Get a 5-cell (6V) NiMH pack if you're worried about low voltage brownouts. With standard servos and a properly sized 4-cell pack, I doubt you'll have any problems (I haven't). I use NiMH packs made from 4 or 5 A cells in my planes. The A cells have a much higher current draw capability than AA cells. I get my A cell packs from cheapbatterypacks.com . They have a custom battery pack maker on their website, you can choose the cells you want, the configuration (brick, stick, side-by-side, etc), the wire gauge and the connector type.