Some myths seemingly persist... - which are those Mr. Bare? MYTHBUSTER fans everywhere want to know!

Well, not the greatest day in my motor testing career Got the motor all bolted up to a test stand, threw a 10x7 prop on the front shaft, plugged a 3 cell pack in and powered her up.

Everything went fine until about half throttle. After that was the most violent vibration I have ever seen in a brushless motor. So much so, that my test stand started bouncing off sideways. Which is no mean feat as it's an old work bench!!

Went and balanced the prop (which was fine), and tried again. Same result. The end of the prop shaft was vibrating at least a centimetre from one side to the other.

I haven't had time to do a thorough inspection, but it seems like I have at least broken one of the bearings, and possibly bent the shaft.

I must have a bearing from another motor laying around that will fit. I just need to check the shaft and see if it's ok.

WOW! Bet your happy it was on the ground and not in the air! Good thing all us experts were warning you about........everything except what happened!

Were you running both motors at the same time or just one of the motors? Also consider starting off with smaller props and have you thought about using a 3 blade prop, at least for testing purposes. I mention this since I have seen 3 blade props cure "resonance" issues (severe shaking/vibrating). I hope that the damage is not too bad and I also hope this is not an instance of "less expensive parts not being up to the job!"

One final thought, hard mounting a contra rotating system for testing may not yield the best results. Some sort of rubber pads or padding might provide some degree of elasticity which might help reduce the effects of vibration. Just mentioning as a thought since I do not know what your test cell looks like and you might already have some degree of damping built in!

Michael (if your not fryin, your not tryin!)

I imagine you'd need to have props on both motors... to balance the system properly. I'd start off with a couple of 6x4's and work up!

Thanks Michael. I only had the rear bell/front prop going. My next test will have both front and back going. I will try and mount some rubber pads in there for the next test.

Thanks Dr. Kiwi. I'd try 6x4's if I could. One of the problems with the motor is the shaft size attached to the front bell is 12mm!! My APC 6x4 is less than 13mm across at the hub. Might be pushing it a bit I think the minimum will need to be an 8" APC prop. The hub on those is closer to 20mm.

I'm looking for a place locally that has the right size bearings. Will change them over for the next test. The bearings might cost more than the motor

How did this end up? I was thinking of trying one of these out but with the first result above I'm hesitating a bit. It might have been the prop shaft was already bent in some way during your first test? I assume you balanced the bells?
/Håkan

Could u guys help me please in this issue:

What´s the thrust generated by this contra rotating set?

I´m building a XB-42 Mixmaster bomber (pusher contra-rotating) and it will weight aprox. 2,5 kilos.

I´m searching for a contra rotating brushless set and may be this set would be able to fly my plane.

Any clues PLEASE

I have shelved this motor for the time being. It needs a lot to get it back into running order. Would be easier to buy a new one and test it with both props.

I'd check out the HIMAXX contra rotaters CESARNOGUEIRA. http://www.maxxprod.com/mpi/mpi-266.html

Guys, I put a video on you tube testing this motor, take a look at:

HobbyKing counter rotating or contra Rotation Motor cr23l Test (1 min 29 sec)

Thanks

Originally Posted by CESARNOGUEIRA

Could u guys help me please in this issue:

What´s the thrust generated by this contra rotating set?

I´m building a XB-42 Mixmaster bomber (pusher contra-rotating) and it will weight aprox. 2,5 kilos.

I´m searching for a contra rotating brushless set and may be this set would be able to fly my plane.

Any clues PLEASE

Nice work! It sounds great. Im really looking forward to some flight reports. Good luck!

Anyone have an idea what the no-load current is?

So, how has everyone done with this motor? I put one on a 56 inch stick type plane this week, "just to see". I used APC 9X6-e and -ep props and a 3 cell pack for about 45 amps total. Seems to run fine, although quite noisey. I had to pay particular attention to prop balance as the front prop will do nasty things if it is even a tad off (I had to balance the hub).

Alternately, I have benched it on 4S with the same props and it pulls up in the 70 amp range (1000w input). It seems to be OK but I've not flown it there. I can't really see it lasting all that long up there. But then, maybe that's OK, this was just an experiment anyway.

My setup has the motors out in the breeze and they never even got up to ambient temps, which leads me to believe I could use larger props. Anyone tried anything up in the 11X? range?




Chris

No one? Everyone moved on?

Chris

After I busted my first one, I never ended up getting another one, so can't offer any advice!

OK, I went ahead and set up an 11X7 prop on the front and an 11X8 on the rear. One thing I hate about this motor system is the 12mm hole that you have to drill in the back prop...who does that? No such thing as a prop reamer that big that I know of. So it's the drill press, not fun to center. Nonetheless, I carefully balanced both the blades and hubs and mounted them up. On the bench, on 3 cells, they seemed great. Smooth, around 8K rpm, nothing getting hot and lots of thrust. Power...well, over 800 watts, yup, better than 80 amps total.

Took the plane to the field and it flew much better than before, loooooong legs now. But the noise. Good grief, it's like flying nitro again. But not everywhere, it's like there is a resonance that kicks in at about 3/4 throttle. At low throttle settings, nice a quiet. In fact the sound of a power off dive is incredible, it makes the neatest "wind up" sound as the two props spin up.

I'm not sure but I think the problem could be in that long, small shaft that runs from the rear motor to the front prop hub. I'm thinking that it may be "whipping" when the system unloads in the sky. that being the case, maybe I need to swap the props, higher pitch in the front than the rear, hmmmm.

I flew it several times, short flights, just a few passes around the field and then down to check temps. Then back up for a few more laps, probably averaging half throttle. Nothing got even warm, well, the batteries got a little warm, but with that kind of current drain it's not surprising.

I think this power system has potential, it certainly seems to be able to handle some stress. But the noise is a definate issue. I'll try a couple of Xoars and maybe 3 blade props (I read somewhere that they can sometimes overcome odd resonances). And I haven't even begun doing the whole differential prop comparison thing yet. This is the true shortcoming of the system, props.

Chris