Aerodynamics discussion

[mavisky]

Now I know this is an area that not alot of dsm'ers venture into, but with road racing taking a bigger and bigger hold on this site I think it's an area worth discussing. Now I don't claim to be an expert by any means. I spent roughly a semester 6 years ago working in a wind tunnel at Tri-State University doing some modeling of the effects of drag and frontal design on a new design of aerodynamic "bullet-train" shaped semicabs and that's about it. I understand the basics of fluid dynamics, but all the equations and laws have since escaped me.

Coming into this racing season I decided its time to start getting serious about my aerodynamic situation. I entered the season with 5 goals.

1. Better cooling for the engine. The intercooler is working fine at my power levels, but the oil and coolant need some help. To that end I've already installed a large aftermarket oil cooler from www.diamondstarmotorsports.com in the driverand it should be more than enough for what I'm doing. The radiator however is still a work in progress as it needs replaced itself (old stock radiator in rough shape) and I'm still not sure I've got enough airflow getting to it. Here are some pics of the radiator and the area it's sitting within.

Fmic in front of radiator

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Oil cooler inside bumper (has since been re-located even further into the airstream

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New small radiator ducts (spots around "eagle badge" are now cut out for airflow as well)

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I'm hoping that's enough airflow to keep everything cool, but if not I may be adding some extra ducting to the sides of the core support and top of the hood latch area. Of course adding all that airflow into the front bumper causes an increase in air under the hood. This has to exit out the bottom of the car which then causes both extra drag and adds front end lift which contradict goal #2. To help remedy this I'll be installing a carbontrix 18x10" hood vent directly above the turbo to help let the air from the radiator back out of the engine compartment.

2. Increase front end downforce.

This one is rather simple. A new fiberglass front splitter is on its way. I'm shooting for about 3" total projection from the front of the bumper. This should equate to a fairly heft amount of front end down force hopefully. Based on some simple and basic testing with the foilsim software (which admittedly isn't the best for this type of work, but it's the only free thing out there) the splitter alone at a 1 degree rake at 100mph should create approximately 11lbs of downforce before even considering its effect with the bumper there as well to trap air above the splitter. I've got the upper strut mounting points fabricated for the splitter rods to attach to, all I need now is to finish the lower mounts (hopefully wednesday) and order up the splitter itself. The upper support alone is strong enough that I can give it a hefty tug and make the car rock back and forth.

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3. Decrease drag.

Drag slows you down of course, but slow moving air underneath the vehicle also creates lift. Increasing the speed of the airflow underneath the vehicle will actually result in a loss of lift. Now I haven't gotten too drastic on this as I know some sanctioning bodies limit just what sort of stuff you have done so until I figure out the exact rulings on that area I've kept it rather simple with a line of 2 1/8" holes drilled into the rear bumper to keep it from turning into a parachute for me at speed.

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4. Better brake cooling.

see my other thread - http://www.dsmtuners.com/forums/showthread.php?t=224768

5. Rear downforce.

Now they always say you should approach downforce in equal doses both front and rear so you don't drastically upset the overall balance of the car. Now some people such as Greg Collier and Leon Reitman have went with the large rear spoilers to achieve their downforce. I've considered it, but by the time I remove the rear spoiler, fill the holes on the fender and rear hatch and paint them I'll be out a good chunk of change that could be spent on something else for the vehicle. So I've been considering a slightly different approach. I've been contemplating a 4" high protrusion of plexiglass off of the rear of the hatch at about the same angle of attack as the bottom of the spoiler. Mounted with a brace and 3 spoiler struts to the sheetmetal ahead of the rear spoiler it may work. I've still got alot of thinking and figuring to do about this, but what are your thoughts about a situation like this. Here's a crappy rendering of what I'm thinking about with the new section in green.

 

[terefic181]

5. Rear downforce.

I think that's a great plan Kyle!! , but maybe it's just me.

about 1/2 way down on the page
http://www.dsmtuners.com/forums/showthread.php?t=203956

...and my picture was way worse than yours.

I like the holes in the rear valance, I think that realy helps. There was a guy in a 240Z that said he got 500RPM's more on the straights with a couple of holes in his rear deck area.


Tom

 

[mavisky]

haha, wonder how I missed that.

