Incorrect Lift Theory

[mingalababya]

Ugghh, these NASA people! First, they tell us Pluto is no longer a planet, and now this!! What should we now teach student pilots on the theory of lift? It appears that the "Long Path" or "Equal Transit" theory is incorrect??
Check out this page from the NASA website (http://www.lerc.nasa.gov/WWW/K-12/airplane/wrong1.html) and also conduct the experiment given below ... interesting stuff. If you've been teaching the "Equal Transit Theory" or the "Venturi Effect (http://www.lerc.nasa.gov/WWW/K-12/airplane/wrong3.html)" theory for explaining "the force of lift" to your students, will you now change after reading this?

[Captain Oveur]

Very interesting point. I notice the website doesn't give any particular answers saying that the Equal transit method is appealing because it is mostly correct. As an graduate of aerospace engineering I can see where they are coming from but as an instructor I ask the question: which explanation does the CAA expect the student to use? Obviously many thiings are simplified, such as the four forces (no mention of resultant force!), but we have to think of this practically. :ugh: Everything is much more complicated that the simple explanations and science fact is derived from a method of deduction and constants based on repetition through experimentation. All liquids flow downhill? not quite.. glass and lead are solids etc..

Good point well made tho!

Oveur

[portsharbourflyer]

This is not a new idea, it has been known for years that the simplistic " Bernoulli" type explanation of lift used at ppl level and even cpl level theory is flawed. It can't explain why a flat plate produces lift.

Try reading Barnard and Philpotts Aircraft Flight book, this readily explains why this theory is flawed; furthermore it explains some alternative theories on why a wing produces lift.

Captain Oveur, as an aero graduate then you should have encountered Kutta Joukowski (incorrect spelling I know) wing lift thoerems.

[Captain Oveur]

Ha ha Touche!

Sorry long day at the office... You're absolutley correct. Was trying to make the point about CAA aerodynamics and theory and the real world! Not unlike CAA paperwork and the real world.

Kutta-Joukowski subscribed to the principles of conservation of momentum and newtons 3rd law. As such downwash from their rotating cylinder resulted in an opposing force pushing it skyward. This still doesn't explain flat plate sufficently. A good explanation is here: http://hyperphysics.phy-astr.gsu.edu/hbase/fluids/kutta.html.

To be honest i'm digging deep here so i'll have a think and see what I can come up with!

Oveur

[portsharbourflyer]

Why a wing produces lift? there several theories, however I am probably not the best qualified to comment which is most valid. Anyway whichever explanation you believe there will always be someone who can provide a counter argument and this is something that could be discussed forever.


Mingal, the theory the ppl technical exam recognises for wing lift has always been incorrect since ppl training began. The way I approach the subject is to explain this is what you are required to learn for the exam, but please be aware that it isn't strictly correct, but I would avoid explaining any other theory due to the complexity involved and it would just generally confuse the student. I prefer teach to accurate facts rather than just teaching people to pass an exam, but in this instance it is unavoidable.

[LD Max]

I was beating out a similar tune in this thread.

http://www.pprune.org/forums/showthread.php?t=237796

Last e-mail I got from the FAA was to the effect that they're re-writing the book.

[BigEndBob]

As far as exams are concerned, questions should not be asked about theories only facts.
Who is to say that a students "answer" is not a valid theory?

[MD900 Explorer]

Seems there is another member of the aviation community who disagrees with this. The person behind this claim is a Swedish guy by the name of Martin Ingelman Sundberg.

His thoughts are as far as i understand them that the "bernoulli" principle is correct in the fact that the principleis correctly defined, but does not apply to the theory of lift.

He theorise that the air over the top of the aerofoil is not the part that creates the lift, but the underside. Find it hard to believe myself, but thought i would throw a fox in amongst the chickens...:E

MD :E

[theresalwaysone]

This is not a new idea, it has been known for years that the simplistic " Bernoulli" type explanation of lift used at ppl level and even cpl level theory is flawed. It can't explain why a flat plate produces lift.



A flat plate produces lift in the same way as an aerofoil does, it just produces a lot less becuase the flow over the plate is virtually completely non laminar when a small angle of attack is presented. Bernoulli's Theorem is not flawed at all and does explain why a flat plate produces lift but his theorum was written in regard to streamline flow, a point missed by many authors and comentators.

There has been no change in the way CAA (or knowledgeable instructors) teach or examine basic aerodynamics and the NASA site is correct.

