odd number of pistons?? in my internship at the aircraft accessory overhauling shop, instructor asked us why there are odd number of pistons in all type of pumps used. All pumps, be it engine driven pump, power transfer unit, ram air turbine etc etc(variable delivery pump installed in A320), they all had odd number of piston. The obvious question is why is it so?? So far i think it has something to do with the rotation direction(clockwise or counter clockwise), but that may explain such case in RAT and PTU which are supposed to work as motor also, but EDP always works as pump. Does it have some design significance or operational significance??