downforce v weigt/drag

[SC-targa]

Originally posted by Eagledriver
I think that the idea of keeping airflow attached to decrease drag is deeply rooted in the past. Think about the ideas of the 30's with cars that had tapered rear ends and tapered fairings behined the driver.

My guess is that modern aerodynamics favor the wedge shapes that we see on modern cars. The idea being less frontal drag and less lift being more important than keeping the airflow smooth and attached. -Andy

I disagree. Wind tunnel tests still show that a teardrop shape is the lowest drag form. There are many other factors involved in the selection of an automobile shape besides low drag. The long tail can move the aerodynamic center of pressure to an undesireable location. The weight can be excessive, and upset the handling balance.

The Kamm tail is an attempt to obtain most of the benefits of the tapered, tear drop shape by chopping off the tear drop taper, but having a clean, laminar seperation of flow from the body. This gives nearly tear drop performance with an often more useable package. Many of the tiny little spoilers on street cars, like the little ones used ov VW jettas in the late 80's, are thee to provide clean seperation and low drag. They also may reduce lift slightly, and improve stability.

Regards,

Jerry Kroeger

[tom1394racing]

Here's some additional information on the subject of the Porsche rear spoiler.
Many of us have read the stories about how the sports prototype cars from the late 60's and early 70's were so difficult and scary to drive. This is because the engineers, in an effort to reduce drag, had gone to the near teardrop shapes like the long tail cars designed for top speed at Lemans. While these cars did have low aerodynamic drag, they also experienced lift forces that tended to pick the car up off the track leading to handling and stability problems. Top speed did improve but in many cases lap times suffered as the handling deteriorated. On the 917, Porsche solved this lift and stability problem by chopping off the tail on the car and making it a spyder with no roof. Does not seem too aerodynamic but it worked.
When Porsche began the development of the 911 for GT racing, in the early '70s they realized that they needed to address this same problem of aerodynamic lift and instability. They had seen that other GT cars like the Ford Capri and the BMW's were running with rear spoilers to their apparent advantage. So Porsche decided to experiment with spoiler shapes on the 911 in the Tech University of Stuttgart wind tunnel.

Here's some resullts of the flow visualization using surface tufts both with and without the rear duck tail.






The flow visualization showed that the spoiler forced the flow to separate at it's traling edge and that the upward deflection of the air streamlines to pass over the spoiler caused the pressure to increase on the roof, rear window and decklid. This siginificantly reduced the aerodynamic lift without hurting the drag, This was the same effect that had been acheived with the 917 spyder. The lift and drag data for the duck tail showed the following:

Note that rear lift was reduced without sacrificing drag.

The race engineers did not trust the wind tunnel data and had devised their own test to determine if the aerodynamic aids would improve handling and stability. They took a prototype to the test track and acceelerated it to 125 mph and then had the driver input a predetermined hard steering input to destablize the car. They then measured the number and extent of the steering corrections needed to bring the car back into control and moving in a straight line again. With the spoilers in place, the corrections to stabilize the cae were minimal and the spoilers were then deemed race ready.

[Rex Walter]

I am an Aeronautical Engineer with over 30 years wind tunnel testing experience, so I like to think I know what I’m doing – with airplanes at least. I still have a few things to learn regarding automobile aerodynamics though. I have been doing some research on the subject of Porsche drag, and I must start by apologizing to Island911 and Eagledriver, as they had most of the story correct. I have found many sources that say that the drag of a 911 is reduced by adding a spoiler in the rear – but not much explanation. As Eagledriver explained, drag can be dived up into two parts, induced drag associated with lift, and everything else, called parasite drag (friction drag, pressure drag, profile drag, whatever you want to call it). The induced drag can be approximated by CL (lift coefficient) squared divided by (PI * aspect ratio). From the numbers given by Bill Verburg and from Frere, I calculated that at 100 mph, the lift loss due to adding a rear spoiler was about 100 lbs, which translates into an induced drag loss of only about 6 to 7 lbs, which is not a lot of drag. Frere says that this is only a 3% drag loss, which is confirmed by the data given by tom1394racing.

