How to Size Electric Motors for Airplanes - RC Groups
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Apr 22, 2011, 09:31 AM
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How to Size Electric Motors for Airplanes

I have an airplane that a friend gave me 10years ago (never flown) and I want to convert it from Nitro to Electric. The plane weighs almost 5lbs including the Nitro engine. It has a 48" Fus and a 68" Wing (see photo).

Can someone please help me size an Electric Motor, ESC, Battery, and Prop for this plane?
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Apr 22, 2011, 01:33 PM
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I converted a Telemaster 40 a couple years ago which is similar in size and weight.Basically,you want to shoot for 100ish watts per pound. That would be 500ish watts.

I used a motor from - Gforce 32 - 4250-800kv -720 watt ,which has the power of a 40-46 nitro engine.

70amp Align esc (40-50 amp will do)
3cell 5000mah lipo
14/7 APC e prop

This combo provides plenty of power for this plane,I have run it on 4 cells,but it's not needed.
Apr 22, 2011, 03:29 PM
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Thanks a lot for the info, I really appreciate it!
Apr 22, 2011, 04:08 PM
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Harry H's Avatar
This would work well and you cant beat the price. I have used a few of these motors and have been very satisfied.

You would need a 75-80 amp esc and either 4-5S battery.

Apr 22, 2011, 04:35 PM
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Apr 24, 2011, 04:19 PM
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Be aware that this motor requires 4S (4 cells) minimum.
Also - check very carefully where your CG is with a glow engine. An electric setup has a tendency of moving the CG backwards (since less weight is concentrated right at the nose). Be prepared to add ballast to the nose and to emplace the battery as far forward as possible.
Apr 25, 2011, 06:14 AM
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scirocco's Avatar
Given the short nose of the Cub? and the relatively high weight of a glow engine (presumably originally 46-60ish??) allowing for engine, muffler, mount and servo, I would be looking for a nice heavy outrunner of at least 10oz, Kv of about 400 (say 390-410), and 6S battery (or 2 x 3S in series). You will probably have to buy a new ESC suitable for 6S, but the Plush 40 and 60 from Hobby King can take 6 cells and have ample capacity switching BECs, which is a good thing in its own right. $23 and $35 respectively. You can also get cheap series connectors if you have some mid size 3S parkflyer packs and want to use them in pairs.

6 cells, 400Kv and a 12x8 prop will give you around 450W at about 20A, which on any half decent motor will be at least 70-75% efficient so most of those "only" 90W/lb will be going to the prop, which at 12" is also likely to be a lot more efficient than the original glow prop. You'll only need about 2200 mAH packs for at least 10 mins of mixed aeros , cruising around, and touch and goes, and probably use only 1200-1500mAH in that 10 mins. Or on a nice big Cub wing, go with 3000-3300 MaH packs; it will barely notice the extra weight. Basically choose your power system component sizes to balance without ballast if you can. Much better IMO to use a bigger motor and more cells than a smaller motor and ballast.

If you've got the ground clearance a 13x8 or 13x10 would fly it most un Cub-like!

Candidate cheap 400Kv motors include the Turnigy 50-55A, at 300g, and the Turnigy G60 at 360g. Also E-Flite's Power 60 at 380g. Or the Monster Power 60 at 420g, if the original engine was particularly heavy. You'll see plenty of 6S reviews on the Monster Power pages, most with much bigger props and much higher power levels - don't worry too much about perceived underpropping - just enjoy knowing that you have a reliable efficient power system, noting that max allowable current is always well above peak efficiency current.

Going this way will give you a nice understressed system (max current under 10C!), efficient use of the motor, even cheap ones, short charge times (not using many mAH), sustained power throughout the flight (low voltage drop at low C rates), and greater inherent system efficiency (higher voltage is a better way to develop power than higher current.

Although I use a geared inrunner, (because I've had the motor a long time waiting for the right project), the setup I describe is what I am using in a Sig Kadet LT-25, 63" span, 75oz - Not dissimilar to your acft. I use about 2/3 to half throttle for takeoff and big lazy loops from level are just a matter of progressively adding throttle during the pull up, and can do that throughout the flight. Last flight was 11:30, with a mix of everything, and I put 1200mAH back into the pack. The most I've put back was 1800, and that was after 10 mins of mostly full throttle trying to do fast flypasts (which I eventually concluded was pointless in a fat wing trainer). I don't have sustained vertical performance, but definitely quite sporting - and I could always step up a prop size; another benefit of building an understressed system - plenty of margin to increase power.

You probably won't see many recommendations for 6S setups, but with a 6S 2200 pack often cheaper than a 3S 4500, the ready availability of cheap high voltage ESCs with built in switching BEC, and plenty of nice low Kv outrunners at varying price points, the cost barriers of higher voltage setups have evaporated and we can take advantage of the technical benefits of higher voltage over higher current.
Apr 25, 2011, 12:43 PM
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Thanks for the great information everyone! This is my first post here and I am very new to RC-Air so I really appreciate the response I got. It will save me lots of money and time so THANKS.

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