Just another crazy idea I have
I was thinking as there are many aircraft with contra-rotating propellers, and since today a lot of people seem to be making home-brew brushless motors, why not combine the two, and make a contra rotating brushless set up.
Here it is, my first design, using two single-stator motors. The design would have to use two seperate shafts, with bearings between all rotating components( sorry, these are not in the drawing)
Is it feasable? Is there a better way? (I'm sure there is)
Thanks

The basic idea is proper.. getting the bearings might be expensive.
The propellor end will need to be quite stiff to prevent flexing due to the loads.
You may need an intermediate fixed shaft between the two rotating shafts. The forward prop shaft will be somewhat extended to clear the aft prop, so additional support might be needed.
The intermeidate shaft would help with the bearings.. the outer race on the bearings supporting the inner shaft would be fixed and the inner race on the outer fixed, eliminating the problem (?) of having both races on each shaft rotating opposite each other.

in recent article in ama mag. they discussed a plan for rubber powered plans with counter rdotating props also gave address for this type motors also said had one made for electric.

It's not as far fetched as one might think. Check out the March issue of Quiet Flyer magazine.

Fellow built one on ''the other'' electric site using two Gobrushless motors back to back that worked fine. Was thinking of doing this for my Convair Pogo but it works fine with a single CD Rom motor with no torque problems. Might be cool though.

My main problem is the width of the shafts. Since the inner one is about 2mm(thinnest brushless motor shaft) then the outer one would be about 9mm (the bearings for 2mm shaft are 1/4 inch + almost 2mm at least shaft width) Now the shaft is getting pretty thick. Now the second motor tube would have to be 16mm (14 mm bearings) THat's kinda thick. I don't if that might bea problem. THe stators for the 2nd motor would have to be drilled out, and I don't know if that will fit. But the most serious problem would be making the shaft. It would probably have to be custom made unless I can find a ready-made , and custom making it would really cost a lot, and since you want it to be stainless steel, it would be pretty hard to machine it at home. Any of you guys know where to find a 1/4 inch ID Stainless Steel Shaft? (The OD isn't as important, but it shouldn't be too large) Another Idea for the shaft was using Carbon fiber. Would that work?
Also, I don't think there should be too much trouble with having the two shafts having bearings directly between each other. The only problem I can see is that the bearings have to be able to withstand 2x the rpm, but with a 15,000 system (the max I'm estimating for this, it should be lower), this shouldn't be a problem. If there is one I can't see, please point it out.
Thanks
Mitek

Instead of inline motors, why not side-by-side, geared to the shafts?
The Double-Mamba, Allison V-3200., Allison XT-40 used this method.
Shorter but wider, without the need to drive thru the front motor.

like this one.
jimbo

That's the one I was thinking of,Jimbo. Any details on the shaft setups?

Wow!
Beautiful work
How does it run? Are there any weird vibrations with having two propellers?
Could you tell me details on the construction?
And could you post a video of it running?
And about the side by side idea
Might be interesting, I guess it would simplify things a bit. You wouldn't have to have one shaft running through the other. I just wonder how would you link up so that two motors would drive two propellers on the same axle?

Can someone enlighten me on the advantages of a contra setup. Does it offer any significant advantage over single prop motors.

Pehaps the torque due to a single motor will get cancelled but is this really worth the expense and the complications ?

eljimb0 - please do let us know your experience with that neat piece of engineering.

I don't know if "the other forum" is a bad word. Please excuse me if I step into the Hot zone. I have found great things and great folks here and there. The motor I made is about 2 months old. The stators came from tape drives the magnets from "Strong Motors" . A Swiss fellow (A. Gausser) made a similar motor about two years ago , His motor weighs 600 grams. I never saw his motor. I'm sure he is a great guy and a terrific inventor. Here is a link to a video of my motor running http://www.rcgroups.com/gallery/show...cat=500&page=1

I don't know if there is any practical reason to do this with a model aircraft. The issues of efficiency can be debated forever. A steeper pitched rear prop seems to be part of the formula to take full advantage of the pre-accelerated air.
Pairs of opposite pitch props are almost non-existant. To do this contra thing with gears is complicated to the extreme.
Lots and lots of tiny little things to break and get lost.
However>>>>
Two direct drive electric motors with one speed controller driving both motor is butt-simple. This unit pulls like a Morgan horse. It has a great sound. The cool factor is 10+.
The inside prop is threaded to fit the aluminum end bell. There are 4 bearings (2 for each motor) all of them bought from my local hobby store. I hope to kit a version of the motor .

Thanks for the info eljimb0

While no significant advantages seem to accrue from the contra job - I can understand your sense of achievement - not be mention pride putting it together.

I guess we cannot always go about trying to justify whatever we do in this hobby in dry engineering terms. Its mainly about having fun, doing different things and you seem to have had a swell with this project

Great looking piece of work . ......Happy Flying

Andreas Gasser's contra-rotating coaxial twin lrk outrunner for his F5A pattern ship:
http://groups.yahoo.com/group/lrk-torquemax/
-> files
-> twin lrk & contrarotating prop
-> contrarotating_tandem_lrk.doc (in English & German, with Andreas' contact information)

It would seem to me that you woild need to have a higher pitch prop on the rear to take into consideration that the air is moving. Seems like Tom Hunt had this problem wiyh his Spitfire,

Eljim0's design, and discussion on pitch:
http://www.rcgroups.com/forums/showthread.php?t=246407