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downforce with Carrera Tail

any graphs out there with/without tail?

Last edited by on-ramp; 07-05-2004 at 06:20 PM..
Old 07-05-2004, 07:22 AM
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I would imagine the graphs that exist won't take a cab into consideration. Air runs differently over a ragtop vs. a coupe. That said, using the data purely for comparison is fine. I believe Bill V. has this chart as well as several other members.

Cheers, James
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Old 07-05-2004, 02:37 PM
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Most of these were originally scanned (from Paul Frere's book) and posted by Bill Verburg.

Rear Lift:




Front Lift:




Effect of propping up the rear decklid:



From another book:

Old 07-05-2004, 05:00 PM
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On Ramp,

We have no specific graphs to offer, but a few first-hand observations, born of many hours of attempting to reduce drag and lift on our G-Stock 911 racer.

(1) Considering lift (or downforce), without drag coeffecient, is nearly meaningless, since these are the "trade-offs" we continually attempt to balance.

(2) It is fairly well accepted that the stock Carrera (late 1980s) body style, WITHOUT tail, generates drag coeffecients of 0.38 to 0.39 at 60 MPH - slightly MORE DRAG than an early 1980s Camero!

(3) Since aerodynamic loads increase by the square of speed, a given tail might reduce lift by a few additional pounds above 100 MPH, but the penalty in DRAG is huge. Unfortunately, none of the above graphs plot C/D.

(4) If reduction of lift, and reduction of drag is the goal, large gains can be made by examining ride height, and managing the air flowing under the car.

(5) In any case, if the car in question is well balanced to start, any aero changes at the rear (more effective tail, addition of a wing, etc.) must be accompanied by complimentary aero adjustments at the front.

I hope this proves helpful. We would welcome any data on DRAG coeffecients of the various tail configurations.

Ed LoPresti
RacePro Engineering
New York
Old 07-05-2004, 09:24 PM
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Actually the graphs I posted were from a discussion by Robert White and originally presented at 'War Bonnet Tech' and based on work that he did at Weissach.

The info is old but I have yet to see better on the SC/Carrera body.

Please read the original in it's entirety
in the 5/78 issue of Pano, also in 'Up~fixen' vol V

Some snippets from the above
Ed is correct in the aerodynamic sense a spoiler will decrease lift and increase drag, however the the aero aids that Porsche uses onthese cars are not spoilers in that sense. They do not increase drag but they do decrease lift.

if you look at this graph



It clearly shows the accedental discovery of the reduction in drag caused by raising the engine lid. This serendipitous discovery led to the development of the production 'spoilers'

Unfortunately though I have seen the actual drag #s for the different spoilers I cannot remember where, but I am under the impression that the #s show a reduction in drag w/ Carrera type spoilers, so I cannot say w/ certainty that Ed is wrong about the effect of the spoilers on increasing drag.

I do have the 993RS #s handy and will throw them out only as a guide
base version w/ small fixed wing and stock body aids @270kph
Cd = .33 FA lift in N = +178 RA lift in N = 0
Clubsport(biwing) w/ 4 adjustments
Cd = .34 FA lift in N = 0 RA lift in N = -119
Cd = .34 FA lift in N = +59 RA lift in N = -297
Cd = .35 FA lift in N = +119 RA lift in N = -535
Cd = .36 FA lift in N = +178 RA lift in N = -713


I do agree w/ Ed that ride height reductions and controlling air under the car are the best first steps.
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Old 07-06-2004, 08:34 AM
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Bill, what AOA does each measurement correspond to?

I guess I dont understand the presentation. At various angles of attack, the Coefficient of drag increases, front axle lift in Newtons increases (greater AOA on rear spoiler forces rear down, nose up, increasing lift on front axle?) and rear axle download in Newtons increases?
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Old 07-25-2004, 09:12 AM
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Quote:
Bill, what AOA does each measurement correspond to?
I'm assuming that this is in re the 993RS biwing? If so the upper wing is adjusted to 0, 3, 6 & 9 degrees
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Old 07-26-2004, 01:54 PM
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I'm searching for data comparing the turbo tail to the Carrera tail. In my organization's (POC) rule book, I get docked 1 piont for not having a stock tail. I have a narrow body Carrera with a turbo tail. This is preventing me from changing gear ratios. I would imaging if I put the Carrera tail on I won't notice much difference and the benefits will out weigh any disadvantage.
With all that said, the graphs above do not show the Carrera results. Does anyone have anything?
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Old 03-09-2005, 04:45 PM
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Armando,
I would be really careful about saying you won't notice a difference between the carrera tail and the turbo tail... there is a huge difference. I run 3 different wings depending on the track and how much drag vs. downforce I want, a stock RSA tail, early turbo, and late turbo, and I can definitely feel the difference between the three of them.
I think you could be safe in assuming that the carrera tail is between the ducktail, and the turbo on the graphs, but as Ed said you've also got to consider ride hight, and how aerodynamically clean the car is. The reason Porsche changed the turbo tail with the 964s was that they'd gotten the aerodynamics worked out well enough that they could use a lower drag tail, make a little less downforce, but still get the same overall amount of lift on the rear axle that they wanted.
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Old 03-09-2005, 06:52 PM
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I didn't mean there wouldn't be a difference. According to my class, if I put a different year wing, I get penilized. My point was am I better off installing a correct year/type wing and rework my gears or keep the stock gears and keep the turbo wing I have now (or even upgrade to a 3.8RS wing).
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Old 03-09-2005, 07:07 PM
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Bill V.

