Value of aerodynamics on small RC cars?

[Flummo]

After a discussion on MSN I started thinking about the importance of the aerodynamics on small radio controlled cars, say 50-60cm long and top speed 60-100km/h. They generally have no serious aero equipment, except for a rear wing at best. A flimsy plastic body looking like some real world racing car, but air inlets/outlets aren't even open, the bottom of the car is open and so on...

Back to the MSN discussion. A guy showed a picture of a RC-car looking like some old DTCC Mercedes or something like that, he said the top speed was close to 100km/h. Not sure about how big the car is. He asked if I knew how to make it faster, both in a straight line and around turns. We started talking about aerodynamics, and he said the car became undrivable if he removed the rear wing, so obviously it does something even on so small cars and moderate speeds.

How do you guys think such a car would react to modifications along those lines:
*Flat bottom added, with venturi tummels at the rear (size probably limited by rear suspension)
*The "intercooler air inlet" on the body is opened up and air is ducted from it to the engine to cool it. Possibly to the engine air intake too.
*The rear of the body gets a few ventilation holes to evacuate hot air into the low pressure zone behind the car. Possibly opening up the louvres above the front wheels and air outlets behind the front wheels too.
*Maybe adding dive plates/splitter plate to the front of the car.

Waste of time or interesting test?

[PNSD]

You can buy the external shells for not high prices. I think it would be worth while experimenting changing a few things before doing it to his main model. I wouldnt risk trying to give the engine moer power though. Im not sure but I thought they already rev quite high, and are quite often not the most reliable.
Underbody aero seems a good way forward but like you said you may need extra cooling with closed off underbody ??

[vyselegend]

I'd say for such a light thing as a RC car, the weight of the rear wing has far more to do with the car's behavior than the (probably inexistant) aerodynamic downforce generated by the wing's surface...

Maybe you friend finds the car undriveable when the rear wing is removed because it just made the car a little oversteery before (because it may represent a resonable percentage of the overall weight, and is placed at the rear and high). Your friend should try adding the same amount of weight than the removed wing, but near the floor, like ballast. I bet it won't be faster, but hopefully it should turn slightly better... My 2cts anyway

PS:In F1 ingineers usually say that aerodynamics effect becomes perceptible (or cease to be negligible) at approx. 100 km/h. And that is with great wing surfaces.

[flynfrog]

Aero does matter with rc cars. One big gain is to cut out the back of the body. They tend to build up pressure in this area making the back end want to fly. If its a nitro car you will want to keep as much ventilation to it as possible. These little engines run hot the cooler you can keep them the leaner you can run the faster you can go. Some people play around with different bodies as some have different characteristics some with more front DF some with more rear. Some lower drag ect.

I'd say for such a light thing as a RC car, the weight of the rear wing has far more to do with the car's behavior than the (probably inexistant) aerodynamic downforce generated by the wing's surface...

some what true but also the car is so light even small changes in DF are a major percentage of the weight of the car.

[rjsa]

Knowing the downforce generated from a real life wing similar to the one you are using in the RC car you can estimate the downforce generated with this method:

http://en.wikipedia.org/wiki/Similitude_%28model%29

Which is the procedure used the way around, I.E., guessing a racing car downforce from a scale model in a wind tunnel.

I'm sorry but I don't have time to run the maths right now.

[Belatti]

Long before modern 1:1 wind tunnels, the majority where made at scale 1:4 or even 1:8 wich is the actual scale of many rc cars. So I would say aerodynamic counts.

To do aero research under that scale and take valid conclusions in other scales teams used dimentional analisys.
So, there may be things you simply can´t extrapolate lineraly from 1:8 to 1:1 or backwards.

[rjsa]

That's exactly what the example above does. I would calculate the reynolds number for the known case, establish a speed coefficient and get a downforce coefficient.

Most likely a nonsense speed for the model will be needed, a rough approx of real running downforce can be done then by multiplying the downforce by a Vm^2/Vc^2 (model velocity, calculated velocity).

Anyone has an idea of how much DF a hear wing in a LM car will generate at let's say, 200Kph?

[Flummo]

Thanks for the replies!

PNSD:
There are other bodys available at low costs for sure, but I believe this is a ultra low budget project, if anything gets done at all. The guy used to race the car, and just thinks it's fun to get it out and do something with it again. And about the engine, there are no modifications planned. Nothing beyond getting it to run, that is...

vyselegend:
I thought that weight was a bigger factor than aero on those toys too, but I am having doubts now. A wing as light as a couple of credit cards shouldn't have a huge impact on handling because of the weight, when a fuel tank containing 150cc or so methanol doesn't affect it too much. Sure, not weight in the same place, but nevertheless a massive diffrence in weight on such a small car.

flynfrog:
Yes, it is a nitro car, and they sure need all the cooling they can get.
Cutting the back out of the body you say? Does make sense, and I had ideas along those lines, even if I didn't think about making any huge cutouts.

rjsa/Belatti:
I know many wind tunnel tests are made scaled down, and the good and bad sides of that. But the formulae are beyond me, (I like books with pictures and diagrams ) and I do not have any data to convert anyway. The wing on the car looks to be made equal thikness all the way, i.e. a simple curved piece of plastic instead of a proper aerofoil.

[G-Rock]

Forget wind tunnels, just make a little platform that hangs out of your road car window. On that platform, attach a digital weigh scale and set your RC car on it. Drive a 100km/hr and voila. If you see a gain in weight, you have downforce.

