FAA Aerodynamics

[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/aviation/pilot_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
... etc



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_research/testing/airmen/test_questions/media/cfi.pdf

-------------------------
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.

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

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

Bernoulli's principle states that in fluid flow, an increase in velocity occurs simultaneously with decrease in pressure. This principle is a simplification of Bernoulli's equation which states that the sum of all forms of energy in a fluid flowing along an enclosed path (a streamline), is the same at any two points in that path. In a fluid flow with no viscosity, and therefore one in which a pressure difference is the only accelerating force, it is equivalent to Newton's laws of motion. It is important to note that the only cause of the change in fluid velocity is the difference in pressures either side of it. It is very common for the Bernoulli effect to be quoted as if it states that a change in velocity causes a change in pressure. The Bernoulli principle does not make this statement and it is not the case.

One common and correct way of understanding how an airfoil develops lift relies upon the pressure differential above and below a wing. In this model the pressures can be calculated by finding the velocities around the wing and using Bernoulli's equation. However, this explanation often uses false information, such as the incorrect assumption that the two parcels of air which separate at the leading edge of a wing must meet again at the trailing edge, and the assumption that it is the difference in air speed that causes the changes in pressure.

[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]

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.

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.:ugh:

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]

LD
A bit of common sense is alot more important in many respects than overdosing on theorums.
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.

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.

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

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... :eek: 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:


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.


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".


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.

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 :ok: 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. :ok:

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 (http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html)

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! :oh:

Many thanks for your input.

[LD Max]

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"


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 (http://www.av8n.com/how/htm/spins.html#sec-coanda-fallacy)seems to put the Coanda Effect in its proper place.


:-)

[bookworm]

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.

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]

With thanks to Unfazed above, who steered me to another thread, this link (http://www.av8n.com/how/htm/spins.html#sec-coanda-fallacy)seems to put the Coanda Effect in its proper place.
:-)
Denker's critique (http://www.av8n.com/fly/lift.htm) 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]

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?

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. :ok:

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:

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.

[LD Max]

With your symmetric aerofoil, I'd agree that the average Cp is negative. But for the flat plate it's most definitely positive.

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

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]

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 :)

Most explanations of Bernoulli will agree with you. But have you seen the MATH! :eek: 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:

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

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.

[LD Max]

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.

Please accept my humble apologies if that's what you thought this is about. It's really not what I'm saying at all... and in fact to support my complaint about the FAA theory, I'm referring to the NASA website which is, I believe, a US institution? Not once have I said the JAA teach it better than you do, and I wouldn't dream of suggesting such a thing.

I'm not trying to get "heavy" on the theory, I just want it to be correct - not more complicated. Do you agree that an aeroplane flies because of the wind whacking into the underside of the wing? If not - would you tolerate it being taught?

As far as "We're superior to you" attitude, I think you have an unfortunate chip on your shoulder. In fact I think the FAA system as a whole is MUCH MUCH better than the JAA system. It IS far more practical - and enjoyable too. But if the theory is that unimportant - then why teach it all?

I could list a whole host of complaints against the JAA system - and their question banks, but that's not why I started this thread.

[boogie-nicey]

In that case I have misunderstood your original comments... please accept my apologies.

I feel that at the progressively higher rungs of the licencing/certification ladder we should indeed induce a greater depth of knowledge and training. Someplace between JAA and FAA would be ideal, best of both worlds means best system :)

Remember: we're all in it together too!

[LD Max]

Boogie: Accepted, and Yeah I'd go along with that! :ok:

[John Farley]

LD Max

If there is one thing I admire and applaud it is somebody why does their best to ensure that student pilots are not taught incorrect theory. In that context I will live with simplified and incomplete (because that is necessary in the practical business of a attaining a PPL) but incorrect no.

So please allow me to start by saying a sincere ‘Well done you’.

The trouble is, when thinking and writing alone, it is very easy to make mistakes oneself which can degrade the value of your (however very well intentioned) output. I think this has happened in your first post, I honestly hope you can show I am wrong.

When dealing with Q240 you state (not suggest by the way) that answer C is correct. This is despite the fact that it seems to me answer C includes a really classic silly intended to flush out those that jump to conclusions about what they have read as opposed to carefully checking what is actually written. (…my word don’t we all do that from time to time - as examiners are only too well aware)

My problem with answer C is that it talks of larger and lower Centres of Pressure. A centre of pressure according to my education is a point. If I am correct a point cannot have magnitude - only position. In which case two different ‘positions’ of anything cannot give rise to a force.

