Usually alkalines don't have a very high current drain rating compared to NiMH or NiCd batteries, so you'll get more voltage drop with alkalines under load. In general it's not a good idea to have the spring loaded battery trays for airplanes, I'd recommend getting a battery pack. If you just have standard servos, a pack made up of AA cells is probably fine. If you have high torque servos, you probably should go to A cells or larger in your packs.