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Sep 28, 2010, 03:29 PM
busy bee
Vickie's Avatar

How do I calculate size of motor+esc+battery

I am new to RC flying, and have had a few lessons with IC engines, but really want to fly electric.
I have a few models ready to fly, but have recently bought a couple of half built models, both are Black Horse models, one a Piper Cub 72" wingspan, and the other is a Chipmunk with a 64" wingspan.
Both of these are supposed to have IC engines, and I want to finish both & fit electric motors. Can anybody explain how to work out the size motor I am going to need, how to work out the best size of ESC and battery? Is there a formula to work it out
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Sep 28, 2010, 03:44 PM
Time for me to Fly...
Mr. Wiz's Avatar
The quick way is to look at a similar size and type of electric plane and copy that power system.
Sep 28, 2010, 04:23 PM
eye4wings's Avatar
Start from the estimated AUW of the model.
For scale flying assume that you will need about 60 watts of power per pound weight.
If you want the models to look right on the ground as well as in the air, next work out the scale size prop that you will be fitting.
Go shopping for motors armed with the wattage required and desired prop size.
Find motors that will give maximum power about 20% above your requirement and spin a prop at or near to the size you want.
Find out what voltage these motors will draw using the desired prop. (If they aren't recommended for your prop but one an inch or so smaller you can always run on a cell less so as to keep the amps within recommended limits for that motor) This will probably eliminate some motors from your short list.
I find that motors with rev/volt constants (kV) in the area of 800 - 900 work best for my scale models and give adequate thrust for scale flying on 2s or more usually 3s LiPos, but I normally use props of x8" pitch to get the speed of airflow on low cell counts.
To arrive at the size of ESC you need, when you know how many cells in series, divide the maximum wattage of your chosen motor by the voltage of your power pack. This will give you the current (amps) that your ESC will have to handle. A small safety margin will already be included because I suggested you chose a motor that is more than adequate for you expected wattage, but some would add a further safety margin just to be on the safe side.

What will always help when converting to electric power from IC is any lightening of the structure that can be done. A heavy airframe is bad news as it wastes power in additional and unneccessary drag shortening your flight times and may be less pleasant to fly.

I scratch build to my own designs so I can build in lightness from the start, which you won't be able to do with your part built kits, but do what you can. Every ounce saved with give benefits.
As one example the model in my avatar spans 112", weighs 12 pounds and flies on two packs of 3s 8,000 mah LiPos for more than 30 mins. I could give others, but hope that you will see the benefit of not having to haul around a lot of weight.
Hope all that helps.
Sep 28, 2010, 06:10 PM
Registered User

check here
Sep 28, 2010, 06:22 PM
Just having fun
Fredriksson's Avatar
+1 on what D B said above. The tutorial he points to is outstanding.

Another option in going from gas to electric is the Grayson Hobby EZ-Glow to Electric line of combos.

Sep 29, 2010, 03:56 AM
Registered User
There's a whole forum dedicated to Glow to Electric conversions

Plenty of good ideas in there. And depending on which bit of Staffs you're in your local shop might be useful. The Hills at have plenty of experience with largish electric models and are very helpful.

Sep 29, 2010, 05:09 AM
busy bee
Vickie's Avatar
Thanks for all the replies, I did find noflyzone's blog last night, and spent the rest of the evening reading and working out the stall speed of my chipmunk, and looking at the webocalc site. Still not sure what I'm doing though. Will talk with my local shop, and the people at the flying club are very helpful too, I think the more people I talk to, the more I will understand.
Sep 30, 2010, 11:30 AM
Registered User
Busy Bee,

Find the total weight of the Chipmunk, which may be mentioned at web sites that sell them. You'll need over this much thrust, to fly it straight up.

Go to a motor calculator (such as at DiversityModels on the web), and try out different motor/battery/prop combinations, to get an idea of what size motor and prop and battery voltage you will need to get that amount of thrust.

Try a range of motor/prop/battery combinations that put out the needed thrust, and look at the ones that give you the longest flight times. Save the continous amps that each combination draws. You will need a speed controller that is rated at maybe 30% more than this continual amp draw. Price speed controllers.

Price the motors, and eliminate anything beyond your budget. Look at prop length of each combination: eliminate the ones that will not fit on the Chipmunk. (Look at ground clearance on the Chipmunk.)

Find the weight of the best combinations, by searching web sites that sell those batteries and motors. Make sure that the weight is acceptable.

Example: if the plane is 7 lbs, you need maybe 120 oz of thrust, or about 80watts x 7lb = 560 watts to the prop. Some options to try on the calculator are:
Scorpion 3020/14 600 watts to prop, 46A, 3-5 cell LiPo $70.00
Monster Power 32 615 watts 58A 4 cell LiPo 12x8 prop 115 oz thrust $35.00
46 692 watts 41A 6 cell LiPo 10x8 prop 122 oz thrust $40.00
60 646 watts 38A 6 cell LiPo 14x10 prop 141 oz thrust $45.00

You'll probably need 4000-5000 mAh of battery power for decent flight times. Try pricing the batteries at, and the speed controllers (60-80A). Turnigy plush 80A at HobbyKing are about $40.00, but may take a month to be delivered.
Good luck.
Oct 04, 2010, 04:00 PM
busy bee
Vickie's Avatar
You have been very helpful, the model is moving on nicely now. Time will tell whether it flies well - or not!
Oct 04, 2010, 08:42 PM
Balsa to the Wall
Chuck's Avatar
Here's a quick guide from Horizon Hobby:

Determine a Model’s Power Requirements:
1. Power can be measured in watts. For example: 1 horsepower = 746 watts
2. You determine watts by multiplying ‘volts’ times ‘amps’. Example: 10 volts x 10 amps = 100 watts
Volts x Amps = Watts
3. You can determine the power requirements of a model based on the ‘Input Watts Per Pound’ guidelines found below, using the flying weight of the model (with battery):
• 50-70 watts per pound; Minimum level of power for decent performance, good for lightly loaded slow flyer and park flyer models
• 70-90 watts per pound; Trainer and slow flying scale models
• 90-110 watts per pound; Sport aerobatic and fast flying scale models
• 110-130 watts per pound; Advanced aerobatic and high-speed models
• 130-150 watts per pound; Lightly loaded 3D models and ducted fans
• 150-200+ watts per pound; Unlimited performance 3D models
NOTE: These guidelines were developed based upon the typical parameters of our E-flite motors. These guidelines may vary depending on other motors and factors such as efficiency and prop size.

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