Aerodynamics with R/C racing cars

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Peter-RC
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Joined: Tue Oct 14, 2008 1:03 pm

Aerodynamics with R/C racing cars

Post by Peter-RC » Tue Oct 14, 2008 1:21 pm

Hi All,

Currently am I racing with 1:8 scale nitro R/C cars (3,5cc engine, 2,5kg, zero technology) which have a separate mounted body.
This body is just a piece of polycarbonate that resembles a real racing car, like the pug 908 for instance.
Altrough the rear of the body features a spoiler at around 50 degrees angle of attack.
I know this creates some downforce and a huge amount of drag. see below
Image

Now the regulations say:
Everything behind the rear wheel centerline is free
that is to a maximum overhang of 153mm behind the rear wheel centerline that is free to modify.

I was thinking to cut out the big inefficient spoiler and to replace it with a more efficient wing (double or multi-element) and mounted directly on the car. (thus when you remove the body, the rear wing remains attached to the car.

I have TurboCAD on my MAC at home, but to start making a rear wing... which profiles are mostly used?

the regulations say:
max width 267mm
max chord 77mm
min angle of attack: 35 degr.
max angle of attack: 45 degr.
max overhang from rear wheel centerline: 135mm
max height (measured from the ground): 170mm

and thats it, they say nothing about max elements, endplate size etc.
remember that these car's go around 110km/h... so I guess aerodynamics can play a role.

For a single element wing, which profile should i use to start with?
multi-element, which profiles? (let's keep it to a 3 element, mainplane + 2 flaps.)

regards,

Peter

benjabulle
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Joined: Thu Aug 26, 2004 8:53 pm

Re: Aerodynamics with R/C racing cars

Post by benjabulle » Wed Oct 15, 2008 7:39 pm

Interesting stuff, I'm also beginning to work on a 1/12 electric body with some ideas, one of them being the same with a real rear wing. For the wings profiles you can find NACA profiles used in racing cars in some books. "Competition car downforce" by S.McBeath has some of them with also multi elements, be careful, they're in the first edition (1998) not the last one from 2006. The best book on that topic is called "ALI WINGS" by Enrico Benzing but you won't find it.
I think you could start with a single element, knowing R/C, the more difficult will be the mounting of the wing and how to keep it in one piece if you go out.

Ciro Pabón
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Joined: Tue May 10, 2005 11:31 pm

Re: Aerodynamics with R/C racing cars

Post by Ciro Pabón » Wed Oct 15, 2008 9:43 pm

Wow. I only hope this thread catches the eye of the aero "fans" around here. This is a small (maybe) project that I would take with all my heart if I had the knowledge. Please, guys, anything contributing to a design for this thread would be a first in this forum.
Ciro

Aerodynamicist#1
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Joined: Thu Oct 09, 2008 10:32 pm
Location: UK

Re: Aerodynamics with R/C racing cars

Post by Aerodynamicist#1 » Wed Oct 15, 2008 10:16 pm

For a single elementthe best way to go is a 'circular arc' aerofoil, although i'm not sure where you could get coordinates for this. A good general aerofoil to use might be a NACA 64(3)-618, although i'm not sure how well this would work on such a small scale?? Also you should be using gurney flaps, if you need help with this, i can dig out an explanationa and formulae for them if you are interested.

nas
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Joined: Sun May 11, 2008 6:01 am

Re: Aerodynamics with R/C racing cars

Post by nas » Thu Oct 16, 2008 4:23 am

I apologize if this has already been posted, but it may be of some help:

http://www.mh-aerotools.de/airfoils/index.htm

Peter-RC
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Joined: Tue Oct 14, 2008 1:03 pm

Re: Aerodynamics with R/C racing cars

Post by Peter-RC » Thu Oct 16, 2008 7:30 am

benjabulle wrote:Interesting stuff, I'm also beginning to work on a 1/12 electric body with some ideas, one of them being the same with a real rear wing. For the wings profiles you can find NACA profiles used in racing cars in some books. "Competition car downforce" by S.McBeath has some of them with also multi elements, be careful, they're in the first edition (1998) not the last one from 2006. The best book on that topic is called "ALI WINGS" by Enrico Benzing but you won't find it.
I think you could start with a single element, knowing R/C, the more difficult will be the mounting of the wing and how to keep it in one piece if you go out.
1:12 scale cars should have a bigger advantage i think... as they go just as fast or faster than mine 1:8 car (but they miss a combustion engine :D )

