Propulsion Systems

A number of propulsions systems are available for model aircraft, with one setup being used far more than any other.

Tractor Configuration

The tractor configuration air-screw is by far the most common configuration and used on 22 of the 24 aircraft considered. The commonality of this configuration is it's own advantage, as there is a wealth of resources to assist in the design and use of a forward mounted air-screw. It is also agreed that these aircraft are more stable than the alternative propeller based configuration, the pusher. While majority of the aircraft researched used a single propeller, one aircraft used a twin-propeller configuration, with one mounted on the leading edge of each wing. Given that the BMFA rules for the payload challenge stipulate a single motor must be used, the performance advantage of a twin-screw configuration would have to outweigh the reliability and weight penalties associated with the addition of a gearbox to drive two propellers.

Pusher Configuration

The pusher configuration sees the propeller attached rear of the motor, driving the aircraft forward through the air. The application of this driving force from the rear is what is commonly attributed to making these aircraft more unstable, though, it can be argued instead that the aircraft is more manoeuvrable. The remaining two existing aircraft researched used a pusher configuration. With the propeller located behind the majority of the aircraft, there is no increase in drag from the turbulent propeller wash interacting with the airframe and additionally, no loss in lift due the entire leading edge of the aircraft receiving clean air. A rear mounted propeller is also more protected during landing than a nose mounted air-screw, leading to fewer breakages in the event of a poor landing.

Electric Ducted Fans

Though not seen in any of the aircraft researched above, the remaining propulsion option for UAVs is an EDF. EDFs work by using a mechanical fan housed in a cylindrical duct to reduce the losses associated with a typical propeller. The reduction in losses means that an EDF is more efficient than a propeller of similar size. This could be useful for a smaller UAV as required here, as a smaller diameter EDF would be capable of producing the same thrust as a propeller much larger. As with the pusher configuration, on the ground, the mechanical component of a ducted fan is very well protected, resulting is fewer breakages. Consideration must be given however, to the higher operating speeds of the fan within an EDF, and whether the advantages of the EDFwould warrant the addition of a gearbox to allow a given electric motor to reach the required operating speeds.