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Fender Vents, General aero Questions

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I'd gotten these scoops from John Washington when he was making them in CF and had planned to use them on the fenders, so I finally installed them.  I chose to go lower for the extraction point.

 

922858_645066602176434_276221131_n_zps61

img_7349_zps18641221.jpg

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Those vents are meant to be used in smooth airflow. You'd do better taking them off, as they essentially cover 90% of the hole, which is positioned in a low pressure turbulent flow area, as you can see in the 27th test of the last wind tunnel outing.

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Is your point that you can test airflow with a shop fan in your garage, or that those vents work, or that it would work as well as an open vent, or that the area behind the wheel isn't a turbulent low pressure area of the car? We can prove that it is a turbulent low pressure area by using the photos from the last test. Since we know what the flow looks like there, we can make some assumptions about what kind of vent to put there. FWIW, I wouldn't put a NACA there either. 

 

http://i51.photobucket.com/albums/f380/rontyler/WindTunnel/PICT0288.jpg

http://i51.photobucket.com/albums/f380/rontyler/WindTunnel/PICT0570.jpg

http://i51.photobucket.com/albums/f380/rontyler/WindTunnel/27HoodVentsUntaped.jpg

 

 

This spot needs a big open hole to let the air out. Here is a good example:

 

http://www.google.com/imgres?imgurl=http://upload.wikimedia.org/wikipedia/commons/a/a8/Pikes_Peak_Toyota_Takoma_709762748.jpg&imgrefurl=http://commons.wikimedia.org/wiki/File:Pikes_Peak_Toyota_Takoma_709762748.jpg&h=1200&w=1932&sz=343&tbnid=5wb2fz0ZQklvKM:&tbnh=90&tbnw=145&zoom=1&usg=__G27RsXV_8DRVjXfk6LAgf6JvHVM=&docid=-C1StHtprmh-mM&sa=X&ei=vK6iUb_lG4PUiwLYpIDACA&ved=0CDMQ9QEwAQ&dur=356

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My point was that the vents went where the airflow showed they would work based on the data I had and what I could mimic in my own shop. I will be happy to test these in the wind tunnel and prove your point for you, but they are on the car now and wont be coming off until I have a better solution to cover the now rather LARGE holes in the fenders. My eventual goal is to replace the fenders with fiberglass versions, so I wasnt concerned cutting them up. That said, I placed them where I saw the need in my shop with my crude testing.

My goal was to help out the fenders by removing the trapped air in those sections behind the doors since I dont have fender liners currently.
Mike

Edited by Mikelly

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This setup didn't work. Huge aero understeer at speed - almost drove off the track on the second lap. A splitter needs to be no more then 2" off the ground if you want it to work as a splitter. You can alway angle the splitter down and just have it work as a spoiler.

 

btw410paddock1.jpg

 

(The car looks like its about 6" off the ground but there's a jack under the car behind the door)

 

Coming from a long way back but, as I have now worked out through experience, a splitter should be just part of an aero package which at the front must include an undertray, at least. And if you have an undertray on a S30 then, if it  all is working properly, engine bay vents will be essential for both cooling and aero purposes.

Edited by 260DET

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This setup didn't work. Huge aero understeer at speed - almost drove off the track on the second lap. A splitter needs to be no more then 2" off the ground if you want it to work as a splitter. You can alway angle the splitter down and just have it work as a spoiler. btw410paddock1.jpg(The car looks like its about 6" off the ground but there's a jack under the car behind the door)

John,

Which part didn't work?

Was that frontal plain airdam a problem

or the splitter?

 

The fascia looks just like what the Miata guys do and I was going to try.

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At Bonneville we call that 'the snowplow'! The big opening effectively lets the air the spoiler up front dams up go straight into the hood, and then  pressurize the underhood and lift the car up (adds to understeer.) 

 

It may work better with about a 4" slot and the entire front covered with the 4" slot properly ducted to the radiator...

 

The key is getting air out from under the hood once it's in there. One way is to limit the amount entering, another is to duct the underhood area to low pressure to let that air out.

