mark kibort

This discussion took off a bit on the (how to go faster thread) so instead of further getting off topic there, I thought it would be good to move it back here where the discussion never really ended.
There were still some that thought that the Gurney flap works at lesser angles of attack (or angles of incident).

so, here it is again. This was a response to Mike who had a wing GF fly off and it hurt his cornering speed due to lack of downforce even though they had set the wing at near 0 angle of incidence. my point was that they would have been better off by not even using a GF, and using wing angle alone, because the GF hurts in Drag, at lower Coefficient of Lift values. He had the wing set at 0 degrees (flat) and the gurney flap was probably .5", which is about 4-5% of cord in most cases.

some assumptions: GF flap about 3-4% of the cord length. wing set at 0 degrees with GF vs no flap and wing set at 10 degrees. wing is a flat bottomed, 4412 NACA type. (very basic common asymmetrical shape)

This is what I said:

Did you see the lift (downforce ) values?? even at 0, you get a Coefficient of lift of near 0.4 and drag of .04, However, with the gurney flap of about 1/3" in height, you get a lift value of 1.5 and a drag of .12. ..... in order to get 1.5 Coefficient of lift, you need the wing set at 10 degrees without a Gurney flap, but the drag is only .08. at that same Cl with a GF, you need 0 AOL and it produces .12 coefficient of drag. that's 50% more drag to use a GF.

Ok, follow along to where im going here:

In your situation at RA. if you put down 500lbs of downforce (with 50lbs of drag) at 0 AOL with a GF, and it flew off, suddenly your wing was only putting down 250lbs, and 25lbs of drag. (good for the straights, but a problem for you in the turns) the net gain for the straight, was 25lbs of drag, or about 8ft-lbs of torque (near 8hp) at the engine gained. (due to a 3.2:1 gear ratio at 150mph)

The point of all this is that if you (your engineers) decided to use wing angle INSTEAD of a Gurney Flap, the drag would have been 50% less. meaning, instead of making 500lbs of downforce at a cost of 50lbs of drag, you could have made 500lbs of downforce at a cost of 25lbs of drag.
you would save 25lbs of drag, or 8ft-lbs of engine torque or near 8hp.

With gurney flap , L/D 12:5 (Lift 1.5, drag .12)
Without gurney flap L/D 18:1 (Lift 1.5 , drag .08)

(gurney flap at 3-4% of cord or near 1/3-1/2" high on a 12-13" width wing )

SAME DOWNFORCE, LESS DRAG

The same gains you saw down the straight , you would see by not using a GF and using the wing angle to give downforce.

If anyone here doubts this, especially the ones that have taken out of context the summaries in the GF links on the internet..... please speak up and show why you don't agree with this information.

So Mike, you want to gain 8hp, or 8ft-lbs of torque at the engine at road America..... And you like the way the car felt at 0 angle of incident with a Gurney flap when going down the straight??? Lose the gurney flap and put the wing at 10 degrees........


For reference, here is the the initial part of the discussion with a couple of graphs:

Now the charts. Dont want to confuse the situation with fact, but here I go........

I like the graphs below because the NACA 4412 is a very similar wing to the cup car wing. (like the Clark Y air foil below as well) the gurney flap size is probably about 1/4" which is the 2% of cord GF. (the bottom two charts are for a higher lift wing Re 110,000 range)
Look at an a lift coeffieint of 0.5 look at the angle of a Gurney flap wing vs a non gurney flap wing. its 1 degrees vs 5 degrees.


NOW, look at the drag when both wings (with and without gurney flap) are making 0.5 Cl. Drag with Gurney Flap vs drag of non- GF. Its .025 vs .04. near 40% difference.

what do you see? I see that there is about a 40% increase in drag for a gurney flap on a wing producing ths same amount of downforce(or lift).
And, if you had a 3% gurney flap, which is pretty common, that increase in drag is DOUBLE over a non gurney flap wing at a Cl of 0.5.


To get 0.5 Cd, you need 5 degrees of AOA for the non-gurney flap wing, and only 1 degrees for the gurney flap wing. (0-1 degrees depending on the height of the GF)
all gurney flaps at 0.5 Cl have an increase of drag over a non GF wing. .

An interesting twist to all this, is that at no lift (0 lift) ..... the gurney flap produces so much drag as that if you wanted to match it with a wing angle, with no gurney flap, you could create enough lift with the non gurney flap wing as to produce .5 Cl with about 5 degrees of AOA, which is huge!!

