Zephyr: A High Altitude Long Endurance Unmanned Air Vehicle ...

which are then insulated. In order to minimise weight and stabilise the aircraft, theyare span loaded thus distributed along the wing and are arranged into groups of superpacks.2.3 PropulsionIn the past the propulsion system used contained a DC motor driven by a speedcontroller that provided power to the propeller via a mechanical gearbox. Howeverduring the Zephyr 6 flights in 2007, there was a motor failure on the third day andanalysis of the motor and the gear box showed that the failure was due to a gearboxseizure (see figure 6).Since then a new propulsion system has been developed that still includes a motorcontrolled by speed controllers, but instead of a gearbox they provide power directlyto the propeller through the direct drive motor that is optimised for flight up to70,000ft. By removing the gearbox the efficiency is increased by reducing losses frommechanical drag, and it also increases the propulsion system reliability.Figure7. The original Zephyr 6-1 motor gearbox that failed on day 3. [1]19