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View Full Version : Mini-Z "Gurney Flap"


color01
2010.09.14, 07:02 AM
I tried out something "new" today at Inside Line, didn't want to fabricate a giant spoiler for my GT1 but needed just a touch more stability at speed, hence I crafted a small Gurney flap and glued it to the very end of the GT1's rear lip spoiler.

http://img189.imageshack.us/img189/1258/img0456zi.jpg

Basically, Mini-Z's are small, so a lot more of the airflow around the car is laminar than compared to a 1:1 car; assuming the airflow is nearly laminar underneath the car and separates once it exits the rear bumper, a Gurney at the tail end of the Z could theoretically reduce flow separation behind the car, 1) pulling air upwards out from underneath for downforce and 2) reducing drag at the same time. I don't know if what I'm theorizing is correct, hence I might as well try it out.

Reading papers I learned that a Gurney flap's height should be about 1-2% of its wing's total chord length. I took the entire GT1 as the chord length (about 6" or 150mm) and calculated that a 3mm high gurney flap should be sufficient to extract air from underneath the body (more downforce) without a large drag penalty.

http://img40.imageshack.us/img40/7815/img0458ov.jpg

Now, for sure a 3mm Gurney flap like this is going to remain within the boundary layer of air around the Mini-Z: this is a tiny car and a tiny Gurney -- real race cars have Gurney flaps around 1/8-1/4", they're within the boundary layer and those wings have a cross-section about the same as a Mini-Z! Boundary layers don't scale down with car size, so if my tiny Gurney doesn't work, then that would mean my theory is wrong, and if it works I would have some strong evidence that my theory is headed in the right direction.

Long story short, it works very well! At Inside Line the PN wide 8 slicks don't hook up, so the slow, technical turns were a nightmare to navigate; but surprisingly I had traction on the fast back sweeper and fast S-curve in the middle of the track, where I usually had the most trouble. That tells me that the Gurney flap is working, and not only that, it works as well as a conventional spoiler while being 3-4x lighter. So it works, and it's definitely worth some further investigation.

Next test, Mod power (PN 43t) and proper tires for the Inside Line track (K30/K20). :)

byebye
2010.09.14, 08:16 AM
Good read! This past weekend the DC crew ran HFAY and I noticed coming out off a turn and on too the straight my car would drift to the left. Not sure if it's tweak(mr-03 chassis) or torque from my diff settings or toe. I didn't have time to really shake the car down. I did well but the only way to compensate for it was to adjust my steering center to the right. That kind of helped. I run a Mosler body so I wonder if this gurney flap might help me a little.

Kris

dxm2
2010.09.14, 11:27 AM
Great Info and great innovation. Are you running your GT1 at 94mm?

color01
2010.09.14, 04:04 PM
Yup, I cut the car out to 94mm to use the chassis setup I'm used to. Might actually consider 96mm once the new PN motor mount comes out. :)

If you guys want to try out the "Gurney flap" -- I hesitate to call it one for sure since I can't prove that it's working on the same mechanism as a real one -- then you need to make sure there's air available to pull out from under the body. In other words, the rear of the body needs to have some fairly substantial holes that air can obviously escape from. Then the flap will, at the very least, create a small area of low pressure behind the car that will pull the air out and up. If it's working as theory dictates, then it actually creates some small vortices that pull air up by their swirling motion, which would increase downforce much more than having just a low pressure wake.

lfisminiz
2010.09.14, 08:11 PM
Brian, pretty neat idea. That body needs a little something.........maybe.:)

herman
2010.09.14, 10:34 PM
great read... great theory... now does anybody have a smoke machine? lay it down on the track and see how the smoke reacts when the car moves around for a couple of laps... can you post more pictures of that flap? :D

EMU
2010.09.14, 11:12 PM
I ran mine at 96mm, even 98mm. I preferred 96 or 94mm. I felt that the rear was very stable (removed wing). The only thing I didnt like, was that the big hips on the car mean any rub on a board points your nose to the wall. Same goes for contact with another car. It does look really good on the track, and drives the part...

Interesting Gurney flap... I may try that. I have used lip wings, but never a gurney.

color01
2010.09.16, 08:08 AM
Yeah, the Gurney flap actually works quite well, but as I found out last night it's VERY sensitive to height changes.

