Question

The total drag coefficient for a wing is given by______

(a) cd=\(\frac {D_f+D_P+D_i}{Sq_∞}\)

(b) Cd=\(\frac {D_f+D_P}{Sq_∞}\)

(c) Cd=\(\frac {D_f+D_P+D_i}{Sq_∞}\)

(d) Cd=\(\frac {D_f+D_P}{Sq_∞}\)+Di

The question was posed to me in an interview for internship.

My question comes from Downwash and Induced Drag in division Flow over Airfoils over Finite Wing of Aerodynamics

Answer 1

Right choice is (c) Cd=\(\frac {D_f+D_P+D_i}{Sq_∞}\)

To explain: For wing, the total drag comprises of viscous drag plus the induced drag. The coefficient of total drag is given by Cd=\(\frac {D_f+D_P+D_i}{Sq_∞}\), which is nothing but Cd=\(\frac {D_f+D_P}{Sq_∞}+\frac {D_i}{Sq_∞}\)  i.e. Cd=cd+Cdi.