So, in summary, you don't want a gurney flap on a wing if your AOA is less than putting out near its max lift. as a safety margin, I would install one if I was near 50% or 75% of its max lift capabilities based on its AOA and lift/drag characteristics.

I do like all the papers that had been linked too. I have an emotional attachment ot the cal poly paper because I was Aero there for a while. However what has been quoted was the negligible effect of the increased drag for gurney flaps less than 1.25%, which on a 13" wing cord length, is near 1/8" . most wickers or Gurney flaps are around .25" which is near the 2% of cord.
on that Poly paper, the GF plots don't even go down below lift levels of 1.0, but it's shown below... again, strengthening my case that at low lift settings, drag can be much higher with a GF.

The net net of all this , based on the facts, is that more lift (downforce) is possible with a GF. a normal wing might max out at 17 degrees, with a gurney flap it can create 20% more downforce. This is the point for using the GF. as you get up in the Cl levels, the drag differences for a given Downforce are very small. at the low end, there is a significant difference, and if I was racing at a fast track, I would pull the thing off and use wing angle vs the GF to give the appropriate downforce... in the case of Mikes experience at RA... that GF fell off, I would give the wing 5 degrees and in the end, he would have benefited by having the same downforce and less drag after a pit stop and a wing adjustment without the GF.

And by the way, a cup car wing is 12" wide and most gurney flaps Ive seen are near .5" which is OVER 4% ..... This DOUBLES the drag at low wing angles using a Gurney flap....... just to keep things "real" here.

Here is some reference for a more symmetrical wing and gurney flap comparisons

http://www.aoe.vt.edu/~mason/Mason_f/AIAA2007-4175.pdf



__________________

mark kibort

I put a lot of effort to bring forth an interpretation of the graphs. Interesting that there is still belief that Gurney flaps work at lower wing angles. The justification here has been really based on interpretation of some articles, that were written about completely different applications.
winders , Sunday, and others were quoting them when most of information was referring to GF used on wings when they were near max lift conditions.

Ill go about this a little differently. If I'm wrong on this one. (as several have said), then please find me an example of a GF down force data, vs a standard wing, at the same downforce, that doesn't have more drag. Isnt that a simple request for this discussion?
What im talking about here, Is the lift to drag ratio. with a Gurney flap, the L/D ratio is less than a standard wing at lower downforce levels, especially vs what a 0 AOA set wing with Gurney Flap, vs a standard wing with no GF at the same Downforce.
Its classic for almost any technical discussion here and its a little too bad. As soon as someone disagrees with a common belief, the common "camp" just says, "your wrong " and then evaporates.

Lets see some evidence to the contrary....... if there is any. some great evidence has been provided, but it seems nobody has been able to understand what they are looking at so far.

Dr911

Mark et al
Let's just chill a bit and let dudes/dudettes like me try and absorb what you're saying here.
Some interesting applied physics here but for starters I need to find the durn reading glasses before I can interp. your graphs . For now I'll just pretend to be as smart as you and say"yeah dude ! Right on!"
I mean isn't that what folks do to coast on Facebook?
Ps
I'm not in Facebook so I might be wrong. Peace out man .

mark kibort

Im good with that too. Sure, I hope people can just keep it clean and somewhat polite. The trolling is just toxic in my opinion. As you can tell, I did spend some of my time putting this together and interpreting the data. It seems very clear to me, and it is just as clear that many in this thread have jumped to conclusions before really understanding what they were looking at or reading.
its a perfect technical discussion interesting technology, that works only in a very narrow , but still very effective range. (from what I can see)
If I was to summarize again......... the GF works when the wing starts to stall. it extends the lift abilities when the wing alone cant do the job. But, if it can with wing at low AOAs,, leave the cute, high tech, racer part off the car, all it can do is add drag.

Juha G

I totally get what Mark is saying and I think he has proven the point beyond what anyone else here has provided.

So a Gurney flap is a band aid fix to pushing out more downforce from a wing that is already maxed out (at maximum angle).
i.e. if your wing is not at maximum angle, you should adjust it instead of adding a Gf.

winders

http://www.cirodesignracing.com/Ciro...20research.pdf

Juan Lopez

I'll say thanks MUCH Mark for taking the time to break this down. Very interesting topic and perhaps the biggest attempt at a technical discussion here on RList which we can use on our track escapades. I will print and study carefully.

