Properties of a Downforce Fan

[marcush.]

had not realised the 2J had its skirts connected to the uprights ...of course an obvious thought after all.

the squatting of the car when the fans kick in is just insane.

I have heard the Galmer Indy car did the same when you squeezed the throttle in standstill .the Galmer had its exhaust pipe forming a horizonatal flute over the entire difusser exit ..a hot air gurney ...

[xpensive]

The Chaparral 2J was actually a marvel of its time, not only technically so, but also in sheer speed. At the two last races of 1970, Laguna Seca and Riverside, Vic Elford was more than two seconds ahead of Denny Hulme's McLaren M8D in qualification.

[marcush.]

say crude technology but hey if it works and my guy is 2 seconds ahead ..I love it .Mind you any driver including hamilton and Alonso
would eat a strawbale if you can demonstrate this will make them 2 seconds per lap quicker..

[xpensive]

Just imagine if the 2J ground-effect technology had still been allowed for the 1972 Can-Am season with the advent
of the 900+ Hp Porsche 917/10 turbo, around Road America, mindboggling.

[MIKEY_!]

Or if this tech was still allowed when those 1200hp turbos were in use in F1.

[thisisatest]

marcush.,
your idea of connecting the frame ends to the uprights or arb drop links is brilliant. why didnt i think of that?

[marcush.]

marcush.,
your idea of connecting the frame ends to the uprights or arb drop links is brilliant. why didnt i think of that?

not enough sleep ,maybe?

but many thanks for the compliment...even I have my moments it seems ...

seriously when you have some decades of work under pressure under your belt you will happily apply the straightforward principles to solve the issues before they
pop up ...and not look for a sophisticated solution for the hell of it.

[machin]

Its interesting to note from those Chapparall 2J videos actually how poor the "sealing" is, even though the skirts are moved by the suspension... the gap looks (relatively) huge in places...

I guess that's not surprising considering the bottom of the skirts are straight and roads generally aren't perfectly flat... diagram below illustrates what I mean, showing "gap" in yellow:-



Now obviously the 2J arrangement was good enough (we don't need a perfect seal, just a restriction to flow to enable a pressure difference to occur), but the better the seal the lower the fan power required to give a certain downforce level... perhaps a combination of suspension actuated skirts and my earlier "draught excluder" brushes is the way forward?

[MIKEY_!]

Yes I was beginning to think the same thing. We did talk about brushes earlier.

[xpensive]

I would go with sliding side-skirts anyway, we're only talking about 5% of an atmosphere underpressure at best,
like 4-5 mm Lexan or Makrolon with some sort of durable wear-strip if you can find something fitting and affordable.

http://en.wikipedia.org/wiki/Polycarbonate

Front and rear might be more of a challange, brushes might be to prefer there?

[marcush.]

Machin that´s what crossed my mind yesterday already.the bristle type skirts can accomodate surely minor road irregularities well enough and the link to the uprights would look for things like vehicle roll ,heave and pitch and rideheight changes as such...low tech but sure very effective.the bristle type skirt could well work in double row to really create a seal ..
In the end you could ask the track owner for money wiping the dirt from the racing line...
I´m sure good sealing beats bigger fan ...for weight and packaging alone

[munudeges]

That Chaparral 2J is absolutely amazing. 1970? I'm going to have to re-write my own history of ground effect.

The seal is a little poor on it however and will be rather wasteful of engine fan power, but the drop in height when the fan is turned on should give anyone encouragement of the effect. Even just a small drop in pressure is going to be noticeable. The seal with the track surface is obviously the big area of development once you've found a suitable fan.

Looks like few people have dabbled on a budget:

http://www.perpetualdownforce.com/2009/01/plan.html
http://forums.24hoursoflemons.com/viewtopic.php?id=8022

Reliability would seem to be an issue, however. How practical would it be to power the fan off the main engine? They seem to have been thinking about a Lotus 88 style inner chassis - basically an upturned boat attached to the suspension.

[MIKEY_!]

Fan could be run of the main engine (brabham used that) but it also means when you lift off for the corner you loose DF unless you change your driving style. Type 88 came up earlier but don't think we will run with it. Unless it offers some significant benefit over skirts attached to the suspension mounts.

[4G63AWD]

Unless you are on smooth pavement, this type of device becomes difficult:
1) Sand/gravel/FOD entrainment is a serious problem, and one of the reasons the original Chaparral 2J was banned. Anyone at GRM $2007 challenge saw the Cheaparral blow sand 15-20 feet in the air. The cast aluminum fan blades showed noticeable abrasion, with sand all over the interior, and the glass windshield was pitted.One of the junk science Discovery channel shows did an example of a vacuum system test on a dirt road, intended for preventing logging truck rollovers. It worked, but was a huge dustball.
2) The side skirts coming off the ground is not a big deal. Its a more gradual loss of suction than expected. But the difference in static pressure between 10 & 15mm gap height is significant. Even 600-1,000 lbf of downforce on a street car is worth up to 40% higher lateral acceleration. Much bigger differnce than street tires to race tires. Hence 2J2 won FTD on its first run of day, with cold race tires and a dirty track. I assume you can make a quick simulation of 1.14g's for race tires vs 1.4 g's with sucker system and figure out your lap time difference. If you are using slicks, the difference is proportional. And light weight becomes ever more important, as it maximizes the advantage of downforce.
3) Noise is a real problem. Well over 100dB. Ear plugs, helmet, and 1/2" lexan safety plate make it tolerable short term, but not safe long term. Hearing damage is a real problem, and happens with no notice. Even if personal safety isn't a problem, most tracks/courses have rules, to prevent neighbors from shutting them down. Not something that could be run on the street. Much of the noise is from a 2 stroke engine running wide open, but even the fans are 100dB or so.
4)Jim Hall did it right with the articulated skirts on the 2J. 2J2 did not, for simplicity of build. Any articulated skirt system needs dampening, as they can bounce a lot. His cars are on display in Texas.
5)Wings are much more efficient and effective at track speeds. The people saying powered undertrays are less effective than venturis are wrong of course. Any race designer would love to have a more effective means of powering the undertray than simple venturis, at least at low speeds (all aero is easier at high speeds). And the fan powered systems do not generate aero drag. However, the engines for the fans are gasoline and hp hungry. Figure ~40hp continuous duty for even a basic system. And small engines generally have higher bsfc at full load, high rpm. If you drive it off the engine, then you have a huge problem with fan speed varying with engine speed, and generally being slow in mid corner where you want the fan maxed out. Refilling the gas tank every 8 minutes of run time gets old fast.
6) a second engine is a reliability and safety problem. More things to go wrong. And a second fuel system that really should have a fuel cell. Or its own fuel pump. The weight is not that much of a problem, maybe 200#. The cost is debatable, but will depend largely on the fan selection and sourcing, which in turn depends on the skirt design choices.
7) center of pressure of skirt system should be balanced with the cm and relative roll stiffness of the axles to prevent unpredictable understeer/oversteer. This may be a problem, if you are rallying on dirt/gravel roads, where a rally cornering manuever of deliberately fishtailing is the fast line. Downforce might prevent this, if wheel torque is insufficient.

You should also look at what Cornell did on a FSAE car. It would be great fun to make a streetable sucker car, but the problem would be where to use it. In almost any form of racing, the sucker systems are good for 10% improvement in lap times. (Wings are even more effective, at high speeds). Which means either everyone is compelled to run them to be competitive, or the rules ban all such devices.

That should be enough to get you thinking.

[MIKEY_!]

Thanks for all that but the project was canned a while back due to a lack of funds.