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View Full Version : Bratco Skybaby to electric?


WaldoXerxes
10-20-2006, 12:35 PM
Hi
I posted this on rcgroups under control line, but it doesn't seem to be getting much response so I'll try here.




My cousin gave this to me because he never got the engine to work. http://www.bratco.com/brat100.htm Would it work with electric? I have some RC experience with a Beginair but not much. Any motor advice? I don't know anything about control line, but I'd like enough power to do decent aerobatics.

Thanks

Sky Sharkster
10-23-2006, 02:52 PM
Hello Waldo, Welcome to Wattflyer! There are several controline models flying with electric power, including AMA and FAI competition Aerobatic models. There's no reason you can't convert this model to electric but here's some considerations;
Can you fit an equivilent-sized motor and a battery pack in the fuselage? The model in the photo looks like a 1/2A glow engine. This is roughly like a "400" size electric on direct drive (no gearbox). To power this sized motor for a reasonable flight time you'll need a battery pack of 1000mAh to 1500mAh, either 7-8 conventional (round) cells or a 2s-3s (2 or 3 cells in series)) LiPoly. If you have a sample of either of these packs, see if they'll fit inside the fuselage without fouling the bellcrank/pushrod or leadouts.
If that works, you'll need to fabricate some type of firewall ahead of the battery pack to mount an electric motor. There are lots of motor mount types but the most common are the GWS "Stick" type, the "Clam shell" and the "X" rear mount, often used for "Outrunner" motors.
For this application I'd suggest an "inrunner" with the clamshells. This is very much like a "beam" mount on conventional glow motors and will keep the nose streamlined. One reason I'm leaning towards an inrunner is they wiill use a 5" to 7" prop, much like the glow motor. An outrunner would use a larger prop and may not have clearence with the supplied landing gear. Multiplex, Mega, Himax, Warp, Medusa all make direct drive "400" replacement brushless motors. You could even try it with a brushed ("can") 400 to start and upgrade later, but with direct drive, can 400's aren't very efficent.
Also, if you use a brushless motor, you'll need an ESC to hook up the 3 wire motor lead to the 2 wire battery. This means you'll also need a receiver. I've heard of flyers "setting" the receiver/ESC to full throttle position before the flight and letting the low voltage function cut the motor. I've added a few ideas for this after the links:
http://www.multiplexusa.com/motors/Brushless%20Motor/400/6Direct.htm
http://www.nesail.com/detail.php?productID=2926
http://www.nesail.com/detail.php?productID=1494
http://www.nesail.com/detail.php?productID=3405
http://www.nesail.com/detail.php?productID=3530
Last, if you plan to use LiPoly batteries, this is very important! If you don't have any "flight-adjustable" speed control, the motor will be running full speed from the time you hook it up until it fully discharges the battery. You MUST have some sort of low voltage-detecting device inline to shut off the motor before it "cooks" the battery, which it surely will. These are sold under the name "Batt-saver" or "Lipo-saver". They're about $20.00. If you don't use this, or another LVC cut-off you will get one flight per battery and likely burn up the plane and anything it comes in contact with.
It is also possible to use a conventional ESC (with receiver onboard) and have a transmitter to control the motor, with the control lines for normal U-Control. This is not allowed in AMA competition (R/C assist of a controline model) but is OK for sport flying. Another way to save the LiPo is to have some sort of motor shut-off and time the flights, cutting the motor before the battery reaches low voltage. There are "onboard" timers sold which do exactly this but I don't have the link handy. I believe these allow 3-wire brushless motors to run "full Throttle" and connect to two-wire battery packs.
So, it won't be a simple and easy process but it is possible and is being used by controlliners already.
Good Luck!
Ron

WaldoXerxes
10-24-2006, 04:31 PM
Thanks for the info. The engine is a NORVEL CLX .061, it says right at the side of the webpage, you might not have seen it. The trouble is that I can't actually get to the inside of the model. It is plastic and sealed. All there is on it is the engine on the front. Could I just attach a motor to the front and velcro everything else on the side? Would this make a big difference in flight? Or I could cut open the canopy to get in. There looks to be enough space for a battery but I have no experience with motor mounting so I have no idea if I'd be able to put a motor in there. It looks easier to just attach the motor to the front of the aeroplane where the engine is now.

Sky Sharkster
10-25-2006, 12:44 AM
Hello Waldo, you're right, didn't notice the engine size on the side;The .061 is slightly larger than an .049 but I still think a "400" electric will be plenty of power.
Well, if you can't get to the inside of the model easily, it makes mounting the electric "bits" more difficult. It will still fly with the battery and wiring outside (lots of 3D's with profile fuselages use this type mounting) but it won't help the overall performance.
After thinking about this model and RTF electric controline models in general, it might be better to use a brushed 400, with a 6volt "Speed 400" you could use 7 or 8 cell NiMH battery packs, much less expensive and not too tempermental! This will save the work and worry of converting the brushless motor leads and fitting a LiPo-regulating ESC.
As far as mounting the motor with a flat "Firewall" mount the only "400" mounts I could find that will permit this are a couple from Hobby-Lobby.com. Part numbers HLAN2094 or HLAN2096.
Guess that's all the ideas I have, hope some others post with suggestions!
Good Luck!
Ron