winders

I would think Jack is right only when the wing is in the laminar airflow attached the car surface. Get the wing high enough and it will be in airflow that is not attached to the surface and is flowing parallel to the direction of travel. Of course, the farther forward you have the wing, the more likely the airflow is affected by the car shape.

For example, on my race car, the wing will be right at 4' 10" high. This should get it out of the attached airflow. So, with my car on a level surface, I should set my wing angle relative to horizontal.

Scott

333pg333

The thing that perplexes me is the prohibitive cost of renting a windtunnel. I realise that they're big technical items, but they are just big fans after all. I can't imagine the cost of making those fans turn is reflected in the cost of hiring their services. Sure I'm looking at it in an ignorant way, but I bet the fact that these facilities are booked out by big companies allows them to charge so much which is way out of reach for most mere mortals.

The wing I use has a steep trailing lip that allows the angle to be set very flat but still have some effect....or it's just crap. Any guesses?

Larry Herman

Now this is my point. From a purely observational aspect, I would think that the majority, if not all of the air that flows across your wing comes from over the top of your car. Therefore, to get maximum efficiency, the wing should be straight across, not flattened in the center. With all of the air hitting it from the same direction, it should probably not be stepped, curved, flattened or any of that stuff.

Now on a 911 where a significant portion comes from around the sides of the car, having a wing where the shape changes to reflect this would be preferred, as long as the flow pattern is known.

IMHO, unless there is some sort of wing size limitation, it is only about drag vs downforce. Whether or not the wing is maximized in terms of its size vs its downforce is immaterial. That is why I think that without a lot of testing, a straight wing is probably best in most applications.

JackOlsen

One method was to string a wire in front of the leading edge of the wing.



I also did some tests with multiple vertical(-ish) wires to get a picture farther ahead of the wing. In this picture, I was also using compressed air to shoot confetti through a tube to see how it moved under the wing.



I have better images somewhere, but this is what I was able to find right now.

(Question from my current perspective: how did I ever have so much spare time? )

Not according to my testing. The air moving right over the surface of the car and the air that's even higher than the roofline all dip because of the low pressure area over the rear window.

I think you'd have to get at least a couple of feet above roof height to get something close to horizontal airflow.

Although as a side note: I did see a point with my 72"x9" wing where the rear was coming down enough so that the front was lifting. This was with 100-mph testing, and -- again -- a very big splitter in front.

mark kibort

Ive done some of the same tests as jack, but not as elaborate.
the angle off my 928 rear deck, is about 7 degrees deflection. its pretty clean air and basically gives the wing a higher angle of attack than which would be measured off horizontal. rear lips, curves, etc, are complicated for their effect. basically, the kick tail on a lip is like a gurney flap in a way, but really simulates flap on an airplane wing, but fixed. the take a straight edge and bring it to the leading edge, and that plane vs horizontal becomes the new effective angle of attack.

If you look at wing NACA numbers and profiles, you can see what happens with drag and lift characteristics. many racers are not putting down any more downforce with these styles of wings, but are increasing drag. most optimal AOA of most wings is near 15-20 degrees for max lift. with max lift, you get drag, its just a fact of life. the good news is that the drag is only usually 10% of the lift value.
with some very low wing heights, all the compound curves are needed to equal lift rates across the span. (like low BMW wings with their blunt rear window angls) if it is near clear air, the wing should be the same shape across.

mark kibort

jack is right, and the angle might be 7 to 10 degrees depending on the car and the height that the wing is placed.

Here is a generic wing graph for a wing similar to a Cup car wing from the '04 year.

sure, the lift doesnt go up but the drag starts to skyrocket after a certain point. now, is it stalling? not really, but you are getting a lot more drag. at stall, the lift goes way way down, and you could then measure or see some "burble" with some tufts, but that would be pretty extreme to be able to measure.
the rising curve that peaks and then heads down after 20degrees AOA, is lift CL(or in our case, downforce)
the one going to the stars is drag Cd. the straight line across is center of pressure Cp.
L/D is the outer graph scale and shows it go right up to a peak and then go down. Lift to drag at near 20 degrees is about 10:1. (e.g. 100lbs of dowforce is about 10lbs of drag. 10lbs of drag, has the HP cost effect of about 1-2hp in 3rd gear at near 100mph. or for you torquies out ther, 10ft-lbs at the tires, but 2ft-lbs at the engine.)

Larry Herman

But as we all know, flaps on airplane wings are only to generate maximum lift. Once the plane is aloft, they are retracted so that the wings can generate lift with minimal drag.

97C2s911

Why doesn't someone have wings that adjust with speed and or angle of car ?
How couldn't F-1 have this ?

