- Current electric propulsion technology intended for manned atmospheric flight involves replacing the internal combustion piston-propeller engine or the turboprop engine (turbine-compressor + shaft output to propeller or rotor) with electric motors to drive the propellers or rotors. Propellers are more efficient than jet engines for subsonic propulsion [NASA source and another external source]. At such low velocities, electric motors are much more efficient than internal combustion engines or jet engines and have better torque-to-weight ratios as well. (See the linked Wikipedia article with the comparison of electric motors to IC engines.)
Consequently, there is a move afoot to design and build all electric propeller driven aircraft, primarily in the small aircraft category, by such big name players as NASA (check out the Puffin one-man concept aircraft), Cessna, Sikorsky and EADS.
One could ask, why not Electric Jet Propulsion? Electric space propulsion systems have been around for a few years, used mainly for satellite attitude control, in the form of plasma jets, such as Hall effect thrusters, ion thrusters, VASIMR jet, etc. But the thrust levels of electric plasma jet engines are very tiny, in the range of a few milliNewtons, although their specific impulses are in the range of 5000, compared to about 500 for chemical rockets. In the low-gravity, zero air friction environment of outer space, spacecraft can use electric jets to gradually build up to extremely high velocities, in the hundreds of 1000s of km/hr. This idea was proposed to be used for the mission to Mars and was first tested on the Deep Space 1 mission. Electric jet propulsion systems are therefore currently relegated to low-gravity space applications only.
Many may be familiar with the older, widely adopted concept called More Electric Aircraft (MEA) which was intended to replace mechanical actuators and control devices with electromechanical actuators. Instead of having a large hydraulic pump located centrally with miles of high pressure plumbing laid out all over the aircraft, smaller electromechanical actuators, which have a small pump and hydraulic actuator all built into one unit, could be mounted directly at the point where actuation is required, such as in the wing flaps for example. This move not only brings about safety from totally removing pipe bursts or leaks, but also reduces weight, power consumption and response times. Many military and civilian aircraft manufacturers have adopted the MEA approach, especially with the wide-spread use of the fly-by-wire technology. The MEA method is a big step towards bringing the all-electric vehicle into a reality. Moog is a well known supplier of aircraft components that follow the MEA concept.
Back to current electric airplanes.
Here is a link to an article on Boeing’s single prop airplane, powered jointly by a hydrogen fuel cell and lithium-ion battery pack, which cruised for 20 minutes at a top speed of 62 mph during a test flight in Spain in early 2009. The Dimona has a 53.5 ft wingspan and uses a single PEM fuel cell along with the Li-ion batteries for storage.
Cessna (a division of Textron, which also owns Bell Helicopter, Lycoming Engines, E-Z-GO_golf carts, trams, etc._ and other companies) has teamed up with Bye Energy to develop an electric version of the Cessna 172 single-engine airplane. (Here is a link to an article on that project.) Bye Energy is headquartered in Englewood, Colorado, with offices at the Arizona State University SkySong Innovation Center in Scottsdale, Arizona and Albuquerque, New Mexico. The first flight of the electric airplane is expected to take place by the end of 2010. (Here is a press release from Bye Energy.) Bye Energy is involved in developing biofuels for aviation (alternatives for 100LL fuel for IC engines and Jet A for turbine engines) as well as a collaborative effort to create electric hybrid propulsion systems for aircraft, called the Green Flight Project.
Sikorsky Aircraft company (a division of United Technologies Corp. which also owns Pratt & Whitney a/c engines, Hamilton Sunstrand aero sytems, Carrier air conditioning, Otis elevators, etc.) plans to fly an electric motor powered light helicopter (the S-300C 3-bladed main rotor, used for training and agricultural purposes) by the end of 2010 under project Firefly. They will replace the 190 hp avgas powered Lycoming engine with a 190hp electric motor and Li-ion battery pack, giving it a estimated flight time of 15 minutes. (Here is a press release from Sikorsky.)
The European Aeronautic Defence and Space Company (EADS) has the Cri-Cri (Really? What a creative name indeed!), a four-propeller & motor light aircraft with high density Lithium polymer battery packs (22 kW total power). The Cri-Cri made its debut flight in August 2010 in the French city of Royan. The motors powered by the battery pack give the airplane the ability to takeoff, climb, cruise and land for a total autonomous flight time of about 20 to 30 minutes. The Cri-Cri has a wingspan of 4.9 m (16 ft) and a fuselage length of 3.9 m (12.8 ft). This article has some photos and a 3 minute video clip.
Sonex Aircraft company, based in Oshkosh, Wisconsin, launched their E-Flight Initiative in 2006 to explore alternative energy systems to power sport aircraft. Their main focus is in the sport class of aircraft. Sonex is working on developing an electric motor propeller system as well as ethanol based IC engines. Sonex sells kits for various light sport airplanes. (Here is a press release from Sonex.)
I think that the best approach for an electric aircraft would be to go hybrid. Battery technology is the limiting factor that is holding up long duration endurance flights. Fuel cells still have a long way to go before they are reliable and cost effective. Therefore, although electric propulsion is very viable in terms of improved efficiency, lower weight and lower costs, an onboard electric power generator will be required. The most compact fuel sources are chemical fuels. Just as in the case of the Jaguar C-X75, the best option for onboard electric power is to use a constant speed turbine engine rotating an electric generator. The turbine engine provides the versatility to use a variety of fuels, gaseous and liquids, both biofuels as well as conventional fuels. The turbine is a reliable prime mover that is compact and light weight. Large battery packs, which are not at all environmentally friendly, can be eliminated. Having a liquid fuel tank with a fuel gauge gives one the ability to confidently predict range and trajectory.
Here is an article from Popular Mechanics explaining why current Federal Aviation Administration rules for light sport aircraft (LSA) are holding back the progress in electric propulsion for that class of aircraft.
This article from Aviation Week has some updates on new projects in the Electric Airplane frontier.