cetom

ok so here is one of those dumb questions . for the gt3 wing that we are talking about mounted on gt3 uprights how do you (or how can you ) measure the AOA
thanks

mark kibort

With a level, and then with a ruler on the leading edge. measure the distance of the leading edge of the wing, to the level set at "0".

take that distance, divide it by the wing cord distance (11.75") and then grab a calculator and input: INV then TAN. that will give you the wing angle of incidence. (angle relative to the car on level ground)

the AoA is that angle PLUS the angle of the air deflected from the roof line

add: 12.5degrees for the 911, 8 degrees for a 928 and near 0 for the Ferrari. This is approximate for a wing that is fixed at or below roof line level. (the ones higher than roof line, might have much less roof lind air flow deflection.

cetom

thank you very much . the uprights that i have are the standard gt3 uprights that couple with the wing . there are a number of predrilled positions . so if i can figure out which position is "0",and set it there then the AoA would be 12.5 . is that correct
again thanks

ProCoach

ALWAYS measure!

333pg333

Congrats Mark on getting the email from Nasa Tom. You are persistent and have been rewarded. :-)

mark kibort

I think it is. I took the GT3 uprights as well and modeled them for my adapters for the 928. However, your right. It doesn't matter what they might say if they are marked, as a few things can change that as well. (rake, etc). So, measure and when you find zero, 12.5 degrees should be the angle off attack, which is the actual max lift of the wing...... based on all the data. So, if you need more downforce, you should put on a GF. but remember, almost instantly you will get the equivilant of 7 degrees wing angle of incidence. so now an equivalent true 0 AoA with a GF might be at -7 degrees (negative) angle of incidence

mark kibort

and look at the data... measurement, in the case of the Porsche's, is a little misleading as we have seen...... but your right, at least it's relative.

Thanks! We have had several discussions since as well. He is head of the NASA Manned Space Program as well, so he is a busy guy. Pretty honored he spent the time to reply and continue the our discussion. He gave me some interesting concepts to think about, as well as information on how the simulations tools are made. Turns out he is a big racing fan, with Dan Gurney being one of his heros, as he is mine, for being an engineer as well as a driver!

333pg333

I was talking to the Aero guy we use and asked him to generalise about Gurneys. A very brief reply for what it's worth.

"Gurney on the rear wing is too complex of a question to answer blanket. You have to test but usually I can get a little less drag with a gurney and a bit less flap angle. We always use a small gurney even on the Indy cars when drag matters huge. But that said, its a small drag number for a time attack car, we deal in big swings of an axe not really threading the needle like we had to do in Indycar"

mark kibort

Makes sense, as it seems all the race cars we deal with and see at the track, with only a few exceptions, have a pretty significant roofline deflection.... That known, it points directly to most folks being at or well over the max lift coefficients. In fact, it would be hard to find most any racers without a GF, that is not at or over the max lift capabilities of their wing, so backing the wing off and putting on a GF will almost always end up with less drag compared to how most all racers have their wings set up. (unknowingly, beyond max lift capabilities). In fact the only car ive seen so far without a 10 to 15 degree roofline deflection has been the Ferrari. in that isolated case , it might have more drag with a gurney flap at 0 angle vs a gurney less wing at 7 degrees having near the same downforce......... This was brought up in beginning. thanks to some putting down some simulation data for our GT3 cup wings and wings like it, and all the with and without GF wind tunnel data, its pretty clear that their sweet spot with sports cars is much broader than I was giving them credit for, based on the narrow and low angle max lift points of most racing wings and the extreme roofline flow deflection of most cars.
Even my car with 8 degrees deflection, would be well served if I want any more lift than I already have, to use the GF... at the point im at now. 16 degrees calculated AoA, backing off 5 degrees gives me 11 that puts me at slightly more lift, and a lot less drag. a GF at this point might equal the drag, and provide the same downforce, but I would have to make the wing totally level to do this. I have the 11mm GF that came with the GT3cup wing, standing by, incase the car is loose this weekend on the high speed turns and I need more downforce.

