Gurney Flap Study
03-11-2011, 09:26 PM
#31
Race Director
If Trakcar saw a 1 sec difference I believe him...Peter is an excellent driver and he know his car very well.
03-12-2011, 07:57 AM
#32
Lifetime Rennlist Member
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
"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
A 0⋅5% chord, full-span Gurney flap on the wind-tunnel model of the DC-10 created a 20% increase in total aircraft lift, and practically no change in total aircraft drag during the second segment climb configuration. Obviously, such an increase in lift instigates an increase in lift-induced drag. The aircraft drag being constant was deemed a spectacular result.
How in the world anyone can conclude that this study says a Gurney is a bad choice amazes me. But everyone is a genius in cyberspace.
03-12-2011, 11:40 AM
#33
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I was in the Robertson Racing (ALMS Ford GT's) garage the other day and looked at their Gurney flaps. they are HUGE, like well over an inch. I asked them why so big, they said it was the required height for the GT2 cars and it slowed them down considerably.
I just put a 10mm gurney on my 63" wing, interested in seeing how it works when I test it out next month.
I just put a 10mm gurney on my 63" wing, interested in seeing how it works when I test it out next month.
03-12-2011, 02:14 PM
#34
Nordschleife Master
The gurney flap originated as a band-aid solution to get more downforce out of a surface that was not adjustable (or already adjusted to it's maximum) as Kibort mentioned. It remains as such today: a band-aid solution. However, that does not mean it's a "bad thing," as it does have other benefits.
It's ease of addition/replacement and low cost are the two primary advantages. A proper wing that would accomplish the same thing would have to be either multi-element or radically curved in a short space, which raises the cost of production excessively. You don't see gurney flaps in aviation because aircraft use movable surfaces that are as drag free as possible when neutral-position/cruise, whereas racecars always need the downforce more often than not. If we could have wings that self adjust to low drag for straights, then nobody would use gurney flaps.
It's ease of addition/replacement and low cost are the two primary advantages. A proper wing that would accomplish the same thing would have to be either multi-element or radically curved in a short space, which raises the cost of production excessively. You don't see gurney flaps in aviation because aircraft use movable surfaces that are as drag free as possible when neutral-position/cruise, whereas racecars always need the downforce more often than not. If we could have wings that self adjust to low drag for straights, then nobody would use gurney flaps.
03-12-2011, 06:48 PM
#36
Rennlist Member
)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
"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
as far as lap time improvement. really?? a gurney flap, worth 1 second? Hey, do you believe in the tooth fairy too? com'mon. are we that naive and uneducated to not ask the question, what is REALLY going on. To stay on topic, the gurney flap didnt create this lower lap time, its the downforce, which, if he wasnt maxed out in wing setting, could have been achieved with a higher AOA. does that make sense.
Agree but some folks see this as a place to try to show their superior knowledge at the expense of others and often choose to ignore the facts. Also from the same study:
A 0⋅5% chord, full-span Gurney flap on the wind-tunnel model of the DC-10 created a 20% increase in total aircraft lift, and practically no change in total aircraft drag during the second segment climb configuration. Obviously, such an increase in lift instigates an increase in lift-induced drag. The aircraft drag being constant was deemed a spectacular result.
How in the world anyone can conclude that this study says a Gurney is a bad choice amazes me. But everyone is a genius in cyberspace.
A 0⋅5% chord, full-span Gurney flap on the wind-tunnel model of the DC-10 created a 20% increase in total aircraft lift, and practically no change in total aircraft drag during the second segment climb configuration. Obviously, such an increase in lift instigates an increase in lift-induced drag. The aircraft drag being constant was deemed a spectacular result.
How in the world anyone can conclude that this study says a Gurney is a bad choice amazes me. But everyone is a genius in cyberspace.
