Question

What is the coefficient of pressure at the minimum pressure point for an airfoil with critical Mach number as 0.6?

(a) 1.53

(b) 1.66

(c) 1.42

(d) 1.15

The question was posed to me by my school principal while I was bunking the class.

My question is based upon Critical Mach Number in chapter Linearized and Conical Flows of Aerodynamics

Answer 1

The correct choice is (b) 1.66

Easiest explanation: Given, Mcrit = 0.6, γ = 1.4 (air)

The critical Mach number is calculated using 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

On substituting the values, we get

(Cp)crit = \(\frac {2}{1.4×0.6^{2}} \bigg [ \frac {1 + \frac {(1.4 – 1)}{2} 0.6^{2}}{1 + \frac {(1.4 – 1)}{2}} \bigg ]^{\frac {1.4}{1.4 – 1}}\) – 1

(Cp)crit = 3.97\(\big [ \frac {1.07}{1.20} \big ] \)^3.5 – 1 = 1.66