Gurney Flap for Rear Spoiler (S-5)
08-06-2009, 04:11 PM
#1
Rennlist Member
Thread Starter
Gurney Flap for Rear Spoiler (S-5)
All,
As many of you know I added a front splitter to my 85 street/DE car and it is working very well and even survived a recent unexpected impact with a dead racoon at over 55 MPH.
The front of the car has a lot more bite and I wanted to see if I could mirror the success on the front of the car with a gurney flap or wicker bill on the rear S-5 wing that I have. I hope to gain back a little of the neutral balance the car had prior to the front splitter. ( I can adjust that out with a sway bar....but want to explore other ways)
Here is a little information for those that are interested, although I refrenced the book Competition Car Aerodynamics by Simon McBeath for most of the information.
http://www.modified.com/tech/0610scc..._2/index1.html (about 1/2 way down the page)
and
http://en.wikipedia.org/wiki/Gurney_flap
[Finally
http://www.ae.uiuc.edu/m-selig/uiuc_...5bulletin.html (my rear spoiler looks something like the chord of a trainer as referenced)
So, the main aspect of all of this is to put a 90 degree angle at the trailing edge of the wing in the boundary layer. As noted this adds down force by adding a vertical force at the trailing end and slowing down the flow ahead of the gurney with little increase in drag. The height of the gurney should be in general about 5% or less of the chord of the wing. The one pictured below is 1/2 inch or just 4%.
It's made of 1inch wide 1/16 thick aluminum strip and fastened to the wing via 3/16 diameter anchors epoxied in place after careful drilling. I've made one smaller and one larger.
The finish is made with a self etching primer and textured 3M paint that closely resembles that rubber components.
I know this isn't for the purist, but a small tweak on a non-factory spoiler.
I expect to give it a test at Mid Ohio in two weeks when I'm back up there to instruct.
Regards,
Ken
As many of you know I added a front splitter to my 85 street/DE car and it is working very well and even survived a recent unexpected impact with a dead racoon at over 55 MPH.
The front of the car has a lot more bite and I wanted to see if I could mirror the success on the front of the car with a gurney flap or wicker bill on the rear S-5 wing that I have. I hope to gain back a little of the neutral balance the car had prior to the front splitter. ( I can adjust that out with a sway bar....but want to explore other ways)
Here is a little information for those that are interested, although I refrenced the book Competition Car Aerodynamics by Simon McBeath for most of the information.
http://www.modified.com/tech/0610scc..._2/index1.html (about 1/2 way down the page)
and
http://en.wikipedia.org/wiki/Gurney_flap
[Finally
http://www.ae.uiuc.edu/m-selig/uiuc_...5bulletin.html (my rear spoiler looks something like the chord of a trainer as referenced)
So, the main aspect of all of this is to put a 90 degree angle at the trailing edge of the wing in the boundary layer. As noted this adds down force by adding a vertical force at the trailing end and slowing down the flow ahead of the gurney with little increase in drag. The height of the gurney should be in general about 5% or less of the chord of the wing. The one pictured below is 1/2 inch or just 4%.
It's made of 1inch wide 1/16 thick aluminum strip and fastened to the wing via 3/16 diameter anchors epoxied in place after careful drilling. I've made one smaller and one larger.
The finish is made with a self etching primer and textured 3M paint that closely resembles that rubber components.
I know this isn't for the purist, but a small tweak on a non-factory spoiler.
I expect to give it a test at Mid Ohio in two weeks when I'm back up there to instruct.
Regards,
Ken
Last edited by Vlocity; 08-06-2009 at 04:26 PM. Reason: Fixed Links
08-06-2009, 06:24 PM
#4
Inventor
Rennlist Member
Rennlist Member
It was only 13mm tall, but certainly less pliant than the old S-spoiler.
Is your flap slotted, so you can raise or lower it?
