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Old 08-14-2008, 04:39 PM   #1
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Lift/downforce physics info??

I'm doing a little research on the downforce produced by different Porsche rear spoilers. I know the lift equation: Lift=(lift coefficient)X[(air density X velocity squared) /2] X (wing area); however I cannot find a formula for the lift coefficient that doesn't simply change this formula around, requiring you to know the lift as one of the factors. Anybody have an easy lift coefficient formula?? Or, just to make things really simple, does anyone have downforce numbers for Porsche spoilers? In particular, I'm looking for the Boxster spoiler, the 930 whale tail, the 996 GT3 wing and the 997 GT3 RS wing.
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Old 08-15-2008, 03:10 AM   #2
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these are not simple equations when it comes to complex shapes; these things are usually determined emirically or using CFD software on Cataia models. there are no good simple kinematic equations here; everything is a diff eq. read up on the bernoulli effect.

essentially, to know the lift on the entire car, you need to know the difference in airflow velocity between the top and bottom of the car. this is of course a dynamic relationship. additionally, once you know this, it would be necessary to determine lift at the front vs. rear. i don't know of any way to calculate this for a complex shape.
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Old 08-15-2008, 12:16 PM   #3
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I'm not really looking for the whole car. I'm just interested in info about the spoilers/wings. I know that to get the exact answers, the math is complex, but approximations can be made, particularly if you are using airfoils with similar shapes but different surface areas. A method for determining coefficient of lift for different shapes is the element I'm really after.
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Old 08-15-2008, 12:57 PM   #4
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Quote:
Originally Posted by j.fro
I'm not really looking for the whole car. I'm just interested in info about the spoilers/wings. I know that to get the exact answers, the math is complex, but approximations can be made, particularly if you are using airfoils with similar shapes but different surface areas. A method for determining coefficient of lift for different shapes is the element I'm really after.
well when you're talking about lift, you have to consider the entire car. a spoiler 'spoils' airflow to reduce the lift generated by the shape of the car's body. to figure out how much a spoiler reduces lift, you have to consider the shape of the car both with and without the spoiler, calculating lift in each configuration.

calculating downforce generated by a wing is much easier, but depending on the shape of the wing, it can still be rather complicated. the easiest equations treat the wing element as a simple inclined plane. downforce is calculated by finding the resultant -z vector of the incident airflow. for example, if air flowing at 100mph is incident on a wing with a force of 200lb and the wing is a 45deg angle, then 1/2 of that force is down and the other half is drag. downforce would be 100lb.

most wings use a combination of inclined plane and bernoulli. for non racing wings, the bernoulli effect is probably a small component of the total downforce, so using kinematics and trig will probably work well enough to compare relative downforce between two elements.

there is another component to consider: drag. well designed wing elements can produce tremendous downforce with very little drag component.
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Old 08-16-2008, 03:08 AM   #5
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Expanding a bit on Insite's explaination, the car is part of the lift. The air flow pattern is influenced by the shape of the body work. The lift created by an airfoil is heavily influenced by that flow. If you look at the airfoil on the boxster, it's basicaly just a nearly vertical wall which would have almost no lift if it weren't attached to a car.

There are countless tables of airfoil lift and drag coefficients developed for the aviation industry, relating to Reynolds number typically, but I'd be very surprised if you ever found coeficients that you could plug into the simplified form of the navier-stokes equation you've got there relating to a specific car.
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Old 08-16-2008, 07:00 AM   #6
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all true. also, it might help to explain the difference between a wing and a spoiler. a wing is used to actually create downforce. a spoiler is used to reduce the lift generated by the shape of the car itself.

if you think about the car in simplified form, the bottom is flat and the top is curved. air must follow a longer path to get over the car than to go under it. this means that air flowing over the car must move faster than the air flowing beneath it; the bernoulli effect causes the low pressure area above the car responsible for the lift. to reduce this lift, we can EITHER reduce the speed of the air moving over the car, or increase the speed of the air moving beneath it. a spoiler does the former. venturi tunnels under downforce cars do the latter.

in effect, the spoiler on our car is used to simply interrupt the flow of air moving over the car to slow it down relative to the air moving under the car. once this happens, the pressures above and beneath are closer and lift is reduced.
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Old 08-16-2008, 08:49 AM   #7
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Tach and shift light

Shift Light

Somebody can tell me where I find place to plug wire to get RPM pulse for my just added tachometer and shift light. Boxster S 2000.
Thanks for help
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Old 08-17-2008, 10:17 PM   #8
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There used to be some 986 wind tunnel data floating around on the internet. It had pretty good data on the effects of the OEM spoiler. I checked my tech bookmarks but could not find it. Google is your friend. If you find the site you will also get lift/drag data for many cars, spoilers and wings. Good luck. Hope it helps.
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