The data given by tom1394racing also shows about 100 lbs of lift loss at the rear axle (but no speed, I assume it is still about 100 mph). The drag reduction given is also about 6 lbs. The total drag (parasite + induced) is a little over 211 lbs before the spoiler, and 205 lbs with the spoiler. This matches the 3% drag reduction mentioned by Frere, and seems to be almost all due to induced drag (as Island911 and Eagledriver were trying to tell me earlier). But this seems like a very small amount of drag – too small to notice. It could be that the increased stability that was mentioned earlier has a lot to do with the improved “feel” of the car.

The flow visualization shown by tom134racing, shows that without the spoiler, the bottom row on the back window and the engine cover have attached flow. With the spoiler added, both these areas are separated. Anytime you have separated flow, the surface pressure will increase, reducing lift on the back window and engine lid, and increasing the airflow into the engine compartment.

SC-Targa is correct, in that the teardrop shape has the lowest drag, and I believe that adding a spoiler to the back of a Porsche is “fooling” the airflow into thinking there is a teardrop shape there. The Whaletail does a little better job of this than the Duck Tail.

Rex

[tom1394racing]

On the long tail cars, the elimination of the teardrop shape and incorporation of a spyder/spoiler body style did increase the drag. The story goes that at LeMans, Herr Bott (the Porsche engineering boss) was fixated on top speed on the Mulsanne straight. When the spyder body style was used, the car was much easier to drive and had faster overall lap times but was slower at the end of the Mulsanne. Bott did not want to use the spyder body style and Norbert Singer had to convince him that faster lap times would win the race not top speed.

Going from the teardrop (long tail) to the spyder, forced flow separation at the termination of the body resulting in a larger wake and more induced drag. However the region of low pressure on the back of the tail was eliminated reducing the lift and making the car easier to drive fast through the corners.

Today's race cars use efficient wing shapes at both the front and rear to provide down force with minimal (but always increased) drag. This technique is fundamentally much more effective in stabilizing the cars and increasing the cornering speeds as compared to the simple spoiler which only acts to reduce the induced lift from the body shape.

BTW...I spent the first part my career working on Gas Turbine aerodynamics. This discussion is reminding me of how much fun I used to have with it.

[randywebb]

"the bottom row on the back window and the engine cover have attached flow. With the spoiler added, both these areas are separated"

For the many who are viewing this thread: Look at how the strips are pointing in different directions in the pic with the ducktail, and are not pasted down onto the body surface like in the pic w/o the ducktail.

[tom1394racing]

The upward streamline deflection caused by the spoiler increased the pressure all the way forward to the leading edge of the roof near the front windshield. There was even some discussion among the engineers at the time that the sunroof would heave to be strengthened to withstand the increased pressure.

[Rex Walter]

Tom,

I would think that the change in pressure at the sun roof would be small. The fast moving airflow over the top of the car creates a low pressure area (vacuum), and lifts. With the tail in place, the flow over the roof will be slightly slower, and the vacuum will be relatively less (as you said, the pressure is lower all the way up to the windshield), but I would not think it would be much less.

Rex

[tom1394racing]

The potential flow effect of the duck tail combined with the front lip spoiler does cause a very significant change in the pressure distribution across the top of the car. The aerodynamic center of pressure moves from 0.88 m from the center of the car to 0.73 m from the center with the spoiler combo. This is another reason why the car becomes so much more stable with the aero package.

