Quote:
Originally Posted by aerohead
The ideal teardrop form(about 2.7:1 fineness ratio) outside of ground effect has a Cd 0.04.I believe Morelli developed a low drag form of which Aptera borrows from.It was never tested in ground-effect.---- Any lengthening of the ideal teardrop body of revolution exhibits an increase in drag do to skin friction.I believe Cd 0.04 is the limit.Any environmental structure within close proximity will degrade the Cd.
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Do you have a reference showing attached flow on such an abrupt shape?
According to "the Theory of Wing Sections" by Abbott & Von Doenhoff, the NACA 66021 has a cd of .0035, but the 66009, twice as fine, gets .003. Even the 63 series is only listed at up to 21% thick, although I think I did OK with a 64025 for a strut.
For a good example of balancing volume with frontal area, I'd look at the Zeppelins. It is unfortunate that so much data on shapes pertains to wings. The earlier NACA series 0010-35 shape, with 10% thickness and a continuous convex curve to the back edge, more like a zeppelin outline, got to .003 cd, but had trouble with pitch instability as a wing.
To modify a pure shape for running near the ground, the bottom is squished in proportion to how low it is. This produces a wing shape, but the lift can be cancelled by a bit of rake and the venturi effect underneath. The overall effect on drag is not as bad as the addition of wheel exposure. Successful LSR cars are not jacked up. One such HPV was made, but it embarrassed the builders.