An RC helicopter using a contra-rotating propulsion system will have two rotors spinning in opposite directions. This allows the torque generated by the rotor blades to be canceled out, and removes the need for a tail rotor to keep the RC helicopter from spinning. Without a tail rotor, the rc helicopter’s tail boom can be eliminated, saving weight.
One possible contra-rotating layout has one rotor blade mounted at each end of a long fuselage, as it is for the full scale Boeing CH-47 Chinook, and is known as a tandem rotor. This concept is explained well by the Tandem Rotor article on Wikipedia. Alternatively, both contra-rotating rotors can be placed on the same shaft, resulting in a coaxial design (sharing the same axis). Another good Wikipedia article on Coaxial Rotors explains the theory behind this.
Coaxial rc helicopters are very stable and easy to fly. The Bladerunner series of indoor rc helicopters made by Interactive Toy Concepts, and the ESky Lama V3 and Lama II V4 electric RC helicopters are examples of designs using co-axial rotor blades.
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Posted in Coaxial RC Helicopter Design