Min. ride height for diffuser?

[Tuepflischiiser]

Hi

I'm thinking about making a diffuser for my rc-car (10th scale). Some friends think it works, but some think it won't because of the low ground clearance (about 5-7mm). This is the car... http://www.rc-gallery.net/images/use...ce9dbf248d.jpg

What do you think?

[phoenix]

I don't have an answer for you, I'm afraid, but a couple of things crossed my mind.

First is that the behaviour of air, and particularly it's density and viscosity, is not scaled down to 1/10 - only your car is, so I'm pretty sure that the air will behave in the same way as on a 10/10ths car (pardon the intended pun!) which would probably mean that the 'gap' is too small. You will need a flat bottom on the entire car - up to the diffuser - for it to have any effect though, and that will presumably add a weight penalty.

However there must be some air passing under the chassis and it must be doing something under there.

The next thing that I thought was the costs of experimenting on a 1/10th model is probably 1/100th that of experimenting on a real car - both in time and money - so I suggest you give it a try - and let the stopwatch decide if it works or not.

I'd be keen to know the outcome if you do decide to test a diffuser though....

[JohnD]

Agree with pheonix - air does not behave the same on a scaled down model as on the full size, and that it will appear more viscous and dense. So the interaction of model and air will NOT be the same (I think phoenix had a temporary grammatical aberration) though I agree with his result - the gap will be too small.

Water tunnels have been used to try to make scale models give full-sized data, but that would be hugely more difficult to achieve in the garage than an air tunnel.

What is the absolute top speed of your model? I'm sure the scale speed will be up to 200mph, but how fast will it actually go over the ground? 30mph? That is achievable with a home made wind tunnel, and there are plans and threads on the 'Net about these. As you want to race the model, it doesn't matter that your data will not scale to a full sized car. You might discover that wholly different aerodynamic aids are appropriate in the smaller scale and become the Ross Brawn of scale racing!

You might like to try some home-brew CFD - Computational Flow Dynamics.
CHAM, a company that provides CFD services to industry, has long offerd a free download of their PHOENICS software.
No relation, I suppose to pheonix!
See: http://phoenics.software.informer.com/
This is two dimensional but can show what is happening.

John

[zac510]

What do they do on the scale F1 cars?

edit: I mean the R/C cars not the models the teams use in wind tunnels.

[Tuepflischiiser]

Ok, I will give it a try. The car reaches a topspeed of 40 - 60mph, depending on motor choice and track.
Other drivers use high downforce GTP bodies with molded-in wing that acts and looks like one big wing.

[JohnD]

Tuepflischiiser,
May I call you Tuep?

Suggest invest in Simon McBeath's "Competition Car Downforce" published by Haynes. Lots of ideas for full sized cars, that may or may not be translatable to models. In particular some basic theory, or if you want more of that, Prof.Joe Katz's "Race Car Aerodynamics".

That the usual method is a large built-in wing, rather than splitter and flat floor, may mean that others have been here before, but try it! The cost of your experiments may not be a tenth of full sized trials, but surely a lot less.
As pheonix says, the ultimate wind tunnel is the stop watch!

John

[phoenix]

Originally Posted by zac510

What do they do on the scale F1 cars?

edit: I mean the R/C cars not the models the teams use in wind tunnels.

F1 hardly use scale models any more as they have found better methods. They explore a concept on the computer using CFD, then move directly to full scale wind tunnels - mostly if not all with a moving floor too - to test and evaluate.

p.s. thanks for correcting my error, JohnD - you knew what I had meant to say!

[JohnD]

No probs, pheonix.

I think F1 uses full scale and CFD exactly because it is so difficult to scale the results from models, rather than have a better method of modelling flow. Full-scale needs a wind tunnel as big as a block of flats and all the teams have their own for security. True 3D CFD needs supercomputers, so that's a lot of where the millions of dollars go.
But I think you can get useful, interesting results from Foenics.
John

[phoenix]

Originally Posted by JohnD

I think F1 uses full scale and CFD exactly because it is so difficult to scale the results from models

I think I need a lesson in plain English, as that was what I was trying to say! However, I'm not sure that models have been completely dispensed with, so I didn't want to be quite so emphatic. Clean upper surfaces - i.e. those not subject to turbulence or ground effect - are more predictable in scale models than elements close to the ground or with slots in for example because, as mentioned by JohnD above, by scaling wind speed so that the air behaves as it would at racing speed in terms of compressibility etc meaningful results can be achieved. Once the air is being forced to compress (or expand) at a greater rate by such thing as scaled slots the linearity of predictions goes out of the window.

