Greetings E-Flyers. I have been flying RC on and off for over 30 years. Strictly Glow. I would like to try converting one of my planes to e-power, but am at a loss as to what to use. I want to convert a 10 pound aerobatic plane that presently uses a 1.20 four stroke for power. From what I have read, I think I need about 100 watts per pound. What kind and how big a motor? What kind of speed control? What kind of and how many batteries? Do I need a gear reduction unit? How big a prop? These questions are where I get lost. I know that volts times amps equal watts. Can anybody out there shed some light on thes questions for me? Do any of the web sites have a chart that will help? Thanks for any help.
Al Burman

Take a look at Hobby Lobby's AXI 4130 series motor and speed control combos. Also check out Modelairtech for large belt drive motor systems. You will find that converting .40 and larger size glow models to E-Power is quite expensive and challenging. I just spent $102 on one 16 cell Sanyo 1950AUP pack. The equivalent LiPoly series/parallel pack would be around $200 or more but would be much lighter. Glow or gas power for large models is by far cheaper and simpler than electric power but if your budget can stand it , go for it. E-Zone has a good conversion forum that may help you.

Thanks for the info. It helps. I know that at this point, glow or gas is less expensive, especially since I already have so much equipment. But, it's something new and like many of us, I am always curious to try something new.
Thanks again,
Al

Al,

First thing is to stop thinking in terms of electric motor = glow engine, and battery = fuel tank. The battery is where the chemical reaction takes place to release energy, much like the combustion chamber on an engine. Bigger battery means bigger power.

You got the Watts per pound concept, but skipped right over the important part and went right to questions about the motor. Motor comes last if you're powering using Watts per pound rules. Watts is Volts times Amps. Volts and Amps are the battery's department. The key to electrics is to work around the battery, if you haven't kinda deduced that by now

Throw the notion of engine size out the window. You have a 10lb aerobatic plane, period. Watts per pound rules say 100W/lb for aerobatic planes. A little simple math tells you that you need at least 1000W.

Okay, let's translate that into Volts and Amps. 100 Volts and 10 Amps would give you 1000W. Let's see, at 1 Volts per cell under load, that's 100 cells. Shopping around, you won't find a motor that's rated for 100 cells or 100 Volts. Yes, nominal voltage for NiCd and NiMH is 1.2 Volts per cell, but that's at rest. Under heavy loads, their effective voltage is 1 Volt per cell. This also makes for easier calculations, and a pleasant surprise as far as power goes.

Let's try 50 Volts at 20 Amps. Still nothing, no motors rated for 50 cells. Besides, if you think about it, even 50 cells at 2.25 ounces per cell is over 7 pounds! Long flight times, if the fat pig can even get off the ground.

After a couple of conversions, and/or reading about some in our Glow to Electric Conversions forum, you'll see that most guys are using current levels between 40 and 60 Amps. Let's shoot for the middle and go with 50 Amps. At 50 Amps, you're talking about 20 Volts, or 20 cells. Hey, the battery load is only about 3 pounds for NiCd or NiMH. Now we're getting reasonable. It's a compromise between weight and flying time. You could increase the Amps, use fewer cells, and end up with a lighter airplane, but it won't fly as long.

Now we need a motor that can handle 20 cells and 50 Amps. That's a question of shopping. I know motors from AXi, Hacker, and Aveox that can handle that. From here, you can now start answering those questions of yours.

What kind of speed control? One that can handle at least 20 cells and at least 50 Amps. Castle Creations is a good brand, as are Jeti and Hacker. Again, a matter of shopping.

How big a prop? and Do I need a gearbox? kind of go together and depend heavily on the motor. The AXI is pretty simple. You just go to www.modelmotors.cz, and look at the speficiations and application tables for each motor. The big 4130/16 swings a 16x10 on 20 cells at ~44 Amps, direct drive. Roughly guessing, I would say a 16x12 or maybe a 17x8 or 17x10 would get you to 50 Amps. Conventional motors like Hackers will need a gearbox. In that case, you want to start with the largest diameter prop you can fit on the airplane, then fiddle with the gear ratio and motor wind to get the right Amps through the system. A program like Motocalc helps tremendously with this stage of the design. You can tweak the prop pitch, gear ratio, and motor wind in the virtual world until everything balances out.

Thanks Matt, You have answered just about all my questions. I haven't seen any of the larger planes here that were e-powered and I want to try one. If it's OK with you, Im going to publish your reply in our club newsletter. I know the interest in e-power is here, but most don't talk about it.
Thanks again,
Al

Glad I could help. Feel free to use any of my posts.