Drag Coefficient

[rphillips]

Does anyone have a ballpark figure of the drag coefficient of a FV?

Ray

[SR Racing]

Here is a good article to make some guesses from and links to calulate it based upon measurements.

http://en.wikipedia.org/wiki/Automobile ... efficients

[SR Racing]

http://www.fvansw.asn.au/docs/July%20Ma ... 0FINAL.pdf

Some even better info...

Jim

[24b4Jeff]

First off the calculation of the drag coefficient for a shape as complicated as a FV is an extraordinarily complex undertaking, which requires mastery of CFD codes and a supercomputer, even today. So if anyone tells you that they have a code that works on your PC they are talking about a very ballpark estimate! Think about this: to do the drag coefficient of a Boeing 777, which is much simpler in shape than a FV, they exceeded the capability of the computers available to them; with the multi-gigaFLOP machines then available, all they could do was a wing.
BTW most of the aerodyamics work today concentrates in the high Reynolds number regime; our cars operate at decidedly low values of Re. The folks who are most interested in our regime are hydrodynamicists, such as the designers of submarines. Unfortunately it is difficult to get any of them to talk much about it. I know this from experience years ago working on the SSN-21.

Some time ago some folks actually put some Vees in a wind tunnel. The cars involved were a Caracal, a Citation, and one or two others. I have a tape and copies of the data. But the data are not especially reliable, because the wind tunnel was not equipped with a moving ground plane, and there are some issues as to the angles of attack. For that reason I am not giving out the data, because I fear that some physically unsophisticated folks will interpret them incorrectly.

That being said, if you are looking for a ball park figure, use something in the range 0.3 to 0.4.

[rphillips]

Thanks!

Ray

[hardingfv32-1]

A wind tunnel test of a FST (Euro Vee) in Race Tech mag has the Cd at about .640. I have a research article on wheel areo for a moving wheel about our size that shows a Cd of .560 . I include the rear wheels in the frontal area calculation when doing drag numbers.

Ballpark drag est: 35% body 65% wheels

Brian

[24b4Jeff]

Earlier I mis-spoke. The range of measurements for various configurations of a Citation were from the low 0.6s to the high 0.7s. That is a lot of variability for a small range of configurations! Also, as mentioned previously, the wind tunnel did not have a moving ground plane.

Sometimes, at some values of Re and some car configurations, it is ok to assume that the wheels and body are essentially independent of each other so that the composite Cd x A ~ Cd (tires) x A(tires) + Cd(body) x A(body). In general though, one does so at ones peril.

[hardingfv32-1]

Would you agree that the rear wheels should be included in the frontal area calculations? All the CFD simulations of open wheel cars I have seen show the flow as normalized before reaching the rear tires after clearing the front.

Brian

[Larry Bradley]

It is somewhere near: a cement block with barn doors hung on the side.