Question

What does the Prandtl – Glauert rule relate?

(a) Shape of airfoil in transformed spaces

(b) Incompressible flow to the compressible flow for same airfoil

(c) Coefficient of lift to coefficient of pressure

(d) Coefficient of drag to coefficient of pressure

I have been asked this question by my college professor while I was bunking the class.

Question is from Linearized Subsonic Flow topic in section Linearized and Conical Flows of Aerodynamics

Answer 1

Right choice is (b) Incompressible flow to the compressible flow for same airfoil

The explanation: The Prandtl – Glauert equation is given by:

Cp = \(\frac {C_{p0}}{\sqrt {1 – M_∞^{2}}}\), Cl = \(\frac {C_{l0}}{\sqrt {1 – M_∞^{2}}}\), Cd = \(\frac {C_{d0}}{\sqrt {1 – M_∞^{2}}}\)

This equation relates the pressure/lift/drag coefficient in incompressible flow Cp0 to the pressure/lift/drag coefficient in compressible flow (Cp) for a two – dimensional airfoil with the same profile.