Question

What is the mach number of aircraft flying in air where the pressure ratio is 1.893?

(a) 2

(b) 1

(c) 3

(d) 4

This question was posed to me in an interview.

This intriguing question comes from Measurement of Mach Number in chapter Atmosphere and Air Data Measurement of Aircraft Performance

Answer 1

Correct option is (b) 1

To explain: The answer is 1. Given σ=1.893 and we know that γ for air is 1.4.

On substituting the values in the equation \(\frac{p_1}{p_2}=\Big\{1+\frac{\gamma-1}{2}(M)^2\Big\}^\frac{\gamma}{\gamma-1}\)

We get 1.893=\(\Big\{1+\frac{1.4-1}{2}(M)^2\Big\}^\frac{1.4}{1.4-1}\)

On solving we get M=1.