Quote:
Originally Posted by RobertSmalls
True, but we don't (and maybe can't) know the contributions of profile drag vs skin friction, nor how much of the profile drag happens at the rear of the vehicle.
Come to think of it, maybe you can estimate skin friction's contribution to Cd, by looking at the Cd of a flat plate in turbulent flow. That would give Cd=0.03 from skin friction, for any object that is roughly shaped like a compact car, assuming 100% turbulent (and perhaps also 0% detached) flow.
If a tail increases surface area by a third, you'd be looking at Cd=0.04 from skin drag, and the rest is form drag. But don't expect to reduce profile drag to zero by reducing transom area to zero. Basjoos achieved Cd=0.17 with a tail and other mods, and any car shaped just like GM's race prepped Impact with tailcone will have a Cd=0.153.
Darin, did you record before and after transom area and CdA on your Metro?
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No doubt things have changed since the 1970s.Back then,profile drag constituted 55% of drag,skin friction was 7-12%,the remainder locked up in cooling sys. drag,interference drag, and induced drag.
With respect to the rear,if you're finished modifying the front,then the rear is where the rest of the drag is.The only way you can eliminate it is with a full boat tail.If you reduce the transom to zero you will reduce profile drag to zero if you stay on the template.
With respect to basjoos car,it can still go 0.08 lower.
The EV-1 could go 0.063 lower.