I'll try to work up some better pictures on that rear wing setup. My biggest thing though is still figuring out how to attach the lower half the best. My thoughts are a C-channel along the bottom about an inch or so on the edge neares the spoiler and may 2 inches along the back of the spoiler to keep it from being a shearing point. I've also been considering using 1/4" - 1/2" plexiglass for its major construction in order to not hinder visibility too much.

These guys sell 1'x4' sections and luckily the rear hatch opening is just a hair over 44".

http://www.estreetplastics.com/category_s/32.htm

Of course I'm not too experienced in lexan (did some work with acrylic back in 7th & 8th grade) so I'm not familiar with it's inherent flexibility/strength given the varying thicknesses. The 1/4" sections are fairly cheap too at around $27 but the 1/2" jumps to $61.

I figured about 3 evenly spaced out wing struts (same ones I'll be using for the front splitter) should hole the top half fairly stable. These guys have a good selection.

http://www.pitstopusa.com/SearchResult.aspx?CategoryID=1288

I'm also wanting to talk to a local guy who just finished his bad arse mustang and uses a lexan rear spoiler to get an idea of how thick and stiff his is as he obviously uses many different supports than I would be. Also hoping to talk to him concering angle of attack measurements. He'll be at the autox on Sunday so I'll be able to ask him then.

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[terefic181]

Now, that's some definate down force on the rear of the Mustang!! I'm most interested in what you come up with regarding the specifics of the adjustable spoiler extension. ( number of struts needed etc.)

 

[mavisky]

Yea it's a brute of a machine. Those sticky hoosiers are 26"Dx16"x14"W

There's also a C4 corvette that runs a plexi spoiler with 3 large struts at the base of his hatch, and last weekend I passed another C4 corvette with a similar wing heading to another autox. Must be working for them. No reason it can't work for us.

 

[Greg Collier]

Man that thing is huge If you put something like that on the back of your car and there's a good wind blowing... you may not move

I've been through three wings to date. The first one was aluminum and way to heavy but I was a little hesitant going speeds over 140 mph on the big tracks. I was also the brunt of a lot of jokes from the RRE boys

I'm pretty satisfied with the one I'm running now and I'm using very little adjustable trim. I'm not so concerned at the beginning of a race when I'm running a full load of fuel, but it's the last five or so laps when my butt gets a little squirrelly running turn two at Fontana or turn nine at Willow Springs. Power drifting over 150 mph you really need to make sure your ass is following you...

As far as my Intercooler placement I think the "V" mount concept is hopefully going to do the trick. It was so cold and wet at Fontana that it wasn't a true test for engine temps... we'll see. I really like the configuration of the PTE I/C I'm using, it was also a pretty easy piping job.

As far as the rear end sucking up all the air like a sail, I've been planning a lightweight aluminum pan under the fuel cell to divert the air from flowing up into the rear bumper. Also having the front splitter so low to the ground helps to keep a lot of air from traveling under the car (just more points to keep me in SU of course):rolleyes

 

[mavisky]

Agreed Greg, that wing is strictly for Autox and will never see speeds of more than 70mph. Hard to believe some people build such monster cars for 50 second blasts around orange cones, but so long as they're having fun, who am I to point that out. My wing was going to have a roughly 3-4" rise above the rear decklid and at a much less agressive angle.

I'd thought about the rear undertray, but just didn't have the time or money to invest in fabricating one up at the time given my other projects. (hell I'm still not sure where the brake money is going to come from) Hopefully this solution will work for now.

Which wing exactly are you running right now if you don't mind sharing?

 

[Greg Collier]

APR Performance came through with a beautiful carbon fiber unit. It's truly a sophisticated design and weighs next to nothing with infinite adjustments. It's bad ass stealthy looking...

 

[terefic181]

Kyle,

The last time I looked these were the point assessments for NASA's PT series, for aero.

AERODYNAMICS:
1) Add, replace, or modify front splitter or front fascia for aerodynamic or ground
effect purposes +2
2) Add rear wing +2
3) Replace or modify OEM rear wing +1
4) Add or modify canards +2
5) Add or fabricate flat bottom/belly tray +5
6) Add rear diffuser +2
7) Replace or modify OEM rear diffuser +1
8) Add rear vertical panels +2
9) Add side skirts +2

undertray: I've looked at other NASA classes before and, I've seen rules for front splitters that do include the pan as far back as the front most portion of the front wheel well. We probably need clarification for this series, though. If you go with anything more than that it looks like a big "5" points assessed, for sure.