[mazzy1026]

Bernoulli's Theorem is not flawed at all and does explain why a flat plate produces lift but his theorum was written in regard to streamline flow, a point missed by many authors and comentators.
Exactly why aircraft don't just have 'flat plates' for wings!

[theresalwaysone]

Why a wing produces lift? there several theories, however I am probably not the best qualified to comment which is most valid. Anyway whichever explanation you believe there will always be someone who can provide a counter argument and this is something that could be discussed forever.


Mingal, the theory the ppl technical exam recognises for wing lift has always been incorrect since ppl training began. The way I approach the subject is to explain this is what you are required to learn for the exam, but please be aware that it isn't strictly correct, but I would avoid explaining any other theory due to the complexity involved and it would just generally confuse the student. I prefer teach to accurate facts rather than just teaching people to pass an exam, but in this instance it is unavoidable.

With respect, what a bizzare statment! You say in one line you only teach what is required for the exam and then on the next line you state you prefer to teach accurate facts rather than just teach people to pass an exam, I would say that is a gross contradiction wouldnt you!?


How is the technical exam incorrect, please explain what theory it supports that makes you say QUOTE the theory the ppl technical exam recognises for wing lift has always been incorrect since ppl training began. I cannot see any questions in the exam papers that supports this statement.
(Just a small point but PPL training has been going a lot longer that the technical exam!)


Teaching basic aerodynamics dosnt requires COMPLEXITY just a spoon under a tap,a garden hose or the example of a door slaming shut with a draught. Teaching top surface pressure velocity relationship teaches students to respect clean wings in icing conditions and thats when lack of understanding of aerodynamic theory can bite very hard! Do you teach that?

[mazzy1026]

I think he means he would much prefer to teach the accurate facts, but due to the complexities involved and conflicting reading material - he can't! :ok:

[portsharbourflyer]

Mazzy,

Thanks for that, my point exactly, I don't think I would have many students left if I started lecturing Kutta thoery or Navier stokes equations.

I should correct myself, Bernouillis principles are not flawed, it is the explanation used in ppl/ATPL texts to explain how these lead to wing lift that are flawed.

Theresalwaysone,

I would suggest that you read one the trevor thom technical book or any similar ppl level technical text ( even your cpl/atpl technical notes), then read Barnard and Philpotts Aircraft Flight (it is a non mathematical text), you will soon see what I am referring too.

[Genghis the Engineer]

Personally I think that Glauert's 1926 book "Aerofoil and Airscrew theory" is far superior, but a little less accessible.


Seriously, I really don't see a problem. I've learned so many different explanations of how an aeroplane flies, that I long ago ceased to believe that I know how an aeroplane flies - I lost track of that somewhere in the middle of my doctorate (in aerospace engineering).

What I do know, which is much more important, is how to predict whether it will or not. Which brings us back to the grossly simplistic, and largely innacurate model which has been used for training pilots for years.

The fact is, it's good enough. It does a fair job of explaining about the stall, the relationship between speed, lift and g, and how the controls work - it does a good enough job for any private and the majority of professional pilots, without loading them down with theory that is frankly of no practical use to them whatsoever.

But, seriously, if anybody really wants to see the theory - get hold of a copy of Glauert - it is beautifully written, and in 1926 had already shot down the theory we all learned when we did our PPLs.

G

[mazzy1026]

Lets get back to basics - stick your hand out of the car window on the motorway and angle it upwards into wind - your hand wants to go up.

[GroundBound]

If the aerodynamcis of lift can't be properly explained, then how do engineers design wings for aircraft - especially functionally different wings (e.g. fighter or big transport)?
:sad:

[mazzy1026]

If the aerodynamcis of lift can't be properly explained, then how do engineers design wings for aircraft - especially functionally different wings (e.g. fighter or big transport)?
:sad:
By building a wing, sticking it in a wind tunnel, testing it, and changing it until it works at it's best :p

[Genghis the Engineer]

As I said, it's not important that you can explain lift clearly - it's important that you can predict it accurately.

Through a combination of computer modelling and wind-tunnelling, fortunately we're pretty good at that nowadays.

G

[policepilot]

None of you have any idea what you're talking about,,, it's money that keeps you flying. More money, bigger plane- no money, no plane.