 

It's in the aero testing about taping up the front end.

 

Actually, http://forums.hybridz.org/topic/112012-looking-for-a-few-good-candidates-and-their-money/page-2

 

Go to post 35 about the comments, it's encapsulated there pretty well, and references the test runs in the original aero data tests so you can refer to it.

Edited by Tony D
  • Like 1

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Over the last several weekend events my hood has become looser and looser at the pins.  We will have to adjust the pins, and probably re-mount the rear pins lower since I'm out of thread to adjust down further.  We will likely have to extend the spoiler lower to try and remove as much of the clearance ot the ground as possible.  I think our hood vents will work properly, but as with any piece that is constantly take off the car, constant attention ot fit is mandatory.

 

Mike

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Thx TD, good links set me on course again.

 

Its funny how little time I spend on Aero for my Aeroplanes versus my car. I guess the planes are closer to begin with.

Controlling air through our air-cooled motors is key. Looking at the TINY 1.75" annular duct around the spinner on a 4000hp Unlimited like Rare Bear with 3,350 cubes and max-performing each molecule even to the point of trying to gain thrust from its newly acquired heat while exiting the cowl.

 

So, answer to my question seems to be: its what happens AFTER the airdam (which is kinda funny in a "Ya, N/S" way).

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I remember helping construct some Honda CVCC Powered Pushers back in the mid 70's that used (get this) 1974 Chevrolet Impala Harrison Air Conditioning Evaporators in the wings as radiators! 

I was in the 7th grade, and the way it was explained was the speed vs the density of the exchanger allows the evaporators to cool the engine just fine at speed. the holes in the wings were DINKY compared to the size of the cores, and of course a duct opened up to let the air through the core. Same as your 'what happens after the air dam' comment. Small opening to control inlet air, and get efficient flow through the exchanger, then blocking the rest as best you can.

 

I always wanted to use smaller, thicker radiator on a Bonneville Car with minimal opening but Class Rules make it prohibitive. And for a street car, unless you are making a Vegas Runner dedicated to live about 80mph all the time.... I doubt it would be practical on a street car. Though we can dream, right?

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Not sure where else to put this and it's under fender aero related so lets see if this thread works. I just lowered the splitter in an attempt to make more downforce. Now am working on the vertical fences that cover the front tire. Here is what I have been using for several years and as you can see it's more of a blended flare shape rather than gurney strip type of device.

 

image_zpsd2d47cac.jpg

 

I'm thinking of moving to more of a true vertical 'trip strip' or gurney flap type of design thinking this might make more downforce by adding a stagnation point just above the splitter. The splitter lip is 4" wide all around which puts it roughly 1.25" wider per side than the tires (15x10). For drag reasons this seems excessive but am hesitant to give up the spitter width. Here is what I have so far:

 

image_zpsa98989f1.jpg

 

Goal is to maximize downforce for hillclimb. Trying to decide how to finish off the new fences. Narrow them by maybe 3/4" including the side of the splitter? Leave the bottom and taper it so that it blends into the flare at the top? Leave it late model style like the picture? By the way, the top where it meets the flare is flat to the world although the pic looks angled. Thought or suggestions would be appreciated to help think through this.

Edited by heavy85

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RaceCar Engineering did a test on a couple different setups on a Honda Civic in 2006. Best result was with no slope or ramp at the bottom. McBeath seemed to want a ramp to work and sounded surprised that the lip worked better than the ramp. Would help on yours to have the inner fenderwell removed or have holes in it to let the pressure out of the engine bay.

 

As to how to finish them off, I'd be inclined to make them fit to the top of the flares, otherwise you'll generate a vortex at the tip that hangs out. That vortex might be helpful, or it might hurt airflow further back... ???

 

 

Originally Posted by 7/2006 RCE

The final configuration showed further that the splitter ramps had a perhaps unexpected detrimental effect on efficiency. Adding small ramps again in front of the full height vertical fences not only reduced downforce at front and rear but also increased drag. Oddly then, what looked like a device that might have made the vertical fences more efficient in fact had the opposite effect.

