Contra Rotating Propellers using Six Helical Gears

Contra rotating propellers are mainly used in aircrafts for propulsion. Contra Rotating Propeller is a system of two propellers placed one behind the other on a single input drive shaft of the same engine spinning in opposite directions by employing suitable gear drive system. Contra rotating propellers have been found to be between 6% and 16% more efficient than normal single propellers. However they can be very noisy, with increases in noise in the axial (forward and aft) direction of up to 30 dB, and tangentially 10 dB.