[boguing]

I've done a bit of fluid dynamics too, and was always troubled by the number of approximations and fudges needed to make the theory work precisely

But, in the case of the NASA 'experiment', surely they are missing the obvious? The symmetrical wing section does have a long path (and a short one) as soon as an incidence is applied. The static point moves swiftly down around the leading edge, whereas the theoretical separation point is fixed at the sharp trailing edge. (In the case of +ve lift, and vice versa). Which is why Bernoulli still works.

Edited. it to is.

[sidtheesexist]

Fascinating stuff!! I'm relieved that I'm not alone. Aeronautics graduate and ex-instructor - I always struggled to find a definitive explanation - probably because there ISN'T one!!! As as a previous poster has already pointed out, what's really important as an instructor is to explain when an aerofoil doesn't produce ENOUGH lift and the warning signs!

[rodthesod]

As as a previous poster has already pointed out, what's really important as an instructor is to explain when an aerofoil doesn't produce ENOUGH lift and the warning signs!

I've always found venturi and Bernoulli principles to be adequate for pilots although they may not satisfy the geeks. I learned my aerodynamics in the RAF from Kermode (?) and I never broke an aeroplane because of my ignorance. It was also a satisfactory expanation for the many hundreds of glider and jet pilots I have subsequently taught to fly.

Everyone must know that an aerofoil fails to produce enough lift when the angle of attack becomes too high for the bernoullis to hang on and they roll off the trailing edge, often causing a buffeting through the airframe as they roll and their lift arrows thrash the wing surface.

[jabberwok]

What about the trick of suspending a tablespoon under a running tap? There's no underside here, just water running over the curved surface - and that produces plenty of "lift".

[mazzy1026]

What about the trick of suspending a tablespoon under a running tap? There's no underside here, just water running over the curved surface - and that produces plenty of "lift".
What's that all about then? :uhoh:

[Bendo]

When I get a new student for a TIF I ALWAYS make them a cup of tea or coffee first and get them chatting while the kettle boils. I have a big spoon in the kitchen for just this purpose.

The point of it is that the laminar flow accross the back of the spoon causes the water to deflect , the same way as the wing causes downwash at the trailing edge. Newton's Third will have it that every action has an equal and opposite reaction.... and so the spoon deflects the water, and the water pushes the spoon.

Similarly, the wing forces the airflow downwards... and the downwash pushes the wing up.

It is useful for explaining ground effect and also for demonstrating the separation point and loss of lift in the stall. :ok:

Having a brain fart and can't remember for the life of me what its called :ugh:

[rodthesod]

The point of it is that the laminar flow accross the back of the spoon causes the water to deflect , the same way as the wing causes downwash at the trailing edge. Newton's Third will have it that every action has an equal and opposite reaction.... and so the spoon deflects the water, and the water pushes the spoon.

Similarly, the wing forces the airflow downwards... and the downwash pushes the wing up.


Do it again - I think you'll find the water pulls the spoon, as indeed the pressure reduction of the airflow travelling over the upper convex surface sucks or pulls the wing upwards.

The other similar demo using flow over the top surface only, is the one where you hold a piece of paper at the near corners so that it droops away from you and blow gently (laminar flow) over its curved upper surface.

Does a carb type venturi need downwash at the trailing edge to produce suction? Of course not - after the waisting of the tube it reverts to same old tube.

Regards,

rts

[BigEndBob]

Would be interesting experiment to suspend a spoon inside a large glass vacuum flask, then play water over back of spoon then see what happens?

Think no one theory explains lift, they all do.

I also noted one day a good example of the venturi effect whilst standing next to a canal near a bridge.
The canal narrows to pass under bridge and leaves floating on the surface speeded up as they passed under the bridge and slowed down on the other side as the canal widened.

Also the QEII had a problem some years back where it travelled fast off the US coast and scrapped its bottom in shallow sea.
Theory was that the bottom got sucked down and hit some rocks that it should have easily sailed past.

[bookworm]

It is useful for explaining ground effect and also for demonstrating the separation point and loss of lift in the stall. :ok:
Having a brain fart and can't remember for the life of me what its called :ugh:

You're thinking of the "Coanda effect". How do you use it to explain ground effect?

I think you'll find the water pulls the spoon

"Push" and "Pull" are pretty arbitrary when it comes to spoons, aren't they?!

[rodthesod]

"Push" and "Pull" are pretty arbitrary when it comes to spoons, aren't they?!

Not in the context of this discussion.

[BillieBob]

All very interesting but the original question was "If you've been teaching the "Equal Transit Theory" or the "Venturi Effect" theory for explaining "the force of lift" to your students, will you now change after reading this?" and the answer, in my case, is - Certainly not.

