Coefficient of Lift, Does anyone know it? Options

[Matt Romanowski]

I've done tons of searching, but I haven't been able to find if anyone knows the coefficient of lift for a 914? I've found lots of Cd and Ca numbers, but I need the Cl for some data work.

Thanks!

[ThePaintedMan]

Have you seen this site?

http://www.cassidy-online.com/porsche914/a...aids/index.html

I'm not sure if Chris (the author/instructor) is a member on the site, but he has some pretty interesting figures on there. Yes, its mostly Cd, but there are L figures as well. I don't have all the data to do the derivation for Cl, but you might be able to contact him to find out what he used for A or if he already has Cl values.

[Matt Romanowski]

Have you seen this site?

http://www.cassidy-online.com/porsche914/a...aids/index.html

I'm not sure if Chris (the author/instructor) is a member on the site, but he has some pretty interesting figures on there. Yes, its mostly Cd, but there are L figures as well. I don't have all the data to do the derivation for Cl, but you might be able to contact him to find out what he used for A or if he already has Cl values.

I have gone through all of that but couldn't find anything for Cl. There were values for wings they put on and how that affected overall drag. I'll see if I can email Chris - maybe I missed it.

[ThePaintedMan]

Could you take the 124.76 N he calculated for the lift values and determine Cl if you knew the frontal area of the car? I'm sure he has that stuff, judging by the other interpretations they calculated.

[Matt Romanowski]

Could you take the 124.76 N he calculated for the lift values and determine Cl if you knew the frontal area of the car? I'm sure he has that stuff, judging by the other interpretations they calculated.

Frontal area is 17.22 SF, but I have no idea how to derive the Cl from that info.

[Dave_Darling]

It's not the frontal area, it's the planform area. More or less, the area of the shadow of the car when the sun is directly overhead.

One formula for lift is:

L = (1/2) d v2 s CL

* L = Lift, which must equal the airplane's weight in pounds
* d = density of the air. This will change due to altitude. These values can be found in a I.C.A.O. Standard Atmosphere Table.
* v = velocity of an aircraft expressed in feet per second
* s = the wing area of an aircraft in square feet
* CL = Coefficient of lift , which is determined by the type of airfoil and angle of attack.

Formula from: http://www.grc.nasa.gov/WWW/k-12/WindTunne...ft_formula.html

For our purposes, the L would be the 124.76N, d would be the density at sea level (29.92 inches of mercury), v would be whatever speed the 124.76N of lift was derived or measured at, and s would be the planform area mentioned above.

You'll have to do a mess of unit conversions to get all of the terms to agree, then plug the numbers in and solve for CL.

--DD

[stugray]

CL = Coefficient of lift , which is determined by the type of airfoil and angle of attack.

Well I sure hope that the lift comes out to be a negative number in all cases for a street car. Now a formula-1 going 180MPH and coming over a rise is a little different....

I dont believe that these calculations will apply to our cars because of the ground effect. We should have a vacuum under our cars at speed. A free-flying aircraft does not experience ground effect until feet from the ground. And in that case it works opposite to the ground effect of a vehicle (it helps to hold it OFF the ground)

Stu

[Dave_Darling]

Actually, almost all cars generate positive lift at some speeds. Fortunately, that lift is very rarely equal to or greater than the actual weight of the car. Generating 100 lbs of lift doesn't come close to counteracting 3000 lbs of weight. It can make high-speed corners more interesting than they have to be, though....

It is certainly true that a car is in ground-effect while an aircraft is not. But the equation should still give at least an approximation of the Cl for the car, since we do know a lift force.

--DD

[mittelmotor]

Actually, almost all cars generate positive lift at some speeds. Fortunately, that lift is very rarely equal to or greater than the actual weight of the car. Generating 100 lbs of lift doesn't come close to counteracting 3000 lbs of weight. It can make high-speed corners more interesting than they have to be, though....

It is certainly true that a car is in ground-effect while an aircraft is not. But the equation should still give at least an approximation of the Cl for the car, since we do know a lift force.

--DD

Yes, and the distribution of that lift is quite important. Ask the owners of early Audi TTs that went off the Autobahn tail first. Audi countered with a rear lip spoiler, and dumbed down the steering a bit with more lateral compliance for the front control arm bushings.