Reviewing and investigating the use of co-axial rotor systems in small UAVs.

Co-axial propeller systems have been used successfully in fixed-wing aircraft for many years due to their inherently good performance. Likewise, several pioneering rotarywinged
aircraft projects used co-axial rotors, such as the Westland WG-25 Mote, Gyrodyne DASH, and a range of rotorcraft from Kamov in Russia. There has been much
debate over the years about whether the co-axial rotor arrangement is more efficient than traditional layouts. Our findings point to the fact that although the co-axial arrangement has a reduced power output of up to 15% when compared to an equivalent single rotor system, this can be offset by the elimination of the need for a tail rotor, which could save up to 20% of the required power. It is only recently that this type of technology has reached the small UAV market in the form of a series of semi-autonomous and autonomous rotorcraft which are starting to make their mark in the military, homeland security and civilian fields. This paper investigates the use of such systems and discusses their advantages, disadvantages and performance metrics.