Question

What is the reason for a sudden increase in drag beyond drag divergence?

(a) Transonic region on top surface of the airfoil

(b) Supersonic region on top and bottom region of the airfoil

(c) Prandt- Glauert compressibility effect

(d) The shape of the airfoil

This question was posed to me in an online quiz.

My question is based upon The Supercritical Airfoil in division Subsonic Compressible Flow over Airfoils : Linear Theory, Linearized Supersonic Flow of Aerodynamics

Answer 1

Correct answer is (b) Supersonic region on top and bottom region of the airfoil

Explanation: When the Mach number is increased beyond the point of drag divergence, the drag decreases by a factor of 10 due to the formation of supersonic region in both top and bottom surfaces of the airfoil. This supersonic region is followed by a shock wave which leads to flow separation causing higher drag.