mark kibort

and that was my complaint about the responses and quoting of the results on the "DC10" paper. boundary layers change with speed. also the wing they were working with was thin, high lift. much different characteritics to a cup car wing.

Dont confuse vortex generators, which work on a different principle and function than a gurny flap. they keep flow adheared to the wing, so that the control surfaces have flow over them at the expence of some drag. gurney flaps, without question helps a given wing, become more effective at max angles.

Now, also agreed to is that a stalled wing. (still similar to the comment above), can become "unstalled" at that same angle of attack with the gurney flap, again, showing that it changes the standard airfoil to a high lift airfoil by adding it. now, is a semil stalled wing vs a non-stalled wing producing enough drag to equate to 1 second a lap. probably not. I guess it would depend on the wing downforce and speeds. at sears or laguna, probably not, as the downforce might only be 200 to 300lbs. but at Daytona or Sebring, or other high speed tracks, that 1-2 ftlbs of engine torque lost might be more like double or triple that, (if the semil stalled drag is doubled or trippled) and that could cost a second a lap if so.

Now the charts.

Look at an a lift coeffieint of 1.0 look at the angle of a Gurney flap wing vs a non gurney flap wing. its 5 degrees vs 12 degrees.

NOW, look at the drag of 5 degrees with GF vs 12 degrees of non- GF.

what do you see? I see that there is about a 20% increase in drag for a gurney flap on a wing producing ths same amount of downforce.

In other words:

with gurney flap drag is .075 to .1 Cd
without gurney flap, drag is .05 Cd Both are the same coeficient of lift , of 1.0Cl

To get 1.0Cd, you need 12 degrees of AOA for the non-gurne flap wing, and only 5-7.5 degrees for the gurney flap wing. (5-7.5 depending on the height of the GF)
all gurney flaps at 1.0 Cl have an increase of drag over a non GF wing. the lower heights are almost the same, but anthing from 2-5% will increase drag for the same LIFT OR DOWNFORCE, to .075 or .1 Cd. this is 50% to 100% increase of drag for the same downforce.

Ill have to check your numbers as they dont look right.

EDIT: Yep, if you put on a wicker of near 1/6" , you get increased lift and not really any increased drag. 1/6" is about 1.25% of a 13" width cup car wing. that woud be near a bump on the end of the wing. Oh guess what, its already installed stock on the cup car wing! My boys from Cal Poly said so!!

mark kibort

where do you see this? what am i missing?

at 5% GF curve, shows 2x the lift. it shows at 5 degrees, .5 Cl to 1.0 Cl and at 10 degrees AOL, its .7 Cl to 1.4 Cl. how do you get 6x????

There is no confusion, unless I am missing something here, but I poste the Lift to drag curves that show the drag differece for any given Cl.

say we are looking at 1.0Cl with a 5% wicker , its 1.0Cd without a wicker the drag is .07Cd. a 25% reduction of drag for the same lift or downforce with a standard wing, no wicker (GF)

is that where youre confusion is coming from? or mne. let us know. Kind of an important point!


EDIT: oh richard I see your 6x problem. you cant use something near 0 and expect that a % gain, might mean anythng. using that same logic, i can find a point where the drag goes up by 600% with a wicker too.
in otherwords, the wicker adds about .05 Cl, from beginning to end. sure, if you start at 0 cl, adding a wicker adds infinite % lift, but does that mean anything???

Rich Sandor

In racing applications, wing sizes and dimensions are often limited. When you have maxed out the length and width of a wing, and it's angle of attack, as per class rules, you can still get more out of it by adding a gurney flap (if allowed) with the resultant drag penalty being negligible.

However, if you have the choice of adding a gurney flap to a wing to get an increase in downforce, or redesigning the wing completely and correctly to get the same increase without a gurney flap, the latter will have the lower coefficient of drag. It's just that the nature of aero is so complex that you may design a wing that meets 99% of your needs, and the only way to fill that 1% gap is with a band-aid solution like a vortex generator or stall strip or gurney flap.

