911 rake front/rear

[masraum]

The only question that I have is about the tail. According to the longer blue lines at the tail there is more lift there (lower pressure. And you are saying that the turbulence behind the tail is what is actually reducing the lift?

[350HP930]

The actions of the tail are a sort of compromise.

The tail by deflecting the air stream is going to increase the localized lift right at the area of the tail, but in return it reduces the drag behind the car and also makes the rear of the car more stable by throwing stable vortices into the turbulence so that you do not have a lot of buffeting from random vortices shedding off the air flow at the rear of the car.

At the back of the car it is by then not so much of a lift problem as it also a drag problem since the pressure forces are acting on the rear of the car.

As the diagram shows most of your lift occurs over the window and roof area of the car.

The tail is just doing clean up duties and will not add much more lift than a regular lid would.

[69911e]

Mad: "The tail is just doing clean up duties and will not add much more lift than a regular lid would."

I am pretty sure the Turbo tail pictured reduces (not increases) lift when compared to a regular decklid; If my memory is correct by up to 50 lbs at 100MPH. I will look for the graph, can't remenber where it is now...

[Bill Verburg]

If you want to see what the factory engineers found by experimentation read "Up~Fixen" vol V, pg 309 -316 by Prof. Robert White. The substance of the artical was aerodynamics of Porsches in general and the 911/930 specifically. The article was based on his work and findings when he worked at the factory. The lift graphs which I previously posted were copies of factory work. The 'spoilers' as used do not increase drag, they do decrease lift.

The other link was from a magazine that actually took the time to investigate the aero qualities of 993tt, Did any one actually bother to read it repost of aero article
A small sample


In addition I posted a lot of work on the wings used on 993RS clubsport and in particular the effect on lift and drag of the D angle of incidence of the wing

Repost
Here's some interesting facts for those of you running the stock biplane wing w/ 4 adjustable positions

1)angle = 0° C_d = .34 Rear lift = -119N

2)angle = 3° C_d = .34 Rear lift = -297N

3)angle = 6° C_d = .35 Rear lift = -535N

4)angle = 9° C_d = .36 Rear lift = -713N

All data @ 270 kph

Those things actually produce downforce! and w/o increasing drag too much

Note that the 993Rs clubsport uses a true wing
link to the original discussion

Here's an interesting link to a page totally devoted to

aero

[masraum]

k, yes, I am still less than totally clear.

930, are you saying that the upturned edge of the wing won't cause any force in a downward direction on the rear of the car (not necessarily net downforce, but a vector downward fighting the upward lift that is caused by the shape of the car)

[350HP930]

As I show in my diagram there is both an increase and decrease in pressure created by the tail.

The tail grill will also act like a duct which throws a big question mark into exactly what the pressures and net forces will be on the deck of the tail.

It could work out that the increase in pressure would outweight the decrease and result in a net increase in force on the top surface of the tail would result.

Its also possible that the change in post tail aerodynamics is effecting pressures under the tail of the car which would also alter forces on the rear of the car.

I have never put a model of a 930 in a wind tunnel so I can't answer exactly what is the net force at a particular speed, but I hope that everyone is beginning to see what kinds of forces and problems are at work here.

What I have stated about the tail on a 911/930 are generalisms about air flows over automobiles and the purpose of functional spoilers on cars.

No one knows the answers to these questions without the use of computers and wind tunnels so we can't beat our selves up on any of these questions since thats what millions of dollars of R&D money is for.

[Eric Coffey]

Here is a pretty good site/page for a "quick n' dirty" (for us lay-people) explanation on (car) aero effects.

[Eric Coffey]

And here is a pretty cool (.pdf) Christophorus article (with pictures) on the Porsche Weissach wind tunnel.

