I have a cylinder (L is the length, D is the diameter) and a fiber braided around the cylinder; the angle between the fiber and the longitudinal axis of the cylinder is $\theta$. I have some difficult to understand how the triangle on the right (see the below figure please) has been built.
If I say that b is the fiber's length, who can tell me why the catheti are L and n$\pi$D?
Thank you for your time.