There's a new type of brushless motor showing up with external rotors. Supposedly they are much better suited to driving props at high torque and don't need gearing. Sounds useful to our application.

Some of them aren't shaped right to mount in a sailplane's nose, but others are shaped just like a regular can. Even so, the external rotor might create a problem for routing wires...

Anyone tried one?

If you are refering to the outrunner types like an Axi, then I can comment on them.

I used to have an Axi 2820/10 (the one I put in my brothers plane) which I ran a 13x7 prop on 7-8 cells. It had great performance for sailplane use! They aren't quite as efficient as some of the more traditional motors that are used with a gear box, but you do save weight and money on leaving that aspect out of the picture all together.

You can get an axi setup with brushless speed control for around $160. This setup will long out live your model (unless you crash right on the motor) and will be at home in alot of ships.

-Kelly

p.s. You can see a video of the performance on my website in the video section under the Kyosho Stratus. The all up weight of this plane was about 45oz.

http://kbw99.rchomepage.com

Search this forum for 'outrunner' 'lrk' 'axi' 'torcman' 'flyware' 'high-torque', plenty of threads.

If you want to build them yourself, completely from scratch or from kits:
http://www.yahoogroups.com/group/lrk-torquemax/

Met vriendelijke groet Ron van Sommeren
• diy outrunner brushless e-motor discussion.
• Electric fly-in & lrk meet, June 27th, 2004, Nijmegen, the Netherlands.

Originally posted by kbw99
They aren't quite as efficient as some of the more traditional motors that are used with a gear box, but you do save weight and money on leaving that aspect out of the picture all together.

A motor with gear drive for the same power level will be lighter than the outrunner. The advantage the geared has is that the motor can run at very high RPM to develope the needed power.

Steve

A motor with gear drive for the same power level will be lighter than the outrunner

Well, I'm not so sure about that, I hava a torcman TM-350-20, and a Kontronik drive 502 ( FUN 500-27 with 5.2:1 gearbox ), and they weigh about the same.
I don't have the exact numbers right now, but will post them later.
I have used them both in my Cumulus E-sailplane, driving a 18x11 prop on 8 cells, and the torcman seemed to have more power.
But that is subjective, I can't compare the 2 systems in motocalc, since I don't have all the specs for the torcman.

The big difference is in the price, the torcman was about 60% of the cost of the drive 502.

Michael

Steve / maybe so up in the rareified air you breath (f5b god), but here on the ground, outrunners are seriously interesting ;-) 3 advantages (in my order of preference) 1. Silence 2. Cost 3. simple torque (Ok thats 4 ;-). Out runners seem to be catching on much faster (again at ground level) in europe, than the USA .. a I guess thats because they aren't yet properly sold/distributed/talked about in the US ... a similar thread is here..... http://www.rcgroups.com/forums/showt...hreadid=149456
or just click on 'search' under Rons post ;-)

Kelly / nice vid .... good music too, adds much to the movie.
Mark

Originally posted by Mark Sanders
Steve / maybe so up in the rareified air you breath (f5b god), but here on the ground, outrunners are seriously

The fact are the same regardless of whereyou fly or live. To make high torque and power in electric motors requires bulk and weight. On the other hand high RPM and high power is easy to make in a small, light and compact motor. By attaching a reduction drive the 60K RPM can be reduced to something more useful with lots of torque--simple math. When you build a outrunner on the same quality as a Kontronik it will cost much more than a LRK motor--just look at the two! I have tested a few of the LRK type motors and the efficiency was not too impressive. The big Kohlor out runner motors on the other hand are quite nice and efficient--bit they are big , heavy and cost quite a bit.


There are no right answers--just options.
Steve

Steve, what could be the reasons for this lower outrunner efficiency?

Groeten Ron

sneu's right - It's the same as for infernal combustion engines. Horsepower = torque x RPM x constant. Small motor at 1/3 the torque but 6x the RPM makes twice the power. Having said that, I do own a couple of outrunners, and in some cases I like them. They're shorter than a motor/planetary combo, and need no offset like spur-gears or belts, so they fit into things that might otherwise have to use direct drive.

I've bench-tested my AXI with sailplane props. Watt for watt, they can't measure up to a normal brushless with 4:1 or higher gearing. Not even close - 20-30% more watts per ounce of thrust. +

Originally posted by Ron van Sommeren
Steve, what could be the reasons for this lower outrunner efficiency?

Groeten Ron

Not all out runners are created equal. several I have tested had very good operating efficiency. In looking at them it was clear that they made better use of the available winding area by putting in more wire. The next thing that set the good motors apart was the low "noload" current. I can't be sure what the exact reason was but it might be that the stator material is a better quality and design.

In the large motors the Kohlor looks pretty good--the Plettenberg orbit is a good one in the smaller size area.

Steve