Turning to the general issue of how flat plate and symmetrical aerofoils generate lift (as well as ‘conventional’ ones when flying inverted) I was taught that consideration of the position of the stagnation point provided the complete explanation of how the Bernoulli ‘notion’ was of equal value to pilots when applied to these cases. Since I do not want to teach granny to suck eggs I will not post diagrams regarding this unless requested.

So my bottom line is please keep up the good work – but be careful with the detail.

JF

[LD Max]

LD Max
When dealing with Q240 you state (not suggest by the way) that answer C is correct. This is despite the fact that it seems to me answer C includes a really classic silly intended to flush out those that jump to conclusions about what they have read as opposed to carefully checking what is actually written. (…my word don’t we all do that from time to time - as examiners are only too well aware)
My problem with answer C is that it talks of larger and lower Centres of Pressure. A centre of pressure according to my education is a point. If I am correct a point cannot have magnitude - only position. In which case two different ‘positions’ of anything cannot give rise to a force.

Many thanks for your kind words. :O You're quite right of course. I must apologise for my forthright approach to this. It is a character flaw I must confess, and when I originally wrote that piece - it was drafted as a rather indignant "pull yer finger out" type of message to the FAA. It was watered down a bit for posting on here. :mad:

You will find by scrolling down to my later discussion with Bookworm that I discovered there was indeed an anomaly in this answer all by myself. But that of the two answers given I'd thought it more correct than the one the FAA said it was. In the end I was forced (reluctantly :uhoh: ) to revise that position. I hear what you're saying about the CP merely being a point... But I still stick by the CP being a point of FORCE though, since it is represented graphically by an arrow of variable magnitude and represents the net sum of all the pressure forces acting through that point.

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).


Turning to the general issue of how flat plate and symmetrical aerofoils generate lift (as well as ‘conventional’ ones when flying inverted) I was taught that consideration of the position of the stagnation point provided the complete explanation of how the Bernoulli ‘notion’ was of equal value to pilots when applied to these cases.

I'd agree with that John, except that the theory being taught in the "FAA Handbook for Pilots" doesn't seem to do that. There's a lot of good stuff in there, but also a whole host of inaccuracies which seem to have spawned some of the Questions I took exception to. IMHO Bernoulli explains everything quite nicely, but he is often misquoted and conclusions jumped to which provide incorrect explanations "under his banner" so to speak.

In fact the FAA questions I've posted so far do not mention a flat plate - they all talk about "aerofoils" or "aircraft wings". This is why I took exception to so-called "correct" answers which would apply most correctly to a flat plate - but tell you little about how an aerofoil works.

Many thanks for your input. :)

[IO540]

"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"

The student doesn't have to learn it. The student needs only to pass the exam. As has been amply demonstrated here, the questions are often rubbish. The subject matter is rapidly forgotten. A working ATP will have a table of Vrefs versus loading and will look up the Vref to fly. He needs to know zilch about how the wing works. Just as well, since the exact way the wing of a modern jet transport works is pretty nontrivial.

The whole basis here is that the candidate is expected to have "grown up" in the same little universe in which the exam paper writers live, in which the standard textbook writers live, in which the FAA FAQ writers live.

Aviation is an incestuous little game where you queue up for a burger at the Fly show at Earls Court and in that queue you meet three people: one is a prolific aviation magazine journalist, another is a prolific aviation book publisher, one is the owner of an aviation magazine.

When you sit the exams you are expected to have lived among all this. Then and only then you will speak with the "correct terminology, young man" :)

If somebody who knew everything about everything came to Earth and sat the JAA exams, he would fail them all :yuk:

Incidentally, lift comes from redirection of the airflow (downwards). This brings about the pressure difference between top and bottom. The pressure difference can't be created without changing the direction of the airflow. Chicken and egg. Both physical processes are the same thing! Can't have one without the other. Aerofoils etc are irrelevant (unless you want efficiency) - a barn door will work just fine.

[bfisk]

I agree with a lot of what you are saying, threadstarter.

However, let's think about it for awhile; the level you are analyzing this at is far beyond what would be expected for a PPL or CPL candidate, at least in the US. By all means; we must all seek to increase our knowledege, but let's keep in mind to whom we will be teaching this.

