Distinguish the 'Cd' for Aircraft and Car !

Irshad22 · December 15, 2011, 23:33

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?

truffaldino · December 16, 2011, 02:56

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

truffaldino · December 16, 2011, 13:03

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.

December 15, 2011, 23:33	Distinguish the 'Cd' for Aircraft and Car !	#1
Irshad22 New Member Irshad Join Date: Dec 2009 Location: Bangalore Posts: 19 Rep Power: 16	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?

December 16, 2011, 02:56		#2
truffaldino Senior Member Join Date: Jan 2011 Posts: 249 Blog Entries: 5 Rep Power: 17	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 f-w likes this.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Car park ventilation and impulse fans	guillaume	Phoenics	9	October 27, 2015 05:57
Car park ventilation and impulse fans	guillaume	Main CFD Forum	12	June 27, 2011 11:16
Car park ventilation and impulse fans	guillaume	Siemens	0	May 11, 2008 20:00
Car park ventilation and impulse fans	guillaume	CFX	0	May 11, 2008 19:59

December 16, 2011, 13:03		#3
truffaldino Senior Member Join Date: Jan 2011 Posts: 249 Blog Entries: 5 Rep Power: 17	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.