For those of you who have tried it, and thought it failed for you, here's an interesting tidbit of information about it's origin: Dan Gurney first used it on Bobby Unser's car, and when Bobby went out, his lap times were just as bad as before. Everyone thought the idea had failed. In secrecy, Bobby told Dan that the car had so much more rear downforce, that it was now understeering in all the corners. Dan gave the car some more front downforce and the lap times dropped like a brick.

Rich Sandor

As I said above, in some cases, the wing's AoA and/or dimensions (and/or your wallet) are maxed out, you have no other avenue to increase downforce.

mark kibort

Scott, this is exactly what the discussion is about. Is this true?

from the graphs, it isnt.

If it is, can you show me on point besides at near stall AOA, where it is true?

Richard J also made a few comments to the effect of the confustion of this point. he mentioned that there was no graph showing where you could compare drag at equal downforce, but actually,it is on the graphs provided.

If you look at the graphs, you can see that at all points of equal Coefficient of Lift. (meaning the wing is making the same downforce or lift) that the drag will always be greater when you use a Gurney flap. this different in higher drag, maybe up to 50% more, but in the end, drag due to lift (or downforce in racing)
is so little, it would be a rounding error in performance effects, unless you are running at a really fast track. Its not that Im suddenly discounting the drag for reasons of being in favor of the GF for all wing settings, Im just pointing this out to be fair.

Im not going to look at my competitor with the exact same downforce rear wing, and think to myself, "wow, he has a GF, i have an advantage due to drag", nor will my competitor be able to say, wow, I have a gurney flap so Ill be a second faser a lap (all things being equal) ".
Again, the real reason for the gurney flap is to get more out of a wing, when AOA is maxed on a given wings performance. It basically effectviely gives you 7 more degrees of AOA past the wings natural max point, without worrying about stalling the wing.

mark kibort

Well Rich, for some reason, you and I are the only ones that interpret the data this way.

The problem is the verbage is mixed with airplane applications which might lead the reader to misinterpret the the test results that are so clearly shown in both studies.

Here is the bottomline. you use a gurney flap, not at an angle that the normal wing will be near stall, and your drag goes up near 35% for the same downforce. (i was saying 20-25% before but its actually higher in most cases)

Another good reason to use a GF, might be in ranges where you dont know where the stall angle is. that way, at 35% drag cost, at least you avoid doubing drag, if you did end up stalling, with a setting near the edge. this "edge" is hard to find, as we have a roof line angle that can increase dramatically the wing angle from the horizontal plane. Im at 7 degrees now, but with a 9 degree air flow deflection, that is near 16degrees actual AOA.
you can see how some wings might actually be operating at the stall angle if you see their settings at the track. those would be great candidates for a GF.

Rich Sandor

I think source of contention is we all have differing opinions on when to use a gurney flap. Some would use one without question, others would weigh the need carefully, and Mark will probably never run one.

Mark, bear in mind, as you raise a rear wing from the trunk up to the roof line, you have to increase it's angle of incidence to keep the same angle of attack.

mark kibort

I would absolutely use one, only when ive maxed out the effectiveness of the current cup car wing. Strictly based on the curves, for which I am making my assesement of that choice . I have one, ready to go. BUT, did you know we all HAVE a 1.5% gurney flap, in the shape of the Cup car wing anyway??

So, actually, I am using a GF today, its 1.5% of cord and it gives me some safety margin should I deciede to raise the wing to roofline heights.

Did you see my last post? I talked about the deflection of most of our cars roof line as the flow is forced downward to the lower wing heights gving higher than horizontal effective AOA.


Here is a pic of a stock cup car wing from 2004-5

It has a GF! well, its attually, a DTE. (Divergent Trailing Edge). its effects are the same as a GF.

J richard

Mark,

Forgive me but I'm not going to follow you down the rabbit hole...

