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Old Dec 24, 2007, 07:46 AM
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Joined Apr 2007
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Suitable motor for 12ft thermal glider?

I'm looking for a suitable motor to launch a thermal glider which I am converting to electric. Without motor this 12 ft model weighs 5lb 10oz. The model will only require the motor to take it to thermal height, then hopefully not be used too much again in that flight.
I have considered a MVVS 4/6/1120 sport but it is pricey.
The other consideration is a Cyclon PR52 (750W). This is considerably cheaper but the retailer says it would suit the weight of the model.
I am new to electric flight, having been pure gliders for years. Advice please?
TIA
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Old Dec 24, 2007, 09:05 AM
Bill Mohrbacher, MECA Fox Guy
G WILLIE's Avatar
Beaver Falls, PA
Joined Dec 2006
638 Posts
CAS141, I'll be watching this thread. I'm an old soaring retread, too. I have a 12' Astro Jeff and want to put an electric power pod on it. I haven't decided how to go yet. In all the data I've seen, 2 numbers stand out. First, Larry Dudeck, designer of electric ducted fans planes, says you need at thrust to weight ration of AT LEAST .33:1 for a handlaunched plane. Next, Bob Aberle had a table in MA which said for a powered sailplane, your setup had to produce 40-50 watts per pound.

This year I had scaled up an old Jetex PAAload design, the Peter Pan from FM. With the original setup, it produced 7.9 oz of thrust and weighed 18 oz for a thrust/weight of .44. It was capable of ROG and a steady, but not spectacular climb so Larry number seemed correct. My fan metered 49 watts (11.1V x 4.4 amps), my setup used 43 watts/pound. Bob's numbers were also correct.

I have a 72" Spook Oldtimer I'm in the process of converting to electric. With a really Mickey Mouse test setup, I flew it twice last year with a Master Airscrew bushed setup, 12 x 8 folding prop, 7.2 volt NiMH battery. The Spook wighed 3# 8 oz and the MA setup produced 19 oz thrust for a thrust to weight ratio of .34:1. Power was about 86 watts for 25 watts/lb. The plane would JUST ROG and climb VERY slowly. Unquestionably underpowered!

Based on these two planes, I would shoot for a minimum of 50 watts per pound and at least .5 thrust to weight. By coincidence, my Astro Jeff weighs 5# 4 oz.
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Old Dec 24, 2007, 09:46 AM
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Joined Apr 2007
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I've looked further into cyclon and i am a little dismayed- why is it that things seem to get" improved worse"? I thought i'd found that answer in a Cyclon 40. The info pointed to all the right parameters. But I can't seem to find one on sale in the UK- It seems they are no longer current ( pardon the pun ) - Now the equivalent seems to be the Cyclon premium 32. But, if i understand correctly, this thing has to be mounted arse about face because of a rear attached mount and you are informed that you can push the arm out the back.
But from the photos it seems that makes you have to bend the output wiring back over the motor etc etc.
the 40 had a conventional method of attachment which was great for gliders!! GRRR.

G Willie - thanks for the reply. Don't use ducted fans - all i know about them i that they use tons of revs and motors with KVs that have no relevance to my gliders -
Anyway, google in Cyclon and you will see recommended power parameters and weights of models etc. The premium 32 would seem to do me ( dimensions are suitable too ) but I'm disappointed re that fixing.
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Old Dec 24, 2007, 12:21 PM
Speed cost money.
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United States, FL, Marathon
Joined Dec 2002
3,044 Posts
You first have to make 1 of 2 choices.
1) Inrunner with Gearbox, Somthing like a Hacker would be great
2) Outrunner with Low KV. The later is my choice, but be carefull they don't fit well in tight places. I personally selected one of the hyperion motors, My wing span is much smaller then yours 9', but then again I should have plenty of power to spare.

while you can get away with 50 watts lbs pounds, 75 watt lbs would be better

You say empty weight is 5 LBs, add 1 lb of battery and .5 lbs for motor (just rough guesses)

So ball park power is 6.5 X 75 = 487 watts (thats like almost nothing in my book)

Assuming you are going to run Lipo's and a 3s pack. Thats like 45 amps you need.

Here is a setup that would work just fine.

http://aircraft-world.com/shopexd.asp?id=4582
Get the 8 turn, Plan of using 1/2-3/4 for climb and full throttle for launch.

a 12" prop would be right for this motor.

Put the a 45-60 amps esc with it.

And get some 20C 4000 MAH lipos and you are set. You could find a cheaper motor at Hobby-city, But I am assuming you want ok quality.
A hacker b50 with gearbox would swing a huge prop and yank that thing up into the sky.

Good luck
Highflier
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