Question: On the basis of Bernoulli's equation, discuss the assumption that the compressibility of air may be neglected for low subsonic speeds. A symmetric airfoil at zero lift has a maximum velocity that is 10% greater than the free-stream velocity. This maximum increases at the rate of 7% of the free-stream velocity for each degree of incidence. What is the free-stream velocity at which compressibility effects begin to become important (i.e., the error in pressure coefficient exceeds 2%) on the airfoil surface when the incidence is 5 degrees?