Question

What is the coefficient of pressure at minimum pressure point a function of?

(a) Critical Mach number

(b) Freestream Mach number

(c) Chord/thickness ratio of airfoil

(d) Length of the airfoil

I got this question in an online interview.

This intriguing question comes from Critical Mach Number topic in section Linearized and Conical Flows of Aerodynamics

Answer 1

Right option is (a) Critical Mach number

Easiest explanation: Coefficient of pressure at minimum pressure point which is present at the upper surface of the airfoil is given by the relation:

(Cp)crit = \(\frac {2}{γM_{crit}^{2}} \bigg [ \frac {1 + \frac {1}{2}(γ – 1)M_{crit}^{2}}{1 + \frac {1}{2}(γ – 1)} \bigg ]^{\frac {γ}{γ – 1}}\) – 1

In this the value of gamma is constant depending on the medium. The only varying quantity is critical Mach number. Thus, the coefficient of pressure at minimum pressure point is a function of only critical Mach number.