Originally Posted by John Farley
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.
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.
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
) 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).
Originally Posted by John Farley
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.
