Electric Age: The next generation of aviation is here

Let’s get technical. Battery energy in this context is expressed as energy to weight in Watt-hours/kg (Wh/kg), and while a decade ago that was in the 100s, it’s now in the mid-200s. One example: the Bye Aerospace eFlyer 2 two-seat trainer, for which the value is 260 Wh/kg. A key question is whether these energy to weight improvements will be linear or exponential over the next decade compared with the last ten years.

But it’s not just batteries: the motors themselves must be light. At the level of trainer aircraft like the eFlyer 2 this is not problematic: this aircraft uses a 90kW motor, weighing 360kg, characteristics well within what has been achieved by the technology. Until recently, the most powerful electric motor powering an aircraft was a Siemens unit producing 260kW and weighing just 50kg. This motor was used to power an Extra 330LE aerobatic aircraft that went on to set Fédération Aéronautique Internationale (World Air Sports Federation) electric aircraft speed records in 2017.

Obviously, scaling this up to even moderate general aviation missions will be complex. For example, an aircraft lighter in weight than the eFlyer 2, the Pipistrel Alpha Electro two-seater, developed from the piston-engined Alpha Trainer (and having identical maximum take-off weight), has a flight endurance of one hour compared to three hours for the Trainer, and cruise range 75nm compared to 324nm.

The endurance of the eFlyer 2, which is in flight testing, is projected to be 3 hours, range 234nm, with the company indicating to the World of Aviation that the flight test results so far have supported the projections.

The industry continues to work on improving the energy to weight ratio, though. In January, Oxis Energy, which is collaborating with Bye, has successfully tested battery cell prototypes at 471Wh/kg and is confident of achieving 500Wh/kg in the next 12 months. These batteries are of the lithium-sulfur type, compared to the lithium-ion type currently used in the electric aircraft that have flown and are also used more widely, including in electric cars.

Oxis states that the cell technology has been successfully integrated and ground tested in an aircraft battery and is due to achieve Technology Readiness Level (TRL) 9 in flight trials in the USA. The TRL scale, developed by NASA, assesses the maturity level of a technology. TRL 9 is the highest level and signifies a technology that has been proven in flight.

Other than Pipistrel, established in light aircraft manufacture, and the relatively new Bye, one manufacturer that could enter the electric trainer field is the also relatively new Hungary-based Magnus Aircraft. It has the eFusion two-seater, developed from the piston-engined Fusion 212. An established engine manufacturer, Rolls-Royce, may also join the overall electric light aircraft field, with the Accel single-seat racing aircraft due to fly in the first half of 2020.

The market has been responding to the electric aircraft developments. About 80 of the Pipistrel Alpha Electros are already in service around the world, an international dealer of the aircraft tells the WOFA. Flight testing of the eFlyer 2 will continue for the next two years, but more than 200 of the aircraft and the still-in-development four-seat eFlyer 4 have already been ordered. These include 100 eFlyer 4s ordered by Blackbird, a California, US-based air taxi chartering service, and 60 eFlyer 2s by Norway-based OSM Aviation Group, whose services include training. These orders are starting to prove that the electric trainer and light aircraft segment has real legs.

In the electric commuter niche, we’re starting to see commercialisation

For aircraft of the next weight class, the battery-to-weight ratio becomes more critical. For the Pipistrel Alpha Electro the battery weight is 105kg, while aircraft maximum take-off weight is 550kg, and thus the battery-to-weight ratio is 1/5. This compares with the eFlyer 2 for which the battery weight is 360kg, aircraft gross weight, 860kg (the latter figure being reported early in the programme), and a battery-to-weight ratio of more than 2:5.

Scaling up, an electric modification of a six-passenger De Havilland DHC-2 Beaver was flown on 10 December. The Beaver’s piston engine was replaced by a magniX magni500 electric motor, converted by Harbour Air, a Vancouver-based short-range airline which operates the Beaver. The installed battery weighed 910kg and, with weight of the standard Beaver being 2,310kg, the battery-to-weight ratio is just less than 2:5.

Although this is comparable to that achieved by an electric trainer, Harbour Air tells the WOFA that it anticipates that the current prototype will initially only fit the pilot. The airline envisages that advances in battery technology will allow battery weight to be reduced and the weight made available reallocated to passengers.

Harbour Air, indeed, only requires a relatively limited range from the aircraft, since it is intended to replace the Beaver, which is used on the airline’s shorter routes, including Vancouver–Salt Spring Island, some 30nm in distance, and on tours.

