i have installed a PTG carbon fiber rear wing with three way adjustment. knowing next to nothing about aerodynamics, i have wondered about adding a small gurney flap. from what i've read, it is supposed to improve the drag/lift ratio and improve downforce (or reduce lift). i still haven't dialed in my suspension yet so I'm wondering if i should get that completed first. otoh, i may need to adjust the suspension anyway. in the front i have a standard issue Max Velocity hardcore track splitter with the 2" lip.

just wondering what the more experienced guys think about this and whether it's worth drilling holes in my rear wing.

More downforce is more downforce. Aero on a production car, especially SMALL aero changes are usually not felt, but the times show an improvement. Gurney flaps help both in adding more downforce, and reducing drag slightly. It's a racecar... can't be afraid to cut into it sometimes.

While a gurney flap CAN help, it doesn't' always. Gurneys delay separation when it would otherwise occur. Without knowing if they will help its hard to justify the work. What if your wing isn't stalling at all? Then a gurney probably isn't helpful!

While the likelihood is that it's stalling, the gains from it are small. Typically a few percent. IMO, you'd get more from a fresh coat of wax or by cleaning off the underfloor.

On a selig S1223 (a good airfoil vendors SHOULD be using but aren't) a gurney PLUS vortex generators are worth about a 5% increase in lift with no increase in drag at relatively low speeds. (AIAA journal of aircraft, Feb 1997)

On the other hand, I've been working on the wing below, and a gurney was worth less than 1% downforce and actually added a few newtons of drag.
http://i23.photobucket.com/albums/b397/aerogt3/frontwing.jpg

It's so specific to individual cases it may not be worth it. If you have something else you could do, do that. Otherwise, leave it for later. If it's a two element wing do it on the second element.

Hope this helps.

thanks for the help. with that said, i will just leave it for the time being and see how the wing does without the flap.

The aero guys I work with would sell their mothers for a few percentage points (not automotive industry).

If you do decide to try out the Gurney you don't have to drill holes in your wing. Just have a piece of thin aluminum sheet bent at a 90 degree angle at the length you need and use 3M double sided emblem tape to attach it. You can buy the tape any autobody shop or even Autozone, Napa.

Just have a piece of thin aluminum sheet bent at a 90 degree angle

Or you can just buy angle aluminum at Lowes. ;)

That angled aluminum from Lowes is way too heavy, I was afraid it would slow me down so I went with thin sheet :stickoutt

i bought the aluminum angle at Ace hardware, 1/2" width. i thought about the double sided tape, just didn't know if it would be strong enough.


where's steve j? he always has an opinion on things aerodynamic.

We have used the double sided tape, with both aluminum angle and sheet gurneys ;), for years and have never had a problem.

I don't think you want your gurney flap to be 1/2" tall. I'd say 3/16-1/4" tall MAX.

HI:)

I'm a Mechanical, not a Aero, Mr.M is much better suited to answer your questions.

Which wing did you get, the LTW style or Rolex/Alms style (crawford)?

I'm assuming its the LTW style with the adjustable center...if so just run it, I would not worry about a flap on that thing.

The reality is, in most cases, you can run a bunch of downforce/drag setup and still be competitive. For our cars in CR, we're only depending on aero to balance out the car and add to the mechanical grip (which is what we rely most on).

If this was an ALMS Gt1 car it would be a different story, and you'd be worrying about airflow through the front radiator and odd stuff like affecting your rear wing.

I'm trying a wing design that has a fair amount of drag but also should have a lot of downforce...for the higher powered Cm/Dm/Bm/Sm cars, the drag can be overcome fairly easily.

The S1223 is pretty funky :)

http://www.ae.uiuc.edu/m-selig/ads/afplots/s1223.gif

yea, that is pretty funky. my wing is the tall PTG style wing from MAShaw with three way adjustability. i'll just run it the way it is for now.

Good plan :) What you should look into is a turnk lip, that will make much more of a difference than the gurney for that wing setup.

(Dnd obviously that picture of the profile is upside down, as they are spec'd for aircraft for which they are shaped the other way to produce lift)

Good plan :) What you should look into is a turnk lip, that will make much more of a difference than the gurney for that wing setup.

So a small gurney flap (on the trunk) = trunk lip = wicker bill (on the trunk)???

Well, kind of. He does not have that wing, he has the adjustable mashaw unit, so the center will be angled higher,possibly could benefit from a wicker bill (aka gurney flap). Those are more winglets than gurney flaps/wicker bills.