[Randy Webb]

Yes, Bill, someone (at least me) took the time to read the article & thanks for posting it. But, I thik Rocket is correct that no one is going to be able to answer or understand this very well. I'm on pretty safe ground in that agreement, since I did a fair amount of fluid dynamics for my PhD using heat heat transfer _and_ I don't understnd the air flow around a 911. It really does take a combination of computational fluid dynamics and wind tunnel tests -- lots of them. And big wind tunnels are very expensive. I've built some small ones, but to build a big one is something only a few car companies are rich enough to do. PAG probably wouldn't have one w/o their consulting div. I expect that most who might look at the article could not understand it, esp. re the limitations on such studies (i.e. what they seem to tell you, but really don't). This thread reveals how many folks have opinions w/o much knowledge, IMNSHO. There are some general "notions" re aero that one _can_ assimilate (esp. if you take the right ME course(s)) but it's very hard to apply them to a specific real world situation. Hence, the maxim: follow the facotry unless you really do know better. I wish I had a simple reference book to post but I don't. The closest would be Gaylon Campbell's Into. to Env'l Biophysics, followed by Monteith's Principles of Env'l Biophysics, if anyone wants an education.

[Eric Coffey]

from the horse's mouth.

[Eric Coffey]

from the horse's mouth II.

[MotoSook]

Bill V, I tried reading that article at work, but man is work distracting! I finally read it, and it doesn't go into as much detail as to explain all that affects the 993 aerodynamics.

So! Now after reading more of the articles, coupled with what I know from my classses and wind tunnel sessions, I understand better; however it all still does not fit into any one model or theory. It's an application of various theories!

The most important facts are:

1) Reducing flow under the front of the car reduces lift. But that alone is not good as has been stated above.

2) Flow management, i.e. reduction of separation and wave is good for Cd and reducing lift. Spoilers reduce lift (on top and on the bottom,i.e. diffusers help under the bumper!), and wings generate downforce ....with varying affects for both.

3) smooth underbelly and a tunnel effect under the car are good, very good, but not practical for a road car. Porsche has managed it on the later cars. And others have too!

Back to Tyson's questions, increasing rake WILL reduce lift in the front, but if you do that alone you are asking for trouble. So we run with a rear spoiler to decrease rear lift at highway speeds. And increasing rake will not create ground effect on a stock, pre-993 911. Porsche added front and rear spoilers on the 73.5+ cars as a package.

[Bill Verburg]

Souk, Exactly! you do not need to have a full grasp of the details if you understand the effects of empirical testing.

The more empirical data you look at the more common threads become apparant as you have summarized.

Rake decreases lift primarily because it reduces air flow under the car.

More aero links


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[350HP930]

Hey Bill,

Some of those papers you posted were quite interesting, especially the iterative race car optimization, drag racer aerodynamics and tractor trailor aerodynamics, but why the link to the gurney flap aerodynamics?

It doesn't seem to apply much to what we are talking about here.

[masraum]

I think he was just passing along more aero info, not necessarily related. The Gurney flap has come up here before.

[MotoSook]

I feel like I'm at a flow symposium! Navier-Stokes! NO!N!NO! I promised myself to never read anything with that reference in it after I left college! Damn you Bill!

Great stuff!

[Randy Webb]

"you do not need to have a full grasp of the details if you understand the effects of empirical testing."

Ow! Ow!! I'll have to tell my grad. students -- they will be elated.

[BogieMan]

Alot of great information here. I particularly like the tuft photo series on the 993. No doubt empirical testing is the final word when you do it.

I put a couple CFD (computer aero sim) cases together with a sketched 911 profile. They should finish overnight sometime. There is a basic 911 with a flat underbody and then the same car with the front lowered down. I'll upload some flow viz when the cases are ready. These are 2D Nav-Stokes sims with a ground plane, which is as good as 2D gets. Unfortunately 2D solutions for problems like this are somewhat limited, but my home computer can't solve a real 3D flowfield unless I tie it up for a week or so. The 2D stuff will help explain venturi, tilt angle and ground plane effects, missing spillage and vorticies. We'll know more tomorrow.

-BogieMan

[old_skul]

Man, I knew no aerodynamics/rake conversation could hit 4 pages. I knew Island would start a pissing contest.

[island911]

Start!?

"Enable" maybe.

LOL, oldskul