Question

Which of the following is correct?

(a) Lift produced by an airfoil is always equal to weight

(b) Drag by sphere is more than the drag produced by an airfoil for similar conditions

(c) Drag by sphere is less than that of produced by an airfoil for similar conditions

(d) Drag by sphere is equal to the drag produced by an airfoil for similar conditions

This question was posed to me at a job interview.

This interesting question is from Airfoil Selection-1 in chapter Airfoil and Geometry Selection of Aircraft Design

Answer 1

Correct option is (b) Drag by sphere is more than the drag produced by an airfoil for similar conditions

The explanation is: Lift produced is not always same as weight. Sphere is a 3D object. Drag produced by sphere will be more due to increased friction and 3D effects. An airfoil is 2D shape which will have less drag as compare to sphere for all similar conditions.