probably eco-8 (older electric). it's a "500" size but will fly well on 3s for mild stuff. .50 size (that's a nitro size, it's a "600" electric) is about a 1500 watt bird, so at 3s you'd be pulling a whopping 140amps and fry everything. heck, you can stretch an eco-8 to a 16 (.30 size) and still fly on 3s. it's a very light heli.