Mounting the wing won't be a big problem... my 1:8 car has a nice piece of alloy that hold's the lower-rear-wishbones firmly into place and also connects the chassisplate with the bulkheads.
Just 4 longer screws trough that piece of alloy(plus carbon support for the wing) into the bulkheads and voila... at least in theory :P

Keeping it all in one piece should not be a big issue, it's a racing car.... not a banger racer ;-) ... i'm not supposed to crash it.
Ciro Pabón wrote:Wow. I only hope this thread catches the eye of the aero "fans" around here. This is a small (maybe) project that I would take with all my heart if I had the knowledge. Please, guys, anything contributing to a design for this thread would be a first in this forum.
well small... I was thinking about making some sort of a windtunnel at the house of my parents, at the moment it does not have to give accurate data, I just want to see the different lift values of different bodies/settings.
Aerodynamicist#1 wrote:For a single elementthe best way to go is a 'circular arc' aerofoil, although i'm not sure where you could get coordinates for this. A good general aerofoil to use might be a NACA 64(3)-618, although i'm not sure how well this would work on such a small scale?? Also you should be using gurney flaps, if you need help with this, i can dig out an explanationa and formulae for them if you are interested.
Thanks!
I was thinking to put a good big sized (25x25cm?) JPG image of this profile in the background of TurboCAD and then redraw the lines with some splines... and then later scale it all down to the regulated chord length.
But I guess that would not give an accurate profile...

Ciro Pabón
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Re: Aerodynamics with R/C racing cars

Post by Ciro Pabón » Thu Oct 16, 2008 9:23 am

This kind of flexi-wing (yes, it's a different kind of car, for a different kind of race) could give you some ideas:

http://fastrc.blogspot.com/2007/11/wing-design.html

The chronological pictures are fascinating. I bet you can ask the guy at this site for some directions on simulating the wing for R/C cars.

About the wind tunnel idea: many years ago (I was in high school), when I had an R/C plane, I read something by the Wright brothers :oops: about how to get measures in a wind tunnel in a simple way.

They used a hinged balance, with drag (or lift) plates attached to the wing to measure the relationship between drag and lift. For the science fair, I built a drag balance (in 1975!). I don't have the book I used anymore, but I found also this, that you might find useful (I only hope the aerodynamicists around here don't laugh too hard :)):

Lift balance explained:
http://wright.nasa.gov/airplane/balance.html
Image

Drag balance explained:
http://wright.nasa.gov/airplane/balanced.html
Image

AFAIK, the brothers build their wind tunnel in just one day, and used it only for one day, but it was the base of their profile design.

It took me more or less one week to build something similar to the last contraption, and I ran it at the fair with a regular home fan, without a tunnel. :oops:

OOT, I also had a nitromethane simple R/C car at the end of the 90's. I attached a (rather primitive) web cam to it, to race the car. I thought it was going to be really useful to me, because I'm used to karting, and I found very hard to control the car without a "view from the cockpit". It wasn't too good, the radio link was lousy and the image was anything but clear, but I won a couple of races (against less than professional competitors, just a bunch of kids: I was over 40 at that time).

Oh, man, this post has been really embarrassing. :D
Ciro

yzfr7
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Joined: Tue Nov 15, 2005 11:20 am

Re: Aerodynamics with R/C racing cars

Post by yzfr7 » Thu Oct 16, 2008 2:35 pm

Peter-RC wrote:I was thinking to put a good big sized (25x25cm?) JPG image of this profile in the background of TurboCAD and then redraw the lines with some splines... and then later scale it all down to the regulated chord length.
But I guess that would not give an accurate profile...
You better try using the equations for a NACA profile:
http://www.ae.utexas.edu/~varghesep/cla ... TM4741.pdf

This book has some examples, not many points per profile to be true, but depending on how you will build the thing, you won't need much more than this.
http://www.amazon.co.uk/Theory-Sections ... 773&sr=8-1

Also, search for "naca profile generator" and you will find lots of apps to give you the coordinates. Just don't trust them blindly, specially for 6-series.
pax

Aerodynamicist#1
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Joined: Thu Oct 09, 2008 10:32 pm
Location: UK

Re: Aerodynamics with R/C racing cars

Post by Aerodynamicist#1 » Thu Oct 16, 2008 7:06 pm

There is a good website for aerofoil coordinate files:

http://www.ae.uiuc.edu/m-selig/ads/coord_database.html

You can get data files for most common aerofoil sections on there. You can use the data files with each one to load into a CAD package. I know if you use Solid Edge, you can use the Curve by table command to load the data in, however the data may need to be tidied up b4 your CAD package will accept it.

