Rear wings?

jpaturzo wrote:

924guy wrote: car floor pans are essentially a large lift surface, the wing at the rear counteracts that effect, changing it from a positive lift to a negative lift, or vacuum (in effect anyhow.)

Which is why most rear wings aren't that useful until you add a rear diffuser. Even though the Enzo doesn't have a honkin' big wing on the back, the under body design highlights some good design features:

A few things...

One, wings can work at speeds well under 100 mph, otherwise Piper Cubs couldn't fly. ;) The slower the speed you need it to operate at, the larger it has to be in relation to the mass it has to work on (which might explain why ricer wings are soooo huge... hahahahaha)

Second, underbody ground effects work best IN ground effect mode. Get that car to go over a bump mid corner and separate it from the boundary layer and ALL that downforce goes away at a really bad time. A wing still works when the car isn't anywhere near the ground. that's why even Ferraris with the underbody aero sprout wings as soon as the racing rules allow it.

Lastly, a rear wing is only for accellerative traction in drag race cars. In every other case, it's about stability and cornering force, balancing out the usually low nose/air dam of most sports/GT cars.

On my '70 Mustang, I had the factory adjustible wing. Just for fun, I adjusted it all the way up in the rear once. At 60 mph, the rear fenderwells started rubbing on the rear tires, so even that small one apparently had an aero effect at lower speeds...

Aero can be very effective at lower (autocross) speeds.

Do some digging on Joe Cheng's Vancouver Special.

Also, in Bob Bolles book on stock car setup he mentions that creating 1/2psi pressure differential on the hood of a NASCAR stock car creates nearly 300lbs of downforce.

I didn't do the math to verify his claims but it doesn't seem unreasonable.

Shawn

1/2 psi spread over 600 square inches is 300 lbs. 600 square inches is, say, 5' by 8'. Pretty big area for a hood, but Nascars is big. Doesn't take much pressure if you have a big area.

As for the wing on the rear window, we were talking about the BMW Batmobile after the Mitty. I'm guessing this little guy on the rear window is to keep the airflow laminar over the window and keep things clean for the big wing. We put some holes in the rear window of our race Miata to do the same thing.

You can use ground effects on wings, too. Put the wing 60% of the chord width above the trunk lid and you get a big spike in efficiency without any increase in drag. We tried it, it works.

Note low wing position and holey rear window.

High wing.

'Keith is a ricer, Keith is a ricer....'

There was a good article online somewhere about the windshield things like on the sti and evo. it basically helps the ariflow "stick" to the rear windshield on the down slope. this helps smooth out the airflow higher up for the tall wing.

bunch of cool articles here

pinchvalve wrote: The wing on my Rondo keeps things stable when heading to soccer practice at 150mph.

Not trying to be obvious, but wings and spoilers like the one shown here are for fuel economy more than anything else. The F Edge has a similar one. Pretty cheap for a pretty significant improvement in drag.

E-

An instructor of mine was paddocked with a guy driving an S2000 with a huge sheet metal wing. A wing tall enough that it cleared the roof of the car. I asked him "Is that really necessary?" His reply was "It's good enough for about 3 or 4 10's here at Summit Point." He then related a story where he drove the same car in an effort to beat an informal record at Summit. To get the last bit of speed out of it he pulled the wing off as a last resort. He said it was all he could do to keep it from spinning while making the final right onto the front straight. The car was slower around the course without the wing.

My 16V scirocco had a black rubber wing over the hatch. I could feel the car hunker down over 75 mph.

Keith not to nitpick but...

The "little guy" on the rear window is to keep flow attached. It has little to do with ensuring laminar flow. Actually laminar flow hurts you when your worrying about weather or not the flow will stay attached to a surface. Turbulent flow does a better job of "sticking" to a surface.

I am curious, when you mention putting the wing 60% of the cord length above the trunk lid you get better efficiency --> by efficiency you mean the ratio between down force and drag right?

Yes, the downforce spikes up while the drag remains unchanged. I've seen graphs for it somewhere. I was working with an airplane designer (he did the work on the Nemesis race plane) and he's the one who told me about this. I never got a chance to do blind timed testing on it but at Willow Springs it was a faster setup than the tall wing. I would strongly recommend that anyone who wants to investigate this area double-check the numbers, it's been a while.

So you were saying the shorter wing gave better drag/downforce ratio? Just making sure I don't have this backwards.

Chris_V wrote: One, wings can work at speeds well under 100 mph, otherwise Piper Cubs couldn't fly. ;) The slower the speed you need it to operate at, the larger it has to be in relation to the mass it has to work on (which might explain why ricer wings are soooo huge... hahahahaha)

Trans_maro: Aero can be very effective at lower (autocross) speeds.

I agree with everything said here and all true. See A-Mod:

Moral of all the stories presented here: There are two approaches to aero, at least that I know of: The trial and error method. And the massive computational studies using CFD, wind tunnel testing, and more trial and error method. Both can be very effective, it just depends on your tenacity and budget.

carguy123 wrote: So you were saying the shorter wing gave better drag/downforce ratio? Just making sure I don't have this backwards.

Yes. By placing the wing close to the deck, we were able to take advantage of ground effect forces and improve our drag/downforce ratio.