With NASA announcing its preparedness to roll out the first all-electric aircraft called X-57 Maxwell or Mod II, the spotlight is on the next level technologies for the aeronautics sector. For this latest version of the aircraft, the typical combustion engines have been replaced with electric cruise motors. The development has opened the floodgates of exciting opportunities for eliminating the carbon footprint from flights as jet fuel emissions are largely responsible for an alarming increase in greenhouse gases in recent times.
Though their roots go as far as to 1970s, most manned electric aircraft today are still only experimental demonstrators powered by electric motors driving thrust-generating propellers or lift-generating rotors. A smaller version of electric aircraft is a cheap radio-controlled unmanned aerial vehicle (UAV) or drone, which has become a multipurpose aircraft today.
No doubt, aircraft powered by internal combustion engines carry a much higher payload, range, and endurance than battery-powered electric aircraft. But small electric aircraft are found suitable for initial flight training. They also entail a lower cost of electrical energy compared to aviation fuel, and reduced levels of noise and exhaust emissions compared with conventional engines.
Mechanisms for running an electric aircraft
An electric aircraft is powered by electric motors which can run on a variety of methods such as batteries that can retain a significant electrical charge; power cables that connect to a ground-based supply; solar cells that convert sunlight directly into electricity; ultracapacitors that can store a limited amount of energy for short bursts of high-power use; fuel cells that are similar to batteries but draw their reactants from an external source, and microwave energy beamed from a ground-based source.
Batteries: From early heavy batteries and nickel-cadmium (NiCd) rechargeable types in the second half of the twentieth century, electric aircraft are now powered by lithium-ion batteries. However, batteries have limited endurance between charges and hence limited range.
Power cables: At low altitudes, aircraft can avoid carrying heavy batteries by way of an electrical power cable connected to a ground-based supply, such as an electric generator. However, this method lacks practical utility at higher altitudes.
Solar cells: Though the power output of solar cells is small, their use of freely available sunlight for recharge during the day makes them attractive for high-altitude, long-endurance applications.
Ultracapacitors: Having an advantage over a small battery, ultracapacitors can store energy for short bursts of high-power use, such as when taking off, but they have relatively small storage ability which makes them unsuitable as a primary power source.
Fuel cells: Fuel cells offer a much greater range than batteries as they use the reaction between two fluids such as hydrogen and oxygen drawn in from outside to create electricity. But the technology is yet to reach the production stage.
Microwaves: Microwaves involve the power beaming of electromagnetic energy through a ground-based power source. However, technology awaits practical development.
Then and now
Gaston Tissandier flew an airship on 8 October 1883 which marked the first use of electricity for aircraft propulsion. It was followed by Charles Renard and Arthur Krebs flying La France with a more powerful motor. As an advancement over it, Petróczy-Kármán-Žurovec PKZ-1 electric-powered helicopter, flown in 1917, had a specially-designed 190 hp (140 kW) continuous-rated electric motor that received its power up a cable from a ground-based DC generator. However, the motor burned out subsequent to a few flights.
It was not until 1964 that a model helicopter was flown by William C. Brown who utilized the power needed for flight by microwave power transmission.
Development of Nickel-cadmium (NiCad) batteries led to a full-sized electric aircraft, the Militky MB-E1 flown by Fred Militky and Heino Brditschka in 1973 for 14 minutes. It became the first manned electric aircraft to fly under its own power.
Further, the world’s first official flight in a solar-powered, man-carrying aircraft Mauro Solar Riser took place on April 29, 1979. The photovoltaic cells used in the aircraft charged a small battery, which in turn powered the motor. The German solar-powered aircraft “Icaré II” was designed and built by the Institute of Aircraft Design of the University of Stuttgart in 1996.
During the past decades, a huge number of electric flights have taken off for long durations. The UK budget carrier EasyJet has announced a partnership with Wright Electric to develop an electric 180-seater to fly routes of up to 300 miles starting around 2027. The aircraft will be supported by high aspect ratio wings for energy-efficient flight, distributed electric aircraft propulsion and swappable battery packs with advanced cell chemistry. Wright Electric has already built a two-seat proof-of-concept with 272kg (600lb) of batteries. It plans to develop a 10-seater, eventually at least 120 passengers single-aisle, short-haul airliner and targets 50% lower noise and 10% lower costs. According to Jeffrey Engler, CEO of Wright Electric, commercially viable electric planes will save around 30% of energy costs.
Eviation, an Israeli firm, has designed Alice commuter plane with propellers on the wingtips. The plane is supported by a big battery weighing in at 3.8 metric tons.
Recently, there has been the development of hybrid-electric aircraft too. Aircraft with a hybrid electric powertrain is found to much more effective as the energy density of lithium-ion batteries is much lower than aviation fuel. Out of over 30 projects of short-haul hybrid-electric airliners, the most advanced are the Zunum Aero 10-seater, the Airbus E-Fan X demonstrator, and the VoltAero Cassio. Los Angeles-based Ampaire is working on a project to retrofit a six-passenger plane into an electric and hybrid craft.
NASA’s experiments in developing aerial vehicles
NASA developed a series of solar and fuel cell system-powered unmanned aerial vehicles (UAVs) such as Pathfinder, Pathfinder Plus, Centurion, and Helios from 1983 until 2003. Pathfinder achieved an unofficial altitude record of 50,000 feet (15,000 m) for solar-powered aircraft during a 12-hour flight from NASA Dryden on September 11, 1995. And on August 6, 1998, Pathfinder Plus went on to elevate the altitude record to 80,201 feet (24,445 m) for solar-powered and propeller-driven aircraft.
Not to be left behind, Helios set an altitude record of 96,863 feet (29,524 m) for propeller-driven aircraft. In 2010, NASA proposed Puffin, a concept for an electric-powered, vertical take-off and landing (VTOL), personal air vehicle.
Based in Plum Brook Station, Ohio NASA Electric Aircraft Testbed (NEAT) is a much-needed facility to design, develop, assemble and test electric aircraft power systems, from a small, one-two person aircraft to 20 MW (27,000 hp) airliners.
NASA’s new all-electric aircraft X-57 Maxwell is based on a twin-engine Italian aircraft popular among private aviators and known as the Tecnam P2006T. It has proven technology to reduce fuel use, emissions, and noise. Modified from a Tecnam P2006T, the X-57 has 14 electric motors driving propellers mounted on the wing leading edges.
With this development, NASA intends to embark upon the next phase of testing on the experimental craft, bringing it a step closer to reality.
During Modification 1, the Tecnam’s two internal combustion engines were replaced with 14 electric motors. In its improved version Mod II, batteries and instrumentation were updated. NASA now can move on to test flights at a rapid pace. In Mods III and IV, engineers will keep developing the plane and conduct final tests, including on the wings.
NASA’s aeronautics sector has been built on the solid edifice of X-planes, or experimental aircraft. In fact, NASA has been testing X-planes at the Armstrong Flight Research Center since the 1940s. Till now, NASA has tested most of its aircraft on the yardsticks of high-speed flight. And while the X-57 won’t be fast, it will be the NASA’s crewed X-plane in two decades.