Question

If the lift coefficient for an aircraft at low speed is 0.45, then what is the value of the coefficient of drag at Mach number 0.7?

(a) 0.51

(b) 0.49

(c) 0.63

(d) 0.71

This question was addressed to me during an online interview.

This interesting question is from Prandtl-Glauert Compressibility topic in portion Subsonic Compressible Flow over Airfoils : Linear Theory, Linearized Supersonic Flow of Aerodynamics

Answer 1

Correct answer is (c) 0.63

To explain: Given, cl,0 = 0.45, M∞ = 0.7

The coefficient of lift for compressible and incompressible flow are related by the following relation:

cl = \(\frac {c_{l.0}}{β} = \frac {c_{l.0}}{\sqrt {1 – M_∞^2}} = \frac {0.45}{\sqrt {1 – 0.7^2}}\)

cl = \(\frac {0.45}{\sqrt {0.51}}\) = 0.63