Value of aerodynamics on small RC cars?

[mep]

Does your friend want to drive some real races against others over a long distance?

If yes, than I think there can be a advantage from using aerodynamics.

I have been thinking about this a few years ago.
At this time I was building RC model planes.
But I was more interested in race cars and and wanted to build a race car
with the experiences I got from building planes.

I planned a copy of the MP4 20 in scale 1:4,5 with underbody and working wings.
There should be a old thread about it here.
You can search it if you want.
Actually I am still building the car, but I got stuck in building the suspensions right now.

For the downforce thing.
On this car, weight ca 8kg and over one meter length, the downforce should start to play a little role at ca 80km/h.
The problem is that you need a track where you drive a corner at this speed to have a advantage against the others.
And I doubt there is such a competition track for those cars.
If you only drive in a straight line downforce brings you nothing.
It's not important how high your max speed is, it's important how high your corner speed is.


For the drag thing.
Your car has a max speed. You must know why your car can't reach a higher speed.
- because your engine reaches max revs. But the car could go faster with other transmission

-because you get more drag than your engine brings force

If the second is the case, than you can possible do a lot to reduce drag.
now you are driving at ca 100km/h. Little changes to reduced drag can bring here a noticeable effect .

For the cooling
The better the engine is cooled, the more power it will produce.
Keeping your engine cold, and it can bring extra power and has increased durability.

[Flummo]

Tried to search for your thread, couldn't find it.

I do not think there will be any long distance races against other cars. But I may be wrong. The goal is just to have fun and try to make the car better, instead of letting it sit in the garage or wherever he kept it.

I do not yet know what stops it from going faster, but I think the engine must be close to it's peak rpm at top speed now - after all, it is quite fast for such a small car. But I think less drag can give a small improvement both in acceleration and top speed:
Acceleration, as less power is needed to overcome the drag at higher speeds there will be a little more available to accellerate the car. (No diffrence at lower speed ofcourse.)
Top speed, as less drag means less power is needed to reach any speed. If the engine power has dropped below what the car needs to accelerate at say 100km/h now, with less drag the engine may still have the capability to rev a bit higher. Less power at so high revs, but enough to overcome the reduced drag.

[Scotracer]

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

Jesus, that Mercedes Sauber generated as much downforce as a modern F1 car with less drag

[Cyco]

So it should be able to, the closed wheels increase surface area available to turn the whole car into a wing, whilst at the same time reducing drag.

[mep]

Tried to search for your thread, couldn't find it.

yea maybe you can't find it because it's maybe not one thread but several comments
in different threats. But I must admit I also don't know what all has been written and what not. But I can later post a few pictures here of the car how it looks now.


Back to topic.

I do not yet know what stops it from going faster, but I think the engine must be close to it's peak rpm at top speed now - after all, it is quite fast for such a small car.

Maybe we can get more informations about it just by sitting on the table,
and calculate the car.

What we need are information about the engine the transmission and the tires.
Can you get a power graph of the engine or some info’s like max power and the rpm at where you have max power, and the max rpm and maybe torque.

And the gear ratio,
and rear tire diameter.

[Flummo]

Gear ratio and tire diameter would be pretty easy I suppose, but engine data would be alot harder I think. I will look into it, but I would not get my hopes up.

[British Menace]

Hello all,

I am very interested in this thread and I am hoping it will be well supported.

I have, just recently been working on exactly this concept of improving the aerodynamics of various sizes/ scales of r/c cars.
These include Formula 1 cars in 1/5th and 1/10th scale with gas and Nitro engines respectively. Touring/ Sedan cars at 1/10th scale in both electric and Nitro.

I have raced these cars at a National level both here and the UK so have some experience in R/C car racing. I have also studied Engineering sciences including aerodynamics at Low Re (Low reynolds number .... low speed

I am assembling body parts including undertrays, wings and body parts for these cars along low Re airflow designs......

