Racecar Eng. aero article compilation

[i3igpete]

http://www.rightwheeldrive.com/Aero_articles.zip

pretty cool series of articles, don't know if it's been posted on here before. Some interesting things I've found so far:

-double airfoil wings can be run with really steep secondary airfoil angles and still build downforce (v14n9)
-installing smooth underbody panels have a negligible effect on drag (v15n1)
-nascar-type spoilers are actually pretty effective (maybe someone should try this!) (v15n2, n3)
-splitter-to-ground clearance doesn't matter much at all for downforce, classical theory doesn't work due to high viscous effects (v14n5)

[BigTyla]

Damn it, why did you have to post this? You KNOW posting "aero" in any article is like a magnet drawing me in and forcing me to read the material!

Will have to read at lunch and try to provide some commentary.

[J. Fast]

This is my wing. http://i788.photobucket.com/albums/yy163/JFastMotorsports/Spoiler6.jpg
Bought it from some ebay guy who makes these in his spare time in China. It was $365 with the CF uprights. I picked it for the chord and never knew there was CFD info on it so... sweet! So much more downforce when you combine two different wings? I didn't really get the tradeoff portion of the two wing, I sort of rushed thru the articles since I didn't have much time. Was it increased drag with speed?

[J. Fast]

Another picture that was emailed from the builder http://i788.photobucket.com/albums/yy163/JFastMotorsports/GT5wing-1.jpg It is 64" wide

[i3igpete]

there's really no trade off at all, a dual-element wing can run a ridiculous amount of camber that a single element will stall with. Consequently, he can probably jack up the angle on that second element without stalling it.

btw, is that a voltex knock-off?

[BigTyla]

I have read the first four articles, but V14N5 (talking about underbody venturi tubes) really drew my attention. Some comments:

1) I can imagine that spillover effects would be significant in this problem. I'd really be interested in seeing some 3-D numbers but I understand the reason for staying 2-D here since 3-D is not only more complex but is also significantly more computationally expensive. I'm happy to see the author mention that steeper diffuser angles would be permitted by 3-D effects. He knows his stuff for sure! :)

2) Air speed at the inlet was never specified in the problem. This is a critical variable as it would seem ideal to design the venturi tube with the most frequent air speed in mind (or the situational speed at which maximum possible downforce is desired). A venturi designed specifically for the vehicle's top speed might end up operating terribly during cornering speeds. That leads me to question whether the flow within the venturi tube would ever be anything other than fully-developed turbulence, as a boundary layer in the process of transition will show significantly higher flow separation penalties at the diffuser than the fully-developed case. Of course this can be solved trivially by integrating vortex generators in the venturi design, deliberately tripping the flow to fully-developed turbulence.

3) Why not design the diffuser wall with camber to reduce flow separation penalties? Seems like a pretty obvious solution to me. It's not like cambering the diffuser is hard or expensive, particularly with sheet metal designs.

[DocWalt]

Racecar Engineering is fascinating, I've been subscribing for a while now and it's always a fun day when I get a new issue, lol.

[BigTyla]

Racecar Engineering is fascinating, I've been subscribing for a while now and it's always a fun day when I get a new issue, lol.

How much is the subscription now? I was looking at those issues and $9 per month is pretty steep!

[i3igpete]

I have read the first four articles, but V14N5 (talking about underbody venturi tubes) really drew my attention. Some comments:

He does a bunch of 3D analyses later, but I was mainly intrigued how different reality was from Bernoulli. Very interesting how the downforce is almost constant, regardless of the clearance.

[BigTyla]

He does a bunch of 3D analyses later, but I was mainly intrigued how different reality was from Bernoulli. Very interesting how the downforce is almost constant, regardless of the clearance.

It is very intriguing, but it makes total sense. I think it brings to light the fact that "venturi" is kind of a misnomer here because it isn't a closed system. You'd see results much more in line with Bernoulli's principle if you closed off the bottom. You'd also add weight and have people wondering why you were carrying a ramjet under your car. :D

[Erthmeld]

Been looking everywhere for info on this while deciding how to build a solid front splitter and underbody panels for the Stealth. Fortuitous timing to say the least! :-)

Thanks!

[DocWalt]

How much is the subscription now? I was looking at those issues and $9 per month is pretty steep!

