LD Max

I have some comments about incorrect aerodynamic theory appearing in the FAA question banks. I also refer to the FAA "Pilot's Handbook of Aeronautical Knowledge" which I downloaded here:

http://www.faa.gov/library/manuals/a...ilot_handbook/

Having reviewed this information and the CFI question bank, (which I am currently studying), I am sorry to say that there are some important fundamental errors in both texts, in particular where concerned with the interpretation and application of Bernoulli's theorems.


Between pages 2.1 to 2.8 of the pilot handbook there are various inaccuracies which I have paraphrased for brevity below, together with links to articles from NASA which set the record straight. These include:

• Airflow following the curved path of the upper wing surface must travel faster than the airflow following the flat lower wing surface due to the greater distance it must travel in the same amount of time;
  http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html

• The shape of the aerofoil can be regarded as a constriction in a pipe causing accelerated airflow and a drop in pressure in the same way as a venturi;
  http://www.grc.nasa.gov/WWW/K-12/airplane/wrong3.html

• Lift is developed partly through the reaction of the air rebounding off the lower surface of the wing causing an upward reaction;
  http://www.grc.nasa.gov/WWW/K-12/airplane/wrong2.html

• The air accelerates more slowly from the stagnation point on the lower wing surface than on the upper wing surface, resulting in positive air pressure below and negative air pressure above the wing;
  http://www.grc.nasa.gov/WWW/K-12/airplane/presar.html

• Most of the lift is the result of the airflow's downwash from above the wing.
  http://www.grc.nasa.gov/WWW/K-12/airplane/downwash.html

Some of these inaccuracies have been translated into questions for the FAA question bank, which serve to thouroughly confuse and mislead the candidate. Here are some examples from the CFI Question bank:

http://www.faa.gov/education_researc.../media/cfi.pdf

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EDIT 09 Aug 2006: Due to some very polite feedback I've received in the following discussion, I should mention that The "Correct" answers which I listed below in my original post are actually my own - and I can't say for sure that I am actually correct! It's just my take on them - trying to make sense of a set of questions which don't add up to me. Please read "Suggest" for the reasons given. Apologies for sounding a bit too sure of myself!!! I am always open to (and encourage) others challenging me on this... and indeed this is partly the point of this thread - so we can all benefit from the wisdom of others.

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Q 240, H912 CFI
An aircraft wing is designed to produce lift resulting from:

A) negative air pressure below the wing's surface and positive air pressure above the wing's surface;
B) positive air pressure below the wing's surface and negative air pressure above the wing's surface;
C) a larger center of pressure above the wing's surface and a lower center of pressure below the wing's surface.

FAA Answer (according to Gleim) = B

Correct answer = C. Newtonian (Impact) Lift theory is the only case in which a net positive air pressure (i.e. greater than atmospheric) can exist below a flat plate wing. An aircraft wing (aerofoil) develops negative air pressure above AND below the wing. Lift results when the relative pressure above the wing is less than the pressure below. The center of pressure, on the other hand, is the point through which the net sum of all the pressure forces on the wing is said to act. The center of pressure is NOT a point of pressure, it is a point of FORCE. Therefore lift is developed when the center of pressure above the wing is greater than the center of pressure below the wing.


Q 87, H912 CFI
Why does increasing speed also increase lift?

A) The increased velocity of the relative wind overcomes the increased drag;
B) The increased impact of the relative wind on an airfoil's lower surface creates a greater amount of air being deflected downward;
C) The increased speed of the air passing over an airfoil's upper surface increases the pressure, thus creating a greater pressure differential between the upper and lower surface.

FAA Answer (according to Gleim) = B

Correct answer = There is no correct answer to this question since lift over an aerofoil is is said to be modelled by the Bernoulli equations. Impact lift is not considered independently for aerofoils since the effect is considered as a whole within Bernoulli's theorem. Newtonian lift theory describes lift in terms of a flat plate, not an aerofoil.


Q 96, H912 CFI
Which statement relates to Bernoulli's principle?

A) For every action there is an equal and opposite reaction;
B) An additional upward force is generated as the lower surface of the wing deflects air downward;
C) Air traveling faster over the curved upper surface of an airfoil causes lower pressure on the top surface.

FAA Answer (according to Gleim) = C

Correct answer = There is no correct answer to this question since lift over an aerofoil is not a function of curvature. Symmetrical aerofoils and Inverted flight would not be possible if this were so. Any shape body can develp lift in an airstream. The shape of an aerofoil is primarily to control the separation of the boundary layer, thereby delaying the onset of the stall and to optimise the drag characteristics.