Exhibit A: During pre-race practice I cracked loose the right 10mm of the gurney flap, leaving 10x1.5mm worth of plastic to flop around. The car's handling on the back sweeper went from quick and stable to nervous and prone to washing out (oversteer). The plastic wasn't even gone, just loose and misaligned/flopping around in the air current.

Having fixed that corner with CA glue and accelerator,

Exhibit B: Just before qualifying I cracked OFF the left 10x1mm of the gurney flap, leaving a 2mm high section that tapered into the 3mm Gurney flap (the way I had it initially). Downforce was still there, but the car simply could not be controlled on the back sweeper any longer, and I had to give up that qualifying round, go back and compensate with some suspension tuning.

Conclusion: Gurney flap needs to be made of tougher plastic in order not to get damaged. The aerodynamic effect seems to be very fragile and easy to lose just by breaking off a tiny piece of the Gurney flap. Personally I'm going to trim down the existing Gurney a little bit and use it as a mounting surface for a larger, thicker, and slightly taller one (I'm thinking 4mm, can trim it down later). Seems the car could use a little extra downforce when it comes to Mod, I'm in unfamiliar territory with this new 43t. :)


I should probably start posting these findings in a separate thread as not to muddle this one up.

Cherub1m
2010.09.16, 08:30 AM
Yeah, the Gurney flap actually works quite well, but as I found out last night it's VERY sensitive to height changes.

Exhibit A: During pre-race practice I cracked loose the right 10mm of the gurney flap, leaving 10x1.5mm worth of plastic to flop around. The car's handling on the back sweeper went from quick and stable to nervous and prone to washing out (oversteer). The plastic wasn't even gone, just loose and misaligned/flopping around in the air current.

Having fixed that corner with CA glue and accelerator,

Exhibit B: Just before qualifying I cracked OFF the left 10x1mm of the gurney flap, leaving a 2mm high section that tapered into the 3mm Gurney flap (the way I had it initially). Downforce was still there, but the car simply could not be controlled on the back sweeper any longer, and I had to give up that qualifying round, go back and compensate with some suspension tuning.

Conclusion: Gurney flap needs to be made of tougher plastic in order not to get damaged. The aerodynamic effect seems to be very fragile and easy to lose just by breaking off a tiny piece of the Gurney flap. Personally I'm going to trim down the existing Gurney a little bit and use it as a mounting surface for a larger, thicker, and slightly taller one (I'm thinking 4mm, can trim it down later). Seems the car could use a little extra downforce when it comes to Mod, I'm in unfamiliar territory with this new 43t. :)


I should probably start posting these findings in a separate thread as not to muddle this one up.

WOW!!! that is a pretty drastic change in handling for the Gurney Flap. I've always felt for one to feel the effects of a wing on a Miniz you would need a very big wing because on a 1:1 scale car you need speed to create that down force and for a small scale (1/28) RC car going around ~12 to ~20mph that speed would not create a significant amount of down force that is comparable to the 1/1 scale car. I've always use weights to create down force.

1)So, I guessing after you fixed the right and left Gurney Flap in both occasions you got your stability back?

2)Also, just wondering on one of the your sentences you mentioned using CA glue and accelerator to fix the Gurney Flap did that not remedy the problem?

If that is true you got my attention. I'll have to try it, cause I don't like placing humongous wings on my Z's

dxm2
2010.09.16, 01:45 PM
"Wow" is right. More great info. I've heard lots of guys say that the cars are too small for wings or even body shape to make a significant difference. They are sadly mistaken. At Inside Line I was testing a Nissan 350GT body with no headlight lenses and it was undriveable at full throttle. I taped up the holes with rim tape and it became a different, and better car.

color01
2010.09.16, 07:05 PM
Moved the posts to a new thread, we were definitely mucking up the picture thread. :o

--------------------------------------------------

I didn't fix the break on the left, so I can't tell you about that, but after fixing the right side yes I did get the stability back. Strange stuff.


Anyways, regarding the discussion of aero, there are a couple background items you have to keep in mind when analyzing aero and body changes on Mini-Z.

1) These cars are small, so the shape of the body probably isn't going to matter too much with regards to punching a hole through the air. I don't doubt one moment that the shape DOES make a difference aerodynamically, however I think that its effects pale in comparison to point #2.

2) The shape of the body determines its CG and moment of inertia characteristics, which effect much more change at this scale than minor aero details, IMO.

