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Old 01-28-2010, 05:13 PM   #25 (permalink)
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Originally Posted by ChazInMT View Post
The question I have with your dark red is that it is scaled to the size of the vehicle. I think that as long as we're talking about largish objects like cars, trucks, boats & motorcycles, the 115 flow separation rule applies. So as long as we keep our top radius line from going past that 115 point, it really doesn't matter the shape. The reason we use this particular template is that it has the best "surface area/ wake generation" ratio. The segment we use is very similar to section 5.

This brings up another point that I think people don't often consider. We spend a lot of time trying to make the tops of our vehicles as aero as possible, and often neglect the sides. The air moving around the sides of our trucks and cars has a large impact as well I say. On my Pickup, the sides are about 22" tall on the passenger cabin, just in front of the bed. The roof width at the top in this area is about 52" so with 2 sides at 22", that's 44" of air surface. So we worry about 55% of the equation and neglect the other 45%. This is why I have designed a taper in the sides of my aerocap when viewed from the top. Doing this results in a fastback design.

More food for thought....It is often said that the best possible Cd we can achieve is .18 based on the .09 holy grail of aero shape in ground effect...The sides of the vehicle are not in ground effect. Hmmmmmmmmmm.

From the strut chart you see that the drag minimum occurs where form drag minimum intersects the skin friction drag minimum at t/c 0.255,somewhere between shape #4 and #5.These are 2-dimensional flow devices and in 3-dimensions,the minimum appears around L/D 2.5:1
With respect to the sides,BINGO!!!!!!! This is exactly what Kamm was doing with the roofs of his K-Cars,and is the reason your cap will have lower drag.
As far as the Cd minimum,Subaru,and GM(AeroVironment) have demonstrated sub-0.10 forms.
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