I have taken this small element. This has a pitch; that means, it is not in this plane, but at
an angle. You understood? A pitch is at an angle to the vertical plane. That angle we
have shown here as this phi. Phi is the pitch angle; I have drawn the diagram here. Can
you see that? This is the pitch angle. Now, the propeller is rotating in that disc, this
plane, you understand, but the element is twisted by an angle phi. So, the rotational
velocity is at an angle phi 2 pi n f. Do you get my point? Can you understand what I am
saying?
the propeller is twisted. Imagine that small element we have considered. Suppose this a
propeller disc. I have got a propeller here; a small element is there