In 1972, one of the German magazines, Sport Auto, took a standard Carrera and tested it with several combinations of spoilers. They evaluated top speed and handling characteristics and found the following:

Without either spoiler the car was very unstable at speeds above 200 kph. The addition of the front lip spoiler alone made a big difference in the stability and improved the top speed by 2 kph. The rear spoiler alone caused the car to become unstable again because the rear lift change made the nose rise so much that the feel in the steering wheel almost went away and the car under-steered terribly. Top speed with the spoiler alone did increase by 0.5 kph. They found the best combination was as Porsche had designed it. With both the front and rear spoilers, the top speed was increased by 4.5 kph and the car was much more stable.

BTW...Much of the data I am reporting here comes from a great book I purchased in Germany, "Carrera RS", by Thomas Grubner and Georg Konradsheim.

[Rex Walter]

Tom,

If you had any pressure data across the top of the car or down the back window, I would be interested. I found a web site where a college student was wind tunnel testing a 911 with and without spoilers, but his data only included the baseline 911 without spoilers, and then the test was stopped for an unknown reason.

I'll look for a copy of "Carrera RS", sounds interesting.

Rex

[livi]

I have a '85 Carrera. At approx what speed would I as a newbie notice a difference in stability if I were to add front and rear spoilers (mine lacks both) ?

[Wil Ferch]

livi:
Look at the graphs here...I would say you can "feel" the effects at high-end highway speeds ( USA basis)....but certainly at 80-100 mph, the effects are pretty "real".

---> downforce of carrera vs. turbo spoilers

- Wil

[autobonrun]

Originally posted by livi
I have a '85 Carrera. At approx what speed would I as a newbie notice a difference in stability if I were to add front and rear spoilers (mine lacks both) ?

but what I can tell you from a test I ran when I first bought my car is that at 120mph, there is a substantial difference, both in feel and stability. When I first started researching adding front and rear spoilers to my 79SC (in 1984), I was a intimidated by the cost. Everything I read said there would be an improvement. So, before I added the spoilers, I found a flat two lane Texas road and ran my car up to 120mph and held it there for a couple miles. I tried to center the car on the center line of the road. To hold it there took constant corrections. It wasn't highly unstable, but I did have to make constant corrections, and the amount of movement it took to keep the car on-center was quite a bit.

After I added the spoilers, I ran the same route. At 120mph, the car was so stable, it barely moved from the center line and required minimal input on my part. At 135 mph, the car did require corrections, but not constant and far less than was even required with no spoilers at 120mph. So, although I had already paid the bucks for the spoilers, I was very satisfied that it was money well spent. By the way, I have the front lip spoiler and turbo rear spoiler.

The real question behind the technical discussions IMO is how much top end speed do you give up to get the stability up to that point. At least in my simplistic thinking, that is the bottom line. Since I no longer even attempt top speed runs, stability at lesser speeds was a more important issue. I would hate to be the engineer that has to optimize the car setup for a given track. Being able to find the balance between straight line speed, stability, and cornering is why they get paid the big bucks.

It's interesting to compare the earlier turbo tail design to the Carrera GT tail, much smaller but more functional, notice the shape of the GT's wing in the photo. Sort of an inverted airplane wing.

[Wil Ferch]

autobonrun:
Nothing new here...but you should read the earlier postings within this thread:
1.) unlike what you say...there is fair evidence you don't give up top speed when using spoilers...the top speed increases...
2.) No kidding...inverted airplane wing. An airplane's desired state is to lift it from the ground. It should therefore come as no surprise that an effective downforce-creating device would be an "inverted" wing.....

Best,
- Wil

[autobonrun]

Originally posted by Wil Ferch
autobonrun:
Nothing new here...but you should read the earlier postings within this thread:
1.) unlike what you say...there is fair evidence you don't give up top speed when using spoilers...the top speed increases...
2.) No kidding...inverted airplane wing. An airplane's desired state is to lift it from the ground. It should therefore come as no surprise that an effective downforce-creating device would be an "inverted" wing.....

Best,
- Wil

I did read the entire thread; some of the concepts are too deep for me but that's another issue. There is evidence that top speed does suffer. Nothing is free. Top speed will only not suffer if drag does not increase; and that is a source for discussion. There may be a point where you can decrease lift without impacting drag, but past that point where you get increased downforce, it is at the expense of increased drag and decreased top speed.