3) There's only a really really really really really fine line between fast and uncontrollable. My cars tend to drift just a tiny bit at low speeds when setup the way I like them, So if there's no downforce then I'm screwed at high speed, and if there's too much then I need to crank the wheel farther than where I like to crank it (about halfway to left or right for the back sweeper). Mechanical balance and aero balance must be perfect for me else there will be traction rolling, oversteer, etc. etc.

4) The Gurney flap only makes a small difference, it won't create the same downforce that a massive 1/10 TC-style scoop wing does, it won't make the car super-duper-stable at speed. However, it (when intact, at least) did help me get just the right amount of traction I needed to go around the back sweeper quickly and safely. This is on a now-Mod car no less, I trust that the small Gurney would be more than adequate in Stock.

5) What I'm surprised about is not the fact that it works (it's a bloody spoiler, it had better work) but rather why it's so darn sensitive. When I cracked the right side, the plastic was still on the car -- why the sudden change in stability? I can't explain this at all.

6) I'm also surprised that breaking off the left side hurt the handling so much as well. Honestly, it's a tiny piece of plastic to begin with and I broke off an even tinier piece. I wonder if the sides generate more downforce than the center due to cleaner airflow? Easy to theorize, difficult to test. I'm actually trying to book some time at my university's wind tunnel, but we'll see how that goes. :)

7) Which is why I'm going to try a taller, thicker Gurney the next time and we'll see how it fares. Hopefully it doesn't add too much drag -- this thing points straight up, it's actually quite a worrying point.



dxm, you bring up a very valid point as well in that the GT1 has two fairly large holes in the front bumper that look like air could be forced through them. I think it'd make a difference if I taped them up too, although I'd probably glue some plastic in there instead of tape for durability reasons. :)

EMU
2010.09.16, 10:21 PM
I always analyze bodies aero and mechanical grip properties when I choose them. I typically run large wings for mod bodies... but another thing to consider besides the size of the wing, is the angle that the body sits on the car. If the rear of the body is lower than the front, you are going to have a little lift inside of the body from the low pressure above the rear of the body. If the rear is higher, you are going to have downforce. I typically raise the rear of the bodies if I assemble them myself... and I usually have 2 of the same body, one in ASC and one painted white body. I do all my testing on the ASC bodies before I assemble the painted ones. That way I know how I like my clips...

The gurney flap is definitely a neat idea. It is a minor adjustment... It may be such a noticeable difference because of the lift that is produced in the body. The upper vent and front vents can definitely cause this. I noticed it a bit on my GT1. Also, the rear of the body may be sitting too low, and there isnt much air hitting the rear of the body to plant it down. There is plenty hitting the nose, so that will plant the front, and break the rears loose.

I think I am going to try a gurney flap on my F355. I think a full wing may be too much. Im thinking something a little flexible may work better than something very rigid (which may break). I have to see what I have around...

color01
2010.09.18, 03:35 PM
Kyosho tire packaging seems to be soft enough to withstand some abuse -- here's my new gurney, now 4mm tall.

http://img713.imageshack.us/img713/8619/img0474e.jpg

Edit 9/20/10: Couldn't resist the temptation, I busted out the Dremel and hollowed out the rest of the GT1's rear end to nearly the PNWC-legal limit. The top of the cut is 19mm from the ground, that gives me 1mm to play with ride height *if necessary*.

http://img844.imageshack.us/img844/8907/img0475ur.jpg

Less weight and should help the Gurney do its thing... unfortunately this cut makes a scientific test of the Gurney no longer possible, but if it pays in laptimes and handling "feel" this Friday then the wind tunnel tests later can do the science part. :)

doug01n
2010.09.21, 01:55 PM
Hi Brian... Interesting application of the gurney flap...

I was wondering... I have a Nascar Body, and there's a huge flap in the end, at least twice tall compared to your test... Is this the reason that I felt my car so rear planted at high speed turns?