Lift and other aerodynamic principles can be very hard to visualize, especially for beginning students. It is important for us as CFIs to be able to make the students learn something sensible - and although we need to be conscious about not teaching something that's blatanty wrong - imho we are allowed to, and need to, make simplifications.

Also, as far as effective questioning goes, it's not written in stone anywhere that one answer has to be 100% correct, and the distractors 100% wrong. It is sufficient to have one answer better than, more complete than, or more appropriate than the distractors. This adds a certain level of discrimination to the test. Our primary focus when developing such tests are to make sure that any knowledgeable person will pick the correct answer, whilst making the distractors a feasible choice for the ignorant.

Let's give an example:
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.
Answer C talks about "larger" and "lower" center of pressures. At least to me, that does not make any sense. It sounds very scientifical, but to my knowledge, the CP is the point at where the total aerodynamic force can be said to be accumulated. While as incorrect as saying that all mass of an aircraft is located in is center of gravity, it does serve a purpose. To look beyond the scientific details, however, if we look at the big picture, lift is primarily created by a pressure differential between the upper and lower surface of the wing; correct? While none of the answers are a complete and correct explanation, answer B is the most correct. Mind you also, the question stem specifies "an aircraft wing", which is not a flat plate.



---
As far as professional development is concerned, this is excellent reading for a ATPL student, CFI or whoever is beyond the basics. Let's just keep in mind that most students will be entirely satisfied with just keeping the blue side up, and we must be take great care in not scaring them away by theoretisizing too much about something which is really beyond our, and their, control, anyway.

Just my two cents.

[LD Max]

Ithe level you are analyzing this at is far beyond what would be expected for a PPL or CPL candidate, at least in the US.

I've said this to one or two other respondents, but I'm not trying to teach complex theory... just the right theory. In here amongst friends we can get a bit technical! ;)

...although we need to be conscious about not teaching something that's blatanty wrong - imho we are allowed to, and need to, make simplifications.

I'd agree with that. But there's a big difference between simplification and teaching something that's not right.

it's not written in stone anywhere that one answer has to be 100% correct, and the distractors 100% wrong. It is sufficient to have one answer better than, more complete than, or more appropriate than the distractors. This adds a certain level of discrimination to the test....

It's a good idea to blur the lines to test the candidate's thinking, but I'd disagree that the correct answer can be (say) 60% correct. What about 30% correct? What about 90% correct? Just where is this line drawn? What chance does a candidate have in picking the "most correct answer" if the candidate's thinking has been skewed by an incorrect model?

Answer C talks about "larger" and "lower" center of pressures... to my knowledge, the CP is the point at where the total aerodynamic force can be said to be accumulated.... lift is primarily created by a pressure differential between the upper and lower surface of the wing; correct? While none of the answers are a complete and correct explanation, answer B is the most correct. Mind you also, the question stem specifies "an aircraft wing", which is not a flat plate.

Yep, this has been pointed out before. On balance, I do now agree B is "more correct" than C. But this is a good example of why an incorrect model causes confusion, when neither are correct.

As far as professional development is concerned, this is excellent reading for a ATPL student, CFI or whoever is beyond the basics. Let's just keep in mind that most students will be entirely satisfied with just keeping the blue side up, and we must be take great care in not scaring them away by theoretisizing too much about something which is really beyond our, and their, control, anyway.

I think you misjudge the capacity of our students. Students will learn basically what we teach them.

[LD Max]

The student doesn't have to learn it. The student needs only to pass the exam. As has been amply demonstrated here, the questions are often rubbish.

Actually, you know, I agree with pretty much everything you've said. But I wonder if the emphasis is going back to that old nut about misleading questions.

Yes the questions are often misleading... and as other correspondents here have pointed out, that's not necessarily a bad thing. The purpose, after all, is to test the student's thinking and the model they've been taught.

My real gripe is about the model itself which appears in the "FAA Handbook for Pilots". I put the link to it in my original post, but here it is again (http://www.faa.gov/library/manuals/aviation/pilot_handbook/)for anyone who would like to read through pages 2.1 to 2.8 of the PDF file.

The questions are pretty good on the basis of this theory!

[SD.]

Although slightly off topic, it seems that a common problem is occuring both sides of the pond...........


S**t questions being written by so called experts in the examinations department in both the JAA & FAA.