The report and graphs are from a comparitive analysis of a traditional gurney strip with a t strip. Not the efficiency of the addition of a Gurney to an airfoil. Why that is important is your thesis is outside of the spectum of the study being used. The results are equivilent to putting a carpenters level on the floor and announcing the world is indeed flat.

Why I said you need a graph which compares the drag coefficient of each foil configuration with the angle of incidence is that by making a reverse comparitive you leave out any variables that factor into the CL. Thats a big assumption, remember that these are measured not computational results which are clearly limited by the basic test rig as described in the first paragraph. And as you note the narrower you define the spectrum of the results the easier it is to produce erronious results.

The other issue that makes me question the envelope that produced the CD chart is that the typical result of a stall is a parabolic spike in CD, not a trailing off as indicated in the chart, the standard curve of an airfoil CD in variable angles of attack is a lopsided inverted parabola. Why is this important? because it indicates the limits of the test, they don't care what the results are because they are outside the focus of the test. This is also fundamental in the discussion of why you use a gurney flap to begin with. Thats why I included the CalPoly paper, it is more to the point of the discussion.

There is a basic misunderstanding of the point of using a gurney flap: it makes an airfoil more efficient. It IS a boundry/laminar flow tool, just like a VG or stall strip or strake. Once it protrudes into the airflow proper it becomes another thing altogether.

As you found you have a form of gurney on your cup wing, why would they have gone through the effort to include it if a simple angle change would do the same? because it is more efficient. Also you only have the top element, the lower element on the cups have another taller wicker on the base spoiler. That one is elemental, not blended in. Perhaps Porsche knows something.

Anyway I have to go milk my cows in farmville...

mark kibort

yes, the old rabbit hole of logic.

where did you get the idea that the graphs were comparisons of T strips and gurney flaps??? this a huge error in your evaluation, in that the tests were two independant ones. the graphs I recently posted were very clear print-wise, but the later graphs from the other study say the exact same thing. You make such an emphatic statement that it doesnt address the efficiency of the gurney flap, but yet the graph is identical to a study ONLY analysing the effects of the Gurney flap. you didnt read them did you? thats ok, but dont make conclusions until you. if you did, re-read them. Again, there are separate graphs for T strips and Gurney flaps. not related.

To your second point, you are right. these are actual results and they test the gurney flap on a wing and a wing without one, and measure the results. there are no assumptions. at 1.0Cl, you have an angle of 5 degrees for a gurney flap wing, and this equals 12 degrees on a non-gurney flap wing. you can use whaever values you want. the results are the same. there is EVEN a very clear graph showing with any level of gurney flap, you get an increase of drag. its very low at very low GF heights, but at the usuable height that we would see. 1.5% or greater, the effects are that the GF has near 40% more drag for the same lift.

just on the above, explain further what your objection of this conclusion is. its almost like you are not reading the graphs properly or are missing the fact that the graphs have almost all the data to substatiate what Rich and I are talking about here. the gurney flap alows for a greater range of downforce for a given wing. it has nothing to do with efficiency. It basicallly changes the wing to another effective shape. The gurney flap has already been installed. yes, it is a good idea. its a good idea becuase most cup cars have a need for some very high downforce by their design. a standard wing at that size might now work as effectively enough. since the drag cost is very small, why not give yourself some upside if you need it. If I needed more downforce, i would use one now. when I do, I will.

I posted the drag coefficient for each airfoil and all the different AOL and Lift and drag coefficients. what is missing?

the cup car has an issue with the rear deck lid. by using a wicker or spoiler there, they increase efficiency in drag. just as our little spoilers on the 928 did. (lowering the Drag Coefficient).
Now, the wicker doesnt make the wing more efficient. that would be a better lift to drag ratio. no where in the graphs is this seen. as I said, its just the opposite. with a wicker, or Gurney flap, the drag goes up for a given downforce or lift by near 30%. YOU use the gurne flap when you want a different downforce charateristic. when you want more than you can normally get without getting near or at stall.