 

[DGajre777]

If you are doing all that, why not add some vortex generators on the roof? A big rear spoiler will be more helpful if air is guided towards it.

 

[mavisky]

See according to that my plans only give me three points and since I'm not using a custom eprom chip i saved like 6 points alone with that. Still in PTB, but I've got room for more suspension or tire mods now.

 

[ldstang50]

If you are doing all that, why not add some vortex generators on the roof? A big rear spoiler will be more helpful if air is guided towards it.

The reason the vortex generators work ok with the Evo is because of the steep rake in the rear window with a short decklid. Because the the DSM decklid is less steep and longer I don't think vortex generators would do much since they aren't supplyin the rear section with more air, its just makin the air more stable coming off the roof of the car.

 

[DGajre777]

The reason the vortex generators work ok with the Evo is because of the steep rake in the rear window with a short decklid. Because the the DSM decklid is less steep and longer I don't think vortex generators would do much since they aren't supplyin the rear section with more air, its just makin the air more stable coming off the roof of the car.

I know they don't provide more air, but the vortices caused by VGs increase the flow of air and give it more energy to maintain its path over the rear wing.

I did read on APRs product manual that a rear wing on a car is supposed to sit 6-8" (maybe 10") away from the body of the car to give proper downforce.

 

[drivemusicnow]

Mavisky... Are you going to be running a trans cooler?

I'm going to put together something as that seems to be one of the big problems people have. I've heard 4th gear tends to shear when it gets hot. The problem is the cheapest lube pump I can find is the Tilton and thats 200$. Plus lines, cooler, thermostat etc. Gets expensive quick. I'm just going to use the drain and fill plugs in the trans, however I might try to "aim" the fill plug towards 3rd and 4th gear. (I have some interesting fittings at work for the transmissions we make.. I'm sure I can find something that will work.


As far as aerodynamics goes, I had a question... To me, it sounds like this would work (it goes more towards that belly tray/under tray sort of concept...

If you were to run the front splitter, and lets say at 3 inches off the ground. Then, straight back from that until you're directly under the radiator, it is a flat piece of (insert material here). Then you would angle it up a bit, say 30 degrees from horiz. until you reached the downipe. I think this would help airflow through the radiator, as well as downforce.

You could do the same kind of thing around the gas tank in the back, have it be "flat", and then angle upwards as it gets toward the back... I don't think I'll do the back, but at least under the radiator as that will force any air to go through the rad.


I have been looking for some time of thermopastic (something that you can heat up, mold, and then set) that is LIGHT! but yet decently strong to force air through the rad/brake ducts/oil coolers. I could do this out of carbon fibre, but it is expensive, and hard to work with. Fiberglass isn't as expensive, however its still decently hard to work with and isn't all that strong. Anyone else have any ideas?

 

[alex99gst]

I read a pretty lengthy thread over at evotuners where someone was talking about the vortex generators and he had a degree or something with aerodynamic design or something of the nature. But anyway, I'll try to find the thread and you could talk to the guy and see what he knows that could help.

EDIT: Found it. I know that VG's might not make much of a difference on a 1g because of the body design, but nonetheless, this is good information.

http://www.evotuners.net/forums/showthread.php?t=730

 

[Greg Collier]

alex99gst I read a pretty lengthy thread over at evotuners where someone was talking about the vortex generators and he had a degree or something with aerodynamic design or something of the nature. But anyway, I'll try to find the thread and you could talk to the guy and see what he knows that could help.
EDIT: Found it. I know that VG's might not make much of a difference on a 1g because of the body design, but nonetheless, this is good information.
http://www.evotuners.net/forums/showthread.php?t=730

Wow... that's a lot of information to assimilate, but it's definitely a different areodynamic with the steep incline of the EVO rear window.

drivemusicnow I have been looking for some time of thermopastic (something that you can heat up, mold, and then set) that is LIGHT! but yet decently strong to force air through the rad/brake ducts/oil coolers. I could do this out of carbon fibre, but it is expensive, and hard to work with. Fiberglass isn't as expensive, however its still decently hard to work with and isn't all that strong. Anyone else have any ideas?

www.Extremedimensions.com sells a large fiberglass sheet splitter that may work for this application.