Question

Which of the following is an incorrect relation for a flat plate?

(a) cm,le=-π \(\frac {\alpha }{2}\)

(b) cm,le=-\(\frac {c_l}{4}\)

(c) cm,le=-\(\frac {c_l}{2}\)

(d) cm,c/4=cm,le+\(\frac {c_l}{4}\)

This question was addressed to me in an online quiz.

My doubt is from The Symmetric Airfoil in chapter Incompressible Flow over Airfoils of Aerodynamics

Answer 1

Right answer is (c) cm,le=-\(\frac {c_l}{2}\)

Explanation: The coefficient of moment about the leading edge is given by cm,le=-π \(\frac {\alpha }{2}\). Putting cl = 2πα we get cm,le=-\(\frac {c_l}{4}\). Finding the moment coefficient about quarter chord we get,

cm,c/4=cm,le+\(\frac {c_l}{4}\).