Question

The lift coefficient for a thin symmetrical airfoil is given by______

(a) cl = πα

(b) cl = π^2α

(c) cl = 2πα

(d) cl = πα^2

I have been asked this question in an online interview.

Asked question is from The Symmetric Airfoil topic in chapter Incompressible Flow over Airfoils of Aerodynamics

Answer 1

Correct choice is (c) cl = 2πα

For explanation I would say: The lift coefficient is given by cl=\(\frac {L’}{q_∞S}\) where L’ is the lift per unit span and S = c (1). Now, L’=ΓV∞ρ∞, according to the Kutta-Joukowski theorem. Putting Γ=παcV∞ we get cl = 2πα.