A look at the history and subtypes of electric propulsion.
BETA Technologies A250 eVTOL prototype aircraft, N251UT flying (IMAGE: Brian Jenkins)
Like electric cars, electric aircraft were there from the beginning.
A brief history of electric propulsion in aircraft
First developed during the early age of flight in the late nineteenth century, the earliest electric aircraft were airships: blimps or dirigibles that used electric motors to generate thrust.
However, it wasn’t until 1973 that the first crewed free flight of an electric airplane took place with the MB-E1, a modified Brditschka HB-3 that incorporated an 8-10 kW Bosch KM77 electric motor. It was only capable of flying for 12 minutes, up to an altitude of 380m (1,247ft).
Thirty years later, the first serial production manned electric aircraft – the Lange Antares, a glider with an electrically driven propeller – completed its maiden flight in 2003. Its 42 kW brushless motor gives the Antares a climb rate of 4m/s (790ft/m) and a range of up to 380km (236 mi).
In 2016, a long-range experimental aircraft powered by solar cells called Solar Impulse 2 was the first electric aircraft to circumnavigate the Earth, travelling more than 40,000km (25,000 mi) in just over a year, despite battery issues along the way.
Today, most of the focus on developing electrically powered aircraft involves unmanned aerial vehicles (UAVs) or proposals for urban air taxis, which tend to have similar design elements (e.g., quad rotors for vertical takeoff and landing capabilities).
Even so, there was another major milestone for manned electric aircraft just this month when BETA Technologies announced that the first passenger flight for an electric aircraft had landed at John F. Kennedy International Airport.
Subtypes of electric propulsion for aircraft
The types of electric propulsion available for electric aircraft fall into one of three categories, though arguably only two are commercially viable.
Electric motors are by far the most common, driving propellers to generate thrust or rotors to generate lift. The key benefit here, as is often the case in aerospace aviation, is weight savings. Electric motors typically weigh less than their internal combustion engine counterparts, but the trade-off is that batteries for energy storage weigh more than the equivalent amount of jet fuel.
Another benefit of electric motors is that can maintain the same power output regardless of altitude, obviating the need for turbochargers or other means of increasing the power output of internal combustion engines.
Hybrid aircraft use cleaner, quieter electric power for take off and landing and conventional piston or jet engines for cruising. In this case, the idea is to use electric propulsion to reduce the aircraft’s carbon footprint while maintaining a conventional propulsion system to enable longer flights.
Examples of hybrid electric aircraft include VoltAero’s Cassio, the Ampaire Electric EEL, and the (now cancelled) Airbus E-Fan X.
While technically another subtype of electric propulsion, aircraft that use magnetohydrodynamics to achieve flight are currently little more than engineering curiosities.
In 2018, a team of MIT engineers managed to achieve the first free flight with a solid-state aircraft called the EAD Airframe Version 2. The aircraft is propelled by creating an ion wind, using similar principles to those of ion-powered spacecraft. Unfortunately, its range is incredibly short, and it’s not capable of generating vertical lift without being cabled to an external power supply.
Power supplies for electric aircraft
As previously indicated, one of the biggest limitations of electric propulsion is the relative energy density of batteries compared with jet fuel. This issue is exacerbated by the FAA requirement for aircraft flying under Instrument Flight Rules (IFR) to carry a specific amount of fuel in excess of what’s required to reach their destination which, in the case of electric aircraft, means carrying more batteries or severely limiting the aircraft’s effective range.
In response to this limitation, engineers have developed various alternative sources of power, including solar cells and even power beaming using microwaves from a ground-based source, but these concepts are still largely theoretical.
Ultimately, the future of electric aircraft will be determined by advancements in battery technology. While there has certainly been growth in small, short range applications (particularly training), it will take a significant improvement in the weight and energy density of batteries before long-range, commercial electric aircraft become a viable proposition.
