|Hybrid in flight. Click to enlarge.|
Researchers from the University of Cambridge, in association with Boeing, have successfully tested a light aircraft powered by a parallel hybrid-electric propulsion system, in which an electric motor and gasoline engine work together to drive the propeller. The demonstrator aircraft—based on a single-seat, ultralight Song motor glider—uses up to 30% less fuel than a comparable plane with a gasoline-only engine. The aircraft is also able to recharge its batteries in flight, the first time this has been achieved.
The hybrid system was designed and built by engineers at Cambridge with Boeing funding support. The hybrid aircraft uses 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 delivers approximately the same power as the standard engine for the Song—a 15 kW Bailey V5 single-cylinder 4-stroke.
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.
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(In 2010, the Cambridge team tested a hybrid version of an Alatus motorglider powered by a 2.8 kW 4-stroke gasoline engine in parallel with a 12 kW electric motor.)
During take-off and climb, when maximum power is required, the engine and motor work together to power the plane, but once cruising height is reached, the electric motor can be switched into generator mode to recharge the batteries or used in motor assist mode to minimize fuel consumption. The combustion engine then runs at its most efficient point. The same principle is at work in a hybrid car.
Although hybrid cars have been available for more than a decade, what’s been holding back the development of hybrid or fully-electric aircraft until now is battery technology. Until recently, they have been too heavy and didn’t have enough energy capacity. But with the advent of improved lithium-polymer batteries, similar to what you’d find in a laptop computer, hybrid aircraft—albeit at a small scale—are now starting to become viable.—Dr. Paul Robertson, project leader
The hybrid power system in the Cambridge demonstrator is based on a Honda engine, in parallel with a custom lightweight motor. A power electronics module designed and built in the Cambridge Engineering Department controls 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 gasoline engine is optimally sized to provide the cruise power at its most efficient operating point, resulting in an improved fuel efficiency overall.
Our mission is to keep our sights on finding innovative solutions and technologies that solve our industry’s toughest challenges and continually improve environmental performance. Hybrid electric is one of several important elements of our research efforts, and we are learning more every day about the feasibility of these technologies and how they could be used in the future.—Marty Bradley, Boeing’s principal investigator for the program
Robertson’s team, which includes PhD students Christian Friedrich and Andre Thunot and MEng student Tom Corker, is conducting ongoing test flights to characterize and optimize the system for best performance and fuel economy.