This means that the singularity featured by the transformation near the sonic speed (M=1) is not within the area of validity. The aerodynamic forces are calculated to approach infinity at the so-called Prandtl–Glauert singularity ; in reality, the aerodynamic and thermodynamic perturbations do get amplified strongly near the sonic speed, but they remain finite and a singularity does not occur. The Prandtl–Glauert transformation is a linearized approximation of compressible, inviscid potential flow. As the flow approaches sonic speed, the nonlinear phenomena dominate within the flow, which this transformation completely ignores for the sake of simplicity.