Some Thoughts on Aerodynamic Shapes

[kach22i]

Maybe the French knew something when they added fins to the 917 for Porsche.

(x) Lettering on the 1971 Porsche 917/20 Coupé - "Pink Pig" - Pretorian {Mark} | Typophile

In 1971 Porsche commissioned SERA (a French aerospace consultancy) to refine the aerodynamics of Porsche 917k

The people back at Porsche decided that it looked like a pig and lettered it with the names of the various cuts in German. The car was entered in the race by Porsche Austria whose sponspors Martini refused to allow any Martini branding on the car, which retired during the night, crashing because of brake failure.

It undoubtedly was created in 1971, and the popular version of the story is that the pig "joints" and names were done as a bit of fun when the body came back from the aerodynamicists 5 inches wider (7ft 3in!) and generally much bulkier than its sleeker cousins.

Video with story
1971 Porsche 917 / 20 Coupé, Porsche Museum. CarshowClassic.com

http://www.youtube.com/watch?v=OqDa0...layer_embedded

http://www.carshowclassic.com/listings/cardetails/53

The 1971 Porsche 917/20 Coupé was meant to be a test car for the Canadian-American Racing Cup. It was designed by SERA, a French designer company, and not by Porsche itself.-The result of the design was both amazing and uninspiring. On the positive side, the 1971 Porsche-917/20 Coupé managed to improve on several things. It combined stability and downforce which-they derived from the 917 KH. It was also a combination of the long tail and short tail design, two-designs that the other Porsche 917 famously employed (the Porsche 917 KH was a short tail, which-some say is one of the reasons why it outperformed the long tail Porsche 917 LH). During the test-runs, the 1971 Porsche 917/20 Coupé did perform very well too. ...............................

.......................However, the general design of the 1971 Porsche 917/20 Coupé was meant with-criticism. In fact, it has been said that even the officials at Porsche (then and now) would admit-that the 1971 Porsche 917/20 Coupé was not exactly the most beautiful cars. Critics said it-looked like a “fat pig,” which Porsche took head on by painting the 1971 Porsche 917/20-Coupé pink with pig parts painted on it—hence its nickname, le Cochon Rose, or the Pink-Pig in English. Surprisingly, it qualified for the 1971 Le Mans.

It was used by the Martini Racing Team (who won the race with the Porsche 917 KH-Coupe) after proving at the qualifiers that it is better than its pre-race performance. Still, the 1971 Porsche 917/20 Coupé did not perform spectacularly. It met-an accident during the halfway point of the race, finishing at 32nd place (still, it-wasn’t the lowest ranking Martini Racing Team car that year; that distinction went to the-Porsche 917 LH). Nonetheless, the 1971 Porsche 917/20 Coupé did prove that it had great-aerodynamic qualities and a commendable speed.

So the 917 KH won the 1971 Le Mans and the Porsche 917 PA Spyder won the Can-Am a year later, leaving the 1971 Porsche 917/20 Coupé as one-off car. Although not successful, it remains to be rather popular due to its unusual-history.

http://www.carshowclassic.com/cargal...lerydetails/53

[kach22i]

Originally Posted by autogyro

Virgil Exner did an article on the Dart Concept, replete with tufted wind tunnel model photos.

Styling, fins, aerodynamics, and the Dart concept car

Wow, great article.

Based on the description the side wind force on the fins counter balance the weight of the front mounted engine when that is also encountering a side wind.

Based on this idea, fins would not work on a rear or mid-engine car, right?

Styling, fins, aerodynamics, and the Dart concept car

When fins are added, a larger surface is presented at the rear of the car directing the side wind so that its force is better balanced about the car's center of gravity which acts as the pivot point. Thus, the addition of rear fins equalizes the forces about the pivot resulting in less tendency of the wind to turn the car off its course. By aerodynamic design, then, the side wind itself is made to compensate for its own ill effects.