There are many ways of explaining why an aeroplane flies and no one 'theory' is superior to any other - they are all no more than 'theories', after all. Why make a simple question difficult (other than to massage a nerd's ego)?

[mingalababya]

There are many ways of explaining why an aeroplane flies and no one 'theory' is superior to any other - they are all no more than 'theories', after all.

Theories can be proven by mathematical calculations and it has been found mathematically that the equal transit theory for lift does not hold. That's what the NASA website set out to prove; their java lift simulator available at that site indicates that, yes, there is an increase in the velocity of air flow over the top of the aerofoil, and a slowing down of the airflow beneath the aerofoil, and yes, this does result in a lowering of air pressure over the aerofoil compared with beneath .. BUT, the pressure differential is too low to generate the amount of lift that you'd expect from such an aerofoil.

Therefore, Newtons third law of action and reaction is the better theory mathematically and it also better explains how symmetical aerofoils such as the horizontal and vertical stabilizers work as well.

I've just done my first brief on the theory of lift and have presented both theories for the audience to think about. I don't want to be pushing the equal transit theory even if it's partially correct.

Thanks all for your input on the discussion here ... it's all very interesting. :D

[bookworm]

That's what the NASA website set out to prove; their java lift simulator available at that site indicates that, yes, there is an increase in the velocity of air flow over the top of the aerofoil, and a slowing down of the airflow beneath the aerofoil, and yes, this does result in a lowering of air pressure over the aerofoil compared with beneath .. BUT, the pressure differential is too low to generate the amount of lift that you'd expect from such an aerofoil.

Nope, that's not what the NASA site ("http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html) says.

There is an increase in velocity above the aerofoil and the velocity differential between upper and lower surface is related through Bernoulli's principle to the pressure difference. The pressure difference exactly accounts for the lift.

The piece of the "equal transit time" explanation that is incorrect is the association of the velocity differential with the difference in "path lengths" above and below the aerofoil. The velocity differential is much higher than would be anticipated from "equal transit times".

[mingalababya]

Nope, that's not what the NASA site ("http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html) says.
There is an increase in velocity above the aerofoil and the velocity differential between upper and lower surface is related through Bernoulli's principle to the pressure difference. The pressure difference exactly accounts for the lift.
The piece of the "equal transit time" explanation that is incorrect is the association of the velocity differential with the difference in "path lengths" above and below the aerofoil. The velocity differential is much higher than would be anticipated from "equal transit times".
Yes, I stand corrected. Thanks for that bookworm. After doing those java simulation experiements, I found the pressure differential to be around 0.15 psi at 10 degrees AoA and thought it couldn't be enough to produce the 623 pounds of lift. Well, I guess I was wrong. The last paragrah on that page you quoted clears that up.

Cheers!

[HappyJack260]

Would be interesting experiment to suspend a spoon inside a large glass vacuum flask, then play water over back of spoon then see what happens?
Think no one theory explains lift, they all do.
I also noted one day a good example of the venturi effect whilst standing next to a canal near a bridge.
The canal narrows to pass under bridge and leaves floating on the surface speeded up as they passed under the bridge and slowed down on the other side as the canal widened.
Also the QEII had a problem some years back where it travelled fast off the US coast and scrapped its bottom in shallow sea.
Theory was that the bottom got sucked down and hit some rocks that it should have easily sailed past.

I wrote a paper on Shallow Water Shiphandling when I was on the staff of the Royal Navy Navigation school in the late 80's - I can't remember all the formulae now but there was a rule of thumb which (I think) suggested squat was a risk when depth < 1.4x ship's draught. Also built some reasonably complex spreadsheets to predict the actual amount of squat vs depth for different classes of ship, based on hull form. Funnily enough, this was only a few months before the QE2 incident.

Interaction between ships due to increased pressures at some points and reduced at others has caused quite a few accidents over the years - we always had to be wary of it conducting Replenishment at Sea operations, when ships approached each other and then steamed alongside.

[G-Dawg]

all i can say is wow....makes good reading though, almost makes me want to pick up that old textbook again..hmm maybe not!!

