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.