Did you fail to read the graphs??? I did read this DC10 information. my father was an aero engineer at NASA and flew these jets as well. We both were looking at the two separate studies showing the graphs that would contradict the DC10 tests. So many differences of a Dc10 wing vs a Cup car wing, i wouldnt know where to start. constant cord, variable cord, not to mention NACA number gross differences, and YOU are amazed that I would question that result, vs the actual tests of the devices in question?? Man, you need to open your eyes!
Now, let me let you in on another possiblity. the DC10 wing, has a certain L/D ratio, right? if the gurney flap changes its Lift/drag ratio, its like it changed its shape to a high lift wing. It has little or nothing to do with a flat bottom surface wing, that is constant cord such as our Cup car wings , in question.
this is basic aero 101 . You seem to think you are be better qualified to interpret the tests presented here.
Im using facts. argue the facts. what are the issues with the facts I have presented here???
The gurney flap originated as a band-aid solution to get more downforce out of a surface that was not adjustable (or already adjusted to it's maximum) as Kibort mentioned. It remains as such today: a band-aid solution. However, that does not mean it's a "bad thing," as it does have other benefits.
It's ease of addition/replacement and low cost are the two primary advantages. A proper wing that would accomplish the same thing would have to be either multi-element or radically curved in a short space, which raises the cost of production excessively. You don't see gurney flaps in aviation because aircraft use movable surfaces that are as drag free as possible when neutral-position/cruise, whereas racecars always need the downforce more often than not. If we could have wings that self adjust to low drag for straights, then nobody would use gurney flaps.
It's ease of addition/replacement and low cost are the two primary advantages. A proper wing that would accomplish the same thing would have to be either multi-element or radically curved in a short space, which raises the cost of production excessively. You don't see gurney flaps in aviation because aircraft use movable surfaces that are as drag free as possible when neutral-position/cruise, whereas racecars always need the downforce more often than not. If we could have wings that self adjust to low drag for straights, then nobody would use gurney flaps.
03-12-2011, 06:52 PM
#37
Rich,
The bottom line is that using a properly sized Gurney flap on an existing wing can give you more downforce with little to no drag penalty. For our application, that is a major benefit.
Not at all what Kibort said.
Scott
The bottom line is that using a properly sized Gurney flap on an existing wing can give you more downforce with little to no drag penalty. For our application, that is a major benefit.
Not at all what Kibort said.
Scott
03-12-2011, 07:02 PM
#38
From the paper I posted a link to:
6.0 CONCLUDING REMARKS
Very low Reynolds number performances of a thin high lift aerofoil with and without Gurney flaps have been investigated. It was found that Gurney flaps produce an upward shift in the lift coefficient that is approximately proportional to the flap height. At angles below stall, the drag increase to the Gurney flap with a height less than the trailing edge boundary-layer is less than 20%. For flaps of larger height, or for an aerofoil that has stalled, the drag increase can typically become up to twice that of the plain aerofoil. The maximum lift to drag ratio occurs when the [Gurney] flap height is about 90% that of the trailing edge boundary-layer thickness. This leads to an optimum height equation that enables the efficient optimisation of any constant-chord aerofoil. The experiments have shown that as long as the height remains less than the thickness of the boundary-layer at the trailing edge, the additional drag will be negligible, which is coherent with other data available in the technical literature for high Reynolds number flows. The flap, therefore, can be applied to a number of low Reynolds number systems, including gliders, micro air vehicles, wind turbines, etc.
Before you say anything, please realize the guys that wrote the paper know more about the paper and the subject than you!!
