Joined Mar 2011
Not As Off-Topic As You Think!
Just thought I would make a quick post that might be a little off-topic, but not entirely. For my birthday, my daughter and son-in-law gave me a J-3 Cub. They obviously don't know a whole lot about my hobby. They didn't realize when they ordered it, for instance, that it has a 6-foot span, that it didn't come with servos or a motor, or that it is a gasser - not electric! It is also a plywood, balsa, Monocoat build, with not a gram of foam. I haven't built a wood/Monocoat airplane in 30 years, and haven't ever done a gas-to-electric conversion. I was up for the challenge, though, and didn't want to offend my new son-in-law by asking where I could return it so...where to start?
Specs said it was supposed to weigh 5.8 lbs, and use a .45 to .60 2-cycle engine. I wasn't planning on doing 3D with this, and I didn't want to get a power setup that would require me buying a size of battery that isn't in my current inventory, especially some big $60-apiece, five or six cell. When I did my calculations for a .45 to .60, equivalent electric motors seemed too big, and used much larger props that I wanted. So I got to thinking... My Extra weighs nearly five pounds (4lb, 11 oz.), and with the Aerosky-standard motor and 15-inch prop, it produces 650 to 750 Watts, and generates around 6 lbs of static thrust, depending on which of my 4-cell batteries I use, and their state of charge is. I have no idea what the KV of the Extra's motor is, and trying to find an equivalent motor was going to be a crap-shoot, so I thought, why not? I'll just use the same power system as the Extra, and that should be good.
I ordered the motor and ESC from Nitroplanes, along with a couple of extra props, but there were two other strong desires I had for the Cub: It really needed a wood prop, just for looks, and it needed about a 1-inch smaller prop so that I wouldn't be constantly replacing broken props.
The Extra has a 15 X 7 prop, so it seemed like I needed something around a 13 X 9. I couldn't find anything like that, but Master Airscrew has a 13 X 8 that would look really good. My concern was that an 8-inch pitch wouldn't give me enough thrust. I went down to my LHS and picked one up, though, just to test. I reamed the prop hole to just almost fit over the machined-in spinner mount of the Extra and then forced it onto the Extra prop adapter, using the bolt and washer to press the prop over the spinner fitting. That made for a nice solid mount.
I grabbed the only charged-up 4-cell I had handy - an older 2650 mAH Turnigy that probably wasn't quite fully charged - and plugged in my Watt meter. With the 13 X 8, it pulled 521 Watts, 38 amps, and 5lb 4oz of static thrust at 13.8 Volts. The stock 15 X 7 prop pulled 641 Watts, 45.5 amps, and 6 lb 2 oz static thrust at 14.45 Volts. I figured if worse-came-to-worst, I could still use the 15 X 7, so I decided to press on with the Aerosky motor.
The end product is in the photos below. You can see that I even used the same motor mount! The Cub came out weighing right at 6 lbs. With the MAS 13 X 8 and a fully charged 4-cell, the power system drew 590 Watts, 40 amps, and produced 5 lb 13 oz on 15.1 volts. That comes out to 98 Watts/lb and pretty close to a 1.0 T/W. The plane cruises at a little over half-throttle, and will stay airborne 6 -7 minutes and still leave 3.8 volts per cell. It has plenty of power, but I probably need to put in a little more down-thrust. It flies a lot faster than I expected, and seems a lot more comfortable at the slightly-over-half-throttle setting.
Although it's not the plane I would have bought for myself, I am very pleased with the way it came out. Heck, I discovered my Monocoat skills haven't atrophied even over three decades of not being used! All that, and there is peace in the family - except for my wife, who isn't thrilled with having another 6-ft span airplane in the garage.
Sorry for wandering off topic a bit, but at least subscribers can pull out power data applicable to the Extra! And no. I didn't cannibalize my Extra for any parts!