High altitude motor developed to allow year-long drone flights

Providing the motors for an all-electric aircraft intended to say aloft for a year at a time was the challenge offered to specialist motor design and manufacturer Printed Motor Works by the aircraft’s designers, Prismatic. The PHASA-35 aircraft is a high altitude, long endurance unmanned aerial vehicle (HALE UAV) with twin bespoke electric motors directly driving high altitude propellers.

Arrays of solar panels provide the power by day and recharge its batteries for flying at night. Weighing only 150kg, including a 15kg payload, the ultra-thin carbon fibre craft has a 35m wingspan.

To meet Prismatic’s demanding specifications and deadline, PMW was selected to design a cutting-edge motor, drive and commutation system that would not overheat either at take-off or during the fast climb necessary to reach the 65,000ft cruising altitude in the limited time available.

To do this the design team needed to quickly gain a precise understanding of the thermal performance required for the motor at all altitudes, so that it could model the specific motor duty points. The near space environment meant taking into consideration the extremely demanding ambient temperatures of -80°C and very low air pressure of 5.6 kPa.

All this required a detailed understanding of the three-dimensional heat dissipation from the motor and adjacent components, while ensuring every motor, drive and commutation component selected was both mechanically and electrically rated for all these conditions.

The PMW team had limited time to complete the whole project, which included phases for electromagnetic modelling, mechanical design, finite element analysis, component sourcing and procurement, and selection of specialist subcontractors to work with the extreme materials and precision required, followed by manufacture, assembly and test at both ambient and low temperatures.

“Despite the project's complexity, our leading-edge R&D meant we were able to deliver the motors to fit with the development and build timeline,” said PMW project manager, Ian Matthews Golledge. “It was a challenge pushing the envelope for this technology, but we knew our design team could do it.”

In creating the IR20E brushless DC motor, PMW shaved every possible gram from its structure and electromagnetic components. This had to be done without compromising efficiency, which was crucial in limiting power consumption so that the weight of the batteries and cables could be minimised.

The motor has an open spoked, internal rotor design with an annular construction to produce maximum torque, together with the necessary lightness and stiffness. Specialist aerospace materials were specified for each component, particularly the bearings, cables and commutation.

The goal of the aircraft is to provide capabilities not available from existing aerospace platforms, such as a low cost, persistent service for the delivery of 5G networks.

Prismatic was recently acquired by BAE Systems which received the first two PHASA-35 aircraft during September 2019. Preparation for flight testing has commenced.