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Aero Question..........

 
Old 02-08-2010, 08:35 PM
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Originally Posted by 333pg333 View Post
As a side note, it's interesting to see a lot of the F1 cars go with a narrower rear wing. Sure they're not dealing with a caged cockpit but just an observation.

So higher and deeper to get cleaner air?
Nah, the rules cut them down to try and reduce the turbulence so that cars can follow them more closely.
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Old 02-08-2010, 08:48 PM
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Originally Posted by bobt993 View Post
Jack, I saw how much work you put into your car in an older post. What was your final direction on the wing? If I recall correctly you even attached some tails to your wing to look at the flow.
Yes, I did a lot of research and then a lot of testing. I set up cameras to photograph wool tufts on different parts of the car and with my wing at different angles. I put tufts on wires to see the direction of the airflow relative to the ground as it hits the rear wing. I used front and rear ride height sensors to measure the effects of different aero components in repeated back and forth testing on a lonely stretch of desert highway. I used the same sensors at my local track to validate some of what I found. Most of this wouldn't have happened if a very generous aerodynamicist hadn't contacted me and guided me along through my testing. I had basically no budget for any of it (I made my own wing out of aluminum roof flashing, and my ride height sensors came off of a junked 95 Lincoln air ride system), but I learned a lot and helped my lap times at the aero-friendly Willow Springs raceway with a 4.5 pound wing I made out of aluminum and glue.

My short summary of everything I learned would be this for 911 drivers: run as big a rear wing as you can, legally. Run as big a set of endcaps on it as you dare. Do whatever you can to blunt the front end profile of your car and add a front splitter as low as you can that extends 5" out in front of the car. Do what you can to smooth out the underside of your car. And don't worry about drag.

If most of the turns at your track are 70-mph or less, you're not going to benefit much from aero. It's important to remember that it only slows you down on the straights, and it doesn't do much at all at normal freeway speeds. And if you're already flat on the gas in a given fast corner, then more downforce isn't going to let you get any more acceleration.

Originally Posted by bobt993 View Post
One warning I was given is to avoid not balancing the down force front to rear. eg front splitter needs to be consistent with what wing you install. My car is balanced pretty well suspension wise, but Chris used very high rate springs and it requires fast hands at times.
You're always going to be juggling different adjustments to the front-rear balance of the car. Keep in mind, front aero (like a splitter) acts on both the front and the rear of the car. Keeping air out from under the car (by dropping that splitter down low) acts on both the front and the rear of the car. A rear wing acts only on the rear. And if it's working too well, it will lift your front end with a cantilever effect.

The best way to determine how the front wheels and rear wheels are getting treated by your aero pieces is to run ride height sensors into your data logger and test on a flat stretch of road where you can make opposite direction runs at a set speed. 100 mph works well, if you can find a place where law enforcement won't bother you.

The second best way is to find the fastest guy in your class and copy his setup.

Originally Posted by 333pg333 View Post
As a side note, it's interesting to see a lot of the F1 cars go with a narrower rear wing. Sure they're not dealing with a caged cockpit but just an observation.
That's because of the rules. Most series don't allow end caps to extend above the top of the wing, but that doesn't mean the wings wouldn't work much better if they could. Overall height and width are normally limited by rules, and that means race teams have to make a lot of compromises on aero.
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Old 02-08-2010, 08:51 PM
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So Larry, which wing? EVO 50 or 60 with the winglet side caps?
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Old 02-08-2010, 08:56 PM
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Jack, Thanks very informative. My car is barely at RSR ride height per Chris Cervelli. I have a buddy that has made a bunch of front splitters for Caymans so I may ask him for some help.
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Old 02-08-2010, 09:35 PM
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Jack's post has a lot of good info. On any production type car you simply aren't going to be able to have too much downforce. Run as big a wing and splitter as practical. Any additional drag should be more than compensated for by increased exit speeds.
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Old 02-08-2010, 10:50 PM
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Originally Posted by claykos View Post
Also - drag and downforce are not "exponentially higher" with speed - they go with the square of speed. Go twice as fast and you have 4 times the downforce and drag. And note that power to overcome that drag goes with the cube of speed - so twice as fast requires 8 times the power to push it through the air!
sorry meant to say geometrically, not exponentially
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Old 02-08-2010, 11:11 PM
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No worries - growth proportional to a variable squared, or cubed, etc is actually algebraic or polynomial, but now we're splitting hairs.