--------------------------------------

I am quite alarmed that these myths, (for they are nothing more), are appearing not only in FAA publications, but in examination papers for both students and instructor candidates. Furthermore, these will have been appearing for some time and no doubt are being taught in FAA schools.

For further reading on the matter, I refer to an extract of a (very good) article which can be viewed in full here:

http://en.wikipedia.org/wiki/Bernoulli's_principle

fullrich

Your analysis is very technincal and a bit ott I think for what is required. I would have answered as per gleim on all quetions and even though the wording may not be correct, unlike JAR there is only one answer that could be correct, in my view. If I had gone with 'no correct answer' on all the JAR papers Ive seen I would be still struging with PPL. The fact is ,Pilots dont understand this theory to this level and are only trained(especially in FAA) to understand the practical principles of flight, not theory, as it is just too broad a subject to cover in detail. My view!

LD Max

Thanks for this fullrich.

As an instructor, I would like to ensure that I am teaching my students the correct thing rather than some "dumbed down" version of the truth. If pilot's don't understand the correct theory, it can only be because they haven't been taught it! Pilots are not dumb!

A practical example of why it is a good thing to know, (for example), that a wing develops negative pressure on the underside as well as topside, is if you have a dripping fuel drain on the ground. This can turn into a steady stream during flight as the fuel is sucked out by the negative pressure under the wing!

And of course inverted flight and symmetrical aerofoils are real too - which some of the FAA theories can't explain.

Of course, you are quite correct in that to pass the test "no correct answer" is simply not an option. I just wish the answers were all correct. For example the first question DOES have a correct answer - it's just not the one the FAA think it is.

Safe flying and thanks for your input.

paco

LD Max - I too came across this problem whilst developing my ATPL(H) course.

In the end I included the correct information (in every subject, not just POF) and pointed out what was the dumbed down stuff for the exams. I know from my Microsoft experience how frustrating it can be to learn wrong stuff just for the exams, so I decided to take a stand!

Incidentally, on the helicopter side, at least, the FAA book is well known to be pretty bad since it was apparently written by low time instructors anyway. Go figure.

phil

fullrich

LD

With due respects, if you have a leaking fuel drain then you do not need to understand bernoullis theorum to figure out that you may loose fuel. A bit of common sense is alot more important in many respects than overdosing on theorums.I agree, that there is negative pressure also below the wing but the point is that it is more negative on top and this can only make one answer. I never said pilots are dumb but that alot of them will learn for the exams and forget the gory details a few months later.
It happens in all walks of life and to suggest that pilots might be different is just arrogamce.

LD Max

Firstly, it is not arrogance for me to say that pilots are not dumb... Pilots have a lot to learn in order to qualify and have an aptitude which renders them capable of doing so. I know you never said they were dumb, but you implied in your first post that this was all a bit beyond the average pilot.

Your statement is, of course, the nub of the problem. Common sense -vs- theory. The things we DO remember from our training tend to be the First Things we were taught (the law of primacy). The majority of pilots will tell you that lift occurs because of faster airflow over the top of the wing causing a drop in pressure. This is close enough - even if not strictly true.

Now, "Common sense" kicks in and tells us this must be because the airflow has to travel further over the curved top surface of an aerofoil in the same amount of time that it travels over the relatively flat lower surface of the wing. But this is completely untrue, even though it is supported by the FAA texts and those schools and instructors who refer to it. It immediately fails to explain inverted flight or symmetrical aerofoils and the detrimental effects of even tiny amounts of contamination on the wing surface.

These little nuggets of infomation then get taught as "add-ons" and indeed many students will then struggle with, a) Trying to make it fit with the incorrect model they were given in the first place and, b) Remembering what they've been taught subsequently.

Common sense might tell us that if we have a dripping fuel drain, we're going to lose fuel - but hey... I've gotta fly... it's only dripping at 5 or 6 drips a minute, that's hardly anything. It'll probably dry up in the air when I get some air pressure under it... I'm sure you appreciate the problem. A case of a little knowledge being dangerous - especially when it's wrong!

It is no harder for instructors to teach (and for student pilots to retain) the correct model. In fact I would say it is somewhat easier for students to retain the correct model, because they will find less contradictions arising in subsequent theory.

What is the value? Well of course it depends on the exams being in accordance with the truth too. After all, we've all got to pass exams - and there's little point in leaving blank answers against those we do not agree with.

