Question

The minimum drag mach number is given by ___________

(a) \(\big(\frac{2W}{\rho S}\big)^{\frac{1}{2}}\big(\frac{K}{C_{Dz}}\big)^\frac{1}{4}\)

(b) \(\big(\frac{2W}{\gamma \rho S}\big)^{\frac{1}{2}}\big(\frac{K}{C_{Dz}}\big)^\frac{1}{4}\)

(c) \(\big(\frac{2W}{\rho S}\big)^{\frac{1}{2}}\big(\frac{K}{C_{Dz}}\big)^\frac{1}{2}\)

(d) \(\big(\frac{2W}{\gamma \rho S}\big)^{\frac{1}{2}}\big(\frac{K}{C_{Dz}}\big)^\frac{1}{2}\)

This question was addressed to me in semester exam.

This key question is from Aerodynamic Relationships in chapter Aircraft Force Systems and the Equations of Motion of Aircraft Performance

Answer 1

Correct answer is (b) \(\big(\frac{2W}{\gamma \rho S}\big)^{\frac{1}{2}}\big(\frac{K}{C_{Dz}}\big)^\frac{1}{4}\)

Easy explanation: The correct formula for minimum drag mach number is given by \(\big(\frac{2W}{\gamma \rho S}\big)^{\frac{1}{2}}\big(\frac{K}{C_{Dz}}\big)^\frac{1}{4}\) where W is weight, ρ is density, K is constant, S is span area, γ is ratio of specific heats and CDz is coefficient of lift dependent drag.