I am at work at the moment so not able to stay on too long but the main problems which are associated with this project have already been mentioned....
Very low speed, hence low Re numbers at cornering speed (20 - 45 mph) of 20 - 150K
Obtaining sufficient effects as such low Re numbers is very problematic but NOT impossible. Airflow around these body's and body parts is not the same at such low speeds as our full scale counterparts. So the way we take advantage of this airflow will also be different!
The wings for EX. end-up looking quite a bit different
I do believe though even at such low cornering speeds, a noticeable difference can be felt in the drive of these cars with a carefully constructed aero package.

More to talk about but will have to be later.

British menace/ Anthony

[fussell]

I know that aerodynamics make a big difference to 1/10 cars, I've tested them by just adjusting the angle of a rear wing (so not adding weight) and found differences in handling and straightline tests. I've also played around with other things, like diffusers. I don't have scientific data, but you can visibly see the difference.

I hope to get some videos of my 1/10 car running up on here this week.

[British Menace]

That would be cool to see ....

[synvex]

This is an old topic, but since I am new to the forum, all new to me! I was an active racer in the 1/8th scale gas-powered class for many years and I can tell you that aerodynimics do play an important part on these cars, although not as important as on F1 cars. Let me explain:

On the model racing car it is all about downforce, rather than ground effects. The reasons are:
1. Model racing cars run on foam tyres to get enough grip, so the size of the tyre, from new, to well used is quite large, so to keep a constant gap between road an chassis is not easy.
2. I, for one, built a wing car in 1979, complete with wing sections between the wheels, with floating skirts, et al. The car went quite well, but had a problem. The model car, if raced properly, runs close and over inside kerbs in the corners, thereby losing the ground-effects quite suddenly and where, normally, it would have been just a spin, now the effect of no grip at all, makes the car crash suddenly. The same goes for diffusers at the back or any undertray fitments. If the car is going fine, everything works and to benifit from the change you use less wing at the back to reduce drag, but the moment that effect is broken, it becomes a disaster.
3. Normal r/c cars' wings are basically a drag foil that create weight at the back at speed. What I have done with great success, was to build a proper wing at the back that creates downforce needed at the rear, but the biggest problem is weight. The normal wing is a strip of Lexan, the proper wing must have a double section of Lexan, with winglets and weighs in r/c car terms: A ton! Also, r/c cars must be able to take a crash and continue and the wing being the highest point at the back, it takes most of the impact, so the wing must be strong as well.

For interest sake, the 1/8th r/c engine operates on approx. 40 000rpm and develop over 3HP in and engine of 3,5cc displacement. These cars accellerate 0-100 in 1,27 seconds approx. All cars are 4WD these days with the circuit racers having this system where the front wheel in real terms, have more power on the inside wheel of a corner, plucking the car through the corner. In full sized cars I do not think the driver's bodies will be able to withstand the g-forces created? The traction created at the back of these cars are so high, that they abondoned differentials at the back to aid the car's ability to turn into corners. Normal racing straight speeds are in the region of 110-120km/h, but accelleration is much more important than straight line speeds as the straight on a circuit lasts barely a few seconds. Main races are 45 minutes in which about 140 laps is done. Average lap times could be in the region of 16 seconds.

[Blanchimont]

I, for one, built a wing car in 1979, complete with wing sections between the wheels, with floating skirts, et al.

Do you still own this car and can upload some pictures?


In the following, I'll try to estimate the theoretical downforce of an 1/8 scale car.
Let's say the original car has a lenght of 5m and travels at 90m/s and produces a maximum of 15kN of downforce,
that would give a Re number of = (5m*90m/s) / 15*10^-6 m^2/s = 3*10^7

A 1/8 scale car at 30m/s runs at Re = 1/8 * 1/3 * 3*10^7 = 1,25*10^6.
So the Re numbers differ by a factor of 24 and therefore the drag and downforce coefficients should also be different, even if the original car and the scale model have the same body outline. But if i ignore that point and simply scale the downforce down by (1/8)² because of the scale and by (1/3)² because of the velocity, then the 1/8 car should be able to produce

15000 N / 64 / 9 = 26,04 N or 2,65kg

of downforce. I think that is in the same range as the weight of such a car and could improve the cornering speed dramatically.

