to the list of people you should also contact. He makes the identical point to that in my quote from John Anderson above.

Last edited by Islander2; 17th Jan 2010 at 23:51.
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