Question

The defining assumption for finding the skin- friction drag on an airfoil is____

(a) Airfoil is new low-speed airfoil

(b) Skin- friction acts due to shear force

(c) Airfoil has a zero angle of attack

(d) Airfoil is considered a flat plate at zero angle of attack

The question was asked by my school principal while I was bunking the class.

I would like to ask this question from Laminar Flow topic in portion Incompressible Flow over Airfoils of Aerodynamics

Answer 1

Right choice is (d) Airfoil is considered a flat plate at zero angle of attack

Easiest explanation: The essential assumption made for finding skin- friction drag on the airfoil is that it is considered as a flat plate with zero angle of attack.  It need not be low-speed airfoil only. Skin friction is caused by shear force and it is not an assumption.