But is it realistic?

I just googled a bit and found this picture. Indeed it shows a huge rear wing/spoiler at an angle of maybe 45 degrees and a nice gurney flap. In combination with the vertical plates at both sides, that should really improve downforce on the rear tyres.

[synvex]

Unfortunately that wing car was in 1979 and I did not even have a camera in those days! What I did was to beg Alclad from an aero-engineering pal of mine [0,3mm] with which they manufacture aircraft wings [cladding] with solid ally rivets as they use on aircraft. I constructed two upside-down sections down the sides of the chassis [fixed solid to the chassis]. The sides were flat with a double edge at the bootom in which the skirts made of polycarbonate ride [slotted vertical with two screws to guide them]. I then cut a formula 1 body of that time out to remove the sidepods and leave only the narrow nose and driver's figure to extend like normal over the car. I then also made a proper wing-section wing at the back from the Alclad with little angle ass well as a secondary flap at the back.

As said, the car went very well and was quite noticable quicker than the normal chassis with the F1 body on [the Lotus 78 long body was the favourite in those years], but if the your car 'lose' the downforce by going on top of a kerb or hit a bump and jump, the crash is quite substantial. Also, the cars in those years were flat tray chassis with no suspension.

If you look at the car on your photo, it is a modern suspension car with 4wd. The body is fixed on the front with two pegs and body clips and at the back the same. That body flex quite a lot under the downwards pressure and I always fixed the body more secure to the chassis in more places to get the maximum benifit from the downforce. These modern cars also use at the back a bar on which the body ride that move with the suspension upward as the suspension moves up to keep the body from clearing the ground, but I never liked this system, because the downforce transmitted to the suspension direct at the wheels making the suspension at the back harder and harder as the speed increases. I liked to mount the wing on the chassis direct an let the rear wheels free to do their job.

But that is theory and taste. As said, these cars generate so much grip at the back that finding the neccessary steering, is a problem.

[machin]

I never liked this system, because the downforce transmitted to the suspension direct at the wheels making the suspension at the back harder and harder as the speed increases. I liked to mount the wing on the chassis direct an let the rear wheels free to do their job.

Modern F1 cars mount their wings to the sprung part of the chassis because the rules mandate it... the ideal situation is to mount the wings/downforce developing body directly to the wheel uprights because THAT arrangment means the suspension does not have to withstand the aero forces and can therefore do its job better (allow the heavy parts of the car to move relative to the road surface and thereby smooth out bumps in the road and therefore increase traction).... maybe I'm misunderstanding the paragraph above, but you seem to be suggesting the opposite...?

[synvex]

I understand what you say, but as the downforce increase on the rear wheels at the back as the speeds increase, does this not make the suspension less effective although the grip increase? The pressure on the uprights of the rear wheels could make that you in effect have a stiff axle effect at the back with the chassis moving up and down. On a glass-flat surface surely the higher grip will win, but what about a bump in the circuit where the wheels have to move independantly? Or in high speed cornering? Will the 'binding' of the rear suspension not mean that all the other components of the rear suspension, like the anti-roll bars, etc. not work effectively?

[autogyro]

Interesting.
Mounting the body/wing directly to the wheel hubs creates a dual chassis car.
It does not prevent the proper workings of the suspension.
However it does negate the suspension effect of the tyres.
On a model of this type the tyres are a major part of the suspension because they are very soft and spongy for grip.

It would need a complete rethink of the design.
Harder tyres? No more unpredictable bounce, less tyre wear.
Totaly different suspension settings?
IMO it could result in a faster car.
No doubt they would ban it.