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Mar 04, 2015, 10:52 AM
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New Product

New single motor contra-rotating Kimmo system

2011 was the year when contra rotating systems came to F3P competition. That year Fabien Turpaud introduced his contra system with two direct drive outrunners mounted back to back and I introduced my dual motor geared system.

Mine started as an experiment just for my own use. I never did thought it would be so successful as it went out to be. Donatas used it at the 2011 ETOC to place 3rd and after that it was used by many great pilots to win many national and international competitions like the German championships, IITOP, ETOC and the of course the very first F3P FAI world championship by Gernot in 2013. Of course the success of our national team using the system is also very important for me. Big thanks to everyone who ever used one or is still using one!

Since then the progress has been rapid. The contra-rotating systems have become the norm in international competition. The dual motor contra was originally intended for around 100-110g planes, but by now the weight of the planes has gone down to less than half of that. It’s safe to say the dual motor contra has become a bit heavy for todays planes.

For this indoor season and the world championships in Poland I decided it was finally time for me to make a completely new system. Luckily I had two excellent test pilots Iiro and Janne, they specified what they wanted and it was my job to deliver such a system. The main considerations were less noise and ability to swing much bigger props while retaining the great efficiency of the old system.

We also wanted it to be lighter than the dual motor system, but it was recognized from the start that big props and lightest possible weight are contradictory goals. Braking was more important and it was accepted that it would probably not be the lightest contra system ever. Now I think we have reached all those goals and even more.

The system got its competition debut last weekend at the open Finnish championships, placing 1st, 2nd and 3rd. But the real test will be at the World championship in Poland in less than 2 weeks. Then we will see how it performs in a bigger hall.

In the following posts I will introduce the system and try to explain how it works and why some design choices were made.
Last edited by Kimmo Kaukoranta; Mar 04, 2015 at 11:54 AM.
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Mar 04, 2015, 10:56 AM
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Motor and the main reduction drive

The new contra system is based around a single 5 gram outrunner that is modified and rewound to perfectly match the system. To reduce noise, the main reduction stage is a friction drive instead of spur gears. There is only one reduction stage that transmits the torque for both props, not two separate reductions from the motor to each propeller shaft. This has several advantages.

Because there is only one large friction drive reduction, it can be located front from where the system is mounted to the plane. Basically between the rear prop and the nose of the plane. This way it can be made much larger than if it would have to be embedded inside the fuselage, behind the mounting lugs that would have to reach over it. This in turn allows for much larger reductions to be achieved. The diameter of the friction wheel is 50mm and together with a ~5mm aluminium roller gives a reduction of over 10:1.

With that large reduction ratio we can swing some big ass props with a tiny motor! The prop range basically starts from where the other systems end. We started from 12x3.8 and worked up to 13x4.7. Big props give you more propwash over the surfaces which helps with many manoeuvres, especially the torque rolls.

The big props also brake the plane in the downlines. With the large reduction the props pick up much less RPM when in a dive, further enhancing the prop braking effect. With such a system you get full control of the speed, without resorting to fixed airbrakes that help with the downline speed but just hurt your flying on other maneuvers. Or dynamic airbrakes that add further complexity and weight to the model. Just put the throttle to idle and you have a huge brake up front. You decide how much you want to brake and when. Full control of the speed.

A friction drive always needs some force pushing the two friction wheels together or it will slip and efficiency will be really poor. The amount of force also needs to be high enough that the drive does not slip at the highest torque transmitted. But much more than that will just reduce efficiency and wear out the friction wheels faster. So I incorporated a screw to adjust the spring tension.

If the drive slips at full throttle, you can hear it from the whining noise it makes. By turning the screw out the correct level of tension can be reached when the whining turns to a smooth humming sound. This does not mean it would need constant adjustment. The units we are using were adjusted on a test bench for maximum thrust when I finished them. After a lot of flights I had to tighten mine a bit once, since it started to slip a bit in the half cuban 8, where I think I use the most power. This is due to the o-ring wear and also the aluminium roller surface being “polished” in use.

The geometry of the motor mounting arm is also such that the force increases with RPM. At idle there is basically only the force from the spring, but at high throttle the motor also swings up and in, increasing the force.

The motor mounting arm also acts as an additional heat sink for the motor, increasing it’s power handling capacity. Not that it would really need it, but since the arm is there anyway, it can serve a dual purpose.
Last edited by Kimmo Kaukoranta; Mar 04, 2015 at 11:16 AM.
Mar 04, 2015, 10:58 AM
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Reversing transmission

The main reduction is directly tied to the rear prop and the outer shaft. In the back of the motor there is a reversing transmission consisting of two timing pulleys, a timing belt and a pair of spur gears. This both reverses the direction and provides further small reduction in RPM for the inner shaft connected to the front prop. Thus the front prop swings at a slightly lower rpm than the rear prop. The original idea was to make this ratio configurable by changing the gears to enable fine tuning of the torque balance between the props. However we have been very happy with the ratio we originally selected (15/14) so there might not be any need for that. But it’s possible.

Since the reversing is done using toothed belts and pulleys, it means the RPM ratio of the front and rear props is always totally fixed. With a system consisting of two separate reductions that both can slip, it stands to reason that it’s more likely that one of them is slipping more at any given time than the other, instead of both slipping the same all the time. That could be nasty if it happens during torque rolling. I’m not saying that would be a big problem, but now any chance of that is avoided. Even if the main reduction slips, the ratio remains unchanged, just the total power is reduced.