I think it's about that much. Considering I'm in school to be an ME, it's useful and a lot more interesting than a lot of stuff out there. ;)

edit: $105/yr or so at the current exchange rate. £66/yr to be precise

[Gearhead]

This is my wing. http://i788.photobucket.com/albums/yy163/JFastMotorsports/Spoiler6.jpg
Bought it from some ebay guy who makes these in his spare time in China. It was $365 with the CF uprights. I picked it for the chord and never knew there was CFD info on it so... sweet! So much more downforce when you combine two different wings? I didn't really get the tradeoff portion of the two wing, I sort of rushed thru the articles since I didn't have much time. Was it increased drag with speed?
Can you post a link to his ebay page? I like the look of it and after reading those articles, I want to get one.

[IPD]

i'm too lazy to read this. is there a solution for a spoiler that doesn't make me look like a ricer?

[i3igpete]

yes. nascar spoiler. and i dont mean a shitty little duckbill thing. I mean a legit one made of sheet steel with rod supports.

[BigTyla]

yes. nascar spoiler. and i dont mean a shitty little duckbill thing. I mean a legit one made of sheet steel with rod supports.

And don't forget the end plates!

[IPD]

yes. nascar spoiler. and i dont mean a shitty little duckbill thing. I mean a legit one made of sheet steel with rod supports.

you mean the ones they've been running for the past 2 seasons. i guess. still looks a little ricey though, imho. :p for you pete, it works; i'm just not about to gut my car to make a spoiler like that legit.

[DocWalt]

you mean the ones they've been running for the past 2 seasons. i guess. still looks a little ricey though, imho. :p for you pete, it works; i'm just not about to gut my car to make a spoiler like that legit.

He means the non-COT spoiler, just the standard sheet metal spoiler.

[i3igpete]

you mean the ones they've been running for the past 2 seasons. i guess. still looks a little ricey though, imho. :p for you pete, it works; i'm just not about to gut my car to make a spoiler like that legit.

no, not the COT wing, which is a wing. if you're talking about functional spoilers, then you need the sheet steel one they ran in the winston cup days.

[IPD]

no, not the COT wing, which is a wing. if you're talking about functional spoilers, then you need the sheet steel one they ran in the winston cup days.

wait, there's a difference between a spoiler & a wing? i've heard the terms used interchangeably everywhere.

p.s.
the "whatever you want to call it" thing on the back of the NASCAR cars for the past couple seasons is much more effective than the old "steel-lip". i've read up on it.

[MR2]

Right so how will you be mounting this wing?

http://3000gt.com.au/car/Misc/Spoiler/gallery1829.jpg
http://3000gt.com.au/car/Misc/Spoiler/gallery2201.jpg

One on the front too?

http://3000gt.com.au/car/Misc/Spoiler/large2225.jpg

I figured you could have one on the rear as long as the elements could be electronically adjusted to flat with the push of a button. (reversing, seeing what's behind you...)

[DocWalt]

wait, there's a difference between a spoiler & a wing? i've heard the terms used interchangeably everywhere.

p.s.
the "whatever you want to call it" thing on the back of the NASCAR cars for the past couple seasons is much more effective than the old "steel-lip". i've read up on it.

They got rid of the new style wing because it wasn't any better than the spoiler for the needs of NASCAR. Just sayin :p

I think Pete was busting your balls because you asked for a spoiler... There is a difference, lol.

[DG]

Another picture that was emailed from the builder

That picture is a prime example of the challenges inherent in doing aero.

Aero is pretty simple stuff when you get to consider the elements in isolation, like an airplane can. There is nothing in front of the airplane wing and nothing behind it. It doesn't (usually) get into ground effect, nor is it closely following another wing. The airstream it sees is, for the most part, undisturbed and laminar - so designing it is really simple.

Car aero though is really, really messy, for all the reasons why airplane aero is not. Not the least of which is that unless you crank the wing way up high, it is in the flow turbulence of the body in front of it.

See how that wing changes profile from tip to center? It is strongly cambered on the outboard sections, but nearly flat in the center. And there is an angle of attack change molded in as well. This is all to deal with the fact that the quality of the airflow behind the passenger cabin on an enclosed car changes across the rear cross section. The air farther outboard is cleaner, so you can take a bigger bite out of it. The air inboard is more turbulent (and the direction of flow may be different) so the AoA is different and the wing can't bite as much air.

Awesome, right? Except that the wing profile needed to accomplish this sort of thing is very specific to the car it will be mounted to - and not just the general profile of the back of the car, but where exactly on the rear deck and how high it is mounted makes a HUGE difference. Move the wing up or down a few mm and the whole profile of the wing changes.