10mm (or whatever it is under the plank) becoming .83 mm in a 1/12th model, for example, just won't give meaningful results if re-scaled.

I always think of those old war movies with naval battle scenes; the water still behaves like water even though the ships are maybe 300th scale, and the visual effect is so obvious - the "waves" don't look real, plumes of water from exploding shells don't look real. Come to think of it, neither do the explosions, flames and smoke when a torpedo or shell hits a vessel.

[Tuepflischiiser]

Originally Posted by JohnD

Tuepflischiiser,
May I call you Tuep?

Suggest invest in Simon McBeath's "Competition Car Downforce" published by Haynes. Lots of ideas for full sized cars, that may or may not be translatable to models. In particular some basic theory, or if you want more of that, Prof.Joe Katz's "Race Car Aerodynamics".

That the usual method is a large built-in wing, rather than splitter and flat floor, may mean that others have been here before, but try it! The cost of your experiments may not be a tenth of full sized trials, but surely a lot less.
As pheonix says, the ultimate wind tunnel is the stop watch!

John

Call? By Phone? Sorry, my is English i not good enough to talk by phone. Except you can speak Swiss-German

I allready have the book of Simon McBeath and I will take a look at hte other book.

After reading this thread, "Competition Car Downforce" and a lot of internet sites (especially mulsannes corner) I have tons of ideas that i can think about. Diveplanes, moving aerodynamic parts,... are all allowed. A Chapparal-type sucker car would also be a cool thing I should think about

Thanks all

[bjwedgemtu]

its one of those things that in theory would work. I don't know a whole lot about the world of RC racing but I do know that diffusers are very very very very dependent on ride height. From watching RC cars and stuff it looks like the bounce around a lot. If you can't control the ride height your screwed. let me know if you get anything that works.

[JohnD]

Tuepflischiiser,
My apologies! I could not write in German, let alone talk.

When I wrote, "May I call you Tuep?" my meaning was to ask if I may use that as your name, as a nickname or a familiar shortening of your name, like "Chris" for "Christopher".

I think that a fan-car would work in a model!
Please tell us about it, if you start that project.

John

[CNHSS1]

i worked in the high end RC car trade for many years, doing lots of R&D etc in most scales. I assume from your questions you run Pro10 class or similar?
in the late 90s/early 2000s we did a fair bit of testing with what was then Pro 10 class cars. we tried flat floor lexan undertrays running into diffusers and they tended to make the car more stable at speed on VERY smooth circuits. The trouble was that they were very senstitive to pitch and ride height. if the car hit a bump on the straight at speed the undertray became a wing and launched the car, a bit like Porsche and Mercedes at Le Mans! in the wet, the cars aquaplaned really badly too. with the 'open' underside of the carbon chassis, with all their slots and apertures, the air flow under the car tended to launch the car less than if running a 'flat floor'
Diffusers and flat floors were banned for national UK events, although we still did some research.
Due to the very low ride heights of the cars, the air flow under the cars was very difficult to manage, in fact most cars bodyshells flex enough at high speed on the straights so that you can heard the lexan bodyshells touching the tarmac.
we found that trying to make a full length venturi tunnel (rather than plain floor) was worth a try, but to be honest, the compromises in packaging the cells, motor and RC gear upset the handling and again the 10/10th scale air flow tended to lift the car too much.
we came to the conclusion taht given the light weight of the cars, and the fact that any ripple in the surface could cause enough pitch to send the car airbourne, that under car aero wasnt worth the effort.

The bodyshell shape was a different story! Changing the bodyshell, or extending the rear of the Group C style bodys rear wings with a lexan flap, gurney etc made big differences. Taller longer side fences or end plates on the rear of the wings made big steps in stability at high speed with very little drag costs. Also raking the bodyshell by mounting it slightly higher at the rear also improved stability.
Early open cockpit shapes like the TOJ and Osella bodies produced most grip on front and rear wheels, when the rules changed to closed cockpit it took a while to get the grip levels back.

Hope that helps

CNH

[simb79]

Ciao everybody,
i'm writing from milano.
i'm running a 2wd 1/8 ON road race car.

do you have news about this topic?
i'm trying to build something..

please let me know what you discovered on this topic.

thanks
simone