333pg333

See the video on post #23....

mark kibort

There is drag due to lift and its mostly due to lift. At a greater and greater speed, only a certain amount of lift is needed in an airplane, so ther is no need for flaps to generate lift which creates even more drag. The idea with airplanes is to use an air foil that produces the right amount of lift for cruise speed (level flight), and flaps for drag to slow down, as well as increased lift for takeoff. During cruise, too much lift, and you would need to point the plane down which would create drag. because of disparate AOL's of the tail elevator and main wings.

Flaps create a change of the characteristices of the air foil. it turns the wing into a high lift wing, and effectively changes its AOL. Gurney flaps do this at lower cost of drag than an angle of attack change, but only to a certain point. (a very small angle of attach effect, due to a ver small lip on the rear of a wing, sometimes only 1/4" thick"
Most of all the drag comes from lift at moderate speeds. "aloft" most of the drag is caused by parasitic aspects of the airplane. this is not the case with cars because the air foils we use are designed to produce more and more lift at speed, so we get more and more drag. (and the speeds are relatively slow compared to airplanes.

It all follows those curves I posted.

333pg333

^^ "more and more lift at speed" ??

I hope not.

mark kibort

Yes, more and more lift, thats what wings do. Now, if you want to put it on a car and turn it upside down, then lift becomes downforce, but the principles remain the same, but you knew this.

95m3racer

Do you think the car will be driven to the limit where these few lb differences in drag/downforce will make a difference? Usual answer...spend it on driver training and the wing will become less important

CFD Data on a wing alone is useless...the numbers given by generic shops that manufacture a generic wing are just that...generic. No use to you or anyone really. You need the numbers setup on Your car in conditions You run in. And even then it might be all fluff.

You know who believes least in CFD...the guy who programmed it, because he knows what was "assumed" or estimated to make it work. CFD is a black art and even at F1 levels is known not to work as well as they want it to. more about optimization than what you're looking for.

Look for a well built wing that has a good range, and then adjust as needed.

Suspension is where you will find advantages, aero not so much.

winders

You are assuming that buying a wing will have an impact on what I will spend on driver training. It won't.....

The suspension on my car is going to be quite good. Buying a wing isn't going to affect it's quality one way or another.

Aero is important and I think you are discounting the difference between a good wing and a so so wing. The CFD data in the absence of other data, is better than nothing.

Scott

333pg333

http://www.goodaero.com/JSG/GOODAero.nsf/Products?Open

Just out of interest I have contacted Scott Good of GoodAero who has been very helpful with my enquiries.
While some might say that it's in his best interests to find fault with my wing, I believe he is advising me out of good faith rather than a commercial proposition. In fact it looks like his wing might be too big to run in the series that I'm looking at. However here is an excerpt that might be relevant.

"First, the wing you have is a compound wing. That's what you call a wing like yours where the center section is a different angle than the outer sections. My wing is a "straight" wing, one where the profile is the same all the way across. The point of a compound wing is to adjust the angle of the center of the wing to match up with the differing angle of the air coming over the roof as opposed to that coming around the sides of the car. This is a much bigger deal in sedans with relatively upright rear windows than it is in cars like ours with very gradually sloped rear windows. It is also more effective when the wing is mounted low as the higher up the wing is mounted, the closer you get to air running more or less parallel to the ground.

Your wing looks to be mounted at more or less the upper limit of the rules, 250mm or about 10 inches. Mine is several inches higher than that (about 14" in height) but even at 10 inches you should be in pretty clean air. While there could probably be some advantage to a properly-fitted compound wing at that mounting height, the shape of yours is pretty clearly not designed for your car. Actually, as abrupt and narrow as the center section is, I have a hard time imagining any car it would make sense on. My bet is it was designed more for fashion than function. If it were my car, I'd put a straight wing on it that is as big as possible for the rules and mounted at the upper limit. If the measurement includes the position of the end plates, I would cut the tops of them down a little if necessary to let the wing make it all the way up to 250mm.

However, having done that, you will then need to find as much front downforce as possible. My experience with 944s is it is hard to get too much traction in the front. Because of that, your traction/downforce in the front is ultimately going to limit how much downforce you can run in the rear as the two have to balance out. If you add a more efficient wing you need to also get more downforce in front or the car will simply be an understeering pig. This is a particularly difficult challenge for you because of the rule limiting you to a splitter which extends no farther forward than the forward-most part of the original bodywork. On my car, for instance, I'm running an 8-inch (200mm) splitter plus two canards on each side. I do not see anything in your rules allowing canards (which is too bad because they work great). "