Those curves are accurate for perforance up until stall and afterward. after all, they are actual tests. all 3 of the tests and experiments show the exact same trend of data. the first one examines the GF and also T strips , the next more gurney flap but DC10 specific. (can you get more away from our type of wing? ) but it also shows the exact same results. graphs posted, AND, the cal poly tests also show the same results.

so, I'm confusd Richard, what did you think is left out of the tests I was using in my conclusion? why do you think GFs make the airfoil more efficient? can you show me on any of the 3 tests where you think efficiency was gained by using the Gurney flap???

Why do you think the VG is the same as a GF? a VG increases drag by creating vortexes that keep the flow attached during near stall conditions by allowing the control surfaces to operate. a GF basially is changing the angle of attack, and also keeping flow from separating to keep the wing flying. two different functions. they are similar in the fact that they are bolt on devices.

Here are the graphs of the 2nd study that says basically the same thing as the first study. I also added a near idential wing with its lift to drag characteristics.
I dont understand why you are suspect of the "Envelope of testing" the stall values and beyond look very normal for a wing at and beyond stall.

mark kibort

Richard, you make some claims here that I think many of that are aruging the point of the GF makes the wings more efficient are making.

Let me put some comments directly in your text, if you didnt have the patience to read by longer response in the prior post.

[QUOTE=J richard;8381120]Mark,

The report and graphs are from a comparitive analysis of a traditional gurney strip with a t strip. Not the efficiency of the addition of a Gurney to an airfoil. Why that is important is your thesis is outside of the spectum of the study being used. The results are equivilent to putting a carpenters level on the floor and announcing the world is indeed flat.
>>>>>>>>>>>>>>not true at all. all tests on the "first paper" had individual results for the "T study" and the Gurney flap experiments. they match other studies and tests focused on Gurney flaps.

Why I said you need a graph which compares the drag coefficient of each foil configuration with the angle of incidence is that by making a reverse comparitive you leave out any variables that factor into the CL. Thats a big assumption, remember that these are measured not computational results which are clearly limited by the basic test rig as described in the first paragraph. And as you note the narrower you define the spectrum of the results the easier it is to produce erronious results.
>>>>>>>>First of all , angle of incidence doesnt equal angle of attack .
angle of incidence is a wing's orientation to the longitudinal axis of a fusalage of an airplane for example. angle of attack, is what you probably mean. angle of incidence is a HUGE difference when looking at drag figures of a DC10 model for example, as that will force the horizontal stabilizers to fight the increased lift in a completely different way, grossly changing drag results of the aircraft.

NOW, to your point, the Cl vs drag vs angle of attack values are clearly stated on both sets of curves, nothing is left out, assumed , or calculated. If there is, I sure would lke to know what you are talkng about there.

The other issue that makes me question the envelope that produced the CD chart is that the typical result of a stall is a parabolic spike in CD, not a trailing off as indicated in the chart, the standard curve of an airfoil CD in variable angles of attack is a lopsided inverted parabola. Why is this important? because it indicates the limits of the test, they don't care what the results are because they are outside the focus of the test. This is also fundamental in the discussion of why you use a gurney flap to begin with. Thats why I included the CalPoly paper, it is more to the point of the discussion.
>>>>>>>There is nothing unusual about the data showing the wings performance near and beyond stall. there is an unstable region beyond stall that is unique to the wings design, as aerodynamic center and L/D coeffieints at different angle of attack. even the Cal poly paper, specifically points out that the lift to drag ratios ONLY improve at a lift coefficient of greater than 1.4 for that tested wing.