I must admit thoughout any training I have done in the states, its been a case of breezing through the theory and learning the questions/answers to simply pass the exam. The only exam I really paid any attention to was the CFI. During my JAA ATPL's was a different story, you need a firm understanding in my opinion to pass these exams. Although every ATPL ground instructor I had, constantly reminded us of extremely poor questions that have been coming up from the ATP question bank. "If this one comes up, then let us know and we'll appeal it" was a frequent saying at the brush up courses.:=

[PantLoad]

It's been years since I took an FAA written exam, but I remember the frustration of goofy questions and less-than-correct answers.

If I recall correctly, it all started around the early seventies, when the U.S. government (in its infinite wisdom) decided to involve PhDs in psychology to help write these written exams. Not so much involvement in the PPL...but, an extreme involvement in the CFI written exam...especially in the 'Fundamentals of Instruction' (or whatever they called it then...whatever they call it now). Needless to say, the questions (and what the FAA called the correct answers) were quite a PantLoad! :> :> :> :>

I also recall writing (on a separate sheet of paper) protests...statements of disagreement...to specific questions and the answer choices. In some cases, believe it or not, the FAA gave me credit for my answers, despite my answers differing from, what they claimed, were the correct responses. (Which told me that the FAA knew the tests were a mess.)

Apparently, things haven't changed much. But, I want to assure everyone, that professional pilots in America are aware of these inaccuracies. It's the FAA...what can you do!?!?!?!?!


PantLoad

[LD Max]

Apparently, things haven't changed much. But, I want to assure everyone, that professional pilots in America are aware of these inaccuracies. It's the FAA...what can you do!?!?!?!?!

Well... I guess you could do what I did and write your comments to:

[email protected] ([email protected])

As I've said before, it's not just the questions which bother me, (though they do of course) - it's the theory being taught in publications such as the FAA Handbook for Pilots. See my last post (http://www.pprune.org/forums/showthread.php?p=2766306)for the link to download the PDF.

[slim_slag]

Another thought provoking thread.

From what I remember, most people who walk through the flying school door and announce they want to learn how to fly couldn't tell you how a wing works ten minutes after you had spent an hour explaining it to them. Most people do not have the mind of an engineer, and IMO you need that to properly understand lift. Most of these people become quite safe pilots, some of them even become very competent airline pilots.

It is extremely hard to examine whether you know how a wing works using a blunt tool like a multiple guess question, unless you want 20 questions to be on wings. Given todays desire to get peoples their PPL in minimum time, this would leave no time to teach them how to work out important stuff like much usuable fuel is left in their tanks.

If you are going to examine on how a wing works better to have a short answer, or do it in the oral, but then I would need to know in advance what examiner believed in so I could teach that one to the poor student. That would become confusing.

So, as I think teaching duff info is worse than not teaching anything at all, I reckon it should be taken out of the syllabus - all the way to ATP. Save this stuff for test pilot school or Embry Riddle students.

Those students who do have the mind of an engineer and want to know how it works should be sent to denkers web site. WHen they come back they can explain it to their instructor.

[LD Max]

So, as I think teaching duff info is worse than not teaching anything at all, I reckon it should be taken out of the syllabus - all the way to ATP.

I agree with the sentiment, if not quite the radical solution you propose.

Do you not think that any pilot - even PPL Students - need to appreciate the 4 forces and basic aerodynamics? Avoiding the stall, especially, make a lot more sense in the context of AoA when different speeds equate to different weights or Load Factors.

When I say Basic aerodynamics, I do mean exactly that. Simplified - but not WRONG. It can get more complex at the CPL / ATP levels, but then it would build upon a solid foundation given in early training, without commercial students having to "unlearn" the incorrect bits.

[slim_slag]

Ah, well I concentrated on how a wing works, as that was the subject you brought up in your first post. How does a wing work, Denker gives a summary (http://www.av8n.com/how/htm/airfoils.html#sec-airfoils-summary) and I don't think that is basic, and I find it hard to see how that can be made basic.

Now what you mention, the four forces, is very important. Knowing about the drag curve is very important as getting that wrong can kill you quite easily. Not knowing about circulation is neither here nor there, and you cannot explain lift without bringing that up. IMO :)

A very good thread.

Had another thought. When an airliner computer decides what angle of attack to fly at, does it do this using a fundamental understanding of how all the air molecules are reacting against the wing? Or does it just know that at this temp/pressure/aoa/velocity/power/whatever I will get this performance? If the autopilot doesn't need to know how lift works at an engineering level, why does a student pilot or even an ATP???