How can I use the "lifting the engine lid" trick to my advantage? I'm having troube interpreting that graph. Would raising my ducktail say, 1", then have a positive overall impact?
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Old 03-09-2005, 07:29 PM
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Not sure if this is what you mean but I've seen people at the track put washers between the hinges and the deck. It leaves a large gap leading into the engine compartment. I thought this was for cooling, but now that I think about it....
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Old 03-09-2005, 07:34 PM
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I was thinking of raising the rear by pushing up the latch a bit...
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Old 03-09-2005, 07:55 PM
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Regarding drag on the 911...I think Bill's post gets the idea across. In fluids class we were taught that increased lift/downforce will result in an increase in drag, but that's for a an element that is more traditional w/o ground effect. Sort of!

The large wake behind the 911 w.o. a tail generates quite a bit of drag. By shaping/managing the flow in the top rear 1/3 of the car, that wake is managed and drag is reduced. Concurrent to that, the lift is decreased because the tails help to pull the air flow to the rear of the car for a longer distance after the "crest" of the car....delayed separation....I suppose you could say that we are still following the fundamentals of aerodynamics...we are reducing lift..and reducing drag!

We discussed some of this in a thread when Tyson's asked about rake...I think most of us remember that long thread. We even had a NASA engineer run a basic 2-D model. Although the model was aimed at showing the effect of different degrees of rake, you can see what else occurs around the car. Regions of high and low pressures are generated and if you look at it model long enough, you might go "ah HA!" We actually played around with a model (not a 911, but an open wheel car) in a small wind tunnel using a drag scale and smoke...fun stuff...and yep, the model was pretty close to reality in many aspects.

I've got the results of the 2-D model on one of my websites...I just can't find the exact address to the link or the dang file right now.

Now if you are talking about the lift/downforce of a tall tail like Jack runs, then you are indeed looking at a wing in the true sense of the word. But, that element is in clean air and has minimal to no affect from flow at the decklid...or the spoiler-lid.

But hey what do I know, I'm restricted to internal flow these days..and I don't get paid for rambling on internet sites so I don't have to be accurate
Old 03-09-2005, 08:04 PM
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granted all the real research I've done is in high energy astrophysics, but I think you guys are trying to re-write the laws of aerodynamics to suit your needs and it doesn't really work that way
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Old 03-09-2005, 10:30 PM
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Dude what does reading the stars have to do with all this
Old 03-09-2005, 11:02 PM
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Quote:
Originally posted by wastintime
... I think you guys are trying to re-write the laws of aerodynamics to suit your needs and it doesn't really work that way
Doesn't sound like they are to me... The stock shape of a 911 generates lift at speed. (Almost all cars do, BTW.) They are trying to modify the shape so that it generates less lift. (People keep talking about "increasing downforce", but I believe most of them are actually reducing the lift.) Since most lift is generated at the "expense" of also generating drag, when you reduce the lift you often will also reduce the amount of drag.

....Or am I misinterpreting what you are saying?

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Old 03-10-2005, 06:56 AM
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How can I use the "lifting the engine lid" trick to my advantage? I'm having troube interpreting that graph. Would raising my ducktail say, 1", then have a positive overall impact?
Yes, you need to understand the principle, raising the height of the rear spoiler will reduce lift but aslo increase drag. You could do the same thing w/ a wicker bill or similar appendage.
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Old 03-10-2005, 06:33 PM
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Fortunately, not much has changed in the nature of AIR or CARS in the last eight months, so . . . . .

We may offer a couple of minute contributions: Dave asserts that "when you reduce the lift you often will also reduce the amount of drag."

In the very next post, Bill says "raising the height of the rear spoiler will reduce lift but aslo increase drag." These are both well-respected authorities on technical subjects, giving contradictory advice! So who is right?

They BOTH ARE! Bill's comments are directed at air flowing OVER the car, while Dave's applies much more to air flowing UNDER the car. For years, we amatuers worked with the model that all aerodynamic improvement was made to the air flowing OVER the car - top and sides. Any air which happened to find its way under the car was irrelevant.

The "over the car" model carries with it a fairly standard "trade-off" - downforce comes at the expense of drag. (More downforce = more drag. Less drag = more lift. Etc.) We were overlooking some of the biggest gains of all! Manage the air flowing UNDER the race car, and one can reduce DRAG and LIFT simultaneously!

Thus, Point #4 of our post above.

One more interesting point in reference to the wicker bills (Gurney flaps) which Bill mentions above. If applied correctly, they carry the additional benefit of keeping flow attached beyond the trailing edge of the foil or wing for an incredible distance!

I shall now return to hybernation . . . . .
Ed LoPresti
Old 03-14-2005, 11:58 AM
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