mark kibort

so, back on topic, i reviewed some of the graphs. you all know the "T" graphs dont apply to us. (tail sections for airplanes) BUT, if you look at the lift coefficients, and the drag profiles, it sure looks like .25 " GF on our wings would provide almost double the angle of attack (say 6 degrees normally, going to 12 with the GF) So, is that good?? I dont know. you could just move the wing to 12 degrees too and get the same Cl.

Now, the Drag part. it looks like at the same Cl, the drag goes up almost double. So, unless Im reading the results wrong, the Gurnery Flap increases drag, based on the same lift.*

(see diagram 14 for drag, and figure 4 for lift vs AOA)
* lift coefficient of 1.0 has drag of .05 without GF. with GF, same 1.0 Cl, Cd goes to .06. (20% increase in drag)

So, if that is true, what are we really trying to do with the GF?
What is its true goals.
is there a second per lap lurking there? Doubtful

mark kibort

I dont see it. looking at the charts,

I was looking at 2% GF, becauase thats generally what we see on race cars. about .25" But even still, even at the 5% GF, you increase the AOL by 7.5 degrees (from 5 degrees AOA to 12 degrees AOA.)

At the same lift , say Cl of 1.0, the drag for no gurny is .075 Cd, while with the gurney, its 1.0 Cd. looks like the drag goes up by 33% for the same lift, but a higher angle of attack with standard wing, no Gurney Flap.

I think that is the net net of all this. you max out near 12 degrees or so AOA and you need more downforce, add a gurney as it will increase downforce as the same as it would be to increase AOA, but is not limited to the standard wings max lift levels. It increases the max lift levels.

Interesting paper. thanks for posting!

Mark

edit: another way to look at this as well, is if you care about drag due to downforce, the clean wing can be at Cl 1.2, and produce the same drag as the gurney flap lip'ed wing at only Cl 1.0. 20% more lift, same drag as the gurney flap.
again, i think the gurney flap is to increase downforce when you are near the limits of the wing.

911SLOW

Come on brother let's debate gurney (again).

https://rennlist.com/forums/997-gt2-...ghlight=gurney

I will test a new one on the cup wing and want more data!

Probably both of us are wrong..but still I like the discussion : )

certz

This is totally fascinating. Just to get a little OT, what is legal for PCA club racing?

Gary R.

Legal in Prepared and GT classes, not Stock.

chris walrod

Gurneys are also quite effective placed on the leading edge of cooling ducting exits as they will help evacuate the duct.

Larry Herman

I put a piece of 3/8" aluminum angle on my RSA to make a Gurney flap, but now my screen door doesn't close properly anymore.

TRAKCAR

I could do more testing, but I am to lazy to mess with it.
I already know it is more then a second faster, maybe 2 seconds with the splitter. They probably balance out:

The only research I do is steal parts of 2010 CUP cars when no one is looking and put them on mine. They all just bolt on and the CUP cars seem to go around the track pretty good, so if it works for CUP car, it works on mine.
They do enough research I figure. I scored a LSD front splitter gurney flap (shaped slightly different) and little fender flaps so far. Even my BBS are 3 piece CUP like, just different CL.

I even Vinyl wrapped it as do all the CUP cars! Maybe less Cd?

mark kibort

So, here is the confusion with your claim. It can be your driving. You woulnt be the first one to be surprised one way or another with your time at the track. slower or faster, with NO changes how many of us has has a 1 second slower or faster time. With me, and the tracks i visit, i can usually predict within a .5 second or so, before I see the sheets. but recently, i had a huge improvement, and dont know why. think it was me! also had a slower time that I thought was on a lap that was in the usual range. you cant go by seat of the pants. you go by your markers, and watch it on video or analyse data.

Now, does a splitter and a wing help with lap times? certainly! more at some tracks than others.

Does a gurny flap provide 1 second beter laps times?? certainly NOT. we already have gone over what its effect is. ONLY if you have maxed out the effective angle of your current wing, does it pay to use one. otherwise, you are dealing with 25% more drag for the same rear downforce. make sense?
What you are really saying is that you bolted on a gurney flap and gained 1 second. this means you are really saying you increased the angle of your wing from 5 degrees to 12 degrees and saw a 1 second lap time. both would give you the same effect. (wing angle change or gurney flap added) Is it normal to see 1 second a lap changed from a 7 degree wing angle change?? I suspose you could if the wing was not producing enough downforce and you were skidish around some high speed turns at a big fast track.

The point is, you probably didnt see 1 second from the gurney flap alone. it was more downforce in the rear. how much, it depends on the wing angle. (I have a cup car wing and I know the downforce at many different speeds through testing). You more than likely just hit a hot lap that you nailed .
the only way you will know for sure, is to look at the data, and /or pull it off, run as hard as you can and see what you run timewise.
another big factor as I mentioned, is confidence. if you are not confident, you will run 1 second slower, regardless if the car can do it or not. 1 second a lap is an eternety. if someone pulled something off my car and said , go run a flier, it would be a few laps before I trusted it fully. on the same through, if someone added something that was said to make me 1 second faster a lap. I would push even harder and probably go faster . placebo !!

winders

This would seem to be more specific:

http://www.raes.org.uk/pdfs/2800.pdf

The study you quote seems more interested in T-strip results.

Scott

911SLOW

Cup lover! : )



Thanks Scott! Excellent find.

This one has some very interesting tables with angles of attack.

And a similar conclusion.. "This leads to an optimum
height equation that enables the efficient optimisation of any
constant-chord aerofoil."

Now we just have to find this optimum gurney height for our cars and call it a day..

mark kibort

in looking at the article. same conclusions for the most part. however, there still seems to be more drag associated with the gurney flap, for a given lift coefficient, which would mean, only use it when you have maxed out the wing you have in AOA.

their tests also show the 5-6 degree AOA equivilance based on lift with use of the GF.
I also like the optimization of the GF height based on speed.

net net. if you are not over 10 degrees AOA, then you dont need a GF.

mdrums

got no wings and racey lookin stuff on my lowly Carrera S...so no dog in this fight.

mark kibort

How is it an efficient optimization if the same lift setting wth the GF equates to more drag? There is optimum GF hights, but thats based on target speed range, but doesnt change the basic results of the tests.

I just thought of something. maybe the GF can increase downforce at the slower speeds, but because the angle of attack is lower, at the higher speeds there might be an advantage over a wing that has a higher angle of attack for the same downforce. Ill have to look at the curves some more.

I have one actually that came with my cup wing. Ill have to do some experiementing with it. Its a 3% GF.

mk

winders

Here is a quote for the link I provided:

"The experiments have shown that as long as the height [of the Gurney flap] remains less than the thickness of the boundary-layer at the trailing edge, the additional drag [added by the Gurney flap] will be negligible..."

Saying that Gurney flaps are only useful when you are at high AOA is just plain wrong.

Finally, I don't see how some 928 driver on the left coast can make definitive contradictory statements regarding what someone in Florida saw in lap time improvements when adding a Gurney flap and splitter to their aero package. That's just plain arrogant and has no place on this forum!

Scott