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Old 11-25-2009, 06:21 PM   #140 (permalink)
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Blackfly - '98 Geo Metro
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Originally Posted by Nerys View Post
how steep an angle can you go before losing laminar flow?
Note the difference between attached flow and laminar flow. Very few automobiles (with the exception of solar cars, and maybe the Aptera) would have laminar flow over a significant portion of the body. For a discussion of the different flow types, see this thread:


I've seen figures ranging from 10-15 degrees as the highest initial change you can do while retaining attached flow, and up to and beyond 20 if the transition is gradual (think curve, not sharp angle).

it should be JUST on the verge of going unstable to be at optimum.
Yup, exactly.

FYI, the top of my boat tail curves gradually from the roof to ~15.x degrees from horizontal. For the sides, I aimed for a transition of ~10 degrees, also taking advantage of the OEM curvature at the rear corners. (But since I accidentally made it off kilter, one side is actually less, the other more.)

I aimed for a smaller angle on the sides because I figure the boundary layer there (compared to the top of the car) is already more turbulent - from windows, mirror, front wheels, panel gaps, trim - and less likely to stay attached on a more aggressive taper. Only a guess though. I'm just making this up as I go along

It would take a fair amount of effort to keep refining the boat tail to find the "verge of going unstable" for a particular car. I'm going to be satisfied with my boat tail if the air likes it the way it is with these angles. I could keep refining until the cows come home, or make a best guess, and be happy with it if it appears to work (which it does so far).
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