[Kengineer-130]

ok then, going off the basic theory of lift as taught by the PPL sylabus, how can an aircraft possibly fly straight and level whilst inverted? :bored: , just the simple force digram based on that will show its impossible, as lift and weight are acting in the same direction :bored: .... So there must be another reason why an aircraft can fly inverted whilst maintaining hight? :ok:

[rodthesod]

ok then, going off the basic theory of lift as taught by the PPL sylabus, how can an aircraft possibly fly straight and level whilst inverted? :bored: , just the simple force digram based on that will show its impossible, as lift and weight are acting in the same direction :bored: .... So there must be another reason why an aircraft can fly inverted whilst maintaining hight? :ok:

Am I the last one to realise that this whole thread is a wind-up? Forgive me for thinking for a while that it was a serious discussion.

[BigEndBob]

Does anybody know of any video footage of an aerofoil in a wind tunnel with smoke, on the internet?
Would be nice to show students.

[pilotbear]

try contacting the museum at farnborough, lots of old research stuff there. Very interesting place to visit too.:ok:

[moggiee]

I do love it when "experts" debunk a theory and then don't bother to tell you what applies instead!

If I read it correctly, they seem to say "some Bernouli, some Newton and some other stuff". Sounds very much like what the RAF taught me 20+ years ago. Anyone who teaches combined Bernouli/Newton theory is close enough for the 10% "other stuff" not to matter.

It works - that's good enough. How many pilots design their own aeroplanes?

[moggiee]

Interaction between ships due to increased pressures at some points and reduced at others has caused quite a few accidents over the years - we always had to be wary of it conducting Replenishment at Sea operations, when ships approached each other and then steamed alongside.
Like blowing between two sheets of paper - and if that doesn't substantiate Bernouli and Venturi, I don't know what does!

[PAPI-74]

www.rmcs.cranfield.ac.uk/aeroxtra


This explains all......:ok:

[Angels One Fife]

The NASA report is nothing new under the sun and it is what a lot of us already knew, that the little Italian isn't the only part of the equation. Unfortunately when we did 'professional' exams we just had to give a simplified answer that fitted the CAA's question rather than the whole story of lift that was learned at another institution at another time.
A bit like Georgraphy teachers at senior school never tell you about adiabatic lapse rates and tephigrams yet they teach about cloouds and they just say clouds form at a dew point which is zero degrees C. Always knew he was not so bright but bless. Pilots not knowing what they are actually on about is nothing new.
Lets not forget Kutta, Coanada, Joukowski et al.
What the R.A.F. taught it the pilots doesn't mean it was the whole story. It no doubt suited the purpose enough to give some understanding. The R.A.F. flight engineers maybe got more of the story and then ETPS went further and of the course the Marshall had a brain the size of a planet. That's how it works isn't it?
99% of pilots I ever meet just roll out Bernoulli's (two l's if I remember correctly) when it comes to lift and then dig themselves deep holes when probed.

[flying jocks]

Found this on MS Encarta.
The theory would explain the flat plate phenomenon. It makes a very straight forward parallel between holding your hand out of a moving car window and adjusting the angle at which you present your hand to the wind. The speed through the air is also an important factor for the reaction force placed against the wind thus producing an "equal and opposite reaction" of the airfoil 'upwards'. It then goes on to describe how the bernoulii principle is an effect of lift but not its cause.
From this I understand that airfoils are given various shapes depending on the task at hand. A large slow moving heavy aircraft would benefit from having a concave undersurface of the wing (slats and flaps contribute to this)as this effectively acts like a parachute. With a forward force still being applied the a/c goes up (just like the boat towing parachute rides you get at the beach). To preserve this effect throughout flight, the a/c's wing incidence angle is usually set at around 4 degrees up w.r.t.o the horizontal thus always having a downward force.
Yes, there is low pressure above the wing and this is caused by faster moving air above - but has very little contribution to overall lift of the a/c. The camber of the wing is there to minimise drag as a flat plate would have severe vortices, hence drag above it.
Have a look at the Encarta CD. They quote 'Boeing' as their information source. There is an interesting diagram/video that goes along with it.
FJ

[Angels One Fife]

Now let us just get those wings spinning round and round, about the lateral axis of course none of that helicopter nonsense here, bring in Mr. Magnus, for effect of course, and before you know it, Mr. Flettner will forget all about his strips and stick to ships. :)

[bookworm]

Yes, there is low pressure above the wing and this is caused by faster moving air above - but has very little contribution to overall lift of the a/c.

If Encarta says this then it is clearly wrong. The difference in surface pressure around the aerofoil accounts entirely for the lift the aerofoil produces. The difference in pressure is associated with a difference in speed. What causes what is a matter of philosophical debate.