[vyselegend]

The other think I doubt about aerodynamic efficiency of those RC cars is their potential aerodynamic instability. With so little span and width, the potential range of efficiency of the wing is greatly reduced, isn't it? (I'm not sure of this, it's more a question than a statement.)

Also PNSD says those thermics monocylinder engines rev high, and as the car is very light, it means vibrations must be horrible, considering the scale of the thing... Random pitch variations induced by engine vibrations must be a hurting factor for the aerodynamic efficiency of the model.

Now Flummo, if you're right about the lightness of those wings, obviously they won't have that much impact on overall weight distribution, so it's worth trying extracting some downforce from them indeed.

But is really aerodynamic downforce what you need to make those RC cars faster? That's another question. I'll still stick to weight and power being the determining factors over that. And as for all other thing rolling on wheels, tyre grip is the predominant factor for steering ability. There's always much more "easy" grip to find with better mechanical grip than aerodynamic downforce, principally because it's not as much speed dependant. (and we're talking about max. 100km/h speeding little cars, which means there's not so much air motion round the car over a lap distance.)

[gcdugas]

Go for it! And use full underbody ground effects since there are no rules! The whole bottom of the car could be one gigantic diffuser. There is plenty of gain to be had.

[Belatti]

Anyone has an idea of how much DF a hear wing in a LM car will generate at let's say, 200Kph?

Check this nice page I have found:
http://www.mulsannescorner.com/data.html

1997 McLaren GTR Long-Tail Downforce:
1234 lbs. @ 150 mph, with 561 lbs. of drag
1776 lbs. @ 180 mph, with 807 lbs. of drag
2193 lbs. @ 200 mph, with 997 lbs. of drag
Lift-to-drag ratio: 2.2:1

1999 Toyota GT-One Downforce:
1980 lbs. @ 150 mph, with 596 lbs. of drag
2851 lbs. @ 180 mph, with 859 lbs. of drag
3520 lbs. @ 200 mph, with 1060 lbs. of drag
Lift-to-drag ratio: 3.32:1

1991 Sauber Mercedes-Benz C291 Downforce:
3476 lbs. @ 150 mph, with 695 lbs. of drag
5005 lbs. @ 180 mph, with 1001 lbs. of drag
6179 lbs. @ 200 mph, with 1236 lbs. of drag
Lift-to-drag ratio: 5:1

1993 Joest-Porsche 962
High downforce configuration:
Max L/D:
2971 lbs. @ 150 mph, with 675 lbs. of drag
4278 lbs. @ 180 mph, with 972 lbs. of drag
5281 lbs. @ 200 mph, with 1200 lbs. drag

Aero. Balance @ 200 mph:
F: 1796 lbs. (34%)
R: 3485 lbs.

Lift-to-drag ratio: 4.40:1
Coefficient of lift: -2.655
Reference area: 1.806 meters square

Max Downforce:
3141 lbs. @ 150 mph, with 785 lbs. of drag
4523 lbs. @ 180 mph, with 1131 lbs. of drag
5584 lbs. @ 200 mph, with 1396 lbs. drag

Aero. Balance @ 200 mph:
F: 1619 lbs. (29%)
R: 3965 lbs.

Lift-to-drag ratio: 4.00:1
Coefficient of lift: -2.8
Reference area: 1.806 meters square

[Tifoso]

I've got a Thunder Tiger TSN V2 Pro, a great car for beginners that has been my intro in this RC world. I started with the bodywork without any modifications, but then they recommend me cutting some parts so that the ventilation improved.

I did some wholes in tha back part, in the upper one a whole for the starter and another for the engine-starter-"puller" (Sorry, no idea how to call it). Then I cut off the windows for more ventilation and some smaller cuts for the air.

I don't believe that the aero does have a great value here apart from engine cooling, but I will try as soon as I can!

[flynfrog]

Thanks for the replies!

PNSD:
There are other bodys available at low costs for sure, but I believe this is a ultra low budget project, if anything gets done at all. The guy used to race the car, and just thinks it's fun to get it out and do something with it again. And about the engine, there are no modifications planned. Nothing beyond getting it to run, that is...

vyselegend:
I thought that weight was a bigger factor than aero on those toys too, but I am having doubts now. A wing as light as a couple of credit cards shouldn't have a huge impact on handling because of the weight, when a fuel tank containing 150cc or so methanol doesn't affect it too much. Sure, not weight in the same place, but nevertheless a massive diffrence in weight on such a small car.

flynfrog:
Yes, it is a nitro car, and they sure need all the cooling they can get.
Cutting the back out of the body you say? Does make sense, and I had ideas along those lines, even if I didn't think about making any huge cutouts.

rjsa/Belatti:
I know many wind tunnel tests are made scaled down, and the good and bad sides of that. But the formulae are beyond me, (I like books with pictures and diagrams ) and I do not have any data to convert anyway. The wing on the car looks to be made equal thikness all the way, i.e. a simple curved piece of plastic instead of a proper aerofoil.

I would start by cutting off the area of the bumper but In the end I almost always take a big ugly chuck out of the rear of the car

[Flummo]

Some say aero would not do any good, others say it may make a big diffrence...

Well, I guess the only way to know is to try it. Too bad the car isn't mine, if it was I would have made a fibreglass body inspired by prototype race cars, with all the aero equipment fully functional. If that doesn't work, nothing probably does...