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The hybrid propulsion system was designed and built by engineers at Cambridge University with funding and support from Boeing Research & Technology.
The hybrid aircraft used a combination of a ~7 kW Honda 4-stroke piston engine and a 10 kW electric motor/generator, coupled through the same drive pulley to spin the propeller. The hybrid system delivered approximately the same power as the standard engine for the Song – a 15 kW Bailey V5 single-cylinder 4-stroke.
A power electronics module designed and built in Cambridge University’s Engineering Department controled the electrical current to and from the batteries – a set of 16 large lithium-polymer cells located in special compartments built into the wings.
The petrol engine was optimally sized to provide the cruise power at its most efficient operating point. During take-off and climb, when maximum power was required, the petrol engine and electric motor worked together to power the plane, but once cruising height was reached, the electric motor was switched into generator mode to recharge the batteries, or used in motor assist mode as necessary, to minimise fuel consumption – resulting in up to 30% higher fuel economy.
The demonstrator was based on a commercially available aircraft, the Song single seat all composite ultralight motor glider weighing less than 120 kg.
The team also developed an advanced battery management and state-of-health monitoring system.
Test flights for the project took place at the Sywell Aerodrome, near Northampton. These tests consisted of a series of hops along the runway, followed by longer evaluation flights at a height of more than 1,500 feet.
The Cambridge team included Dr Paul Robertson, PhD students Christian Friedrich and Andre Thunot, and MEng student Tom Corker, and Boeing’s PI was Marty Bradley.
Cambridge and Boeing spent three years getting to flight test, beginning with studying hybrid electric propulsion for a variety of aircraft sizes from small unmanned aerial vehicles to single-aisle commercial airplanes. This led to development of computer models of the aircraft and power and engine systems. In 2012, the team began design and ground testing of the hybrid power and propulsion system for the experimental airplane.