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Old 05-28-2011, 01:40 PM   #1 (permalink)
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Unusual aero--flapping and pumping

Both these mods were mentioned in an autospeed.au article:

Process and device for reducing the ... - Google Patents

Crossflow fans sucking air from the middle of the vacuum behind the car and blowing it into the the boundary layer as it separates.

Motor vehicle with flow-influencing devices to reduce air resistance - US Patent 6378932 Description

A spoiler continually flapping up and down, creating low-energy vortexes that disrupt the high-energy ones.


Has anybody ever seen either of these at work, either full-sized, on a model, or in a computational fluid dynamics simulation? I don't know of any CFD packages that'll allow moving parts like that. Both mods look kinda cool to me, because my TDI Golf's achilles heel is its squared-off hatchback, and these wouldn't add the bulk of a full boat-tail or kammback.

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Old 06-03-2011, 09:41 AM   #2 (permalink)
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If you are going to do anything in this area I would look at a Kammback style mod, a little like the one in the 307sw thread rather than fitting loads of fans a tubes all over the place. The aerocivic does make use of flows coming out of various places to go elsewhere to reduce overall drag and to provide some ventilation - there are some in the rear panel behind the license plate for example.
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Old 06-03-2011, 10:20 AM   #3 (permalink)
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The issue with fans is they need to be fairly big.
Additionally the power to run them has to come from somewhere.
Kart racing tried this for a while. Really added downforce in their application. Problem was the added weight of a seperate motor to run the suction fan. FWIW, they scrapped the idea.
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Old 06-03-2011, 10:32 AM   #4 (permalink)
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Brabham tried it in F1 too - this was after the era of 6 wheels

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Old 06-03-2011, 02:18 PM   #5 (permalink)
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Google up "Chaparral sucker car". It had a fan or two for downforce as well.

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Old 06-03-2011, 03:18 PM   #6 (permalink)
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Quote:
Originally Posted by some_other_dave View Post
Google up "Chaparral sucker car". It had a fan or two for downforce as well.

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Yep - that one too, that was the one I was looking for earlier
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Old 06-03-2011, 04:38 PM   #7 (permalink)
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The way I understand it, fans for downforce were so effective they were banned from F1 racing--but that doesn't say anything about whether they'd be an overall positive measure for fuel economy. I do have less faith in that patent than in the "flapping spoiler" filed by DaimlerChrysler; you're right that to fill in a turbulent wake at 60-80mph with crossflow fans you'd probably need near double-digit horsepower.

I don't see anything wrong in principle with the flapping spoiler, though; and one must admit that it's easier to parallel park with than a full boattail.
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Old 06-05-2011, 02:57 PM   #8 (permalink)
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The Chapparel was a sight to see, and hear. The two suction fans were powered by a snowmobile motor, and the car's engine was a Chevy bigblock. So you heard the constant whine of the snowmobile motors, and the roar of a BBC rising and falling as it came by.
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Old 06-05-2011, 05:29 PM   #9 (permalink)
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instead of a flapping spoiler, how about attaching a flag across the trailing edge of the roof?
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Old 06-05-2011, 10:36 PM   #10 (permalink)
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A flag would flap with a speed that varies with the vehicle's velocity; but would have the opposite of the affect we want. That's why it's going to take a lot of modeling to get the right size and frequency for the rigid body influencing the wake.

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