Upon exiting Avatar I was discussing with my biologist sister some of the flubbs in the film - she biology, me technology. I made a joke about the helicopters but now it's become a topic of discussion and I thought I'd check my facts. I noticed while watching Avatar that the http://james-camerons-avatar.wikia.com/wiki/Scorpion_Gunship" [Broken]. So, the copters have one prop pod per side AND each prop pod has two rotors (for a total of 4 rotors). The rotors in a pod rotate on the same axis but in opposite directions. IIRC, some carrier-based craft used this prop design in WWII. Funny, Wiki doesn't pull up ones that have familair names, but here's the http://en.wikipedia.org/wiki/Fairey_Gannet" [Broken]. Anyway, it is my understanding that the primary purpose for using contra-rotating propellors is that the engine torque is cancelled, i.e. it does not pull to one side. So, it seems to me, on a craft that is symmetrical and therefore can use two http://en.wikipedia.org/wiki/Counter-rotating_propellers" [Broken], the torque is cancelled anyway, meannig there's no advantage to using contra-rotating props.