I was talking about basically putting asmall winglet on the trunk,similar to what the ltw design does, but just more aggrssive, since his main element is higher up than the original one low ltw setup.

Seeptgpic below: The E46's have it easy b/c the CSl trunk has a nice "lip" built in.

Gurneyflap/wickerbill is to accomplish this:

http://www.allamericanracers.com/images/gurney_flap_sketch.jpg

While the extension/lip on the trunk is to act as a mini-spoiler (think nascar spoiler) and produce downforce and help airflow on the underside of the wing. Many SCCA Gt2/gt3 cars use the nascar style spoiler and when setup properly they can work very well.

Edit: Found the picture:

http://jaffster.com/misc/Misc/ptglip.jpeg

I'm going to bed so I just quoted from that website: " the airflow immediately beneath the wing rolls into a small anti-clockwise vortex behind the Gurney. Immediately above this, a second small vortex, rotating in the opposite direction, is formed by the airflow traveling above the wing as it passes over the gurney's lip. together these two vortices form a small separation bubble - a rotating mass of air removed from the main flow - which is somewhat taller overall than the gurney itself.

In clearing this separation bubble, the airflow's vertical deflection is increased and hence downforce increases. Additionally, separation of airflow from the wing's lower surface is postponed, allowing a higher angle of attack to be used before stall, which further enhances the wing's effectiveness. "

ANyone have a part # for the rear gurney flaps????

Or no where to find some??

ANyone have a part # for the rear gurney flaps????

Or no where to find some??

They are NLA from BMW.

Steve, I know there is info on the forum about taking stock trunk lips from various chassis and using those. What would you suggest for an e36 as an easy add on for the simple purpose of reducing lift and adding a bit of high speed stability. As in e46 M3 lip etc.

MJO

Well, kind of. He does not have that wing, he has the adjustable mashaw unit, so the center will be angled higher,possibly could benefit from a wicker bill (aka gurney flap). Those are more winglets than gurney flaps/wicker bills.

I was talking about basically putting asmall winglet on the trunk,similar to what the ltw design does, but just more aggrssive, since his main element is higher up than the original one low ltw setup.

Seeptgpic below: The E46's have it easy b/c the CSl trunk has a nice "lip" built in.

Gurneyflap/wickerbill is to accomplish this:

http://www.allamericanracers.com/images/gurney_flap_sketch.jpg

While the extension/lip on the trunk is to act as a mini-spoiler (think nascar spoiler) and produce downforce and help airflow on the underside of the wing. Many SCCA Gt2/gt3 cars use the nascar style spoiler and when setup properly they can work very well.

Edit: Found the picture:



I'm going to bed so I just quoted from that website: " the airflow immediately beneath the wing rolls into a small anti-clockwise vortex behind the Gurney. Immediately above this, a second small vortex, rotating in the opposite direction, is formed by the airflow traveling above the wing as it passes over the gurney's lip. together these two vortices form a small separation bubble - a rotating mass of air removed from the main flow - which is somewhat taller overall than the gurney itself.

In clearing this separation bubble, the airflow's vertical deflection is increased and hence downforce increases. Additionally, separation of airflow from the wing's lower surface is postponed, allowing a higher angle of attack to be used before stall, which further enhances the wing's effectiveness. "

Also, there is an entrained vortical structure between the trailing edge and the gurney. In addition, the streamlines exit the wing/vorticity with an increased angle (not like the picture where it just shows a kink in the streamline). And from Newton's third law, you can see where the increased downforce comes from.

The aero guys I work with would sell their mothers for a few percentage points (not automotive industry).

Haha, yeah. Aerospace and open wheel are crazy. But for club racing, time spent on a gurney could be better spent elsewhere unless his car is "finished."

HI:)

I'm a Mechanical, not a Aero, Mr.M is much better suited to answer your questions.

Which wing did you get, the LTW style or Rolex/Alms style (crawford)?

I'm assuming its the LTW style with the adjustable center...if so just run it, I would not worry about a flap on that thing.

[quote]The S1223 is pretty funky.

It's an outstanding cross section though. Makes a ton of lift from what is, in your picture, the lower surface. That's a big deal. It's my favorite non-proprietary airfoil :)

ANyone have a part # for the rear gurney flaps????

Or no where to find some??

I have one of the parts numbers... 51 71 2 268 786.

I tried getting the other but unfortunately they are NLA. I even tried a contact in BMW Motorsport in Germany, but they are truly NLA. :(

I have one for sale (http://forums.bimmerforums.com/forum/showthread.php?t=810983) if anyone is interested.

