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Old 05-09-2007, 08:21 PM   #11
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That sucks when ever you go faster than 20 mph.......
Don't forget your full face masks everyone
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Old 05-09-2007, 08:46 PM   #12
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Don't forget your full face masks everyone
Motorcycle helmet Still it was kinda cool driving without my windshield
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Old 05-10-2007, 10:09 AM   #13
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>>Oh for Pete's sake. If that volume of air is going through the car, then the >>interior also has a coefficient of drag!

I was thinking smooth ducting all the way "through" the cab.

But further, drag is caused by redirection and surface friction... I think a volume of air can be pumped into a large space and sucked out of another location without much drag in the middle, as long as the space is large with respect to the volume of air being moved. I am not thinking 20 sq. feet times 60 mph of air though the cab... but maybe 5% or 10% of that, with the resulting drag in the cab not measurable because of the decreased velocity flow.
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Old 05-10-2007, 02:36 PM   #14
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I am trying to think variational here... even as far back as design. Here's a few observations.

There is a lot of unused space in the car... maybe it can be use to channel air. I know you don't want a 30 mph wind in your face, but what about ducting in the doors, some on the floor corners, above the transmission... heck, the exhaust takes up quite a bit of cross sectional area, close it off and channel some bad air through there.

This whole approach could alter the aerodynamics of the car, especially if you add some energy to the flow at key points (ala vortex generator idea, but maybe active instead of passive). On the extreme engineering side, you have those synthetic jet things like on the Renault. On the simpler side, maybe catch the perimeter air right before the stream separates and channel it down through ducting into an electric ducted fan (EDF) that also provides some thrust. I am thinking hovacraft tail sucking air from through body ducting as well as the rear perimeter (to avoid separation), providing say 20 lbs of thrust with positively altered aerodynamics.

Yeah sounds crazyy I know but clearly the idea works in aircraft and already has been minaturized in the radio controlled modelling world (RC Jets... thrusting air only via electric fan... through the body of the airplane)
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Old 05-10-2007, 04:38 PM   #15
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I've started that... a little, not in terms of improving CD though.... I guess unless one makes some really basic assumptions.... let me think on that.

But I started with the (how much energy to produce a certain amount of thrust) angle. For example:

HW750 EDF

31,100 RPM 5133.3 total watts 14.5 pnds static thrust 200MPH Efflux velocity.

http://www.gworksdesign.com/Pages/HW...motorfan3.HTML

So say 14 lbs of thrust at 5 KWh... I've got 6 Trojan T105's in the garage right now.... "I think" that is about 4.2 KWh of power (need to go through that more formally.

So the question I am wondering: A) How much longer could my engine off glides be, no change to aero, with an extra 14lbs of thrust on demand? B) How much longer with improved aero?

I think the crazy goal would be to build a system that could keep the car going steady, on level ground, no wind, etc, at say 45 mph? But I think that is on the order of a 100 lbs thrust needed, so there is a disconnect I need to close (or not if not possible). Certainly that can be closed with additional thrust but that is not practical... but push the Cd way down....???? Or effectively decrease the cross sectional area as well (a few sq feet of "direct through" ducting let's say).

Again, having fun :-)

Having fun anyway.
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Old 05-10-2007, 11:13 PM   #16
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I improved airflow through the interior of my other car once. All I had to do was remove the weathersealing around the rear hatch. It made an obvious improvement, I could feel air flowing out of the dash vents even with the fan off. I never had to use the fan on it's highest setting. The drawbacks are increased wind noise and the possiblity of water getting into the car. I didn't have to worry about rain during a California summer.

You don't have to cut holes and fabricate ducting. Cars are already full of holes, you just need to open them up or remove some seals. Try looking behind the plastic body panels by the tail lights. On my car there's some rubber flaps that seal the tail lights to the body. These could easily be removed to vent air around the gaps in the rear of the car. There's a low pressure zone behind the car with enough suction to pull extra air through the stock vent inlets.
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Old 05-11-2007, 07:05 AM   #17
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You don't have to cut holes and fabricate ducting. Cars are already full of holes, you just need to open them up or remove some seals. Try looking behind the plastic body panels by the tail lights. On my car there's some rubber flaps that seal the tail lights to the body. These could easily be removed to vent air around the gaps in the rear of the car. There's a low pressure zone behind the car with enough suction to pull extra air through the stock vent inlets.
Be careful with this though. Depending on where the low pressure zones are at the rear, you can actually start sucking exhaust fumes into the vehicle.

On a race car I was building, I did this same thing by doing little more than lightening the lower interior surface of the rear hatch around the seal. Unfortunately it was not easily reversible and while the new venting did improve airflow, it was apparently enough to shift other stock venting into a higher pressure range that actually made them work in reverse and suck air in from a lower area behind the car - right over the exhaust by the smell of it.
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Old 05-11-2007, 08:33 AM   #18
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Yeah, I noticed the exhaust with the hatch back open and the windows cracked. Apparently, at speed, the air pressure out the windows is lower than in the back, so I think the trick is to always vent "from" high pressure zones like the cowling and the front bumper area.
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Old 05-11-2007, 10:55 AM   #19
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Soooo, maybe you want to suck cockpit air out of the vehicle into a low pressure, high velocity area. I am wondering if a place like the top edge of a hatch, where the air is whooshing across, would work... Maybe removing the rubber seal only on that top edge. And hope it doesn't rain...
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Old 05-11-2007, 11:15 AM   #20
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What I am really thinking about is just to use fan power to keep the turbulent perimieter flow a little longer....aka vortex generators. Think of the airflow along the roof and sides.... streamlined at first then starting to get turbulent as it nears the back where the flow separates and causes all sorts of drag. Now imagine caching that airflow and redirecting it down and out into what would normally be the separated lower pressure area in the rear. Ideally this would be a boattail design with a single fan in the middle... where the boattail attaches to the body though is not flush but 1-2 inches separated. This should help contour the flow downward (yes a perfect boattail might do this passively as well, but now add in the flow from the high pressure areas in front of the car too). Maybe the analogy is a straw with a somewhat aerodynamic exterior... some flow simply goes through and some flows gets channeled around.... and some thrust is added as well.
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