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lovemysan 04-25-2008 09:08 PM

corvair aeromods
There are some bueatiful cars on this site. This car has extensive cooling re-engineering. I'm most interested it the purpose of the sill skirts.

LostCause 04-25-2008 09:22 PM

Those are what I've been calling side skirts. I've written elsewhere on the site about their potential benefits (as I see them). Don't mean to be lazy by quoting myself, but why repeat? :p

This explanation wouldn't really apply to those skirts as they aren't extensive enough, nor is its underside built up to maximize their benefits.

From the thread "Win $100"


Frank Lee - "If there's a completely smooth belly pan, what good are side skirts? And vice-versa."

There are three primary reasons I can think of: yaw angle, spillover (for lack of a better term), and downforce. When I have more time, I'll describe each in more detail and with greater accuracy.

Yaw angle refers to the apparent air velocity direction under the vehicle. I need to refresh on the exact mechanism, but essentially high pressure air at the front underside of the vehicle tries to escape out the sides rather than flowing continuously back. This air travels obliquely to the direction of vehicle travel (which entails a energy loss in itself) and in doing so hits the wheels. While the front wheels show the greatest effect, the rear wheels are also affected. The wheels, which visually appear to have a small aerodynamic cross section, are much more dirty due to this oblique airflow. Skirts physically block off this phenomenon by removing the "easier" route for air to follow. Skirts allow the air direction to stay closer to the direction of vehicle motion.

Spillover is very similar to yaw angle, but it just describes the mixing of two regions of different pressure. Like the tips of wings, where high pressure air tries to rise up to the lower pressure upper surface, differences in pressure between the bottom of the vehicle and sides cause the direction of airflow to be altered. The more you change the air surrounding an aerodynamic body, the more drag you create.

Lastly, as Lotus had done with their open wheel race cars, side skirts allow the creation of a venturi under the vehicle. While the venturi would create downforce to counteract the lift generated on vehicles, I am not 100% sure whether this counteraction would decrease cL. Lift, as measured by the lift coefficient, brings rise to induced drag. As unintuitive as it might seem, cars generate lift like airplane wings and create the same type of vortexes. I suppose at the very least it would allow corners to be taken at higher speeds, lowering the use of brakes...
- LostCause

MetroMPG 04-25-2008 11:46 PM

I missed that explanation in the contest thread. Thorough.

I wonder if the benefits outlined would exceed the increase in apparent frontal area in cross-wind conditions. Considering that we probably spend more time experiencing cross winds than not, it's an important question.

brucepick 04-25-2008 11:51 PM

Just when I thought I had a plan for Sven's underbelly LostCause has re-educated me. Or perhaps just partially educated me, first time.

I thought I'd make a belly pan flush with the lower edge of side skirts yet to be built. However that would eliminate the "directional" effect of the side skirts that LostCause described. Am I understanding things correctly now?

LostCause 04-26-2008 03:07 AM

Good point about the crosswind issue. The first thing that comes to mind is that the car only experiences apparent velocity (like sailboats' "apparent wind"). The total component is what matters and unless it is really blowing outside or vehicle speed is slow (<45mph), then I'd imagine the wind angle would be pretty shallow. You are right though, it deserves attention.

I've taken the directional benefit from a section I came across in Hucho's book. Everything I write is from memory...and my mind is like a sieve. :p


Originally Posted by brucepick
I thought I'd make a belly pan flush with the lower edge of side skirts yet to be built. However that would eliminate the "directional" effect of the side skirts that LostCause described. Am I understanding things correctly now?

I'd be more worried about increasing frontal area. Side skirts are only useful because tires already take up frontal area. If I were driving a gyrocar (:D), I'd leave the skirts out.

- LostCause

Otto 04-26-2008 05:43 PM

Colibra and Porsche Carrrera GT have skirts in the horizontal plane, almost perpendicular to the sides of the car.

I suppose this is to fair the air that flows obliquely out from under the car, easing its exit, but somewhat fencing off side gusts so they don't go under the car. Sort of a crude flow valve, where air flows more easily out than in.

Perhaps it also fairs and smoothens the flow from the wake of the front wheels.

Your thoughts?

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