There is a basic misunderstanding of the point of using a gurney flap: it makes an airfoil more efficient. It IS a boundry/laminar flow tool, just like a VG or stall strip or strake. Once it protrudes into the airflow proper it becomes another thing altogether.
[I]>>>>>>>>>yes, it is a tool that can be bolted on to any wing to change its characterisitcs. it basially changes its effective form to a higher lift wing . It makes it no more efficient than a different wing is more efficient. what I mean by that, it will be more efficient in some areas and not in other. It is clear is is not "more efficient" in moderate angles of attack. in fact , it makes a wing less efficient, by creating more drag for the same lift at angles of attack less than near stall. it changes the aerodynamic center of the wing and incresases lift coeficient. [/I]

As you found you have a form of gurney on your cup wing, why would they have gone through the effort to include it if a simple angle change would do the same? because it is more efficient. Also you only have the top element, the lower element on the cups have another taller wicker on the base spoiler. That one is elemental, not blended in. Perhaps Porsche knows something.
>>>>> porsche designed the wing, added, effectivlely, a 1/8" gurney flap, (about 1.25% of cord) but really a DTE (or divergent trailing edge for the same effect) and this gives a high lift wing design, even more lift and gives a slight level of safety margin at higher angle of attacks. (at a very negligible drag increase). QUOTE]

J richard

Yes...

multi21

I'm very grateful.

What if Kibort worked on a race team. Imagine he is "the guy" that comes into every meeting with arms full of graphs, spreadsheets, diagrams, blueprints and still photos. Then imagine, he works side by side with you on said race team and argues his point in person to the nth degree.

Fortunately, we have the option of of ignoring him or this thread. Some may not have that option. Stating the obvious? Perhaps, but I'm grateful, nonetheless.

mark kibort

You know its funny, the stuff we are seeing and stating are obvious. why the involved discussion. you say that the gurney flap makes a wing more efficient, so produce one shred of evidence to the point. you cant, becasue it doesent, unless certain conditions are met.
Im only dragging this thing out, because no one except Rich seems to get it, and I respond to statements that seemed to be riddled with errors.
The actual article you choose to use for your opinion basis, even states, L/D ratios are only better with Gurney flaps, AFTER a Lift coefficient of 1.4 is achieved. in otherwords, near max lift of that particular wing tested.

Guys, its a good discussion. Are you that afraid of learning something contrary to your original beliefs? Wow! that is sad if true. even sader that you migh not open your
Certainly if someone could prove my analysis wrong, i would LOVE to see it.
there is one point that you need to think about . Racing at a fast track.
If you have a choice to race your buddy in an identical cup car, and you have 40% less drag down the straight do to the drag due to lift and the SAME lift, would you choose to install a gurney or not and just increase wing angle. That is the discussion in a nut shell.



Pete, yeah, just imagine a guy that comes in with 2 graphs and says to a team at daytona. "hey guys , it seems the Gurney flap only really helps the wing at near stall, so if we remove it and just use the wing in natural form, we can get the downforce we need on the turney sections, and save 40% drag on the straights. anyone arguing by pulling up some stats regarding a study of wind tunnel tests of a DC10 and its glide slopes, would be laughed out of the room. Trust me, what Im saying is not rocket science. The facts dont lie, the verbage message can be lost in the translation.

On a race team, my discussion, owner, eng. or driver would be the same. show me the facts and how its relevant. Its the teams that dont use the facts or interpret the data in the correct way, that end up at the back of the pack.

And VR wouldnt know the differnce between the effect of a gurney flap and a Flap Jack.

J richard

Mark, conclusions from the calPoly Paper:

"In comparison with a clean airfoil, lift coefficient and nose-down pitching moment were
increased by the Gurney flaps. However, larger Gurney flaps will increase lift at the expense of
increasing drag. Gurney flap sizes less than 1.25% of the main airfoil chord will result in an
increased lift coefficient, with very little increase in drag. In fact, at higher lift coefficients the drag is
lower than that of the clean airfoil configuration. The separation point of the NACA 4412 airfoil
with a Gurney flap is farther aft at moderate angles of attack than that of a clean airfoil. Also, the
use of the Gurney flap increases the loading along the entire length of the airfoil, with a large
increase in trailing-edge loading.
The Gurney flap is an intriguing device for high-lift design because of the mechanical simplicity
of the device and the significant impact on aerodynamic performance. Subsonic aircraft could
greatly benefit from the use of this simple flat-plate device..."

I can find you a dozen more if you like, but I have to harvest my corn in farmville...