Cd*A vs. Equivalent Flat Plate Drag (EFPD)
 

I've noticed amateur road vehicle testers (don't know any professional road vehicle testers) use coefficient of drag (Cd) multiplied by the frontal area of the vehicle (A) to characterize the drag of their vehicle.

This method gives you two variables that must be obtained to determine your vehicle drag.

I'd like to suggest a method used in the Flight Test world called Equivalent Flat Plate Drag (EFPD). When we modify our aircraft, we test and note the change in the flat plate drag area (FPA). This allows us to determine performance changes very simply.

To use this method, assume a Cd of 1.0 and allow the drag calculation to determine the "Frontal Area" (A) for you. This frontal area will not physically represent the frontal area of the vehicle but it will allow the user to have an equivalent representation for vehicle comparison.

Example:
(...all numbers are fictional...) :)
LARGE TRUCK - Drag = 6250 Newtons on a day with density of 1.00 kg/m^3 (for simplicity of calcs) @ 25 m/s.

(FPA)*.5*density*velocity^2 = 6250

FPA*.5*1.00*(25^2) = 6250

FPA*.5*1.00 = 10

FPA = 20 m^2

....

A motorcycle might have numbers similarly to 5 m^2

Having these numbers allows someone to see how the fuel consumption on the truck should be more than the motorcycle based on the aerodynamics of the vehicle alone. If you could modify the vehicles to have the same FPA, the fuel consumption difference will be resultant from a different source.

Happy Modding!
Ryan

...for "flat" surface, roughly:

PSI = (MPH^2)/56500

...and:

Force = 0.0042*A*(V^2) = A*(V^2)/238

100 MPH = 42 lb/ft^2 = 0.17699 PSI

Thank you for the information "Old Tele Man".

For the rest of the community that might read this, remember that the EFPD is not how much flat plate area your car physically has. It is the amount of drag that a theoretical flat plate would have of a particular size. It's a theoretical number, not a physical one. But it does allow modders to compare aerodynamics of a vehicle. A modified Hummer might have the same Cd as a Civic but the physical sizes are different. If you use the EFPD, the numbers will reflect the amount of drag on the vehicle, not the Cd.

Thanks,
Ryan

Also, a quick re-read of my first post, I believe I might should have prefaced my calculations.

Coefficient of Drag [no units] = Drag [force] / (.5 x density of air [mass/area] x velocity^2 [distance/time] x frontal area [area])

EFPD [area] = Drag [force] / (.5 x density of air [mass/area] x velocity^2 [distance/time])

...I noticed your wording "...the Flight Test world..." and your Alabama location; are you associated with any Huntsville activities?

...I just retired from 'Flight Test' activities with Hughes/Raytheon and the USArmy Yuma Proving Ground before that.