Wings and Down force

[Vermont]

There are always a lot of questions in regards to the STI wing and if it produces any down force, and if so how does it produce that down force. I figured I would start this up to answer some questions and post up this nice article about it: http://www.mitsubishi-motors.com/corporate/about_us/technology/review/e/pdf/2008/20e_08.pdf

Yes it talks about the Evo X but the idea is the same for any wing.

 

[IGOTASTi]

Good one Vermont. :tup:

(My question is) I wonder how much of a difference the wing does make since the 07 limited STi didn't have a wing. Subaru I'm sure tested it out.

 

[Grinder34]

Pretty sure there's an accompanying aero analysis of the STI somewhere.

 

[Grinder34]

 

[Grinder34]

Good one Vermont. :tup:

(My question is) I wonder how much of a difference the wing does make since the 07 limited STi didn't have a wing. Subaru I'm sure tested it out.

I bet the lack of wing does adversely affect performance, but it was a calculated trade-off. It probably doesnt make the car unstable or unsafe, just not as fast.

 

[IGOTASTi]

I bet the lack of wing does adversely affect performance, but it was a calculated trade-off. It probably doesnt make the car unstable or unsafe, just not as fast.

I wonder how much slower? 1 or 2 mph?

 

[Grinder34]

http://www.mitsubishi-motors.com/corporate/about_us/technology/review/e/pdf/2004/16E_03.pdf

 

[Grinder34]

I wonder how much slower? 1 or 2 mph?

Well, less drag but less downforce. So faster in a straight line, but not able to carry as much speed through a corner. You cant really quantify it except on a particular corner of a particular track.

 

[IGOTASTi]

Well, less drag but less downforce. So faster in a straight line, but not able to carry as much speed through a corner. You cant really quantify it except on a particular corner of a particular track.

Good point.

 

[Batmobile_Engage]

I'm running the very small '07 STi Limited spoiler on my '04. I'm planning in the future to install an underbody rear diffuser,
but after reading the Mitsubishi write up, I think I'm getting an '06/'07 roof vane vortex generator to even out the air flow
over the trunk lid. I got the rear end loose last night on an interstate on-ramp at about 95mph. It was easily controllable
but it still made my nuts shiver. lol

This may be partially do to my current suspension setup, which I don't plan to change much because I'm super happy with
it right now as far as handling goes. I have a large front air splitter that 'feels' like it sucks the front end down a hair over
60-70mph, that MAY unbalance the aero of the car with no rear wing (its in my closet because I like rolling stealth). The splitter
is about 2 feet deep and bolts to my rally armor engine skid plate. I think that helps even out airflow underneath the car.
So hopefully, with a roof vortex generator and a rear underbody diffuser, that may mitigate the lack of wing.

The way I have my trunk done up right now is with a carbon-fiber-look adhesive wrap that I cut out to match the spot where
the wing was as well as seal the holes from the weather. I COULD put the wing back on very quickly for track use, but I'd have
to replace the wrap on the trunk each time.

So I'm considering getting a track wing with 4-6 bolts that I can drill holes into the trunk and plug them until track time.

What do you guys think...? Especially if you read the Mitsubishi data.

 

[Batmobile_Engage]

Last question for you guys. It's a little harder to acquire and install an '06/'07 STi roof vane, than the one I find online--on sale for $309.
It's the type with the small triangles or vortices like on the EVO MR. That's one thing I'm not super crazy about, but without a wing, on
my Batmobile, that might look sweet.

http://www.fastwrx.com/cafivoge.html

So do you think it's as effective as the STi roof vane?

 

[HolyCrapItsFast]

I'm not sure about that. The STi roof vain is meant to direct the air down and into the rear wing.

 

[Grinder34]

the vortex generators and roof vane arent going to give you downforce by themselves:

Try the diffuser and go from there.

 

[Batmobile_Engage]

the vortex generators and roof vane arent going to give you downforce by themselves:

Try the diffuser and go from there.

Of course they won't produce downforce alone. If you really picked apart all the Mitsubishi data sheets, you might
see it like I do... You're not producing downforce, you're using the roof vane or vortex generator to reduce turbulence
and low pressure pockets over the trunk lid due to air flow separation at the edge of the roof.

I'm trying to figure out a tangible way to describe what I seem to feel at high speeds. It's not that I need downforce,
on the rear end, the rear end feels floaty, like the low pressure pocket is 'lifting' the rear end a bit. That is what I want
to eliminate.