I actually built a small wind tunnel myself, so i may be able to offer some advice, it was very crude, but it was just for fun and in no way representative or accurate, to do that u need to spend a long time designing and manufacturing. However one of the most important considerations is the working section. It should have a taper, so that the exit is larger than the inlet for the working section. This is to counter the effect of the boundary layer growing on the wall of the tunnel itself and help maintain a laminar flow condition where the flow will be incident on the model.

Also you can find a computer program called Xfoil on the internet for free and you can analyse aerofoil sections on this.

Regards

Conceptual
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Joined: Thu Nov 15, 2007 2:33 am

Re: Aerodynamics with R/C racing cars

Post by Conceptual » Thu Oct 16, 2008 8:15 pm

Peter-RC wrote: Mounting the wing won't be a big problem... my 1:8 car has a nice piece of alloy that hold's the lower-rear-wishbones firmly into place and also connects the chassisplate with the bulkheads.
Just 4 longer screws trough that piece of alloy(plus carbon support for the wing) into the bulkheads and voila... at least in theory :P
Mind giving the bolt pattern for this? I have an idea on how to make one out of a single piece of aluminum, and with your previous dimensions, the only thing I still need to make the model is the bolt spacing, and where it is located to the rear wheel centerline. The size, length and thread of the screws to be replaced would be very helpful as well.

Thanks!

Shaddock
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Re: Aerodynamics with R/C racing cars

Post by Shaddock » Fri Oct 17, 2008 12:06 am

Looking at the photo the cylinder head sticks up in the middle of the wing. My guess would be that most of the downforce is generated at the left and right extremities of the rear wing and thats why the end plates are huge and run the length of the car's body.

Try experimenting with the exisiting body work by cuttung a slot gap in the rear wing and maybe the end plates.

If you were to make one from scratch get the wing as high up as possible to improve the efficiency of the centre section.

Conceptual
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Joined: Thu Nov 15, 2007 2:33 am

Re: Aerodynamics with R/C racing cars

Post by Conceptual » Fri Oct 17, 2008 4:18 pm

Shaddock wrote:Looking at the photo the cylinder head sticks up in the middle of the wing. My guess would be that most of the downforce is generated at the left and right extremities of the rear wing and thats why the end plates are huge and run the length of the car's body.

Try experimenting with the exisiting body work by cuttung a slot gap in the rear wing and maybe the end plates.

If you were to make one from scratch get the wing as high up as possible to improve the efficiency of the centre section.
SHHHHHHHHHHH...!

I really want to make this wing! I just need the mounting position info to keep it legal!

benjabulle
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Joined: Thu Aug 26, 2004 8:53 pm

Re: Aerodynamics with R/C racing cars

Post by benjabulle » Fri Oct 17, 2008 9:06 pm

in 1/12 Electric RC, same kind of body, you can't cut it and have holes, the rear wing can't be higher than the roof. Photo of the body I'll try to improve and what you can find under it.


Image



Image

Peter-RC
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Joined: Tue Oct 14, 2008 1:03 pm

Re: Aerodynamics with R/C racing cars

Post by Peter-RC » Tue Oct 21, 2008 1:00 pm

Conceptual wrote: Mind giving the bolt pattern for this? I have an idea on how to make one out of a single piece of aluminum, and with your previous dimensions, the only thing I still need to make the model is the bolt spacing, and where it is located to the rear wheel centerline.!
one piece alu might be to heavy (and it is behind the rear axle...not so good)
the exact location I cannot give... for the very simple reason that my 1:8 car is in Holland and I live in Italy! 8)

for now most of the bodies have a max rear overhang of about 10cm, that gives another 5cm space to mount the rear wing more rearwards... but then the rear wheels are about on 3/4 of the full length of the car, i don't know how that will affect the handling.
Front downforce is very little i think, mounting the rear wing on max overhang could be more damaging then it actually gives a performance advantage.

the current 1:8 cars have the body mounted on the rear wheels (thus downforce is transfered directly to the wheels, that is better say the experts in this world) so rideheight adjustments have no influence on the total height of the car.
I plan to mount the wing it to the chassis which gives the possibility to lower the rear part of the body a good 2cm.
But then I need to remount my 3rd spring arrangement.

I thought that F1 also had a time that they mounted their wings directly to the wheels, but it got banned.
But which one is actually better?
I presume that mounting on a chassis will be better because it does not block suspension movement under download.

I will dig into the wing profiles right away, and hopefully I can run a test @ fiorano next month :P