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:
... 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_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.
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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.