Question

What will be the drag to minimum drag ratio when the relative airspeed is 3.33?

(a) 5.49

(b) 1.51

(c) 5.59

(d) 1.82

This question was posed to me in an internship interview.

The question is from Aerodynamic Relationships topic in section Aircraft Force Systems and the Equations of Motion of Aircraft Performance

Answer 1

The correct choice is (c) 5.59

To explain I would say: The answer is 5.59. Given u=3.33. From the equation \(\frac{D}{D_{min}}\)=\(\frac{1}{2}\big[u^2+\frac{1}{u^2}\big]\)where \(\frac{D}{D_{min}}\) is drag to minimum drag ratio and u is relative airspeed. On substituting the values,

We get \(\frac{D}{D_{min}}\)=\(\frac{1}{2}\big[3.33^2+\frac{1}{3.33^2}\big]\)

On solving we get \(\frac{D}{D_{min}}\)=5.59.