Is that the trunklip of the gurney flap?

Is that the trunklip of the gurney flap?
In all honesty, I'm not really all that sure. I have heard people say that it works with both, but like I said...I'm not really sure.

S1223 isn't that funky. Its someting I wouldn't give an odd look. This on the other hand:

http://www.racersmarket.net/gallery2/main.php?g2_view=core.DownloadItem&g2_itemId=1859&g2_serialNumber=2

I guess the big thing was adjustability.

Haha, yeah. Aerospace and open wheel are crazy. But for club racing, time spent on a gurney could be better spent elsewhere unless his car is "finished."

[quote=Steve J.;10306135]HI:)

I'm a Mechanical, not a Aero, Mr.M is much better suited to answer your questions.

Which wing did you get, the LTW style or Rolex/Alms style (crawford)?

I'm assuming its the LTW style with the adjustable center...if so just run it, I would not worry about a flap on that thing.



It's an outstanding cross section though. Makes a ton of lift from what is, in your picture, the lower surface. That's a big deal. It's my favorite non-proprietary airfoil :)

The S1223 in certainly the highest public domain lift airfoil in the Reynolds number range of 300,000 to 600,000 and remarkably it has a fairly large sweet spot where the L/D is better than Lissaman, FX63, Eppler, and other Selig foils. University students use it for the heavy lift aero competitions all the time due to the rules requiring a maximum planform area of wing. It also has readily available wind tunnel (2D) data and contour coordinates for different low Reynolds numbers in the 100,000 to 500,000 range, which is the range used in racecars coincidentily. Usually, however, if you have unlimited planform area available to you where other airfoils might have better performance in any of the major criteria such as moment (really important for airplanes since the moment needs to be countered by another lifting surface such as the tail which means more drag and reduced lift for the entire plane). The moment for racecars isn't so much a big problem since it would be resolved in equal in magnitude and oppositely directed load changes to each axle pair.

There is, I believe, a Journal of Fluids paper on the performance of using gurney flaps on the S1223 written by a PhD student at the time who ended up working at Bar Honda F1.

Is that the trunklip of the gurney flap?

Dave's is not the trunklip, otherwise id buy it :) His is the flap that goes on top of the ltw wing.

The S1223 in certainly the highest public domain lift airfoil in the Reynolds number range of 300,000 to 600,000 and remarkably it has a fairly large sweet spot where the L/D is better than Lissaman, FX63, Eppler, and other Selig foils.

Okay, I'll bite. Who produces a commercial version of this in the U.S. for vehicle use?

Mike

What exactly are you biting on?

Provide the profile to any decent composites shop who has experience fabricating wings, and you'll be good to go. Depending on the width and mounting system, it'll be $1-2k for single element.

on a whim, I had a foam core wing quoted for a selig 1223 profile at a shop that does r/c airplane wings. You'd be suprised at how big some of them can get. Competition wings are well within the sizes of r/c plane wings. The intent is to sheet it in FG and/or CF.

The foam core would cost about $75.

DIY sheeting isn't all that difficult for somebody that has even a little experience with it. There are plenty of web sites for DIY construction of composite wings for homebuild aircraft or r/c planes. It wouldn't suprise me if you could do it all for less then $3-400.

There are a couple of competitors who are running foam core wings out here on the west coast. So far, two of them have failed at speed. I was being pursued by one of them when it failed. It would not be my choice to run a foam core wing on a car doing 150mph. Maybe you could insert carbon rods in the foam core, but I will stick to a professional motorsports composite manufacturer for my racing wing creation.

Good point. I didn't look at the VNE for the r/c stuff, foam cored wings are pretty common in the experimental airplane area, with 200+ mph VNEs.

An AL or Carbon spar would the minimum, if not 2 of them to distribute load. CF for stiffness and Kevlar for tear resistance would be minimum layup, if not a few layers of glass as well. The beam calculations should be pretty simple to do in that regard.

This is just armchair engineering though. I suppose there is also a false economy when skimping on a device that if failed, can cause a lot more damage.

I definitely would advise going with a composites shop that has built components for aerospace/motorsports, so they know the structural rigidity/integrity that is needed for this application.

There is a reason the high end pro's pay $5-20k for wings to be fabricated.

Obviously, you don't need to spend that, but you should not skimp on something that can bite you in the ass late, and end up costing a lot more.

Yeah, but I can't help but think there is a "Costs more, so must be better - even if it's not" - sort of mentality.

Absolutely the design needs to be rock solid.