Aero Question

[marksearls]

I currently have an RS front bumper and duck tail on a 73 with a 3.0. I don't plan on adding the flares for a "RS look". I was thinking of going to an "S" front bumper. Is there an aero difference between the two? I run my car on the track and it is currently very stable at speed.

[Bill Verburg]

Originally Posted by marksearls

I currently have an RS front bumper and duck tail on a 73 with a 3.0. I don't plan on adding the flares for a "RS look". I was thinking of going to an "S" front bumper. Is there an aero difference between the two? I run my car on the track and it is currently very stable at speed.

Here's a summary of the aero characteristics of most of the air cooled cars, look at the E series(1972) w/ S front spoiler and F series(1973)

[marksearls]

Cool, this is very helpful, thanks.

[marksearls]

So after looking at the chart, it seems that with an RS front spoiler and duck tail is a pretty good aero combination, with 75 lbs of lift in the front and 90 lbs lift in the rear. If I were to switch to a front "S" spoiler, the front lift would jump up to 101 lbs. This surprised me a bit because the "S" looks like it would provide more downforce. As a check, am I interpreting the chart correctly?

[Dave Colangelo]

Im a bit of an aero noob, but I have been reading data sheets for years (as an electrical engineer)

Can some one do a 10 sec run down on what the figures in this chart mean. The frontal area and drag are obvious, but the C's V's and Q's not so much. As well does the (-) lift mean that it generates a down ward force?

Regards
Dave

[fbeatty]

Originally Posted by Dave Colangelo

Im a bit of an aero noob, but I have been reading data sheets for years (as an electrical engineer)

Can some one do a 10 sec run down on what the figures in this chart mean. The frontal area and drag are obvious, but the C's V's and Q's not so much. As well does the (-) lift mean that it generates a down ward force?

Regards
Dave

It has been a while, Cl = coefficient of lift, Cd = coefficient of drag and q = dynamic pressure (I believe). At least that is what it was back when I was working with aerodynamics.

[Flieger]

q does not have units listed. Poor practice.

[Quicksilver]

Originally Posted by Bill Verburg

Here's a summary of the aero characteristics of most of the air cooled cars, look at the E series(1972) w/ S front spoiler and F series(1973)

Why would they compile a chart like this with the drastically different velocity numbers? At first I freaked at the lower Drag lb number for the 911C/E cars before I noticed the different V numbers. That kinda throws the lift numbers out the window other then a basic "Yup the early cars had more lift" while being pelted with apples and oranges.

[Bill Verburg]

q is the dynamic pressure of the atmosphere through which a vehicle moves it is a function of speed and density. It was included solely so I could see how the lift curves vary w/ velocity(I should probably have hidden it rather than confuse the issue for the non technically minded). The above is a very tiny piece of a much larger aero project I did. For the purposes of the work I did rho is constant as there is only a very minor delta in rho for a given track.

Where possible calcs were compared to measured wind tunnel results for consistency

q = .5 x rho x velocity squared

all calcs and units were in MKS and converted to English final results solely for the convenience of the readers that don't think metric

If the peanut gallery doesn't like it then do your own. I share w/ others to try to illuminate rather that tear down.

[porschenut]

I'd be very curious to see what the lift figures look like at street speeds. 230 kph is about 142 mph and 245 kph is about 152 mph. I can't get my '86 Carrera with duck tail and S bumper to 142 even on the tracks that we have here.

[Bill Verburg]

Originally Posted by porschenut

I'd be very curious to see what the lift figures look like at street speeds. 230 kph is about 142 mph and 245 kph is about 152 mph. I can't get my '86 Carrera with duck tail and S bumper to 142 even on the tracks that we have here.

That's correct
for a 1976 Carrera 3.0, stock height/wheels/tires & w/ f/r aero, you car will be similar
the curves look something like this





I initially started to look at this to get some insight into some turn in issues w/ my cars at moderate track speeds, the front lift is the culprit there

[Quicksilver]

Originally Posted by Bill Verburg

If the peanut gallery doesn't like it then do your own. I share w/ others to try to illuminate rather that tear down.

No attempt to tear down, just frustration when supplied with variable data. With the exponential change in forces it isn't possible to get a feel for the true relation between the lift values in the different speed ranges.

Would have never suspected you would have been able compile such a volume of data yourself so I assumed you gathered this from another source and I would have expected them to gather information with the V variable at a constant value.

----
I find it pretty amazing that the ratio of front to rear lift was allowed to be biased towards high speed oversteer for so many models. Basically until you get to the 993 RS they all had some level of aero loose component. The only earlier exception was the non stock example of the 911H with out air dam and a duck added.

[RWebb]

Bill, have you done any regression analysis on these data?

[Bill Verburg]

Originally Posted by Quicksilver

No attempt to tear down, just frustration when supplied with variable data. With the exponential change in forces it isn't possible to get a feel for the true relation between the lift values in the different speed ranges.

Would have never suspected you would have been able compile such a volume of data yourself so I assumed you gathered this from another source and I would have expected them to gather information with the V variable at a constant value.

----
I find it pretty amazing that the ratio of front to rear lift was allowed to be biased towards high speed oversteer for so many models. Basically until you get to the 993 RS they all had some level of aero loose component. The only earlier exception was the non stock example of the 911H with out air dam and a duck added.

here you go standardized to 143mph, the coefficients are all well publicized but you have to remember that any change to the car, like lowering, tires, rake, angla of attack etc changes the coefficients. These are all just guides to relative aero characteristics