So I have also forwarded my concerns to the FAA. But in the meantime as an instructor, I am uncomfortable teaching a syllabus which is flawed to this extent. But I am also obliged to ensure my students pass their exams regardless and will therefore continue to do so. Hopefully, I can also convey some aspects of the truth at the same time without confusing the hell out of them!

bookworm

While I agree with your sentiment, I disagree with the detail of your analysis of a couple of the questions:

Positive pressure coefficients often exist, and dominate, below lifting aerofoils, including flat plates. While a positive average pressure coefficient on the lower surface is not strictly necessary for lift, answer B represents the design goal. Answer C is meaningless. As you correctly point out, the center of pressure is a point. It is nonsensical to describe it as "larger".

The question doesn't assert that the curvature is the cause of the lower pressure, but rather that the air velocity is. Answer C, while misleading, is strictly speaking correct, and is certainly the statement most closely related to the Bernoulli principle.

unfazed

LD Max

Looks like your main point of reference for explaining LIFT is Bernoulli and his theory


Although this is the generally used simplified method for explaining lift there are better theories and I would expect that the FAA have based their question bank on these alternative explanations (open to correction on that assumption)


Suggest that you visit NASA's website for a full and detailed explanation of lift theory, then see if the questions make more sense

http://www.turnertoys.com/G1/aeroScience/default.htm

unfazed

Drat

Just read your post properly and realise that you are well aware of the NASA explanations

Apologies !

porridge

I have been intrigued for sometime that why aerofoils develop lift is due to a combination of Bernouli's principles along with Coanda effect. Read this interesting paper: http://www.aa.washington.edu/faculty/eberhardt/lift.htm
- "A Physical Description of Flight"

LD Max

Unfazed: No probs. Actually, as you will see from the NASA website, Bernoulli's theorems hold good.

Bookworm: Yeah...I'd go along with you on Q96. It is close enough. I took exception to it mainly because it implies lift is a function of a curved upper wing and a relatively flat lower wing. But you're right - it's the only question which has anything to do with Bernoulli.

As far as Q240 is concerned, I also "sort of" agree with what you say about answer C (CP). It is, however, a point of Force - (as can be graphically represented), and therefore it is valid to refer to it in terms of "larger" or "smaller". However, the question refers to the CP above and below the wing. In fact there is only one CP, which represents the net sum of all the forces (both above and below).

My argument was that it was "less wrong" than answer B which refers to positive pressure below the wing. So I took your comments on board, and this is what I found out.

Fortunately the NASA website includes a very useful program which can test our respective theories called Foilsim.

You can access it Here

When you enter the page, wait a short while for the Java program to load. You will then be presented with simulation. You can use the program to change different variables, including the shape of the wing and angle of attack. In the top right quadrant of the window, you will see a drop down menu entitled "Input". Select "Shape/Angle" in that menu. This will present you with a set of controls which allow you to adjust those parameters underneath the graphic of an aerofoil.

Select a "Flat Plate" in the "airfoil shape" drop down menu. You can then vary the AOA from zero to a large number either positive or negative. Note in the graph in the lower right quadrant that the pressure above the wing is Negative and the Pressure below the wing is Positive or vice versa for any AOA greater or less than 0 deg, and they are virtually symmetrical and straight line. Very "Newtonian" in fact.

Now Click the RED Reset button.

Reset "Shape/Angle" in the "Input" menu again. The shape should now be an aerofoil, and thickness of the aerofoil should be about 12.5% crd and the AoA is set to 5 Deg. Note that the pressure curve ABOVE the wing FALLS substantially across the first 50% of the chord. Note in particular, that the pressure curve BELOW the wing now indicates a NEGATIVE pressure over the majority of the first 50% of the chord, with the exception of the vicinity of the leading edge due to the stagnation point.

I will concede that AOAs greater than about 7 deg result in positive pressure under the wing, but also remember that this is not the optimum design AOA.

(If you select "stall model" to the right of the aerofoil graphic, the graph will indicate that the wing stalls at 10 deg AOA, but there is no stall data output from foilsim - and anyway I don't think stalls are accurately modelled because changing the shape of the wing doesn't seem to affect the stalling AoA).

Bear in mind this is a SYMMETRICAL aerofoil. Now adjust the Camber to about 2%c and the pressure on the underside rises to equal the free stream pressure at about 20% chord and then rises above atmospheric as it travels towards the trailing edge.

So I must accept that you are right to say that positive pressures often exist below a cambered aerofoil. However, they never do appear to dominate and negative pressures certainly DO exist while lift is being generated at "normal" AoAs.

