When the all-electric E-Fan made its first flight earlier this year, it signaled a breakthrough in the progress of electric aircraft. Although its performance compares well to other contemporary electric designs, the E-Fan does not represent any major technological leaps. More significant is the company behind the E-Fan: Airbus, the European firm better known for producing large airliners. Airbus is a large, multinational company that is deeply entrenched in the business of burning fossil fuels. That such an establishment is willing to invest in the development and production of pure electric and hybrid aircraft is a strong signal that technology may be on the verge of allowing practical electric aircraft for the masses. Much smaller aviation firms and innovative individuals have been shouting that message for years--it’s just that (almost) nobody was listening.
In answering the question of why electric propulsion should even be considered for aircraft, you must look at environmental and engineering aspects. On the environmental front, the obvious benefit of electric power is the lack of CO2 emissions. In fact, very strict European emission standards were the catalyst for Airbus’ development of the E-Fan, a stepping stone to their planned hybrid-powered regional commuter aircraft.
Even if you trace the energy path of an electric-powered aircraft back to a coal-fueled power station feeding the ground-based charger for the airplane’s batteries, the comparative emissions are a tremendous improvement over the exhaust of a kerosene-burning turbine engine. The same is true of hybrid electric systems that would use a small onboard turbine or internal combustion (IC) engine to recharge batteries in flight.
Without the vibrations inherent in internal combustion engines, an electric aircraft can be built with a lighter and simpler airframe.
Another environmental benefit of electric aircraft is their lack of noise. How often do you hear about neighborhoods being built on cheap land near a long-established airport, only to have the new residents complain about engine noise? It doesn’t make much sense, but these squeaky wheels are frequently successful in having the airport closed. According to Dr. Brien Seeley, a representative for the CAFE Foundation (Comparative Aircraft Flight Efficiency Foundation– a volunteer organization renowned for its efforts in measuring and improving the efficiency of small aircraft) noise reduction is the primary motivator for many who are developing and improving electric powerplants for aircraft. He says, “The most significant distinguishing feature of electrically powered aircraft will be their prospects for unprecedentedly low noise and the new operational opportunities that will open when combined with extremely short takeoff and landing (ESTOL).” Perhaps quiet airplanes that are able to operate from short runways will be the key to reuniting general aviation (GA – i.e. your average privately-owned Cessna or Piper) and a noise-intolerant public.
From an engineering perspective, electric propulsion seems to offer several benefits over IC engines. One of the primary advantages is the vibration-free operation of electric motors. A substantial amount of the structure in GA aircraft is dedicated to absorbing the forces caused by having one or more IC engines attached. Just look at the structure of a glider compared to that of an IC-powered airplane and you will see what I mean. Without the vibrations inherent in IC engines, an electric aircraft can be built with a lighter and simpler airframe. When it comes to airplanes, lighter is almost always better.
IC engine vibration is also one of the contributors to airframe fatigue. Without this driver present, it is plausible that electric-powered airframes could make use of simplified and/or less frequent inspections. These inspections, as well as the repair of any defects that they uncover can be a significant portion of the modern cost of aircraft ownership.
A further advantage of electric motors is their relative simplicity in comparison to IC engines. A typical brushless motor has only one moving part, the rotor, which is often attached directly to the propeller shaft. Even the simplest fuel-burning motors are infinitely more complex and require periodic detailed inspections and costly overhauls. The rudimentary nature of electric motors makes them easier (less expensive) to inspect and maintain, while also being intrinsically more reliable.
The simplified nature of electric motors suggests equally simplified operation for the pilot.
The simplified nature of electric motors suggests equally simplified operation for the pilot. There would be no need to worry about carburetor heat, mixture settings, altitude density and the myriad other factors that affect the performance of an IC engine. That is not to suggest that operating a powerful electric motor is not without its own set of concerns. However, those concerns would be relatively independent of ever-changing environmental conditions.
It’s All About the Batteries
With all of the obvious advantages of electric motors, it is natural to wonder why electric airplanes are still an exceptionally rare novelty. The short answer is that battery technology has not progressed sufficiently to allow electric aircraft to flourish. All of the other elements are in place: powerful motors, sophisticated controllers, lightweight airframes, efficient propellers. It’s just those pesky batteries holding us back. Despite huge strides in battery technology over the last decade, there isn’t a battery around that begins to approach the energy per pound (energy density) that gasoline, diesel, or kerosene can provide. That’s probably why Airbus is giving themselves until 2050 to get that hybrid commuter plane project airborne. Perhaps the next 30 years of battery advancements will get them there.
