Question

For which of these conditions is Prandtl – Glauert compressibility equation valid?

(a) Thick airfoil, small angle of attack

(b) Thick airfoil, large angle of attack

(c) Thin airfoil, large angle of attack

(d) Thin airfoil, small angle of attack

The question was posed to me at a job interview.

My enquiry is from Prandtl-Glauert Compressibility topic in section Subsonic Compressible Flow over Airfoils : Linear Theory, Linearized Supersonic Flow of Aerodynamics

Answer 1

Correct choice is (d) Thin airfoil, small angle of attack

For explanation I would say: The Prandtl – Glauert compressibility equation is more or less accurate for small perturbations which occur for thin airfoils kept at small angle of attack. One added condition is that this equation is inaccurate for transonic and hypersonic flows.