So I still take exception to the statement that "an aircraft wing is designed to produce lift resulting from positive pressure below and negative pressure above the wing's surface". Only because the wing is actually designed to produce lift from "a Lower air pressure above the wing relative to the air pressure below the wing". You could write this statement a dozen different ways for exam purposes without implying that the pressure below the wing must be positive for the wing to produce lift, (which is where the general thrust of the FAA manual seems to be going).

But... Hmmm, I guess that now makes answer C MORE wrong than answer B, so I'd have to go with that too!

Many thanks for your input.

LD Max

Thanks for this link. Unfortunately it doesn't start out well, because it accuses the Bernoulli theorems of depending upon equal transit times in order to explain the differing velocities. Actually, Bernoulli's theorem says nothing of the sort, but is often misquoted in this way in popular texts. This is, indeed, one good reason for this thread!

The Coanda Effect you refer to explains the phenomenon that air "Sticks" to the surface of the wing as it moves through the air mass. (Similar, I believe, to a ping-pong ball held against a stream of water).

It is said that this effect causes the air molecules to change direction, and contribute to the lift in some way. I suspect that it is merely this turning effect which plugs energy into the Bernoulli equations to generate pressure differentials and gradients. These pressures may, in turn, be determined by measuring their associated air velocities. Notice I shy away from saying that an increase in airspeed CAUSES a drop in pressure. Bernoulli does not actually say this. He merely says pressure and velocity are related. If solved correctly, Bernoulii predicts extremely accurate models and are used as the basis for modern Computational Fluid Dynamics.

With thanks to Unfazed above, who steered me to another thread, this link seems to put the Coanda Effect in its proper place.


:-)

bookworm

I think I'd concede that. Well argued!

Foilsim's fun, isn't it?!

I would imagine that a typical "design" cruise Cl is of the order of 0.3 to 0.4 for a light aircraft (perhaps less for transport aircraft). With your symmetric aerofoil, I'd agree that the average Cp is negative. But for the flat plate it's most definitely positive.

bookworm

Denker's critique of Anderson and Eberhart is worth a read. Though it doesn't come through strongly in the critique, it does give a clue that their explanation of ground effect is quite wrong -- the sign is wrong for the "upwash load".

IO540

I can't argue with bookworm who I know knows his stuff but going back to the stuff near/at the top of this thread, let me make a few observations:

All the stuff about Bernoulli, the Coanda effect etc etc etc can be derived from Newtonian mechanics. If you apply NM to each particle, you will derive all the other (more convenient, in their intended applications) principles. So, why not simply teach NM and how to apply it to each particle, using finite element simulation on a computer? All the other stuff is irrelevant. Einstein would probably have a similar view, incidentally

I have an FAA PPL/IR and have passed the CPL theory so far. The FAA stuff is not without its faults, particularly in their various FAQs. The calibre of people that respond to intelligent questions is IME generally far higher at the UK CAA than at the FAA.

But FAA flight training isn't about theory, and using massive amounts of it, A-level-style, to separate the men from the sheep which is very much the general drift in CAA/JAA-land. If one assumes, correctly IMHO, that an airline pilot needs a reasonable technical brain, then why not teach him practical stuff he needs to know, and get him to sit a specialised engineering / mechanical aptitude/IQ test. Most members of the public fail such tests (the sort of test which shows pictures of two gears meshing and ask which way they will be turning, etc etc) pretty miserably. Then you would have airline pilots who have a very good engineering common sense. But this is not the way it is done under JAA.

On balance I think the FAA system is better for the intended purpose. Do we have 747s plummetting because of insufficient theoretical knowledge of the pilots? I fly with some working UK/JAA ATPLs and it's clear they have long forgotten most of the stuff.

As for private pilot training for European flight, VFR/IFR, that's different. One needs a lot of stuff which is not in any syllabus: flight planning generally, IFR route planning specifically, internet weather sources and their practical interpretation, engine management principles, you name it. It is taught in the USA to a reasonable degree appropriate to their airspace system and pilot services but the material is less appropriate to flight in Europe. Nothing is perfect, but the Euro syllabus teaches the pilot fly from Goodwood to Popham on a nice day, while giving him the legal privilege to fly from Goodwood to Malaga in 3000m visibility. OTOH a graduate of the FAA syllabus can in fact fly the same distance right across the USA with what he has learnt.