[lunarhighway]

@kach22i

what you want is a shape that when encountering side winds, will "steer" into the wind.
by increasing the winds hold on the rear of the vehicle, by the use of fins for example, the rear will be pushed out more and the car will turn its nose into the wind, countering the direction of the sidewind. a shape that would have the larged side surfaace at the front, such as a flatbed truck, would have the wind push out the front and have the car steer along with the wind, thus increasing the effect of the windgust and calling for a greater steering correction.

[autogyro]

Another BTW:
Ghia also did a one-off Ferrari Superamerica that same year that looks suspiciously similar:

1956 Ferrari 410 SuperAmerica Ghia Coupe - Images, Specifications and Information

Make you wonder whose project the Dart really was..

And..lest I forget, the 1954 FIAT Turbina, which claimed a CD of 0.14, presumably with it's pants on.

Fiat Turbina - Wikipedia, the free encyclopedia
Turbine Speed with Style | Hemmings Motor News

All these cars were run in the University of Turin wind tunnel. Hmmm...

C'mon Sergio, it's in the "corporate DNA" whether its interbred or not.

[Patrick]

Originally Posted by kach22i

Based on this idea, fins would not work on a rear or mid-engine car, right?

No, they would still work. The idea is to get the center of pressure behind the center of gravity. If the CG is further back you may need larger fins, more fins, or the fins may need to be positioned further back to get the same effect. Or you can reposition some other items to move the CG forward.

[3-Wheeler]

Originally Posted by Patrick

No, they would still work. The idea is to get the center of pressure behind the center of gravity. If the CG is further back you may need larger fins, more fins, or the fins may need to be positioned further back to get the same effect. Or you can reposition some other items to move the CG forward.

And according to Craig Vetter and others who have made high MPG motorcycles with high efficiency airfoils, they have moved the mass as forward as possible and added a fin to the rear for stability.

Jim.

[CFECO]

"Based on the description the side wind force on the fins counter balance the weight of the front mounted engine when that is also encountering a side wind."
The 917 engine was "mid" mounted, right at the rear axle, so the fins probably helped stability greatly. 917=Fantastic machine.

[steffen707]

Originally Posted by 3-Wheeler

Another interesting shape is logged today.

This is the second time this winter that this shape has appeared on the car, and this time a picture was taken before it melted and fell off.

The first picture shows a very interesting taper from the front tire, and is about the shape of something that I would expect to see from wind forces alone.



The second image shows just how "narrow" the front edge of the ice really is, and this surprised me a little bit.

Although this "pointy" shape has been seen before, it has not been on a car, but on the leading edge of the Yamaha TZ750 road racer fairing at Daytona in 1977. The leading edge of the fairing looked almost exactly the same. Yamaha must have done some wind tunnel testing on that old fairing.



This shape may make an excellent example of how to do this in foam/fiberglass when the time comes.



Jim.

Did you take a mold of the buildup before you kicked it off?

[aerohead]

Originally Posted by CFECO

"Based on the description the side wind force on the fins counter balance the weight of the front mounted engine when that is also encountering a side wind."
The 917 engine was "mid" mounted, right at the rear axle, so the fins probably helped stability greatly. 917=Fantastic machine.

If you were to spin a car like a top on a slippery surface like black ice,you'd notice that it appeared to revolve around a 'pole' as Earth does.
This 'polar' position would represent the longitudinal position of the car's center of gravity(CG) and the car's polar moment of inertia would pivot about this point.
If you were to balance a car on it's tail or nose,the vertical line passing the the longitudinal 'center' would define the car's actual C.G..
If you were to also direct a strong blast of air from the side of a car when on a turntable,when the car was so positioned that it neither spun from the nose,nor from the rear,but 'balanced',with no yawing at all,the position of the turntable's pivot shaft center would define the longitudinal location of the cars' Center of Pressure ( CP).
For aerodynamic stability the CP is kept behind the CG,then if a strong gust is encountered,the car will simply weather vane itself into the wind,and thereby cancel any yawing moment.