You simply cannot design a multiform wing like this without knowing what the flow over the back of the car looks like. And even then, the wing changes the flow, so it normally takes a couple of design iterations to figure out the final profile.

And that's with access to CFD and a wind tunnel.

These copycat wings may look pretty, and they might even make a little downforce. But there is no "one size fits all" and what might work decently on one car might be a dog on another - or the same wing, mounted 1" farther rearward, might go from dog to decent - or the other way.

If you are going to start playing with aero, you are better off making cheap wings out of aluminum (or even balsa and foam core) and using tufts and inkdrops to try and figure out what the flow is doing, and hold off on the fancy CF stuff until you have an idea of what works and what does not.

DG

[MR2]

That's pretty much what I've been thinking, start with alumium where you can really stuff around with it to your hearts content and not cost too much or be to hard to adjust.

[DG]

More on this stuff:

That A Mod car looks like it might be the Engstrom car... there's a couple built along that pattern. Joe Cheng kinda set the mold with the Phantom. Snowmobile engine w/CVT so no shifting required, but good power/weight. MASSIVE multi-element, deeply cambered wings with the profile uniform across the cross-section.

Notice how the rear wing is way up high, with no part of the airfoil masked by the bodywork (or even the driver's helmet?)

Those wings are as large as they are because they start working at very low airspeeds. VERY low. But the tradeoff is a metric assload of drag - so much drag that those cars are top-speed limited to something like 70 km/h. It just takes way too much power to force those massive wings through the air any faster than that.

The tradeoff is worth it for an A Mod because the wings produce so much downforce at such a slow speed that the cars (almost) never have to slow down for any corner that they are ever likely to encounter. They really are running at the upper limit of human reaction time.

But I have to point this out too: the final word on any auto aero configuration is the rulebook. Sanctioning bodies are notoriously picky about their aero rules (and for good reason) The rulebook drives aero development. Nobody in their right mind wants to run a multi-profile, sculpted wing if they don't have to - they are far too difficult to get right. Given free rein, everybody would hike their wing up high above the bodywork and into free air (with a couple of exceptions - cars with working diffusers and proper attention to detail on the upstream flow profile can use the low pressure area under the wing to pull more air through the diffuser; see the Audi LMPs for examples). Any aero bought off the rack is likely to be illegal for any given set of rules, because they are all so different.

Aero *can* be made to work, but it is non-trival and is tightly bound to the series rules. It isn't something to treat lightly.

DG

[i3igpete]

definitely a CVT on this a-mod car!


http://www.youtube.com/watch?v=TnAmjSUUrUc

[DocWalt]

definitely a CVT on this a-mod car!


http://www.youtube.com/watch?v=TnAmjSUUrUc

holy crap. Pulling nearly 2G constant through the last corner.

[J. Fast]

btw, is that a voltex knock-off?

Yes it is. It's very similar to a Voltex GTPro-Type 3. It would be very difficult to discern one from the other unless you had the upgraded Voltex base supports. The basic base support mount on a voltex pretty much looks identical from what I could gather from the illustrations voltex has posted from their website: VOLTEX_GT WING (http://www.voltex.ne.jp/english/pro_wing.html)

http://www.voltex.ne.jp/pro_images/pro_GT_Type3_640.jpg.

Edit: And it's also roughly $1700 cheaper than a voltex.

[MR2]

God damn..$1700 :(

making my own.

[J. Fast]

Can you post a link to his ebay page? I like the look of it and after reading those articles, I want to get one.

His name is Carl Lan. He is a builder for Zuhai. Send him a message at his Tradeford Listing, Zhuhai Stone Carbon Co. Ltd, China (http://www.tradeford.com/company/81843/zhuhai-stone-carbon-co-ltd.htm)
He also has all sorts of other carbon fiber wings and misc aero parts avail. Parts are functional and affordable :)

[i3igpete]

Even just a 20mm shallow spoiler adds 135 lbf of downforce by changing flow over the entire car (see table in v15n3).

Multiplying that number by the dimensionless delta(CL) numbers in figure 3 (-0.6->-0.2, vs -0.6->1.0) a 160mm spoiler at 60 degrees should give ~400lbf downforce.

I wish I knew the test velocity. Maybe he mentions it in another article and reuses the same test conditions for all of them.

[i3igpete]

if anybody wants to try to spoiler idea, here are some suppliers for circle track racers:

https://www.lefthanderchassis.com/v2a/viewproduct_group.asp?idgroup=2512645