For electric commuter programmes seeking greater endurance and range, aircraft may include one or more turbine or piston engines in a hybrid electric configuration, limiting the battery-to-weight ratio.

As one example, US-based Ampaire’s Model 337, later known as the Electric EEL, flew on 6 June 2019. It was developed from the six-passenger twin piston engine Cessna 337, one of the piston engines being replaced with an electric propulsion system comprising an electric motor and battery. Another hybrid electric design is a Wright Electric nine-seater, now in ground testing.

In addition, Cranfield Aerospace Solutions is designing, manufacturing and integrating a hybrid electric propulsion system into an eight-to-nine-passenger Britten-Norman Islander commuter aircraft. The UK government is providing funding. Thus, the hybrid electric propulsion option extends electric aircraft benefits to heavier aircraft.

Critically, battery technology is one to watch…

Changing the way aircraft are powered and configured may be needed: enter distributed propulsion

Instead of scaling up, though, an answer may be to scale down. As a rule, electric motors and propulsors scale down well compared to piston and turbine engines. So numbers of relatively small propulsors could be installed about the fuselage or wing where they could actively influence the surface airflow, thus improving aerodynamics and flight efficiency.

This has been the approach of Israel-based Eviation with its Alice, a nine-passenger aircraft. The Alice has two wingtip electric motors and propulsors and one in the tail. The wingtip motors and propulsors would counter wingtip vortices, reducing drag, with each motor spinning in an opposite direction to the vortex at its wingtip. In addition, the Alice is also lighter since it is made, not of metal, but of composites.

Range has been projected to be a potentially commercialisable, if niche, 540nm. Battery-to-weight ratio is less than what would be expected from the modification of an existing aircraft in this weight class: with maximum take-off weight 6,350kg and battery weight 3,600kg, the battery-to-weight ratio is just less than 3:5.

Alice is in the ground testing stage and was progressing towards flight but suffered damage in a fire in January, which Eviation says was due to a ground-based battery system.

Looking at the technology more widely, electric aircraft could also have propulsors ‘distributed’ along their wing leading edges, thus positioning improving the air flow over the wing and increasing lift. As a result, a wing of reduced area could be used, leading to a decrease in friction drag and therefore more efficient cruise flight.

Wingtip and distributed leading edge motors/propulsors are being demonstrated by NASA in the Scalable Convergent Electric Propulsion Technology and Operations Research (SCEPTOR) programme on the X-57 Maxwell experimental aircraft, based on the four-seat twin piston engine Tecnam P2006T.

The piston engines have been replaced with electric motors, and the aircraft is being put through ground and taxi tests and will eventually be flight tested. Further, the wing is to be replaced with an experimental high aspect ratio wing, with the electric motors moved to the wingtips. This configuration will be flight tested.

Finally, 12 small ‘high lift’ electric motors will be installed along the leading edge, and the configuration flight tested. These motors would function, along with the wingtip ‘cruise’ motors, during takeoff. They would deactivate during cruise, at which point the propellers would stop and fold into their nacelles to reduce drag. The high-lift motors would reactivate for landing.

Distributed propulsion is also being evaluated by Airbus, Daher and Safran. They are augmenting the standard single turboprop engine and propeller of a six-seat Daher-Socata TBM-900 with six wing-mounted propellers, each driven by an electric motor.

The involvement of companies of the stature of Airbus and Safran and of NASA gives credence to the concept of distributed propulsion.

Overall in electric commuters though, the companies involved up to now in design (Wright and Eviation) and in aircraft modification, (Harbour Air, Ampaire and Cranfield) are small and newly established. The engine/motor manufacturers include a company of similar type, magniX, involved in the Harbour Air and Eviation programmes, and the established Rolls-Royce, an electric motor from which is an option in the Eviation programme.

Considering the market for electric commuters, the electric Beaver is secure, following flight proving and certification: Harbour Air airline itself. It’s a small market though, with the airline currently operating 14 conventional Beavers. But many of the latter type are in service with other operators and it is conceivable that some may be interested in the electric conversion.

Over to the Ampaire EEL. Mokulele Airlines in Hawaii is to start flight trials on commercial routes with the type, while air taxi service Personal Airline Exchange has ordered 50 aircraft. Cranfield’s electric Islander too has an envisaged customer, the Scottish airline Loganair, operating in the Orkney islands. The plan is that, following the modification obtaining an EASA supplemental type certificate, it will be offered to international operators of the traditional version. Eviation’s Alice, meanwhile, has been ordered by US commuter airline Cape Air.

