View Full Version : Aerodynamics Question...

06-17-2007, 12:43 AM
So I used to own a 1989 Chevy Celebrity Wagon. It was a piece of crap, but an unbelievably tough piece of crap. Anyhow, on the rear window it had an odd "wing" sticking out, but it didn't slope up to create downforce like a regular wing, it sloped down very steeply...

Anyhow, I've seen a few other vehicles with these around and began to wonder what they are for. When I first got the car, (8 years ago), I thought is was for directing air to "blow" snow and rain off the rear window in lieu of a wiper...but now I think it's purpose is to direct air down along the rear window in an effort to fill the area of vacuum that follows the car. To try and reduce the induced drag.

I was bored and drew a couple diagrams real quick:


A standard wagon or SUV leaves a large area of low pressure, turbulent air behind the vehicle, a "wake." This wake is the cause of the drag on a vehicle.


With the weird wing, though, in theory is air is pulled down into the area of vacuum and makes it smaller, thus reducing drag and increasing efficiency. That's my uneducated guess, anyway.

It's similar to the reason that the Evo has those points on the rear of the roof, to draw air down along the rear window.

If I'm wrong, someone in the know please correct me, if I'm right about the reason for their existence, then I wonder why I don't see them on new cars anymore, especially since SUVs are so much more popular now and fuel economy is a bigger concern. Is it aesthetics?

Anyhow, just a little bit of my rambling thoughts...

06-17-2007, 01:26 AM
I'm not completely sure, but it would make sense considering a celebrity could never make a real use of a wing lol

06-17-2007, 01:36 AM

06-17-2007, 02:01 AM
You're correct. :D

06-17-2007, 02:51 AM
I had an `89 Celebrity. That car was a tank. I flipped it on it's side one night, rolled it back over on it's wheels and drove on like nothing happened. The 2.5l engine I had in it was a hoss. That thing would move like no one's business. It surprised more than one person.

06-17-2007, 03:01 AM
Take a baseline MPG reading, take the wing off and see if it goes down. If so, by how much?

06-17-2007, 06:53 AM
My Celebrity was a 3.1, and it was surprisingly quick up to about 50mph...I think I only dared 90mph once... I did jump it, though...

Take a baseline MPG reading, take the wing off and see if it goes down. If so, by how much?

Wish I could, but don't have it anymore.

So, does anyone know why I don't see them anymore? Was it just because they discovered it didn't work that well? Or is it aesthetics? Or is it more sinister, like because the gas companies bought the patent and are refusing to license it?

06-17-2007, 06:58 AM
You're correct. :D

Horay! :wiggle:

06-17-2007, 07:43 AM
It's similar to the reason that the Evo has those points on the rear of the roof, to draw air down along the rear window.

Those fins on the evo are actually to stabilize the direction of air more than anything. They are effective even in a straight line in reducing drag but the big reason for them is to influence air movemnt when cornering so that the air moving past the back of the car hits it as if the car was still moving forward; It straightens out the air moving over the rear wing.

Mitsubishi's engineers claim those fins (they call them vortex generators) increase downforce by 35% at 160 mph while adding almost no drag or weight to the car.

Here is a link to a supplier that makes something like these (they're stick on and have been proven fine for track use) but a little more oriented towards just smoothing general air flow to reduce overall drag. '


And here is a post on a Porsche forum talking about the difference those little things make.


06-17-2007, 08:26 AM
I know how they work, wingtip vorticies are a big deal to pilots so we have to have at least a basic understanding of them, I just didn't feel like going into that much detail. :)

The reason they increase downforce without increasing drag is because vorticies naturally sink (that's why small airplanes should never fly below and behind a heavy), because the vorticies sink the pocket of low pressure air behind the rear window is filled and drag is reduced, just like with the station wagon mentioned earlier. In addition to reducing drag, downforce is increased because the spoiler is no longer sitting in an area of low-pressure, turbulent air.

I've never heard of the them acting as "straighteners" to draw airflow over the wing when cornering, but it's possible.

06-17-2007, 07:14 PM
A really good book on this subject is;

Competition Car Aerodynamics
A Practical Handbook
By: Simon McBeath

He gets a bit heavy into CFD and the math behind it. You can see good examples on how air dams, single element wings, multi element wings, splitters, strakes, canards, etc affect air flow. His examples show some passenger car basics. Main focus on examples of Nascar, Formula 1, Prototypes, ALMS cars. You can get some good ideas from it regardless of whether you happen to have a masters in CFD.

As far as your Chevy goes. The GM wind tunnel is one of the most advanced in the US, if not the most advanced. Over the years they have learned a lot from their racing programs and passed it on down to passenger vehicles. They obviously changed enough of the body aerodynamics to deem that part unnecessary.

The small strakes you see on cars, as well as canards and the like are fine tuning devices. Their use is to tweak airflow based on major devices like wings and splitters. Hence why the strakes on the Evo help a lot once it gets to 160mph.