Another coaxial F3P plane - Page 4 - RC Groups
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Nov 17, 2011, 05:01 AM
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Jurgen Heilig's Avatar
Originally Posted by Kimmo Kaukoranta
Hi Jürgen,

Last winter Janne used a geared single motor system. That unit uses a single 5g motor to swing a 10" prop and weights 11g without the prop. This year he switched to the twin motor contra system and that is what he is currently using.

So on the above videos, in the one with last years final program, he was still using the single motor system. On the AP-13 and AF-13 videos he has the (twin motor) contra system.
Hi Kimmo,

perhaps I should have made my question clearer: Did Janne use a single motor contra system (as shown in your prototype video), or a twin motor system in the videos? Your answer however was clear enough.

The "torque-roll" in the F3P-AF schedule with direction change and a "ridiculous" k-factor of 6 (provided this does not get changed), gives contra systems an advantage.

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Nov 17, 2011, 08:48 AM
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Kimmo Kaukoranta's Avatar
I think you mean the "co-runner" concept, with the single motor driving two props in opposite directions without any gears. That is not yet developed to the point it could be used in a light competition plane. It works, but it's too heavy.

I'm currently putting my efforts to the twin motor geared system to improve it further. The main idea is to simplify it's construction.

The ones Janne and myself are using, the one Donatas flew in the EToc and the few units I have sold have been so far built with methods not really suited for larger scale production. For example the gearbox frame consist of something like 16 small pieces. Lots of small fitting required.

I drew the frame in Solidworks in one part, and had it 3D printed in Shapeways. Actually I had the whole system printed, but unfortunately the printed gears did not last, which I was a bit afraid of. However the gearbox frame and the mount came out great. The next version will have the 3D printed frame and mount, but still uses injection molded gears, of which one is modified to a larger bore for the outer shaft. Not having to do the modification was the reason to try printing also the gears.

Here's a couple of links to my galleries in Facebook on what I have been up to lately (these should be public, so visible to everyone):
The original "hand made" version
3D design in Solidworks
3D Printed Contra, first version
3D Printed Contra, second version
Last edited by Kimmo Kaukoranta; Nov 17, 2011 at 08:58 AM.
Nov 18, 2011, 12:02 AM
Eric Johnson
is the 3D printed gearbox lighter? also will the price be a bit cheaper? i love my coaxial unit by the way i just need a hall with a bit more height to practice down line manuevers
Nov 18, 2011, 03:52 AM
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Kimmo Kaukoranta's Avatar
They are very close to the same weight (see the attached photos). There is some variation also in the weight of the props. I'm glad you like the system!
Nov 18, 2011, 08:56 PM
Eric Johnson
i think ill stick with the CF version if you are still going to be selling it
Nov 21, 2011, 11:28 PM
Doug Cronkhite's Avatar
Yeah.. I'm interested Kimmo. Very interested.
Dec 03, 2011, 09:41 PM
Eric Johnson
kimmo, how are you getting your props shiny?
Feb 04, 2012, 07:42 AM
Registered User
Kimmo Kaukoranta's Avatar
There was some discussion about the distance of the props earlier. I have been testing one unit that has a 10mm shorter distance between the props compared to previous ones. Distance of props is now 35mm measured from tips, 25mm shortest distance in the center of the blades. Also the rear prop is a 10x4.7, instead of 10x3.8 for both. Seems to be working fine with no problems, and the higher pitch prop on the rear increases the total power output a bit (not that I need it in a 94g plane).

I think there was a slight increase in the interference noise when bench testing, caused by the props passing each other so close. However it's not audible in the air. Also the 3D printed gearbox frame in the latest versions seems to dampen high frequency vibrations from transmitting to the plane. The previous carbon fiber frame sounds a bit louder.

I put the printed parts on sale on Shapeways, just in case anyone who has one of the units needs a spare part. Obviously they can also be used to build a similar unit by someone who knows how to make the props etc:

BTW. If anyone is wondering how strong is the printed frame, see (same material):
How strong is White Strong & Flexible? (0 min 20 sec)
Feb 04, 2012, 07:48 AM
Registered User
Kimmo Kaukoranta's Avatar
Eric, the shine to the props. The backside of the prop is against the mold, so if the mold is smooth the backside of the prop will be shiny. The front is against the plastic vacuum bag, which again has a smooth surface. I do use peel ply and breather to squeeze any extra epoxy out, but I remove that after ~10-15minutes and for the rest of the cure, the bag is directly against the laminate.
Apr 21, 2012, 04:35 AM
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Lantsov Alexey's Avatar
Hi Kimmo.

As you see my version with one motor?
Apr 21, 2012, 02:29 PM
Full Scale Piper Cub Driver
Piper J3's Avatar
Originally Posted by Lantsov Alexey
Hi Kimmo.

As you see my version with one motor?
This is a very elegant design... Has this been tried successfully?
Apr 21, 2012, 05:27 PM
"Free" in Christ! #F3P
Free's Avatar
Is there any reason stoping you from having one motor, a gear on the shaft of that motor, two gears attaching to that gear on the motor shaft, a short rod coming from each gear attached to the one on the shaft of the motor, and on the end of those rods, a gear, smaller that the one on the other side of the rods, that attach two two bigger gears, that in turn attach to the props? Sorry I do not have a picture of my idea, so if you do not understand my explanation, please ask.


P.S. For the setup above, I would need to make something to hold it together.
Apr 22, 2012, 08:07 AM
Registered User
Vladimir88's Avatar
Originally Posted by Lantsov Alexey
As you see my version with one motor?
Bearings are not suitable as a contact rotating assembly.
Resistance contact is unstable and has a very large value.
This can be seen in the video.

IMG_0073.MOV (1 min 1 sec)

In my view, promising coaxial motor without gearbox must not contain unreliable moving contact node.
Similar this -
Two motors are not the problem, because this increases the thrust twice.

I think that for example on the basis of two motors AX-1304N ( it is possible to try to create a coaxial motor with weight about 12...14 grams for working with props about 8 inches.
Last edited by Vladimir88; Apr 22, 2012 at 11:53 PM.
Apr 22, 2012, 11:42 PM
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Vladimir88's Avatar
I suggest the idea of ​​a coaxial gearbox with one motor.
Apr 23, 2012, 02:35 AM
Registered User
Lantsov Alexey's Avatar
Vladimir thanks for the reply.
1. your bearing to press, then measure the resistance.
2. the gearbox is problems in the red zone.
Do the gearbox at the rear.

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