Scott
6.0 CONCLUDING REMARKS
Very low Reynolds number performances of a thin high lift aerofoil with and without Gurney flaps have been investigated. It was found that Gurney flaps produce an upward shift in the lift coefficient that is approximately proportional to the flap height. At angles below stall, the drag increase to the Gurney flap with a height less than the trailing edge boundary-layer is less than 20%. For flaps of larger height, or for an aerofoil that has stalled, the drag increase can typically become up to twice that of the plain aerofoil. The maximum lift to drag ratio occurs when the [Gurney] flap height is about 90% that of the trailing edge boundary-layer thickness. This leads to an optimum height equation that enables the efficient optimisation of any constant-chord aerofoil. The experiments have shown that as long as the height remains less than the thickness of the boundary-layer at the trailing edge, the additional drag will be negligible, which is coherent with other data available in the technical literature for high Reynolds number flows. The flap, therefore, can be applied to a number of low Reynolds number systems, including gliders, micro air vehicles, wind turbines, etc.
Before you say anything, please realize the guys that wrote the paper know more about the paper and the subject than you!!
Scott
03-12-2011, 07:08 PM
#39
The Penguin King
Rennlist Member
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I concluded the same thing, and had previously to reading the article assumed essentially the same thing.
M.K. in 5,4,3,2....
M.K. in 5,4,3,2....
03-12-2011, 07:08 PM
#40
Rennlist Member
It seems there is 3 sets of facts here.
1. we have the graphs from the experienments, detailing the effects of the GF at different AOLs, with cooresponding drag.
2. We have a separate set of graphs based on differnent experienment, saying the same thing
3. we have this excerpt of the GF effect on a totally different type of wing, swept as well, and completely different thickenss as well, s compared to a cup car wing which is a constant cord, thickness wing. Hmm, sound similar? hardly. Then, you take one value, 20% increased lift, without giving any consideration to the AOLs or other conditions, such as speed tested, and equate that test as being an indication of results you can find at the track?? REALLY? are you kidding me? The only "facts" here are the graphs. the rest are interpretations of the facts with a myriad of conditions.
Now, if you can get 20% more lift, with no increase in drag that would a nice benefit. is it earth shattering? If you have a wing on the car, its lift to drag (L/D) is somewhere in the 10:1 range if you put down 200lbs of downforce, that is 20lbs of drag. 20% more lift would be 40lbs more dowforce and 4lbs more drag normally. you are saving 4lbs of drag, through a 4:1 gear ratio and 2ft diameter tires, that would be 1 ft-lb. So, if by using the gurney flap vs just angle alone, is the mod worth 1ft-lb of torque at the engine if it was true? Its barely a rounding errror and certainly wouldnt be worth any measurable time at the track.
Now, if you could not get the downforce by the angle available. (i.e. maxed out already) then, yes, the gurney flap is a great idea. it does nothing more than change the effective shape of the wing to a higher lift version.
BUT, buy using the graphs based on wings similar to what we use at the track, the data contradicts those results. Do DC10s all have gurney flaps now?? No they dont!
who is ignoring the facts?
1. we have the graphs from the experienments, detailing the effects of the GF at different AOLs, with cooresponding drag.
2. We have a separate set of graphs based on differnent experienment, saying the same thing
3. we have this excerpt of the GF effect on a totally different type of wing, swept as well, and completely different thickenss as well, s compared to a cup car wing which is a constant cord, thickness wing. Hmm, sound similar? hardly. Then, you take one value, 20% increased lift, without giving any consideration to the AOLs or other conditions, such as speed tested, and equate that test as being an indication of results you can find at the track?? REALLY? are you kidding me? The only "facts" here are the graphs. the rest are interpretations of the facts with a myriad of conditions.
Now, if you can get 20% more lift, with no increase in drag that would a nice benefit. is it earth shattering? If you have a wing on the car, its lift to drag (L/D) is somewhere in the 10:1 range if you put down 200lbs of downforce, that is 20lbs of drag. 20% more lift would be 40lbs more dowforce and 4lbs more drag normally. you are saving 4lbs of drag, through a 4:1 gear ratio and 2ft diameter tires, that would be 1 ft-lb. So, if by using the gurney flap vs just angle alone, is the mod worth 1ft-lb of torque at the engine if it was true? Its barely a rounding errror and certainly wouldnt be worth any measurable time at the track.