So, with hybrid electric technology and distributed propulsion, electric commuters show significant promise and are attracting interest.

Electric VTOL aircraft are being offered for city air transport

In the cultural zeitgeist, though, there’s a concept of the future of aviation where small personal electric vehicles flit through the air above towns and cities, transporting us individually from home to wherever we want to go.

Compared to runway-using electric aircraft, these electric vertical take-off and landing aircraft — eVTOLs — would require relatively more power for take-off. Also, to increase the passenger load, and make the aircraft more commercially viable, some designs are autonomous, a capability not previously certified in passenger aircraft. Regulatory authorities though have supported the emergence of the type: EASA in July promulgated airworthiness standards specifically for VTOL aircraft.

Notably, the regulator believes that applying either the certifications for aeroplanes or for rotorcraft and adding modifications as required, would probably result in treating some configurations more favourably than others, and thus “preventing potentially innovative concepts from competing”.

Many designs are multi-rotor or multi-propeller, like China-based EHang’s two-seat 16-propeller Model 216, and earlier single-seat eight-propeller Model 184, both of which are autonomous. So are Volocopter’s two-seat 18-rotor Model 2X and VoloCity, and Airbus’ four-seat eight-ducted-rotor CityAirbus demonstrator, of which a production version is envisaged.

Some designs seek longer range by using wings and tilting propulsors such as Lilium Jet’s five-seat aircraft of the same name, which has 36 ducted fans along the wing flaps, and XTi’s six-seat TriFan 600, which has two tilting ducted fans, in addition to a non-tilting one. Other designs achieve range through having, in addition to wings, a compound configuration, with separate lift and cruise propulsors, like Boeing’s two-seat PAV (passenger air vehicle), with eight lift rotors and a pusher propeller.

The EHang and Volocopter aircraft are more advanced in their timelines. EHang states that it has made more than 2,000 trial flights around the world. They have investigated the infrastructure and operating procedures necessary for operations within and over populated areas. In particular, EHang, with its home city Guangzhou, is establishing a command and control centre to ensure the aircraft could operate safely in the city’s airspace. They have also been exploring the use of eVTOLs in roles other than urban transport: a Volocopter aircraft, for example, has been used in a staged rescue mission in Germany.

The rest of the pack — the CityAirbus, Lilium Jet, TriFan 600 and PAV — have flown more recently, although the TriFan was a 65 per cent scale aircraft. The Lilium Jet, TriFan 600 and PAV are the longest ranged of the eVTOLs in their class: compared to the Volocopter 2X’s 15nm, the VoloCity’s 19nm, the Lilium Jet has a range projected to be 160nm. Lilium Jet points out that the aircraft could not only fly within, for example, New York City, but also from the city to others nearby, such as Baltimore and Boston. The TriFan, by being not all-electric but hybrid electric powered, has a remarkable projected range of 1,200nm, approaching that of commercial turboprops.

Fundamentally, this shows a problem with marketing these aircraft just as “city air taxis”: rather than hopping passengers from JFK to Manhattan, say, they could revolutionise the way our urban, suburban, exurban and rural areas are developed, as well as how and where infrastructure location decisions are made. Imagine the commuter park-and-ride of the future being not where you drive your ground vehicle after a 30-minute 5km trip, but where an on-demand eVTOL drops you after a 30-minute 50km trip. Imagine what that does to cool down overheating city housing markets, and to rural and regional economies, while still being connected to cities.

In commercial terms, eVTOLs have already been delivered to customers: 38 from EHang, to customers in Asia, Europe and North America, and EHang is working with the Federal Aviation Administration to certify the 216 for passenger-carrying flight. Its home city Guangzhou could become one of the first cities over which eVTOLs are commercially flown, with early planned developments including an aerial touring route.

The eVTOL developers are clearly enthusiastic about the market and, in addition to there being commercial interest, there has been support from regulatory authorities.

… it’s not just batteries: the motors themselves must be light.

Hybrid electric regional transports may require a few great leaps

In the size categories we’ve discussed so far, the aircraft are relatively small and even combined constitute a very small sector of the overall aviation market, particularly from the fuel-use perspective.