I've also heard quite a bit about how diffusers work and how they really don't do much unless they are installed on a car
that already has a spoiler. I don't think that's totally true. I also believe that I should be able to adjust the air flow around
the car if I add the right aero parts while NOT having the wing installed.

Does this make sense to everyone? ....or just to me? lol

 

[Batmobile_Engage]

I also believe that I should be able to adjust the air flow around the car if I add the right aero parts while NOT having the wing installed.

Does this make sense to everyone? ....or just to me? lol

For example:

Cars built specifically for racing are generally engineered with gratuitous amounts of skid plates and flat, sheet metal underbody panels.
Sure it helps protect components from damage, but it also smooths the underside surface of the car. When a high speed fluid (air) is
traveling between the road surface and the flat underbody panels, a low pressure zone is created which literally has the effect of sucking
the car towards the pavement, because the pavement sure as hell isn't getting sucked upwards.

Any sailors out there? Remember what happens when two ships 250' apart, traveling at 8 knots, connected by steel cables and fuel hoses
during a replenishment at sea, increase their speeds and/or close distance under 200'? They get sucked in and crash into each other, big
time! I try to brush up on Bernoulli's Principle periodically before considering aero modifications. I wish I had a wind tunnel at home

 

[Grinder34]

Yeah, exactly.

The "float" you're describing is, well, lift. You need downforce to basically add a (-) to the current (+) of lift to balance it out. The roof vane and vortex generators will do virtually nothing without a wing in place.

Flat underbodys and a rear diffuser will produce real results.

You can also balance out the car by removing front downforce...(ie remove any v-limited lip)

 

[finallymysti]

over the past month I have gotten very into race applicable aerodynamics. with out a full flat bottom car you will have virtually no effect from adding a diffuser. also a diffuser in itself doesn't add downforce. it acts to slowly allow the air to expand thusly letting it not separate and stall/cause turbulence that will affect the airflow under the front and midsection of the car which is making the downforce. a diffuser doesn't need a rear wing to work, however a rear wing will create more downforce thru the underbody because you can shorten the length of the diffuser and increase the angle with a rear wing because you have a low pressure that is pulling the air up and keeping it attached to the diffuser. a plane flat pice with a diffuser gets separation at about 10 degrees where as f1 cars run diffuser to I belive 35 degrees because the low pressure created by the rear wing keeps the air attached.

 

[Grinder34]

I agree with most things you said. Diffusers are more of a downforce enhancer than downforce generator. Meaning on a flat-undercarriage (esp with deep side-skirts) a diffuser can add significant downforce, while on a turbulent underbody the downforce will be significantly less but not 0.

trumps the stock, but the stock isn't horrible as far as production cars go:

Look at this one for example:

 

[finallymysti]

ah yes, I forgot to mention sideskirts. because you have a higher pressuer area outside the undercarriage and lower under the car the air wants to leak in. this reduces the downforce. a low sideskirt will let less air in. also if you can make a vortex as the air leaks in this will help reduce the negative effects because the air will have been energized and keep the speed of the flow under the body and not be as turbulent thus keeping more flow attached

 

[Grinder34]

ah yes, I forgot to mention sideskirts. because you have a higher pressuer area outside the undercarriage and lower under the car the air wants to leak in. this reduces the downforce. a low sideskirt will let less air in. also if you can make a vortex as the air leaks in this will help reduce the negative effects because the air will have been energized and keep the speed of the flow under the body and not be as turbulent thus keeping more flow attached

Well the first part is pretty spot on. The main thing is, with a good (low) front splitter you have let only a little air under the car. Then the profile opens up so you have low pressure (downforce). The higher the sideskirts, the more air allowed into that low pressure area, raising the pressure, and decreasing downforce. Its true, the air leaking in will be less laminar but the big thing is the amount of air under there.

As a fun piece of trivia, some older racing cars (i think it was in the 70s) had sideskirts that were designed to actually touch the ground at all times. They were designed to move up/down in a slot so as one side/end raised/lifted (due to cornering/braking) there would still be an almost complete seal. The bottoms were made of ceramic to last the duration of the race.

Anywho, underbody aero can be EXTREMELY effective at both increasing downforce AND decreasing drag. Thats why most high end racing bodies have very significant restrictions on what can be done on the undercarriage.

Here's some good reading (ok, i didnt read it, i just paged down and it *looked* good)!
http://strangeholiday.com/oops/stuff/annurev.fluid.38.050304.092016.pdf

This is a book i've actually read and its a pretty good read, and a rather accessable read:
http://www.amazon.com/Competition-Car-Aerodynamics-Practical-Handbook/dp/0857330071