Not to turn this into a math semantics seminar...
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Old 02-08-2010, 11:15 PM
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Originally Posted by bobt993 View Post
So Larry, which wing? EVO 50 or 60 with the winglet side caps?
I would probably be leaning towards the 60; with the wider fenders it should look right. Question is, do you want everyone else to have wing envy?
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Old 02-09-2010, 08:22 AM
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Hah! I just want to turn a 28 at Tbolt. Actually, I am beginning to like the development process after working with Chris again.
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Old 02-09-2010, 03:45 PM
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Great info guys and especially Jack. So to digress, if you are running on a predominantly 'small' track with tighter turns are you suggesting that the wings/splitters are largely just going to produce drag that will slow terminal speeds on the straights? If that is the case should people be minimising downforce/aero?
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Old 02-09-2010, 04:17 PM
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That seems very counter-intuitive and it seems cars like usually run more wing on slower tracks, not less.

A simple common sense theory is that since you aren't getting to very high speeds, drag is not as big of a factor so crank that wing angle up. Any loss on the straight should be more than made up for in the braking zone. Of which there are likely lots of. On a high speed track (Spa maybe) you don't need as much wing per se because you are going faster so small increase in wing angle has a much bigger effect.

And the time you would gain in just one or two high speed sweepers on a low average speed track, (think Monaco) seems like it would be worth sacrificing the couple of mph straight speed especially when you can brake better and take the corner faster.
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Old 02-09-2010, 05:23 PM
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Bob: FWIW, I run a 60" wing on a car with otherwise similar bodywork to yours. I have shock sensors with my MoTeC that are used to look at the impact of wing angle on down force and balancing the down force front to rear. I've added extra canards up front to get a bit more front down force to reduce high speed understeer at corner exit. (Hard to see in the photo but we appear to have the same Getty nose on our cars.)
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Old 02-09-2010, 06:33 PM
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Tom thanks, my car is actually blue and is setup the same as the pict (it was for motivation when I read the article on the car). Following paint, I will post the new car look. BTW your car looks awesome in every pict you have posted. I see you shaved the gutters and deleted the hood vent too. You also have a little more fender as the GT2 flares are really only good for 1.5 inches a side. Did you add an engine tray to get rid of the trapped air or just cut open the rear bumper?
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Old 02-09-2010, 06:43 PM
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The given angle will produce two different lift values. Most of the drag is due to lift. a given air foil shape will have a given lift to drag ratio, regardless of its aspect ratio. where you get into issues, is in the shape of the curve to create the same amount of lift. That will produce different lift to drag values.

Too big of a wing, and you sacrafice parasitic drag, too small and you get bad lift to drag values.

I had a stock kick tail wing that produced about 80lbs of downforce at 100mph at 10 degrees of attack. (vs horizontal). the cup car wing which was wider as well, produced 175lbs of downforce at that same speed, but at a lower angle as well (near 7 degrees). in the end, the drag is roughly about 1/10th of the downforce. (roughly in the sweetspot of the curves). at near 0 angle i produced that same 80lbs of downforce at 100mph, but the drag would have been less.

mk

Originally Posted by wanna911 View Post
Take two Cup wings, one 63 inches, one 55 inches. All else the same.



- Is it a given that the 63 inch wing will provide more DF at any given angle than the 55 at the same angle?

- Is it also a given that the 63 will provide more drag at any angle than the 55 at that same angle?

- If the answers to both of those are yes then is it possible to have a longer wing with less angle on it and reduce drag from a greater angle on a lesser wing while providing the same amount of downforce. Or will you end up with the bigger drag plane equaling that out?


Hope that makes sense.
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Old 02-09-2010, 06:46 PM
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That is not true, in most cases. In fact, the air off the roof line can be fairly undisturbed and then effectively increase the angle of attack in the center of the wing for more downforce in the middle vs the ends.

Originally Posted by C.J. Ichiban View Post
the real key to the width issue is like claykos said- the wider the wing, the cleaner the air hitting it on the wider parts...because we're not talking open wheel here- aka there's a big fat cockpit in the way of the airflow...so-

in street based cars the majority of the actual downforce is produced at the tips of the wings where the airflow is the cleanest.

aero drag is also exponentially higher at higher speeds, so if you're running a super high speed track you probably won't need the wide one as much since the same aerodynamic downforce can be generated not by more wing angle or surface, but just as a result of the higher speed.

so if you can just change out wings for no good reason/no rules: wide wing for slower/turn oriented tracks and narrow wing for higher speed tracks with longer straights.
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