Results of the official fish-scale test: 
*6lbs to 'seat' the new spoiler (pushed down onto the foam spoiler)
*30lbs to close the hatch to 1/4"
*40lbs to flush close the hatch
So ~10lbs (or more) additional downforce at 100?
thread
*6lbs to 'seat' the new spoiler (pushed down onto the foam spoiler)
*30lbs to close the hatch to 1/4"
*40lbs to flush close the hatch
So ~10lbs (or more) additional downforce at 100?
thread
08-06-2009, 06:38 PM
#5
Rennlist Member
Ken, check out Carl's website (928Motorsports.com)... he did a clear flap on his AIR S5 wing for his race car. I think he had to separate (seam split) the wing surface to install threaded inserts and then reseal the fiberglass. IIRC, he did it that way to keep the inserts from rotating within the wing and for more strength against the mounting screws and to keep the threads from stripping (tapping directly into fiberglass).
08-06-2009, 07:13 PM
#6
Rennlist Member
Thread Starter
Hey Barry,
I did see Carl's but it looked like his wing was more of an extension at an angle and less of a true Gurney. It also looked to be several inches long. Maybe he will see this post and chime in. I plan to keep an eye on mine and see whether or not the inserts are strong enough. I can always double up the inserts if I need to. I'll be seeing 140 every lap at Mid Ohio and it won't take long to know if this thing works or not.
Regards,
Ken
I did see Carl's but it looked like his wing was more of an extension at an angle and less of a true Gurney. It also looked to be several inches long. Maybe he will see this post and chime in. I plan to keep an eye on mine and see whether or not the inserts are strong enough. I can always double up the inserts if I need to. I'll be seeing 140 every lap at Mid Ohio and it won't take long to know if this thing works or not.
Regards,
Ken
08-06-2009, 07:18 PM
#7
Rennlist Member
Thread Starter
HTML Code:
Is your flap slotted, so you can raise or lower it?
It's not slotted but the one you see here is 1" total width with only 1/2 " extending above the top of the spoiler. What I did was just bought three different widths of aluminum. 3/4"- 1" and 1 1/2" so that I only need to unscrew the 6 fasteners and change up or down. Pretty simple and that kept the bottom edge from effecting the suction side of the wing by introducing it into that area, which is what would have happened with a slot.
Regards,
Ken
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08-06-2009, 10:36 PM
#8
Rennlist Member
It's been a while since I spoke with Carl about the flap he has... and honestly, don't remember if it is a true 90 degree angle or angled toward the rear.
Having gone to school for aeronautical engineering (before life intervened), I do know that just as an airplane has flaps that increase the upper surface area to still provide lift at lower air speeds (ie. when landing). The same would be true by flipping the airfoil upside down... but creating more downforce. Downforce is different than drag, and in any sort of racing it would be preferred to all-out drag.
PorKen's idea of an airbrake setup would be just that -brakes. Any sort of measureable downforce would be miniscule as compared to the drag it would produce. All vehicles that travel through the air... be it on land, air or sea -benefit more from a lower Coeficient of drag (Cd) than a higher one. They can go faster and more efficiently with a lower Cd... downforce is merely to gain and/or maintain traction on their operating surface (water or asphalt).
Gurney may have had NASCAR restrictions on spoiler size/shape/depth, and created a 'cheater' device to comply with the rules. SCCA & IMSA type racing may be a little more relaxed with the regs... the average speeds will be significantly less.
You may find that a 90 degree angle simply makes your car slower and the engine work harder...
Having gone to school for aeronautical engineering (before life intervened), I do know that just as an airplane has flaps that increase the upper surface area to still provide lift at lower air speeds (ie. when landing). The same would be true by flipping the airfoil upside down... but creating more downforce. Downforce is different than drag, and in any sort of racing it would be preferred to all-out drag.
PorKen's idea of an airbrake setup would be just that -brakes. Any sort of measureable downforce would be miniscule as compared to the drag it would produce. All vehicles that travel through the air... be it on land, air or sea -benefit more from a lower Coeficient of drag (Cd) than a higher one. They can go faster and more efficiently with a lower Cd... downforce is merely to gain and/or maintain traction on their operating surface (water or asphalt).