[LD Max]

I suspect those airliners which can fly at a specific AoA, have an AoA vane sensor on the side of the fuselage. No calculation required. So if I pull the stick all the way back, the computer will give me all it's got without stalling the plane. The AoA sensor has been a recommended item for all aircraft by the AAIB for years, even if it is just another channel on the FDR for the subsequent board of enquiry!

Sadly, I doubt we'll be seeing one of those in a cessna for a while (even a G1000 equipped cessna), so giving the pilot a good model of what affects AoA is as good as we can get. At least his eyes can take in what's on his 6-pack, and his brain can make a pretty good estimation of how to keep the aircraft flying. There's no Math involved if the mental model is a good one!

[bookworm]

Had another thought. When an airliner computer decides what angle of attack to fly at, does it do this using a fundamental understanding of how all the air molecules are reacting against the wing? Or does it just know that at this temp/pressure/aoa/velocity/power/whatever I will get this performance? If the autopilot doesn't need to know how lift works at an engineering level, why does a student pilot or even an ATP???

Excellent point. It's entirely reasonable to explain lift empirically. Define it, and describe its relationship with speed, density, AoA, camber, AR, viscosity. Leave the physics to physicists.

I do think that anyone being introduced to lift should have it introduced empirically first. Like any other area of physics, it's tempting to descend into all sorts of profound theories about how it works, losing sight of the mere fact that the predictions of that theory bear no resemblance to real-life observations.

As a pilot, is it more useful to be abole to answer Q 96, H912 CFI
"Which statement relates to Bernoulli's principle?" than:

What is a typical maximum lift coefficient for an unflapped aerofoil?
A) 0.1
B) 1
C) 10

Yet if you try the latter around the flying club, you'll get 99% blank faces.

[bfisk]

It's a good idea to blur the lines to test the candidate's thinking, but I'd disagree that the correct answer can be (say) 60% correct. What about 30% correct? What about 90% correct? Just where is this line drawn? What chance does a candidate have in picking the "most correct answer" if the candidate's thinking has been skewed by an incorrect model?

The question must be set up so that any knowledgeable person will pick the correct answer. I agree that all questions should have answers that are defendable and correct, and I agree that some of the FAA questions are unfairly vague.

I did look to the FAA-published PHAK and AFH, and they support the theories to support the questions. Might I suggest that that is where the problem lies?

As far as the rest of the post goes, I think we agree to the same principles ;)

[LD Max]

I did look to the FAA-published PHAK and AFH, and they support the theories to support the questions.

Might I ask if these are online, like the FAA Pilot's Handbook? If so, would you mind posting a link?


As far as the rest of the post goes, I think we agree to the same principles ;)

Yep... for sure! :ok:

[bfisk]

Might I ask if these are online, like the FAA Pilot's Handbook? If so, would you mind posting a link?

PHAK = Pilot's Handbook of Aeronautical Knowledge (theory stuff)
http://www.faa.gov/pilots/training/handbook/

AFH = Airplane Flying Handbook (flying stuff)
http://www.faa.gov/library/manuals/aircraft/airplane_handbook/

:)

[LD Max]

Ah thanks. Turns out I already had the PHAK which I put in my first link (I just didn't recognise the acronym).

Cheers for the AFH too.

This, of course, is where it all originates. The standard of the publications - upon which the questions seem to be based - really need to be looked at.

[bfisk]

Ah thanks. Turns out I already had the PHAK which I put in my first link (I just didn't recognise the acronym).

Cheers for the AFH too.

This, of course, is where it all originates. The standard of the publications - upon which the questions seem to be based - really need to be looked at.

I agree on that. I have started reading PoF in the Nordian JAR ATPL books, now, and it's like... physics reinvented.

(But hey! I can still keep the blue side up even in the US!)

[LD Max]

The (eventual) response from the FAA is fairly positive.

Thank you for your comments regarding the Pilot's Handbook of Aeronautical Knowledge. The funding for a revision of the handbook was just approved and we plan on beginning the revision this fall. I have forwarded your comments on the CFI test to the responsible inspector. Should your studies reveal any additional inaccuracies please let us know.

Regards,

Mark J. Aldridge
Aviation Safety Inspector, Operations
AFS-630, Airman Testing Standards Branch Federal Aviation Administration P. O. Box 25082 Oklahoma City, OK 73125
Phone: 405-954-3654
Fax: 405-954-4748
[email protected]


So now you know who to contact with any other inaccuracies in the "PHAK".

Of course, he hasn't mentioned who the responsible inspector for the CFI question bank is...