Now, if you could not get the downforce by the angle available. (i.e. maxed out already) then, yes, the gurney flap is a great idea. it does nothing more than change the effective shape of the wing to a higher lift version.
BUT, buy using the graphs based on wings similar to what we use at the track, the data contradicts those results. Do DC10s all have gurney flaps now?? No they dont!
who is ignoring the facts?
Agree but some folks see this as a place to try to show their superior knowledge at the expense of others and often choose to ignore the facts. Also from the same study:
A 0⋅5% chord, full-span Gurney flap on the wind-tunnel model of the DC-10 created a 20% increase in total aircraft lift, and practically no change in total aircraft drag during the second segment climb configuration. Obviously, such an increase in lift instigates an increase in lift-induced drag. The aircraft drag being constant was deemed a spectacular result.
How in the world anyone can conclude that this study says a Gurney is a bad choice amazes me. But everyone is a genius in cyberspace.
A 0⋅5% chord, full-span Gurney flap on the wind-tunnel model of the DC-10 created a 20% increase in total aircraft lift, and practically no change in total aircraft drag during the second segment climb configuration. Obviously, such an increase in lift instigates an increase in lift-induced drag. The aircraft drag being constant was deemed a spectacular result.
How in the world anyone can conclude that this study says a Gurney is a bad choice amazes me. But everyone is a genius in cyberspace.
03-12-2011, 07:25 PM
#41
Rennlist Member
Give me your quick summary.
From the paper I posted a link to:
6.0 CONCLUDING REMARKS
Very low Reynolds number performances of a thin high lift aerofoil with and without Gurney flaps have been investigated. It was found that Gurney flaps produce an upward shift in the lift coefficient that is approximately proportional to the flap height. At angles below stall, the drag increase to the Gurney flap with a height less than the trailing edge boundary-layer is less than 20%. For flaps of larger height, or for an aerofoil that has stalled, the drag increase can typically become up to twice that of the plain aerofoil. The maximum lift to drag ratio occurs when the [Gurney] flap height is about 90% that of the trailing edge boundary-layer thickness. This leads to an optimum height equation that enables the efficient optimisation of any constant-chord aerofoil. The experiments have shown that as long as the height remains less than the thickness of the boundary-layer at the trailing edge, the additional drag will be negligible, which is coherent with other data available in the technical literature for high Reynolds number flows. The flap, therefore, can be applied to a number of low Reynolds number systems, including gliders, micro air vehicles, wind turbines, etc.
Before you say anything, please realize the guys that wrote the paper know more about the paper and the subject than you!!
Scott
6.0 CONCLUDING REMARKS
Very low Reynolds number performances of a thin high lift aerofoil with and without Gurney flaps have been investigated. It was found that Gurney flaps produce an upward shift in the lift coefficient that is approximately proportional to the flap height. At angles below stall, the drag increase to the Gurney flap with a height less than the trailing edge boundary-layer is less than 20%. For flaps of larger height, or for an aerofoil that has stalled, the drag increase can typically become up to twice that of the plain aerofoil. The maximum lift to drag ratio occurs when the [Gurney] flap height is about 90% that of the trailing edge boundary-layer thickness. This leads to an optimum height equation that enables the efficient optimisation of any constant-chord aerofoil. The experiments have shown that as long as the height remains less than the thickness of the boundary-layer at the trailing edge, the additional drag will be negligible, which is coherent with other data available in the technical literature for high Reynolds number flows. The flap, therefore, can be applied to a number of low Reynolds number systems, including gliders, micro air vehicles, wind turbines, etc.
Before you say anything, please realize the guys that wrote the paper know more about the paper and the subject than you!!
Scott
"thin, high lift air foil". Hmmm, do we use a thin high lift airfoil on the cup car? no, its a traditional airfoil. not teardropped as he refers to.