For the record, when we developed the PCA aero package for my vintage car, we started placing a GF at the trailing edge of the roof (remember i had no aero at all, just a 911R configuration) it made a difference at T1 and Bishops at Sebring. Next year came back with a proper wing (and a splitter). After testing, added a GF and some strategically placed droplets of oil to see whats was going on. Since my speeds are relatively slow ( compared to a Cup) I concluded that the GF helped a lot.

Now, on the Cup, its a different story. We'll see how we trim her out at Daytona ...!

J richard

I really hesitate going down this rabbit hole...

but,

Angle of attack is always from the chord of the section to the relative wind. On the back of a porsche that wind, even at roof level is decending behind the car. It produces an angle down from 10-15 degrees. so even in a "flat" setting the AOA is already at least 10deg. This also changes dramatically with speed. The flat setting on the cup is 5 deg down. Also the angle of incidence of the wing is through the chord of the wing, from the center of the leading edge to the trailing egde. With the wing "flat" it already has about 2-3 deg built in. Bottom line is the wing is actually operating in a very high angle of attack relative to the charts: 10deg to the relative wind, 2 deg built into the section, and 5 deg for a typical setting on the cup =17deg or better. that is now operating in the range where a gurney flap becomes very useful, and at the edge of stall pretty critical.

The other issue is theoretical analysis is one thing but the fact of the matter is in the real world variablities affect static conditions described in the study, basically the only way to analyze variables. Lift and drag are variables with speed but AOA and stall are not, and speed is a critical variable for a race car.

Jack Olsen did some real interesting work on his car a few years ago:
http://forums.pelicanparts.com/porsc...tack-wing.html

After all of that and several additional years of devleopment he is running a wing with a gurney flap.

Lastly gurney flaps work and are a very effective tool to increase the effectiveness of a wing, look no further than the original post on this thread and just about EVERY highly developed race car. EVERY ONE. There is a reason for that. I think better time could be spent with the analysis of what is the most effective height of a gurney flap with ultimate lap times, but that is an imperical analysis not derived from isolated charts.

mark kibort

Richard, its not a rabbit hole at all..... in fact, I already addressed this SEVERAL times in prior posts.. AOA is not angle of incidence. agreed. and yes, 0 angle of incidence, as Mike was talking about in his car when at RA or Sebring, Is probably more like 7-10 degrees AOA due to roofline deflection. But, look no further than the graphs provided to see what the lift to drag ratios are.

Don't shoot the messenger here. just because you have empirical evidence here, doesn't mean it's nothing more than anecdotal. what I mean is that I can kick your tire before you go out and you might turn a faster lap time. does it mean kicking your tire makes you go faster?

Mike had his wing set at 0.. angle of incident. and yes, its the cord to the tailing edge. 0 is 0... just because it has a built in , kind of , AOA, doesn't mean you don't use 0. when the simulations use 0, they use 0. are we talking angle on incidence? 'flat" meaning put a level on it... true 0 angle of incidence Is "cord from trailing to leading edge. a level is a good approximation here. 0 lift, might be at an angle of attack way in the negative. High lift asemetrical wings have this effect. don't get caught up in the symantics . the effects of higher lift asemetrical air floils are shown by the graphs. some get 0 lift at AoA, and some get .4 Cl, and some get up to .8, at "0" AOA. you can see the effectiveness of the gurney flap. it jumps up with it, BUT the Lift to drag ratio is SOOOOO much higher. 50 to 100% higher depending on the wing.
In our discussion here we are using the 4412 NACA air foil because that is closest to the cup car wing, but the one study used the 23012, which is more symentrical and has much less lift at 0 AoA. regardless, unless you are near stall, the gurney flap always produces more drag.

Look, many teams use gurney flaps, and I contend that they are all (well the smart ones) are looking for more rear down force, more than the wing can provide on its own, and might be in the max lift (downforce) range, or near stall, where higher drag levels might be found . all these teams might be using them but if they are not at max lift for the base wing, they are losing near 8hp due to increased drag vs removing the GF in times where they want a flat wing that is NOT near the stall range of the wing alone.

