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Cambridge team successfully tests hybrid light aircraft; 30% fuel savings

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.

UK-based start-up Faradair is seeking Kickstarter funding to help develop a six-seat, twin-engine, triple-box-winged biodiesel-hybrid aircraft it calls BEHA: Bio-Electric-Hybrid-Aircraft. BEHA it to use twin electric fan motors delivering 200 hp each, in combination with a 200 hp bio-diesel generator that incorporates a pusher propeller protected within a duct to reduce noise and to increase safety.
The aircraft will take off and land under electric power with the bio-diesel engine used in-flight to recharge the batteries and to provide increased cross country performance.
The aircraft will employ solar skin panels and wind turbine technology for energy recovery. These technologies are not the primary power source for the electric motors, but simply additional trickle charge capability.

(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.



Electrified aircraft is at a very early stage of development
but good posibilities exist with future higher performance batteries, solar cells and improved ICEs for small private planes.


This is not a major advancement, but an application of hybrid technology. The idea is before you carried more power than you need for those few times when you really need it.

Now you carry a smaller engine and have electric assist, which is like a hybrid car. The one problem is planes run at 60+% of maximum output, cars do not.


So a downsized engine and battery energy is where fuel savings comes from. That's not a real savings, as energy needs to be put into the battery pack before each climb or takeoff.

Also, there is something missing in the youtube video. The fact that piston powered, non turbocharged aircraft with constant speed props often don't get throttled back at typical cruise altitudes. As the altitude related HP loss typically matches the aerodynamic drag decrease at higher altitudes. What normally happens is the RPM is reduced slightly, from 2700 to 2400-2500 depending on aircraft model and engine/prop type.

The main reason a fixed pitch prop equipped aircraft gets throttled back, once at cruise altitude, is to avoid too much RPM.


This goes back several years when a European company added an electric motor to a Rotax engine. It helps with take off and climbing, but the batteries recharge at cruise so mileage is not a big feature. They do get better performance when they need it with a smaller engine however.


The question is does it scale up to say an ATR72 size turboprop plane. And I would question that. They use the battery to assist the takeoff and climb. I doubt you could get a large enough battery to power a 22 ton aircraft to cruising altitude.

Even if this part of the scheme does not scale, there is still lots to learn from hybridisation, such as taxiing and noise reduction at the start of take off. Even if the system only got you to 3000 feet on hybrid electric power, it might make it much quieter to people on the ground. And thus you could use it more built up areas, or later at night.


I don't think they intend to scale this up to turbo prop transport planes, it is an effort to to see if it can be done. This was already done in 2011 with that Rotax engine.

A large aircraft maker is working on an electric assisted turbo fan with longer thinner wings for greater fuel economy, the wings fold get into the existing passenger gates.

I would like to see fuel cell airplanes for general aviation with on board reformers turning cellulose ethanol into hydrogen. This would be part of "sustainable mobility".


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