1. lift coefficient goes up proportional to the GF height
2. drag going up by 20% for GF less than boundary layer height (this is loaded too).
3. GF of a height larger than boudary layer flow, can create drag 2x that of a normal wing.
so, summary, as long as the GF is lower than the boundary layer, there is an increase of the Lift coeficient and the drag will go up negligibly.
Again, this is for low reynold number high lift, thin wings. (constant cord)
Moreover, we have graphs to examine the effects of GFs on wings like ours.
there seems to be a contradition in results here. what am i missing??
So, why dont you tell me what he is saying? The Gurney flap can be applied to ...(all sorts of vehciles moving through the air) ....... and what would be the effects? any downsides?? Limitations
Going back to the graphs, show me equal downforce and same or less drag. I dont see it on any of the graphs.
03-12-2011, 07:40 PM
#42
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"If a large transition is needed a lip can spoil the air into the low-pressure area lowering drag. Air flowing over the bottom of a wing at low angles of attack speeds up over the thick part of the wing and then slows down until, at the trailing edge, it's at the same speed as the air that went over the top of the wing. As the air slows down, its static pressure rises and the boundary layer thickens. At higher angles of attack, the boundary layer can develop well enough to generate eddies and flow separates from the surface. When you have separated flow, as in the top sketch, drag goes up and the wing can stall.
If the air close to a fixed boundary is swirling, at any moment some of it is moving toward the boundary making separation more difficult. When you see vanes and barge boards on a racecar think about how the air sees this obstacle and tries to move from areas of high pressure in front toward lower pressure behind the feature. Often the result is a turbulent, swirling motion that sweeps along the body panel retarding separation.
A Gurney flap allows a wing to operate at higher angles of attack than the same wing without a Gurney. Look at the streamlines on the Gurney sketch. You can see the air has to move up to go around the Gurney lip. That creates a lower pressure right behind the flap that translates around the corner to the bottom of the wing. This guarantees a high-to-low pressure gradient along the bottom of the wing and prevents separation.
"
If the air close to a fixed boundary is swirling, at any moment some of it is moving toward the boundary making separation more difficult. When you see vanes and barge boards on a racecar think about how the air sees this obstacle and tries to move from areas of high pressure in front toward lower pressure behind the feature. Often the result is a turbulent, swirling motion that sweeps along the body panel retarding separation.
A Gurney flap allows a wing to operate at higher angles of attack than the same wing without a Gurney. Look at the streamlines on the Gurney sketch. You can see the air has to move up to go around the Gurney lip. That creates a lower pressure right behind the flap that translates around the corner to the bottom of the wing. This guarantees a high-to-low pressure gradient along the bottom of the wing and prevents separation.
"
03-12-2011, 07:46 PM
#43
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Gentlemen could you all please be more polite and just exchange ideas? As far as I know none of us is an expert on the subject. And we are not trying to prove or dismiss a theory.
Gurney lips do work that is a fact, of course we can debate all day about how much they work but IMO the interesting part is to found out how to make them work on our individual cars the best, because for example the last time I tried it on mine I think that I failed.
Thank you.
John
Gurney lips do work that is a fact, of course we can debate all day about how much they work but IMO the interesting part is to found out how to make them work on our individual cars the best, because for example the last time I tried it on mine I think that I failed.
Thank you.
John
03-12-2011, 08:21 PM
#44
Rennlist Member
Scott,
Based on your link and the original link, here are the lift and drag values vs wing settings with different height Gurney flap heights, vs angle of attack for the wing.
This is what I have been talking about. I dont see how the gurney flap does anything else but cost drag, unless you are maxing out your wing without one.
Help me understand its effect or advantage vs just plain wing angle at any other capable angle for a given traditional constant cord, wing.
below is your added article based on the "high lift thin wing"
and the normal wing. in the end, the results are near the same, so its puzzling to see your article's conclusion.