The dominant sector is, of course, that of the large jetliner, with seat numbers in the hundreds and ranges measured in the high thousands of nautical miles. These aircraft are unlikely to see meaningful propulsion changes within their next generation.

But regional aircraft, with up to around 100 seats, are also growing in importance, and their shorter range means that alternative ways of powering this class of aircraft are more feasible. In the short term, the problem is that, with the exception of the Avro RJ, developed from the British Aerospace 146, all the aircraft presently used in the category are twin-engine jets or turboprops, so applicability may require a new generation of aircraft design.

Fundamentally, too, in the near term the best option is likely to be hybrids. During take-off and climb, much more power is required from the engines than during cruise and descent. The engines have to be designed to produce the level of power, including with margins around one engine failure, required for take-off and climb.

One option is to lighten aircraft, reducing emissions, by using supplemental power during this phase: either an electrical system to boost a turbine system on takeoff and climb, or vice versa.

Such an electric motor would need to be more powerful than those of aircraft discussed above, the most powerful of which is the Accel’s, producing more than 750kW. It would need to produce power at the MW level.

United Technologies is examining the possibility of hybrid electric propulsion, with a programme, designated Project 804, to replace one of the two turboprop engines of a 36–39-passenger Bombardier (now De Havilland Canada) Dash 800-100 regional airliner with a hybrid electric propulsion system.

This comprises a megawatt turboprop engine, sized for cruise, and a megawatt electric motor for takeoff and cruise. The battery technology is currently available, but the packaging and battery management system would be specially designed for efficiency and safety.

Currently, however, the hybrid propulsion system would increase the aircraft operating empty weight and thus cut payload. Moreover, range would decrease to about 600nm, from the baseline 1,000nm, though according to United Technologies 99 per cent of the Dash 8-100’s flights are shorter than 500nm. But the company predicts that on a one hour flight around 30 per cent less fuel will be required. First flight is planned by mid-2022.

Elsewhere, Airbus and Rolls-Royce are also evaluating supplementary electric power for take-off and climb. They are replacing one of the four gas turbine engines of an 85–100-passenger BAe 146 regional airliner, designated E-Fan X, with an electric motor. The motor is powered by both a power-generation system, which is a gas turbine integrated with an electric generator, and a battery, with both the power-generation system and battery to operate at take-off. The E-Fan X is scheduled to fly in 2021 and has just completed early wind-tunnel testing to validate the engine’s aerodynamics and to update the information available on this nearly 40-year-old aircraft model.

It’s no surprise that the complexities of these systems mean that barriers to entry are high, and that the players are thus the likes of UTC, Airbus and Rolls-Royce. Entry of smaller players to the market, or major developments of aircraft with newly efficient geometries beyond the tube-with-wings format that might lend themselves to including new propulsion methods, could well be a spoiler.

Critically, battery technology is one to watch, as the industry strives to improve the battery Wh/kg ratio. Battery lifespan must also be increased, and cost must come down, the battery being the most expensive component of the electric propulsion system. But with essentially just the improvement in battery Wh/kg, higher capacity and longer ranged general aviation aircraft, commuters and eVTOLs will become possible in the near to medium term.

Larger aircraft and regional airliners will also require high-power electric motors and development of the battery and power management systems. But as the next generation of regional aircraft are developed, new technologies can be included: distributed propulsion, hybrid systems, and so on, perhaps in another generation to flow upwards to increasingly larger aircraft. It would behove the aviation industry to ensure that, in its next generation of aircraft, it begins to design flexibly to ensure that this kind of technology can be integrated within the lifespan of any new aircraft model family.

…scaling this up to even moderate general aviation missions will be complex.

Light aircraft are already in service

The future trainer
Bye Aerospace eFlyer 2, one pilot, one trainee, Englewood, Colorado, US, Rolls-Royce SP70D electric motor, 26kg, 90kW, battery, 360kg, 94kWh, aircraft length 7.3m, span 11.5m, height 2.3m.

The in-service two-seater
Pipistrel Alpha Electro, one pilot, one trainee, Ajdovščina, Slovenia, Pipistrel PEM 60MVLC electric motor, 60kW, battery, 106kg, 21kWh, aircraft maximum take-off weight 550kg, length 6.5m, span 10.5m, height 2.05m.

