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Old 01-08-2008, 12:33 AM   #40 (permalink)
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Spin - '03 Mazda Miata SE
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Hoerner Dr.-Ing. S. F., Fluid-Dynamic Drag, 1965

Originally Posted by AndrewJ View Post
One thing that I feel hasn't been discussed too much yet about airdams vs. bellypans is the point of diminishing returns.
So far we know (at least in theory) that a bellypan is better because of the lower frontal area, and also the probable decrease in Cd from all the hanging bits on the underside.

But what we don't know is how much better a belly pan is. Has anyone seen any data on this so far? The most relevant thing I've found so far has been this refrence (PDF file) to a 1958 study
"Hoerner 1958 discusses the underbody as the biggest single avoidable
aerodynamic drag component and gives an old example where drag goes
from 0.3 for a smooth underbody to 0.6 for an open and rough as
customary car!"

Still no word on how much an airdam would knock off that 0.6

So, I hope eventually when I do get myself a jack, some jackstands and a new (working) drill I can get a belly pan together to test it out and get some numbers.

This list of things to do is getting quite long.
Technically, Hoerner is referencing "Sawatzki-Weiss, Zts. 1941; (2,d)" in Figure 9 (page 12-6) that is paraphrased in your qoute (assuming I'm reading the bibliography correctly). . The 0.3 is for a "completely smooth model" while the "1/2 faired (Tatra or Volkswagen)" is Cd = 0.5, and "Open and rough as customary" is Cd = 0.6. However, I don't think these are meant as overall coefficients. Instead he (seems) to reference the "completely smooth" (with no cut outs for turning of the wheels) is worth a DeltaC_d_o of 0.3 over the open/rough bottom.

Hoerner doesn't describe airdams in particular, but the paragraph on "Cooling Air (also page 12-6)" he seems to blame most of the cooling drag on the turbulence of the air exiting the bottom of the engine bay. There is a factor of C_D_box = 0.4 where "box" represents the "circumscribed area" of the open radiator with that number coming from Kamm's "Automobiltech Zeitscrift 1939, page 447". He also mentions achieving thrust from the cooling air via the proper "exit nozzle" and "energy transferrred to the cooling air by the engine fan (and some heating.)" Which may be harder for us to realize with fans no longer being attached to the crankshaft.

I heard this recently went out of print, so I'd go find a used copy if you can find one.

Last edited by Fuzzy; 01-08-2008 at 12:58 AM.. Reason: mis-spelled "airdam'
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