24 n MOTORS May 2024 www.drivesncontrols.com Flying high with British technology Unmanned aerial vehicles (UAVs) aren’t new. They’ve been around, in one form or another, since the late 18th century. The British De Havilland DH.82B Queen Bee, which ew for the rst time in 1935, has been described as rst modern UAV. The term “drone” is thought to acknowledge this ground-breaking aircraft. Fast-forward about 85 years and another British company is developing an ultralightweight, solar-powered craft that can y at high altitudes for several months at a time, without needing to refuel, oering a costeective alternative to conventional sensing and communications systems. The Phasa-35 (Persistent High Altitude Solar Aircraft) is being developed by Hampshire-based Prismatic, which is part of BAE Systems. It is designed to y at stratospheric altitudes above 20km (more than twice the height of Mount Everest), carrying out duties such as military surveillance, disaster relief, environmental monitoring, border protection, and providing mobile and Internet communications for remote areas. At this height, in the outer reaches of the planet’s atmosphere, the craft is above the weather and conventional air trac. The unmanned craft – technically a High Altitude Pseudo Satellite (HAPS) Unmanned Aerial System (UAS) – has a 35m wingspan, weighs just 150kg and can carry a 15kg payload. Photovoltaic arrays on the wings provide energy during the day, which is stored in rechargeable cells to maintain ight overnight. The UAV’s two motors are a critical element of the design. In 2017, Prismatic approached PMW Dynamics to help develop the propulsion system. PMW has a long history of working on novel applications for customers in a variety of markets, and we jumped at the chance to ex our engineering muscles. PMW and Prismatic have worked closely to achieve truly exceptional engineering. We often meet customers who think they know what they want, but do not really understand what they need. Prismatic was not like that. Its engineers knew exactly what they wanted the propulsion motors to do. Efficiency, mass and losses The challenge was always about eciency. But eciency isn’t just about energy out vs energy in, and the many variables that make up a motor’s loss pro le. In an aircraft, it’s also about mass. Every part of an aircraft inuences its design and performance. In the Phasa-35, mass is even more critical than in a conventional aircraft. Every gramme of mass aects the maximum duration of its missions, or its maximum payload. But there’s more – there’s a correlation between mass and eciency. If mass is very low, eciency may be sacri ced to a degree, or vice versa. A super-light motor that doesn’t have class-leading eciency may be adequate – the same as a super-ecient, but slightly portly, motor could be. As with any application, it’s critical to achieve the right balance and, for this application, to carve out the boundaries that limit the balance. What we needed was a super-light motor that’s super-ecient. Fortunately, the Prismatic team understood the relationship between mass and eciency. That constrained the limits we could work within, which was a good thing. By reducing some of the many variables and focusing on what really matters, we saved time and eort in the development. Selecting the most appropriate materials and components for the application was not easy. The Phasa-35 is designed to y 24 hours a day and it’s extremely cold at night at an altitude of around 20km. Conversely, during the day, solar radiation is signi cant. The thermal management of the motor is complex, but vital, because every Watt that can be shed by good thermal management reduces the temperature of the motor, which in turn reduces the electrical losses and increases eciency. It is common to talk about system eciencies, but that doesn’t tell the whole story. For an electric motor running at low speeds and high torque, the eciency is relatively low. As the speed increases, the losses don’t generally increase much, but the output does, so eciency climbs. That’s why Last summer, a British-designed, solar-powered aircraft ew for many hours at a height of 20km, establishing itself as a cost-eective alternative to traditional airborne and satellite surveillance systems. Ian Matthews Golledge, technical manager at the Hampshire motor manufacturer PMW Dynamics, explains some of the factors that went into designing the aircraft’s custom motors.
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