It isn’t that current batteries make an electric aircraft completely impractical. In fact, new electric airplanes are emerging all the time. It’s just that the typical flight time and/or range limitations imposed by modern batteries eliminate the electric option for anyone who wants to do anything more than putter around the airport on a Sunday afternoon.
The range limitations have not stopped adventurous builder/flyers from exploring the bounds of electric flight. The first all-electric manned aircraft was the Militky MB-E1, a modified version of the Brditschka HB-3 motor glider. The 43 hp Rotax gasoline engine was replaced by a 13 hp electric motor and a bank of NiCd batteries, enabling 12 minute long flights--cutting edge stuff back in 1973.
Since the MB-E1, numerous electric aircraft have been modified or built from the ground up for electric power. Many of these airplanes were built to either prove a new technology or break a specific performance record. Such airplanes are rarely practical to tie down at the local airstrip for weekend joy rides. One notable example is the Long-ESA flown by Chip Yates, a genius/thrill-seeker who holds an impressive mix of patents and world records.
The Long-ESA is an electrified version of the Rutan Long EZ, a popular homebuilt aircraft. Chip set five records for electric aircraft in the Long-ESA, including top speed (201mph) and time-to-climb (3000m altitude in 5 minutes and 32 seconds). That type of performance is envied by many modern IC-powered aircraft…especially the climb performance. The big drawback is that Chip’s hot rod is only good for about 15 minutes of full-throttle flight.
What's Currently Available
Aside from the record setters, there are a few companies producing electric aircraft for mass consumption. Understandably, many of these designs are glider-like in nature, which is a reflection of the limited energy aboard. Even so, several of these designs are able to achieve excellent performance in their intended role.
An interesting example of a commercially available electric aircraft is the ElectraFlyer-ULS from Electric Aircraft Corporation. Since the empty weight (no pilot or fuel) of the airplane is less than 254 pounds, it is classified as an ultralight aircraft. This means that it does not need to be registered, and it can be flown without a pilot’s license. Whereas many ultralights look like overgrown kites, the ElectraFlyer-ULS looks like an airplane and will provide up to two hours of flight time per charge.
Another interesting example is the Taurus Electro by Pipistrel. At first glance, it appears to be an unpowered two-seat glider. But this airplane has a few hidden surprises. Not only does the landing gear retract, so does an electric motor that reportedly provides better climb performance than the gas-powered version of the plane.
Pipistrel is also refining an electric aircraft that resembles a typical GA airplane more than a glider, the WATTsUP. In fact, this aircraft would be classified as a Light Sport Aircraft (weight and complexity between Ultralight and GA) if it becomes certified in the US. This 2-seater is intended to operate as a trainer, playing to the strengths of its electric power system. It has strong climb performance while also being able to recharge the batteries during descent. Although it is intended to stay close to airports for training new pilots, its one hour endurance (with a 30 minute reserve) could allow the pilot to venture out somewhat.
Closing the Gap
If recent history is a reliable indicator, battery technology will continue to improve at a rapid pace. Current Lithium Polymer batteries have typical energy densities in the 100-150 watt-hours per kilogram (Wh/kg) range. Optimistic predictions of future energy density gains (600 Wh/kg) are still far off the benchmark for gasoline (13,500 Wh/kg). When you consider the stifling inefficiency of IC engines as energy converters (about 25%) compared to the efficiency of modern brushless and induction motors (>75%), the bias becomes a bit easier to swallow.
The batteries currently available are adequate to provide suitable range for some flyers. As the pool of available batteries improves, so will the range of electric aircraft and the number of pilots who embrace them. Despite continued progress, I think that a ground-breaking battery breakthrough is necessary before electric powered airplanes can become commonplace in the near term.
With very clear pollution and noise incentives for reducing the use of jet fuels and gasoline, I think we will begin to see other aviation firms follow Airbus’ lead in a meaningful way. As more R&D is dedicated to the cause of electric flight, the odds of finding that golden breakthrough will only improve. Until that time comes, we will continue to rely on Airbus and the handful of lesser-known visionaries to continue pushing the boundaries of practical electric flight.