Distinguish the 'Cd' for Aircraft and Car !
 

Why do we consider wetted surface area for aircraft and frontal area for car while calculating the Coefficient of drag? My question is why shouldn't we consider the frontal area in A/C aerodynamics?

Because wings (and airplane in general) change their angle of attack and shape (control surfaces), thus changing frontal area while unchanging wetted area. While a car does not go at different angles of attack and it does not move control surfaces, so its frontal area is unchanged.

Also for aeroplanes one uses planform or wetted area as a refence area for the lift coefficient (as using frontal area for this purpoce makes little sence, since it changes), so we have to use the same reference area for drag.

Truffaldino

I have also forgot to say that in the case of car the main contribution to drag comes from pressure rather than from skin friction, due to severe flow separation (because of cars non-streamlined shape): This why wetted area plays much smaller role, and frontal area is more important in the case of car.