What is the Prandtl-Glauert compressibility correction and when is it used?

The Prandtl-Glauert rule scales incompressible pressure coefficient to compressible conditions: Cp_comp = Cp_incomp / sqrt(1 - M^2). This approximation accounts for density changes in subsonic flow and shows that aerodynamic coefficients increase as Mach number increases. It is valid for thin airfoils at small angles in the subsonic regime (M < 0.7-0.8). The correction diverges as M approaches 1, making it unsuitable for transonic flow where more sophisticated methods like transonic small disturbance theory or CFD are needed.