Quote:
Originally Posted by Bicycle Bob
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.
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What I take away from their work,is that the wing sections have really low Cds,but they've got all the induced drag associated with lift.The other thing is that with aircraft,the wetted area is equally if not more important than Cd.Lots of skin friction and much interest in laminar flow design.-------- As far as bodies of revolution,I did take another look at Mair's boattail work,and if you look at his data table,his data infers that he achieved Cd0.035 with his body of revolution,with prolate elipsoid nose,and one-diameter length boat-tail@ 22-degrees.I think this is the lowest thing I've seen for free air.Hucho has this in his book.-------- Ernie's 5-1 fineness ratio for low drag in ground effect seems to still hold true.VW has found it to be the "sweet-spot",also Mercedes-Benz,with their record-car projects.Mair's body in ground-effect according to Hucho's relationship,would come in around Cd0.70,and with ther addition of wheels,would final out in Cd0.12 territory.With extreme attention to the under-carriage,they might shave it to Cd 0.10,as Honda achieved with Dream-2.