333pg333

Ha, so in the end I think we can take from all this is that GF's sometimes work well and sometimes they add too much drag.

mark kibort

As I said in the beginning, BUT, I concede that they rarely produce more drag because more often, most wings are at near max downforce. so, we are all right.
Personally, I think the discussion provided a lot of good information..... enough for me to make some changes!

cetom

mark, is stall an abrupt event . ? what i am asking is -- in the pursuit of max rear downforce if the wing AoA is too great is the penalty less downforce or is it stall , . i guess what i am asking is ,is stall an all or nothing event .
i had a nasty spin last year about this time . at a previous event i had decided that i wanted to be able to rotate the car more in the turn zone. this led me to flattening the wing . i have no idea if the 2 are related but a very experienced driver told me that i should always have max down force at the rear of a 911. so now i have the gt3 wing at position 11 on the gt3 uprights which is nmax AoA . but i wonder if in fact i am really fooling myself because from what you say here the wing probably is in stall
thanks for the discussion

mark kibort

That's a great question.... that's why I was always trying to make a point to use the term.. "past max lift" (where the wing starts to show stall characteristics), instead of the term "stall"....... "Stall" is when an airplane or a lifting body produces less lift force, than the gravity pulling it down and the plane starts to nose down or seemingly fall from the sky.
As Patrick or someone mentioned, the GT3 wing has very gradual stall characteristics... this means, past max lift, the lift doesn't fall off that dramatically. In fact, it arcs a bit, (past max about 2-3 degrees, or say max at 10-11 degrees and falls off to about 14 degrees) THEN, it drops abruptly to a 75% of max lift point..... where it seems to produce that same lift, but drag continues to climb.
My point earlier was that in testing, folks might not know if they are on the front side or the back side of this curve, because the lift would be the same at 7 degrees or 15 degrees, but the drag might be 4-500% more at past the max lift point!

So, in your case if your "max angle point" at position "11", actually means 11 degrees angle of incidence..... you could be at an AoA of 24 degrees,which is well past the max lift point... again, its not stalled , but its in the stall area, which gives about 75 % of max lift (at least to 20 ... don't know what happens beyond..... we don't have that data) and the drag is way off the chart. the stall area is the point past max lift where separation of the flow starts to happen on the curved side (underside ) of the wing. it gets worse with angle, but our wings have a little less abrupt stall characteristic, which is VERY good. say you are on a bumpy high speed turn , like sebring final turn....... If you had a sharp response for the wing stalling with angle change, in 1 or 2 degrees, that would mean as the nose dived you could have a lot more lift, and then if it raised on acceleration out of the turn or after a bump, you could lose a LOT of lift (downforce) so that the rear stepped out and could cause a spin. However, the GT3 wing doesn't do that. it has soft stall response and large angle changes in the stall area, doesn't change downforce all that much. However, in the pre -stall area, there is some greater changes of lift... this might be the reason you want the wing at max lift if you can get the nose downforce to balance... its the only area where the change of angle doesn't create larger changes in downforce , if your car was bouncing around in a turn.

So, since you are actually operating at 11 + 12 degrees (at least), or 23 degrees AoA, you might want to back that angle off because right now you are deep in the stall zone, probably 500% more drag than if you were at a similar downforce effect at a lesser angle. (75% of max lift might happen at near 0 AoA, which means you might have to have the wing actually pointing down because -12degrees angle of incidence, would be 0 AoA. AND that would give you more downforce than you currently have.
however.... I would try angle of incidence of "0" (flat wing) this would mean you would gain 25% more downforce, as it would be near max for a naked wing) and your drag would be 25% of what it is today. means and if you needed more, you could just add a gurney flap and tilt it down 2-3 degrees to start.

hope that helps.

edit: ive attached the cup car wing profile characteristics and the lift/drag plots for a less aggressive wing, just to show that rise in drag, and the softness of the stall characteristics.
as a note, in almost all simulated tests, the lift is calculated to go kind of flat after max lift... some fall faster than others based on wing configuration. But, in almost all cases, actual wind tunnel tests show that the lift falls off dramatically (the 75% point they fall to ) and drag continues to rise as in the actual plots below. the cal poly study showed this pretty clearly with the NACA4425 wing study

cetom

thank you very much. this has been very helpful

mark kibort

Just back from a racing weekend at Thunderhill... 5 more MPH down the main straight on both races, and qualifying. two different temp days! one 70 and the other 80F.
according to all we have learned here.... I was able to bring the cup car wing to max lift range by taking 6 degrees out of it where it was well over max AoA with roof line deflection. weight tests show the same downforce which follows all the actual wing curves as well as calulations for lift.
net gain was 50% reduction of drag. at 6:1 lift to drag, drag is about 60lbs for normal drag, and near 120lbs for increased drag for over angled AoA. so, we figure 120lbs of drag, is 40ftbs of torque for the last 1/3 of the straight at thunderhill. sure makes sense that we gained 5mph and kept the downforce in tact.

edit: probably a little less drag gain, as the lower AoA was greater downforce as it brought the wing to max lift, which would give higher drag too, so the net gain might be lower. the calculations that Krodile posts next is a much better approximation, with some adjustments based on some actual wind tunnel data.