Gurney may have had NASCAR restrictions on spoiler size/shape/depth, and created a 'cheater' device to comply with the rules. SCCA & IMSA type racing may be a little more relaxed with the regs... the average speeds will be significantly less.
You may find that a 90 degree angle simply makes your car slower and the engine work harder...
08-06-2009, 11:45 PM
#9
Rennlist Member
Thread Starter
Barry,
I understand what you are saying but the beautiful thing about the Gurney from what I have read is that there is little drag penalty up to the 5% of chord because I believe you are only working in the boundary layer area. The other thing that is mentioned in some of the research that I did was that the Gurney creates two counter rotating vortices immediately behind it. With a big Supercharger on my car I'm not really too concerned about the small amount of drag that it may create on the straights. It's about getting the mechanical grip I need for mid corner off accerlation which will more than make me faster at the end of the straightaway. The third link I provided in the initial post, you will note that the it was a case example on a sail plane and the effect of the Gurney was increased speed.
Additional tests were also performed to estimate the model's average airspeed with and without Gurney flaps. Repeated measurements over a given course indicated that their sailplanes equipped with Gurney flaps had a 23% increase in average speed (20.3 mph with flaps vs 16.5 mph without flaps) when using a 1%c Gurney flaps.
We'll know more in a couple of weeks and I will be sure to report back. The main reason for the experiment is to gain back some of the balance of the car. I had it pretty neutral and could make it oversteer with throttle on and with throttle off (think breathing the throttle just before mid corner to rotate the front to the apex so that I could then pin the rear wheel back down with power).
And I just like to tinker if this works or doesn't I'll have gained some knowledge.
Regards,
Ken
I understand what you are saying but the beautiful thing about the Gurney from what I have read is that there is little drag penalty up to the 5% of chord because I believe you are only working in the boundary layer area. The other thing that is mentioned in some of the research that I did was that the Gurney creates two counter rotating vortices immediately behind it. With a big Supercharger on my car I'm not really too concerned about the small amount of drag that it may create on the straights. It's about getting the mechanical grip I need for mid corner off accerlation which will more than make me faster at the end of the straightaway. The third link I provided in the initial post, you will note that the it was a case example on a sail plane and the effect of the Gurney was increased speed.
Additional tests were also performed to estimate the model's average airspeed with and without Gurney flaps. Repeated measurements over a given course indicated that their sailplanes equipped with Gurney flaps had a 23% increase in average speed (20.3 mph with flaps vs 16.5 mph without flaps) when using a 1%c Gurney flaps.
We'll know more in a couple of weeks and I will be sure to report back. The main reason for the experiment is to gain back some of the balance of the car. I had it pretty neutral and could make it oversteer with throttle on and with throttle off (think breathing the throttle just before mid corner to rotate the front to the apex so that I could then pin the rear wheel back down with power).
And I just like to tinker if this works or doesn't I'll have gained some knowledge.
Regards,
Ken
08-07-2009, 12:13 AM
#10
Rennlist Member
Another way to increase the downforce on the rear is to raise the spoiler out of the boundary layer and 'into the wind' like the Marks (Kibort and Anderson). Even at a 0 degree angle, you may be able to achieve increased traction since the wing is more 'engaged'...
As Kibort has stated in your splitter thread, the front/rear ratio has a variety of combinations to it. What you don't want to have happen is so much rear grip that it will enable lifting on the front. I would also suspect there would be an 'efficacy window' of speed that would yield the greatest results (too slow, nonexistent drag... too fast, excessive drag). Remember, NASCAR speeds are more consistent (and higher) than the lows and highs of sportscar racing... that could prove to be a factor as well.
Just using some 25 odd year old memory cells in this thread...
As Kibort has stated in your splitter thread, the front/rear ratio has a variety of combinations to it. What you don't want to have happen is so much rear grip that it will enable lifting on the front. I would also suspect there would be an 'efficacy window' of speed that would yield the greatest results (too slow, nonexistent drag... too fast, excessive drag). Remember, NASCAR speeds are more consistent (and higher) than the lows and highs of sportscar racing... that could prove to be a factor as well.
Just using some 25 odd year old memory cells in this thread...