I have done the JAA PPL and the CAA IMC Rating and their various exams. Much of the material is utterly irrelevant to flying and so many of the questions are ambiguous (to the point of being blatent word plays intended for faithful readers of Trevor Thom and nothing else) that if one answered them strictly correctly, or refused to answer the unclear ones, one would not pass most of the exams.

I have also closely inspected the JAA ATPL material and e.g. the depth of understanding of met theory in there is way way beyond what any pilot needs IMHO, yet it lacks so much practical stuff, which the much smaller FAA CPL syllabus does contain.

The CAA LAME exams (I've seen snippets) are word plays like I have never seen before.

Nothing is perfect.

P.Pilcher

I seem to recall, many years ago, a conversation I had with one of my university student colleagues. He was reading for a Masters' in music, I was reading for my Masters' in electronics. My colleague was doing a bit of extra work studying to obtain his Licenciate of the Royal Academy of Music (LRAM) - an advanced piano playing qualification. He complained about how much his tutor had told him he had to "unlearn" in order to be successful in the written examination as in fact his musical theory knowledge far exceeded that of his examiners! On the other hand, I have read examination questions in hairdressing science where the examiner writing the questions on electricity clearly did not understand the difference between current and voltage.

It is the same in flying - although it is nice to apply total rigour to the answers to rigorously constructed questions in an examination, the aeronautical knowledge to be reasonably required of a commercial pilot need not be anything like as advanced as that for a test pilot or aeronautical design engineer. From the examples quoted above, IMHO (and only IMHO) the slightly imperfect standard of knowledge required by the FAA is totally adequate for a commercial pilot. Why encourage them to make life more difficult that it already is?

P.P.

LD Max

Well... from the point of view that any difference between MY understanding of what generates lift and YOUR understanding of what generates lift, is going to make no difference to how we fly aeroplanes, it makes no difference at all! You're quite right.

BUT, from the point of view of the student who has to learn all this stuff in order to get his/her licence in the first place, I think it does make a difference. I harp back to one of my earlier replies:

LD Max

I did say this in my discussion of Q240 in my original post. Flat plates are effectively "Newtonian".

Yes, FoilSim's great fun. I particularly like the way it's open source and the user interface can be tweaked to demonstrate specific applications (like the baseball demo).

boogie-nicey

Oh dear, another example of how the centre of the aviation universe is the UK....

It's very true that the FAA system is far more practical and basic PPL students don't understand the concepts in such depth.

This is nothing more than UK ott arrogance of how they're teaching the world how to fly..... The US have a right to conduct their own training affairs based on vastly more research and a thankfully a lack of silly JAA style Aviation Science that seems to make the equation 'theory heavy'

By the way I have trained in the UK and US and like both systems but I don't like this silly "we#re superior than you" approach to training, we're all in it together no matter where we trained.

LD Max

Most explanations of Bernoulli will agree with you. But have you seen the MATH! I'm sure you can work most things out from first principles but as you correctly point out, we don't have 747s plummeting from the sky due to the pilot's insufficient technical knowledge. God forbid we should require them to do a finite element analysis on the back of fag packet during their Landing Checks, because they'd probably fly into a mountain in the meantime!

In my search for "the truth", this is not what I'm advocating at all.

The "watered down" version of Bernoulli's theorems which we get taught as pilots, provides us with a reasonably simple set of equations from which we can derive answers for ourselves which are approximately correct and which give us a reasonable model about the conditions which enable us to fly. From this we can also get an appreciation of why we Stall, Load Factors, the markings on an Air Speed Indicator etc etc. All good stuff for a Pilot to know. He doesn't have to be an aerodynamicist to understand the basic Lift Equation.

I don't have a problem with any of that.

What I DO have a problem with is questions such as this:

This completely skews the "Newtonian" origins of Bernoulli's theorem, to imply that it is merely the action of the wind whacking into the underside of the wing which causes the plane to fly. This is inexcusable IMHO.

It's really just "Chinese Whispers" isn't it? You start off with a set of equations (Bernoulli's), simplify them for "Pilot Consumption", and then Misquote and Misapply them. It reduces the theory of aerodynamics to "Chants and Incantations" which would adequately describe the flight of a witch's broomstick.

I see little point in teaching the theory at all if it's not going to generate a reasonably correct model in the Student's mind about what's going on - and as I've said to a number of previous correspondents, an inaccurate model is harder to learn than a correct one, because when the model ceases to make accurate predictions - the student cannot work out the correct answer. He has to Learn the Answer, rather than rely on his knowlege.

The same applies to the irritating "Word Play" which appears a lot in JAA Exams. I agree with you whole heartedly for the same reason.