Edited by JMortensen

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Thanks John thats good data point. Any idea how wide they were relative to the front tires? Have also though a lot about cutting holes in the tub to vent the engine bay but not sure there either. Devil is in the details making it tough to make it most effective.

Edited by heavy85

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I know they completely covered the front of the tire, I think they were a bit wider but I couldn't say how much off the top of my head. It's really the same theory as the gurney in front of a vent. There has to be a point where you're adding more drag than necessary, but you'd probably need to test it to find out what worked best.

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In the January RCE there's some additional data from the Pikes Peak electric Toyota that is helpful.  This shows the rear s-flap better but there's also a front one just as large.  For your base setup I would cover both the front and rear tires so you don't see any tread from the front.  Then for the flap you can have it extend at least one inch.  For Max downforce more may be better and sharks teeth may allow that to work with less drag.  

 

For your splitter you haven't mentioned what is allowed in the rules.  For more downforce you may be able to extend further out.  A lot of the Pikes Peak cars in the open class use 10 to 12 inch extensions.  You can also add end plates to the splitter to help.   But getting back to max front downforce I think you'd want to first make sure the radiator is ducted and then seal the wheel wells.  On top of the fender add very aggressive louvers and cut out the back of the fender.  

 

Search for time attack aero, splitter, fender vents, etc. and you'll get some decent examples.  At this stage then you have enough low pressure to help drive splitter ramps (diffusers).  And if rules allow you can make the splitter a large wing section and run footer plates on the end.  From looking at a number of the time attack cars that are doing heavy CFD footer plates are starting to show up on them.  

 

Hope that helps,

Cary

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Cary - pretty much open aero rules. I already have well sealed up front end with ducted radiator. Large vents in hood and rear of front fenders open. I havent done anything to the rear yet. I just lowered the splitter and added three diffusers into it.

 

What are sharks teeth?

 

What do you mean by seal the wheel wells?

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It looks like you have the ducting covered.  Forget the comments on the sharks teeth.  Serrated gurneys were darlings in the wind tunnel but never panned out, although everyone had them on there cars at one point.  In the tunnel they reduced drag by half at the same downforce level.

 

For the splitter take a look at Loeb's Pug.  Rather than a curved splitter blade you can run if straight across the front to gain more area for the pressure to work on.  Since you don't have the SCCA 6 inch rule you can make it as long as you want.  I'd guess 10 to 12 inches is about the max that would work.  On the sides you can add fences, also like the pug.  And then on the fenders you have add s-flaps.  For the wheel wells my comments were to make sure there's no opening into the engine compartment above the splitter.  On the top of the fender you can add louvers and all this should make the diffuser ramps much more effective.  The ones in your image can get a lot larger.  Take a look at some of the P2 DSR front ramps for ideas.  Max angle is around 14 degrees.

 

A more difficult to build splitter would be a large wing section for the front.  On the ends use a foot plate so that it won't completely stall when it hits the ground.  The Reynard inverter had a small version of this.  A few older legrand DSRs have used this and needed to double front spring rate to keep it from dragging.  The downside is a lot more work than a flat panel with some ramps.  Perhaps an upgrade for the future.

 

Hope this helps,

Cary

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Here's a shout out to heavy85:  Cameron, can you check your PMs?  I sent you a question on your seats.  Call me at 256-366-4685 or email me at [email protected] if your PM system isn't working.  I'm having trouble with the Intermediate RR seats and am hoping you can render some advice.  Thanks.  I figured I could reach you via this thread since I think you've been lurking on this topic.

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Added flaps in front of rear tire and left the front as pictured above. Couldnt find reasonably priced super aggressive vents for the top of the fenders so am just going with what I could find - standard louvers. From looking at professional racecar pics looks like starting about centerline of the tire and going back seems to be the norm. Before I start cutting the fender anyone have other tech on the subject?

 

image_zps969c4769.jpg

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