The Hungarian option
Magnus Aircraft eFusion, one pilot, one trainee, Pogány, Hungary, Siemens electric motor, 60kW, battery, aircraft maximum take-off weight 600kg (Fusion 212), length 6.6m, span 8.4m. The electric racer, Rolls-Royce ACCEL, one pilot, Derby, UK, YASA 750R electric motor (x 3), more than 750kW (combined), battery, aircraft gross weight 1,200kg (Nemesis NXT), length 7m, span 7.3m.

Practical commuters are being developed

The electric seaplane
Harbour Air De Havilland DHC-2 Electric Beaver, one pilot, six passengers (piston-engined DHC-2), Vancouver, Canada, magni500 electric motor, 560kW, battery, 910kg, aircraft maximum weight 2,310kg (piston-engined DHC-2), length 9.3m, span 14.6m, height 2.8m (piston-engined DHC-2).

The hybrid commuter
Ampaire 337 / Electric EEL, one pilot, six passengers (Cessna 337D), Hawthorne, California, US, Continental IO-360-C piston engine (Cessna 337D), 160kW, electric motor, 160kW, battery, aircraft maximum take-off weight 2,000kg (337D), length 9m, span 11.6m, height 2.8m (dimensions for Cessna 337D).

The distributed commuter
Eviation Alice, two pilots, nine passengers, Qadima, Israel, Rolls-Royce or magniX electric motor (3), 260kW or 280kW, battery, 3,600kg, 929kWh, aircraft maximum take-off weight 6,400kg, length 13.2m, span 16.1m, height 4.2m.

Distributed propulsion is being studied

The distributed-propulsion prototype
NASA X-57 Maxwell (final version), one pilot, three passengers, US, cruise electric motor (x 2), 53kg (including propeller), 60kW, high-lift electric motor (x 12), 6.6kg (including propeller), 11kW, battery, 390kg, 69kW, 47kWh, aircraft weight, approximately 1,400kg, length, 8.7m, span, 11.4m, height 2.6m.

The European experimental
Airbus, Daher and Safran EcoPulse Daher-Socata TBM-900 (above), one pilot, five passengers, France, P&W Canada PT6A-66D turboprop engine, 630kW, turbogenerator (turbine and generator), Safran ENGINeUS electric motor (x 6), 45kW, battery, aircraft basic empty weight, 2,100kg, maximum takeoff weight, 3,400kg, length 10.7m, span 12.8m, height 4.4m.

eVTOLS are maturing quickly

The Chinese hopper
Ehang 184, one passenger (no pilot) (Ehang 216, two passengers (no pilot)), Guangzhou, China, electric motors, battery, net weight 260kg, length 4m, width 4m, height 1.5m.

The German multicopter
Volocopter 2X (above), one pilot and one passenger, or two passengers (no pilot), Bruchsal, Germany, electric motor (x 18), battery, aircraft operating weight empty 290kg (VoloCity, 700kg), maximum take-off weight 450kg (VoloCity, 900kg), diameter of rotor rim including propellers 9.2m (VoloCity, 11.3m), height 2.2m (VoloCity, 2.5m).

The distributed multi-fan
Lilium Jet, five passengers, Wessling, Germany, electric motors, battery, span 11m.

The long ranger
XTi Trifan 600, 1–2 pilots, 4–5 passengers, Centennial Airport, Colorado, US, GE Catalyst turboprop engine, 1MW, electric motors, generators, aircraft maximum take-off weight 2,900kg (STOL), 2,400kg (VTOL), length 12m, span 11.6m.

The pan-European contender
Airbus CityAirbus, four passengers (or one pilot and three passengers), Donauworth, Germany, Rolls-Royce (Siemens) electric motor (8), 100kW, battery (x 4), 140kW, 110kWh, aircraft weight 2,000kg (final version).

Larger airliners are also being explored

The hybrid turboprop
United Technologies Project 804 Dash 800-100, two pilots, 36–39 passengers, East Hartford, US, two pilots, 37 passengers, PW120 (x 2), 1.3MW 1,800shp, turboprop engine, 1MW, electric motor, 1MW, battery, aircraft maximum weight 15,700kg, length 22.3m, span 25.9m.

The hybrid jet
Airbus E-Fan X, two pilots, 85–100 passengers (BAe 146-200), Ottobrun, Germany, Lycoming ALR502R-5 turbofan (x 3), about 5MW, electric motor, 2MW, gas turbine, generator, 2MW, battery, 2MW, aircraft typical operating empty weight, 23,800kg (BAe 146-200), maximum take-off weight, 42,200kg (BAe 146-200), span 26.3m, length 28.6m, height 8.6m.