Thursday, December 08, 2005

Note on the use of BECs in electric gliders from Andre Kilian


Just a few notes for the electric glider guys. I have observed a few cases where the guys are using some of the budget brushless speed controllers on the smaller electric gliders (e.g. Hawk 1400) and have "unexplained" crashes due to loss of control, and after checking the equipment after the crash everything seems to be working fine! The problem is mostly due to the BEC circuit overheating, and results mostly from the use of a 11.1V Lipo with more than 2 micro servos, or with 2 servos that do not move freely (e.g. aileron control horns).

My suggestion is to use a proven brand of speed controller, be very careful when using more than two micro servos (they normally draw more current than standard servos) and make sure that they do not bind. It is also preferable to use micro servos designed specifically for the collective pitch micro choppers, as these normally run on 4 servos and a BEC s peed control. Some controllers also come with an additional heat sink, such as the Align brand (good quaility but expensive) and the Melody (good quality at very reasonable price). It also helps to keep the controller away from heat sources such as batteries and motors, and never velcro the controller to the fuz!

The other problem is that the guys with the 2-3m gliders use the BEC controllers with up to 6 servos (4 in the wing, rudder and elevator) which is a recipe for disaster. The same principles as above applies - with these gliders rather disable the BEC circuit and run a seperate battery pack on the receiver or use a seperate BEC unit.

As these notes are based on practical experience, it would be useful if our electronical boffins can explain why you have to dissipate so much heat on a BEC unit to bring the voltage down from 12.5V (3S lipo) to 4.8V. l have seen that if you stir the sticks on any micro heli with 4 servo s for longer than 2 minutes without running the main motor, everthing stops working (BEC overheats) but if you fly it is fine!



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