Originally Posted by Mr Optimistic
The issue isn't 'maths' its 'concepts'; that's the source of the difficulty. [....] Those concepts are deceptively simple looking: it's not human nature to think that way.
You're right, the difficulty is that some concepts are counter-intuitive.
"What falls faster in a vacuum, a feather or a stone?" The 'intuitive' answer is 'a stone' because it's heavier. It takes a moment's thought (or a demo) to arrive at the right answer.
Think of that other ancient trick often used to demonstrate how a wing works.
Take two sheets of stiff paper.
Fold over one edge of each so you can hang them over two pencils.
. ^ ^
. | |
Now curve the two sheets.
. ô ô
. ) (
(I suppose you get the idea, I'm too lazy to do a drawing and post it...)
Now blow down between the two sheets.
The 'intuitive' answer is that blowing between them will push them apart.
Again, it takes thought, and knowing the airflow in the resulting 'venturi' causes a pressure drop, that pushes the two sheets together, to arrive at the right answer.
Or take a wing... again the 'intuitive' notion is that it's the 'wind' pushing on the
underside of the wing that produces the lift.
It's not until you actually see a picture of the pressure distribution over a typical lift-producing wing, that you realise it's the lower pressure on the
upper surface which produces nearly all the lift, rather than the higher pressure on the lower surface.
With intakes (supersonic intakes in particular) it's much the same story.
The fact that "sucking" air through a con-di intake (whether with ramps or a spike doesn't matter) will result in a forward force on the intake structure (hence thrust) is at first counter-intuitive, yet it's a proven fact.
I was rather hoping somebody here (this is Tech Log, after all!) would come up with a drawing of the pressure distribution in a supersonic intake, much like the wing pressure distribution I mentioned above.
After all, a drawing says more than a thousand words.
CJ