And the inverted wing is something new; at least for Porsche spoilers. Airplanes have been around for a hundred years, but the previous designs for the rear spoiler were not an inverted wing; a wing yes, a tea tray yes, even a duck tail but my point is not that it is a surprise, but that this particular design was chosen for the GT. The "surprise" is the size of the wing and why was it chosen for this particular car and not others.

[masraum]

Motor Trend did a test of a '72 with and without spoilers and noticed a difference as low as 60mph.

[Rex Walter]

At 60 MPH, the lift at the rear axle will be about 55 lbs without a rear spoiler. Adding a duck tail spoiler will reduce this lift to 15 lbs (according to the data given previously). The stability will also be increased due to moving the center of pressure to the rear.

Rex

[Wil Ferch]

autobonrun:

Simply look at the graph that was shown three posts above mine...the one from Tom1394racing.... That, and most of the text written in this series of postings shows that top speed increases with the use of Porsche's aero aids. To say "nothing is free" is too simple a statement... if an aero device is used that converts the aero shape of the basic structure ( the aero affects of the "car" itself)..then it is entirely conceivable that you get overall less drag and higher top speed. And...as shown and proven by the text and graphs. As much as 4.5 kph more. Simply *look* at the graph posted.

Don't want to believe the picto-graph? ....OK... it was also posted the coefficient of drag ( Cd )was reduced from 0.407 to 0.397 after you add the spoilers. OK, so some of the concepts may be over your head, as you yourself say.... but what do you think happens when the drag factor is reduced ( and forget about the numbers, then)? Ayup....top speed goes up !

As to the inverted wing....it's nothing new.... inverted wings were used as far back as the 70's...maybe not for Porsche on street cars... but the concept breaks no new ground here.

Wil

[randywebb]

Steve - do you have the cite for the Motor Trend article?

Also, re the C-GT -- that car was designed from day 1 for good aero and high speeds with 21st Century knowledge and technology. so, it is many years ahead of the 911 - also, it is not constrained by the marketing need to look like a 911, as are the 964/993/996/997...

For a good look at current aero dynamic design, also include the various recent Fcars, the Honda Insight and the Toyota Prius. Note the high rear deck on those cars and the abrupt, Kamm tail type "breakaway."

[autobonrun]

Wil,

In my comment I do state that top speed will not suffer as long as the drag does not increase. And as you rightly pointed out, the figures show Cd did drop with the addition of the spoiler, so top speed can increase. So I think we agree on that issue.

As to the concepts being over my head, I said that jokingly to keep the atmosphere calm. Actually, the equations and concepts are quite clear; probably part of my engineer background.

The area I feel may be open for discussion is where the design of the spoiler not only reduces lift, but goes the other direction and actually increases the downforce at the expense of drag. I know that some teams go for more downforce on twisty high speed circuits but not as much downforce on circuits with long straights.

I'd be interested in your thoughts on going past the point where lift is reduced to the point where there is actually a positive downforce? I'm thinking this can't be done without imposing additional drag.

Anyway, good thread with interesting comments and links.

[Wil Ferch]

No harm intended, certainly....

...and no doubt... if we're broadening our discussion to *any* cars....not just the known aero aids historically used for 911's....then of course, there is a good chance that whatever downforce-generating devices are used...there will come a point where the overall drag created will increase due to effect of the downforce components......

My mistake was that I thought we were talking 911's and their aero devices only.....

I know I've ( un-intentionally) irritated knowledgable guys like Bill Verburg on this board, in a semantics debate we seem to have....but Bill talks about the 3.8 cup wing ( ??...is that right , Bill ??) that actually produces net downforce. I don't recall what the published overall drag ( Cd) effects were when this is employed. Bill....you there ??

cheers,
- Wil