To ignore the graphs, from several different actual wind tunnel and simulated tests , is feeble In my opinion. It makes perfect sense to me.

so do me this favor... find a condition where the wing is NOT near max levels and you would get less drag from a gurney flap. its a challenge and this discussion would be over with one post and one from me saying
" thanks' for proving your point.

mark kibort

thanks!
now, regarding your trailing edge of your roof GF, are you referring to a function of vortex generator?? where it creates a disturbance to get more air flow to the wing, albeit turbulent air flow. breaking up the laminar flow to make the wing get more air? big difference than this discussion about the effectiveness of the GF on the wing itself.
I did a big change on my car when it went from stock kick tail wing to real cup car wing. I did a lot of testing as well. (some more involved than Jack Olsen's testing) spring scales for downforce measuments, etc. what I found is that I could get the same downforce with 7 degree tilt of the wing, vs 15 degree angle of the stock wing, that was in a lower spot too. same downforce, much less drag.... then later, much higher downforce at the same drag.

Thanks for that post as well. yes, and depending on the wing, that angle might be at near 17 degrees in most cases. roof line deflection doesn't do that much, and most of the new wings are positioned much higher and further back to get more clean air, which again, is less deflected and doesn't effect the AOA near as much.

thanks Winders for that posting again.... Hey, its where I got most of my information. its based on the custom wing, (something close to represent the DC10 wing ), and it shows clearly my point. Don't get too misled on the better L/D ratios of the gurney vs no gurney flap comparison. that is NOT the point here. the point is , at what point does a GF installed wing, produce Less drag vs than a wing without one? that is the main issue here. Clearly, without doubt here, all those charts point to more drag with a GF unless you are near stall range of the wing without one.
do you see that??? What do you take away from the article??
attached are graphs from that article. below, graphs from the 23012 NACA wing. (more closely resembles our wing, though the 4412 is best)

winders

You really need a dictionary....

mark kibort

anecdotal vs empirical?

Not enough statistical information makes empirical results, anecdotal.
Im not an English major, that's mom's area. But what am I missing here?

mark kibort

Does everyone see on that second set of very clear graphs, that with the L/D curve for no gurney vs gurney starts out with for the same lift, GF has MUCH more drag , until you get near stall? in no place does the GF produce any less drag, ever UNTIL, you reach stall. then the GF does its magic.

my take away from that is, if you are not using an extreme wing angle, using max lift of a given wing, then don't use the GF. if you need more than it can provide, bolt on the Gurney Flap!

and by the way, most everyone that has a GF has one that is about 5%, so use that figure and the results are pretty telling (i.e. 5% is in the 1/2" high range /12mm for a 12-13" wing cord)

it seems the wing in the graphs, 22012 NACA, maxes out at about a Coefficient of lift of 1.3 and that happens at about 17 degrees. Drag at that point is about .12 Cd. at that same lift, with a gurney flap, drag is about .14. But remember this is at max lift of the wing before stall. still, even at max lift, the GF produces near 10% more drag.
NOW, if you go to a 7 AOA point on the plain wing vs 0 AoA on the GF wing, producing .7 Cl guess what the drag is for both those wings?????
.025 Cd for the wing alone, and .075 for the GF wing..... that's 300% more drag!!!!

Folks, im not making this up, this is wind tunnel testing. and by the way, .7Cl is not that little of a lift value. that's about half the wings potential at full pre-stall angle .

chartersb

Fascinating thread and I appreciate all the time people have spent analyzing and arguing over the data. This weekend we will be at the ACE wind tunnel at the University of Ontario Institute of Technology in Oshawa Ontario testing the NET effects of GF's on actual down force and drag. We are not engineers but regular racer guys and our results will be in real world terms that we understand. For various configurations and at speeds up to 130 mph we will determine down force at each contact patch in lbs; lateral forces in lbs at each contact patch; drag in lbs and horsepower loss.

mark kibort

awesome! Love to be there to see it all! make sure you do a test with a flat wing with GF ... (which might be 7 degrees AoA due to roof line air deflection) and then do a test with 7-10 degrees with the bare wing. my guess is that you will see substantial drag with GF. I bet near 25lbs more drag if you are getting 500lbs of downforce at the rear at near 130mph.
The cup car wing I was testing put down 275lbs of downforce at 120mph, so , based on Richards comments, I don't think I was anywhere near max AoA. Im guessing with the height of my wing and roof line geometry, 7-8 degrees deflection + 7 degrees of wing incidence, equals about 15 degrees AoA total the way I show up to the track. If I needed more downforce, would just bolt on the GF that came with the wing. .4" 4% cord if I remember .