I thought that the main reason was the huge wheelbase of 102mm, but I'm starting to think that this flap makes most part of this effect.

color01
2010.09.21, 09:01 PM
I've seen the NASCAR spoilers and they're practically identical to this Gurney flap. That's why I put "new" in quotes, the idea is old hat but I hadn't seen anyone else trying it. :) I can't say for sure if it's the body or the wheelbase, but the fact that the NASCAR bodies have high tails and the big flap makes it likely that aero plants the rear down at speed. If I can borrow someone's NASCAR sometime I'll get back to you with a better idea of what's going on. :)

hilldebrandt
2010.09.21, 09:53 PM
It was mentioned about the sides of the spoiler possibly having more downforce than the center ?
Take a look at this photo of the new Nascar Nationwide Series Mustang, with the raised spoiler ends...

http://mini-zracer.com/forums/attachment.php?attachmentid=30545&d=1285120368

ruf
2010.09.22, 01:43 AM
We goofed around with this back in the day. We ended up going to full "lip" wings with a more sloped angle and 10-15mm chord length. The downforce was greatly increased with no perceivable effect on drag. Also did the higher sections on the side. I just thought it looked cool, but the reason is that the air is typically most turbulent in the center where the stream coming off the roof meets the streams curling in from the side windows. The sides have "cleaner" air that can be used more effectively for downforce. The center is just used to create a little bubble to make the air coming off the roof flow a little better and reduce drag. That's my armchair CFD analysis...
http://www.eng.fea.ru/spaw2/uploads/images/STAR-CCM-race-CFD2.jpg
http://images.digitalmedianet.com/2004/Week_20/zq4nih6e/story/daimler4.jpg

Also, the reason we made the new wing is that it's more effective for downforce/drag.
http://www.auto123.com/ArtImages/111487/nascar-aero-inline.jpg
The bump is designed to reach up in the center and grab clean air while evacuating the turbulent air.
http://mini-zracer.com/forums/attachment.php?attachmentid=30550&d=1285135644http://mini-zracer.com/forums/attachment.php?attachmentid=30551&d=1285135654

color01
2010.09.22, 04:25 AM
Good info guys, thanks for sharing. :) Did you ever test the effectiveness of the Gurney flap vs. lip spoiler when most of the rear end is open vs closed? I can definitely see why the flap would lose out to a standard spoiler on a "sedan"-type body where the rear is closed: the way I see it, the flap works on the principle of drawing air out from underneath, so if the area below the flap is closed, the flap would simply be a really crude, draggy spoiler. But what if the rear is opened up? Does the air that normally collects in the rear bumper get pulled out by the low-pressure wake of the flap? Does it get pulled out more effectively than it would below a standard angled lip spoiler?

Looking around it seems that popular 1/12 bodies tend to have Gurney flaps at the very back even though the entire body is already a spoiler. I tend not to see the flaps on Touring or World GT bodies that have raised wings and closed-off rears. Is this a rules-driven adaptation by the 1/12 pan cars (no raised wings allowed) or does the flap work as I'm thinking it works?

Looks like there's a lot of things to test... Joe, are you guys gonna have a formed Lexan spoiler for the 911 GT1 or do I need to switch back to the F430? :)

ruf
2010.09.23, 10:13 PM
You should be able to mount our new wing on the 911 GT1. I'd actually like to try that body someday. It's always been a favorite of mine, though I prefer the 98 update.

I've never tried the gurney flap with an open rear Mini-Z body. In 1/12, there are rules against wings. I'm wondering how much air actually gets under the body. 1/10 sedan rules prevent cutting out the entire rear. I don't see why a gurney flap wouldn't evacuate the air better, but I wonder if there is more of a direct spoiler effect on downforce.

color01
2010.09.25, 06:08 AM
So, went to Kenon today with the new Gurney flap and it just isn't durable enough to stand up to racing. :rolleyes: What's more, it didn't provide enough downforce to match up to the new front diffuser I fabricated, which made the front end really stable and planted but the rear end all sorts of wacky -- sliding out, traction rolls, the works.

In the end I cut the Gurney completely off and mounted a scoop wing high up in the original location, in clean air. It's more durable, provides more downforce, maybe a little bit more drag, but best all cured the traction rolling and sliding out all in one go. Consider the Gurney flap scrapped, at least on my GT1. It seems to be best used as a final tuning aid if you need just a *little bit* more traction, so perhaps it may still have its applications for the new PN Lexan pan cars and EMU's F355.

Joe, you win. :p

ruf
2010.09.28, 12:58 PM
Joe, you win. :pHaha! Maybe this round! Your diffusers look very interesting! I only experimented with flat bottoms.

ruf
2010.12.15, 07:50 AM
For a good look at why our wing is shaped the way it is, check out this video. Specifically, watch how the dirty air curls and collects on the rear windshield behind the roof and coming around the side windows.

http://www.youtube.com/watch?v=1O2QtVLyNw0