So, find a coefficient of lift. Say, 1.0Cl look at the gurney flap vs no gurney flap at a height you choose, look at the drag vs the same lift will be higher with GF.
Now, look at your links experiement and tests. same thing. use any lift coefficient you want.
compare with and without GF. I dont see how the GF doesnt do anything but help create more lift when maxing out a normal wing happens, and it always causes more drag if a comparable downforce creating wing angle is available.
If Im wrong, let me know what im missing here.
http://www.aoe.vt.edu/~mason/Mason_f/AIAA2007-4175.pdf
Based on your link and the original link, here are the lift and drag values vs wing settings with different height Gurney flap heights, vs angle of attack for the wing.
This is what I have been talking about. I dont see how the gurney flap does anything else but cost drag, unless you are maxing out your wing without one.
Help me understand its effect or advantage vs just plain wing angle at any other capable angle for a given traditional constant cord, wing.
below is your added article based on the "high lift thin wing"
and the normal wing. in the end, the results are near the same, so its puzzling to see your article's conclusion.
So, find a coefficient of lift. Say, 1.0Cl look at the gurney flap vs no gurney flap at a height you choose, look at the drag vs the same lift will be higher with GF.
Now, look at your links experiement and tests. same thing. use any lift coefficient you want.
compare with and without GF. I dont see how the GF doesnt do anything but help create more lift when maxing out a normal wing happens, and it always causes more drag if a comparable downforce creating wing angle is available.
If Im wrong, let me know what im missing here.
http://www.aoe.vt.edu/~mason/Mason_f/AIAA2007-4175.pdf
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
http://www.raes.org.uk/pdfs/2800.pdf
The study you quote seems more interested in T-strip results.
Scott
03-12-2011, 10:03 PM
#45
Rennlist Member
And yet another study as to the usefulness of the wicker, this one is more specific about the height of the strip related to the chord width of the airfoil less than 1.25% has no increase in drag. This is much easyer to figure on a racecar than determining the boundary layer thickness (it changes with speed), since I left my windtunnel in my other coat pocket:
http://digitalcommons.calpoly.edu/cg...ntext=aero_fac
"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"
Mark, the graphs you posted show the same results, the 5%gf curve is producing 6 times the lift at the same incidence as a clean airfoil, and while the drag increases with the addition of a wicker, the additional 6.5deg of incidence required to produce the same lift will be substantially more that the wicker. There is no chart for that which is probably where your confusion is comming from.
Gurney flaps, vortex generators and stall strips are all devices that work in the bounary layer to improve adhesion while minimizing drag, their use is well proven and you can find them on just about everything that flys.
The other thing to consider is the addition of a gurney flap to a wing that is already stalled will dramatically increase lift coeficient while substantially reducing drag. Remember that drag increases exponentally as an airfoil stalls, the drag from a stalled wing is huge. So in the case of a car putting down laps with a semi stalled wing, a wicker could in fact restore the downforce and dramatically reduce drag...could easily be a second of improvement on a fast track.
just sayin...
http://digitalcommons.calpoly.edu/cg...ntext=aero_fac
"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"
Mark, the graphs you posted show the same results, the 5%gf curve is producing 6 times the lift at the same incidence as a clean airfoil, and while the drag increases with the addition of a wicker, the additional 6.5deg of incidence required to produce the same lift will be substantially more that the wicker. There is no chart for that which is probably where your confusion is comming from.
Gurney flaps, vortex generators and stall strips are all devices that work in the bounary layer to improve adhesion while minimizing drag, their use is well proven and you can find them on just about everything that flys.
The other thing to consider is the addition of a gurney flap to a wing that is already stalled will dramatically increase lift coeficient while substantially reducing drag. Remember that drag increases exponentally as an airfoil stalls, the drag from a stalled wing is huge. So in the case of a car putting down laps with a semi stalled wing, a wicker could in fact restore the downforce and dramatically reduce drag...could easily be a second of improvement on a fast track.
just sayin...