Thanks nthfinity for providing some information about aerodynamics. I can tell that you are knowledgeable concerning automotive aerodynamics. Since I last asked the question I have been reading to try and understand the concepts better. Maybe you can provide some critic and see if understand the venturi tunnel concepts.
Lately I have been really interested in aerodynamic down force as an aid to increasing cornering speeds. It started when I learned that the main difference between prototype classes and others based on street cars is their aerodynamic down force which allows them to corner much faster. As a result, I have noticed that many of the newer car designs incorporate the venturi tunnel as a way to increase down force.
From what little I understand about the subject it sounds like it is based on principles similar to those used to create lift on airplane wings. Many of you are most likely familiar with these principles but for those that aren’t the way an airplane creates lift is through a relative air pressure imbalance between the low pressure on top and higher pressure on the under portion of the wing. This results due to the shape of the wing which flat on the bottom and curved on top.
The curvatures on top is longer than the flat under portion. As a result the top portion of air has to travel faster than the lower portion since the distance is greater. This higher velocity of air on the top is associated with lower pressure due to some physics that I can’t explain here.
The ventrui tunnels used on CGT, Ford GT and Ferrari’s 360 CS uses the same principles. St-anger provided some incredible pictures of the CGT to help me demonstrate these aerodynamic features. The first picture is the bottom view of the underneath side of the CGT. This is the undertray and I labeled the front portion and the area of the rear diffuser. The rest of the pictures are a view of the undertray attached to the CGT from behind and again shows the rear diffuser. It is the rear diffuser that acts like the curved portion of the airplane.
As air flows underneath the car, as it comes upon the rear diffuser where the cross section of the undertray increases and the distance the air has to travel also increases. To see this imagine taking the undertray off and pounding it flat, although this isn’t possible because the undertray is made of carbon fiber and while strong isn’t malleable. The curvature of the undertray up towards the bumper is longer than if the undertray were flat and ran straight across to the bumper as normally would occur. Because the distance of the undertray increases the distance the air has to travel increases, the velocity of the air increase and the associated air pressure decreases. As a result, the air pressure above the car is greater than beneath the car and this creates a net downward pressure, increasing down force and lateral grip. Pretty ingenious. It is also sometimes referred to as an inverted wing.
IMO the 911 shape is one of the most beautiful in automotive design. I especially like the shape of the G-series or the 1974-89 shape, if you couldn’t tell from my earlier posts. Sadly these aerodynamic aides don’t appear to be compatible with 911’s as it appears to me that the placement of the engine low at the extreme rear of the car prevents creation of a rear diffuser. Additionally, the overall shape of the 911 resembles the shape of an airplane wing more than any other car. The end result is that the shape of the car creates lift especially over the rear axle at high speeds.
If you follow the changes that have been made to the design over the past 10 years you will notice that the rear portion of the car use to drop from the roof line to all the way to the bottom of the rear bumper. This long line relative to underneath the car meant air traveling over this section of the car was traveling at a higher velocity and therefore had less pressure associated with it, creating the lift over the rear axle.
Tea tray spoilers help by shortening the distance the air has to travel and therefore the speed, reducing the pressure differential and lift associated with it. Spoilers also created down force, so they accomplished two objective to decrease rear lift. The changes that have been made to the shape of the 911 have been to decrease the long drop off from the roofline to the bumper by raising the bumper line and to add popup spoilers to further shorten this line and create more down force.
So unfortunately the 911 not only has the challenges associated with the rear engine weight bias but a shape that creates lift at high speeds. The end result is that the 911 is even more prone to spin in high speed turns. But regardless of its faults there is something that evokes passion in the 911. Maybe it is its reputation and the challenge of driving it. Maybe it is the pure simplicity of its design. But what ever it is for the Porschephiles there is nothing else like it
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I know this post was a little technical so I hope that it was helpful and interesting. I really enjoy this type of stuff and hopefully there are others out there that enjoy it as well
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