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Old 11-15-2009, 08:19 PM   #21 (permalink)
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Quote:
Originally Posted by aspera View Post
Still...I don't think it would be that difficult to duct the inside of the rear wheel wells to the back of the trunk. That should be a straightforward HIGH pressure to LOW pressure job.
http://ecomodder.com/forum/showthrea...lights-89.html

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Old 11-16-2009, 12:16 PM   #22 (permalink)
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On the race cars, it's not really to reduce drag down straights. It's for cornering. If you look at the Chaparral 2J or the Brabham BT46B cars, they have skirts that basically seal off the bottom of the car. If you take the small vaccum generated by the fans & multiply that by the surface area under a car, you get huge amounts of "downforce". It may have reduced drag a little bit, but you could go around a corner like you wouldn't believe.

Backyard aerodynamicist here.

Volumetric requirements. take surface area of rear of your car in ft^2. Multiply that by your desired speed in ft/min. This would roughly give you the cfm required to fill that hole. 55 mph is 4840 ft/min. Turns out to be a whole bunch of air.

You could try to use an electric cooling fan ducted down to the underside of the car.

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Old 11-16-2009, 03:59 PM   #23 (permalink)
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Is that a cubic foot per minute? That is a ton of air.
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Old 11-16-2009, 05:25 PM   #24 (permalink)
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But if the air came out of a slot going around the rear edge of the car and the slot was angled such that the air formed a virtual boattail (or kammback), it would need much less air and therefore energy.
I think it's a bit excessive to try fill the void behind the car with air flowing the same speed as the car.
What's needed here is a Blown Kammback. Much like the blown flaps we hear about on planes. I haven't even got around to making a regular kammback but some of you guys here seem to have the time/abilities etc.!!

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Old 11-16-2009, 05:27 PM   #25 (permalink)
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I just played around with the calculator and then accidentally deleted my post.
I tried using drag area (because it is easier to find), speed and distance. I'm not sure what it means but I think I found a fun way to compare a Hummer and an Insight. I call it DRAG VOLUME.

The drag area for a Honda Insight is 5.10 sq ft.
The drag area for a Hummer H2 is 26.5 sq ft.
60mph is 5280 ft/min.
5.10 x 5280 = 26,928 (cubic feet/min?)
26.5 x 5280 = 139,920

The Hummer moves 26,928 (drag volume?) of air in a mere 1016 feet. If the Hummer goes 1016 feet per minute that's just 11.5 mph.
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Old 11-17-2009, 03:06 PM   #26 (permalink)
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Quote:
Originally Posted by aspera View Post
I just played around with the calculator and then accidentally deleted my post.
I tried using drag area (because it is easier to find), speed and distance. I'm not sure what it means but I think I found a fun way to compare a Hummer and an Insight. I call it DRAG VOLUME.

The drag area for a Honda Insight is 5.10 sq ft.
The drag area for a Hummer H2 is 26.5 sq ft.
60mph is 5280 ft/min.
5.10 x 5280 = 26,928 (cubic feet/min?)
26.5 x 5280 = 139,920

The Hummer moves 26,928 (drag volume?) of air in a mere 1016 feet. If the Hummer goes 1016 feet per minute that's just 11.5 mph.
Sorry. I'm too dense to figure out what you are saying here.
Are you saying that at a mere 11.5 MPH a Dummer moves 26,928 cu.ft of air ?

( Imagine you are explaining yourself over again to a six year old. )

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Old 11-17-2009, 03:22 PM   #27 (permalink)
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Staying on topic here but veering off course a little -
I have a Civic hatch with the little mini tailgate.
Other posters here have found that opening the windows in their cars actually does not hurt the FE as much as they thought.
If I were to open my tailgate, as well as my windows, air would swoop into the cabin and find its' way out the back of the car through the opening - hopefully filling in my wake some.
What do you guys think will happen ?
A. I will die from all the fumes coming into the cabin from the stupid hole I made
B. The drag will actually be worse.
C. I'll be fine because the air ( and fumes ) will exit the cabin out the back. Possibly a very slight increase in FE.
D. Nothing. ( A wash )

Here is an example of the tailgate down ( Note that in the picture the hatch is open. For the test, only the tailgate would be open ):
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Old 11-17-2009, 04:07 PM   #28 (permalink)
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Old Suburbans that had roll down glass in the tailgate exibited the same properties. Kept you cool too. As far as asphyxiation goes, Most trucks & SUV's have the exhaust turned to the side for this reason. A simple clamp on extension with a 90 degree bend should keep you consious.

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Old 11-17-2009, 04:54 PM   #29 (permalink)
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Without making any guesses on the drag, I think your handling will be pretty squirrelly. I'm visualizing it without any real aero-ducation, but I think the air flow off the bottom of the car will be quite significantly altered relative to the air flow over the car. Induced lift since the low pressure zone will be close to the top of the vehicle and farther from the bottom?

But then again, I'm pretty clueless.
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Old 11-17-2009, 05:43 PM   #30 (permalink)
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I'll try this idea out next time I go visit my relatives ( for X-mas)
Extremely easy test to set up, but I'll have to relocate my plate.
I don't have cruise control, so I'll just try and keep it a steady as possible.

I noticed that the truck that has the airflow setup in the image above has a raised section that creates sort of an air pocket. This makes me think of the air pocket that was on the back of the UCDavis Taurus.

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