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Old Aug 27, 2011, 08:06 AM
Meh close enough.
minivation's Avatar
United States, TX, Longview
Joined Dec 2010
558 Posts
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Would 800W motor with 1150 do for a 2-3kg-ish RCM trainer?

Hi,
I'll be starting a restoration project of an old RCM trainer sometime later, and along with it I'll be performing a gas to electric conversion.
I bought a 890Kv, 800W motor (originally designed for 4S, but I'll be using 3S) and will be using a 11x5 prop with it.

Sorry for acting like an power-systems noob, but would the motor power be enough for some gentle flight (no rolling, no loops - just takeoff, fly, land)? As noted, although the motor is rated 800W, I'm running 3S, so I don't think it'll be as much. The RCM trainer used to fly with a .40 equivalent GAS engine, and the plane, empty, weighs in at 2.5 kilograms.

Thanks!
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Old Aug 27, 2011, 09:34 AM
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scirocco's Avatar
Australia, ACT, Kambah
Joined Feb 2001
3,057 Posts
Kv of 890, 3S and 11x5.5 is going to be pretty marginal on over 3kg as you'd only be looking at a bit over 200W.

If you can fit 12", I think you'd be a lot better of with a 12x8. if 11" is the max you can fit, then a APC 11x7 would give a bit better thrust and bit better pitch speed and should just fly it as you describe. Better still, a 3 blade 11x7, and you should have a bit more margin to play with.

ps as you can see, the 800W rating is kind of irrelevant in this case. Best not to think of "an 800W motor", but rather a power system that depending on prop and no of cells might be limited by the motor to a max of 800W.
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Old Aug 27, 2011, 09:41 AM
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Dr Kiwi's Avatar
Chattanooga, Tennessee, United States
Joined May 2003
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Even a trainer probably needs 50W-75W/lb.. let's say 3kg AUW (~6.6lb)... absolute minimum would be 330W... better would be 495W.

A rough rule of thumb for converting IC to watts is:

Watts = cubic inches x 2000.... thus a .40 IC is the equivalent of ~800W..... you can see that you have much less power than it was designed to fly with.
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Old Aug 27, 2011, 11:45 AM
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eflightray's Avatar
South Wales U.K.
Joined Mar 2003
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Have a look at this thread, there may be something useful there -

Converting RCM trainer to electric.
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Old Aug 27, 2011, 12:19 PM
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mt_100's Avatar
United States, MI, Holland
Joined Dec 1996
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I have a Great Planes PT-40 ARF converted from glow. Minimal conversion work on the ARF, took the talk out and put a battery tray in so no weight savings attempted. It weighs 90oz ready to fly.

Originally flew on a Turnigy 42-40-750 and 4S 3700 and 4S 5000 pack on a 12x8 prop. Now I fly on a Monster Power 25 from Hobby Partz with a 12x6 prop and 4S 5000 battery. Flights of 10 minutes with aerobatics are the norm.
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Old Aug 27, 2011, 12:22 PM
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snoopy_schulz's Avatar
Germany, lake of constance
Joined Oct 2010
407 Posts
Quote:
Originally Posted by minivation View Post
Hi,
I'll be starting a restoration project of an old RCM trainer sometime later, and along with it I'll be performing a gas to electric conversion.
I bought a 890Kv, 800W motor (originally designed for 4S, but I'll be using 3S) and will be using a 11x5 prop with it.

Sorry for acting like an power-systems noob, but would the motor power be enough for some gentle flight (no rolling, no loops - just takeoff, fly, land)? As noted, although the motor is rated 800W, I'm running 3S, so I don't think it'll be as much. The RCM trainer used to fly with a .40 equivalent GAS engine, and the plane, empty, weighs in at 2.5 kilograms.

Thanks!
Hi minivation

Your RCM trainer will have a weight of about 3.3 kg with the 800W motor and a 3S lipo (~ 4000 mAh). So you should use as minimum a
14x7 Prop with your 800W motor. That gives you adequate power for gentle flight, about 340 W shaft power, 40 A. Using a 11x5
the shaft power is about 192 W. That is truly to low for gentle flight. I would suggest rather a 4S Lipo with a 12x6 APC prop.

Best regards
Micha
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