Because the gears and the timing pulley are turning at the low propeller shaft RPM, not at the high motor RPM, the pitch of the sound they produce is quite low. And since with the bigger props the RPM is lower, the gear sound is a nice and low purr. But the sound still correlates with the throttle position and the pilot gets an audible feedback for every click of the throttle stick, it’s not totally silent. The spur gears and timing belts are machined from POM (=acetal =delrin).
Last edited by Kimmo Kaukoranta; Mar 04, 2015 at 11:14 AM.
Mar 04, 2015, 11:04 AM
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Main frame

The main friction wheel, main frame and the front bearing holder are made using additive manufacturing methods, laser sintered from nylon powder. This is a strong but light material, about third the density of aluminium. The frame needs some torsional rigidity since the motor is mounted on an arm and there is some spring force trying to twist it. This is provided the same way as in our F3P models, using a truss made from carbon fiber rods. The result is a light and rigid structure, better than what I could achieve with some prototypes I machined from aluminium.
Mar 04, 2015, 11:06 AM
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Weights and figures

Weight of the contra system without props is around 12.9g and with the light 13” props around 15.8g. Granted, this is not be the lightest contra ever available, but it is still the lightest one that will swing huge wide blade 13” inch props with authority. There is also no need for additional air brakes, fixed or dynamic, saving weight. Compared to to the dual motor contra, we also save the weight of one ESC (~0.7g + wiring).

The efficiency is very good also. With a 57g plane and a 2S 120mAh GensAce I can fly 6-6.5 minutes or over twice through the preliminary sequence, in a basketball court size hall with 10m free height. A bit less in a bigger hall. Enough for any sequence in any hall and more.

The current draw at full throttle with the 12x4.7 balsa props is around 2.5A at 7.0V constant voltage (YGE-7S ESC). The static thrust is around 135g. With the 13x4.7 balsa props the current goes up to around 3.0A and the trust to around 155g. Almost 3:1 thrust to weight on a 50-60g model, so do not think you need to spend much time at full throttle ;-) The smaller 12” props give a little bit more flight time, but not a big difference, a couple of maneuvers. But the 13” props certainly brought a smile to our faces the first time we tried them ;-)
Last edited by Kimmo Kaukoranta; Mar 04, 2015 at 11:18 AM.
Mar 04, 2015, 11:11 AM
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Videos with the new system flying

Some videos from the open Finnish championships:
2015 Finnish Open F3P-AF Janne Lappi (4 min 26 sec)

2015 Finnish Open F3P-AF Iiro Lehto (4 min 23 sec)

And one from a bigger hall, much like the one in Poland:
F3P Team Finland for the 2015 World Championship (7 min 58 sec)
Mar 04, 2015, 11:19 AM
"Free" in Christ! #F3P
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BEAUTIFUL!!! Truly an innovation!!!

Do you plan on selling these? Or maybe just the parts?

Mar 04, 2015, 11:34 AM
Risto Hölttä - F3A/P Team FIN
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Yes, the planes and the technology in them is truly state of the art, from tail to the tip of the prop axle. The pilots are top notch as well.

It will be very interesting to see if a small Nordic country can beat the big ones like USA, France, Austria and Germany among others.
Mar 04, 2015, 03:16 PM
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Finnspeed, let’s not get carried away, the competition has not even started yet...

Free, right now there are less than ten in existence, just the ones in our team’s planes and a few backups. If there is demand, I could build a batch when I get back from the worlds. So if anyone is interested, please email me at for pricing and details.

About the parts/kit option, I have thought about that also. The motor systems are getting more expensive and that worry has been expressed by many. I guess F3P is still cheaper than many other FAI categories, but it would be great that it does not become too much of an elite sport. It should still be possible for talented young pilots who do not necessarily have the resources to buy several hundred euro motors to get into. Also there seems to be people with interest in building their own systems.

With a basic kit I could provide just the custom manufactured and hard to find parts. The builder could source the motor, shafts, bearings and other easy to get parts by himself. A complete kit could include all the needed parts. I would do all the things that require special tools or equipment. All the fitting, gluing, testing that needs just time, patience and basic tools every modeler already has would be left to the builder. For those who do not want to rewind a motor, a complete modified motor could be provided at extra cost.

Some good instructions would obviously be needed. That could be in form of a build thread here in RC Groups. That could also benefit those who have purchased a complete system, in case they crash and need to do some repairs or change some worn out parts.

I would of course still provide complete, tested and ready to run systems, in addition to the kit. But it the kit would allow also the tinkerers and those not willing or able to shell out hundreds of euros to fly one.

Anyone else interested in a kit option? I have not figured out pricing or anything yet, but if there is interest, I can look into it.
Mar 04, 2015, 04:36 PM
"Free" in Christ! #F3P
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Thank you Kimmo. I know that I am interested in a kit option and I think if the price of your system will be significantly less than comparable systems(<$400) I think the F3P community(especially those just getting into the competition aspect) would be very appreciative.
Yes, a few of us are making our own systems, but they are far from perfected.

Personally I would be most interested in a kit version of the system(non assembled)(or without common parts). We'll see. 😀

Mar 04, 2015, 04:40 PM
"Free" in Christ! #F3P
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What are the size of your motor shafts?? It is 2mm for the rear prop and 1mm for the front prop??

Mar 04, 2015, 09:40 PM
I don't want to "Switch Now"
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Very nice Kimmo, thanks for sharing so much detail
See you in a couple of weeks

Pat MacKenzie
Mar 05, 2015, 12:03 AM
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Kimmo, congratulations on an awesome system!! I would be interested in a kit version.
Mar 05, 2015, 12:43 AM
Risto Hölttä - F3A/P Team FIN
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Hehe, good one, Kimmo. I'm not getting carried away, I just think that there are good chances to win some nice positions at least. We shall see...

Personally I'm happy to have been able to offer some assistance to Team Finland, but my part has really been very small in all this. In any case, I will be there in Poland as a helper for the team.

Mar 05, 2015, 02:29 PM
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