An electric aircraft is a plane powered by electricity. These planes help reduce the environmental impact of flying by producing very few emissions and making quieter flights. Electricity can come from different sources, with batteries being the most common. Most electric planes use electric motors to turn propellers or turbines.

Crewed flights in electric airships began in the 19th century, and a tethered helicopter flew in 1917. Model aircraft powered by electricity were flown as early as 1957, before the modern small drones used today. Small unmanned aircraft systems (UAS) can deliver packages, while larger ones can be used for long missions like taking aerial photos, monitoring areas, or providing communication services. The first crewed flight of an electric airplane, the MB-E1, happened in 1973. Most electric planes used by people today are still experimental.

The Lange E1 Antares was the first electric airplane made in large numbers with special approval from 2006 and a unique battery design. It first flew in 1999, and more than 100 of these planes were delivered by 2004, with over 165,000 hours of electric flight completed by 2022. Between 2015 and 2016, Solar Impulse 2 traveled around the world using solar power. Electric Vertical Take-Off and Landing (VTOL) aircraft and personal air vehicles are being studied for use in city transportation. Electric planes used for commercial travel could help lower costs.

History

By May 2018, about 100 electric aircraft were being developed. This number increased from 70 in the previous year. Of these, 60% were developed by new companies, 32% by established aerospace companies, half of which were major manufacturers, and 8% by academic, government, or non-aerospace organizations. Most of these projects were from Europe (45%) and the United States (40%). The majority of aircraft being developed were urban air taxis (50%) and general aviation aircraft (47%). Most of these aircraft used batteries for power (73%), while some used hybrid systems (31%), mostly for larger planes. By May 2019, the number of known electric aircraft projects had grown to about 170, with most focused on urban air taxis. By 2022, about 100 electric aircraft designs were being developed worldwide. By 2023, the number of aircraft concepts using clean energy (not only electric) was estimated to be up to 700.

The use of electricity to power aircraft began in the late 1800s during the development of airships. On October 8, 1883, Gaston Tissandier flew the first electric airship. The next year, Charles Renard and Arthur Krebs flew a more advanced airship called La France. However, early electric airships had limited speed and range because the batteries needed to store electricity were very heavy.

Fully electric airships are expected to be available by the 2030s.

In 1909, a claim was made that an electric model airplane flew for eight minutes, but this was later disputed. In 1957, the first recorded flight of an electric radio-controlled model airplane was achieved. Even small model airplanes faced challenges with the power density of electric systems.

NASA’s Pathfinder, Pathfinder Plus, Centurion, and Helios were solar and fuel cell-powered drones developed from 1983 to 2003. On September 11, 1995, Pathfinder reached an altitude of 50,000 feet (15,000 meters) during a 12-hour flight. After upgrades, Pathfinder later set a record of 71,530 feet (21,800 meters) in 1997. In 1998, Pathfinder Plus reached 80,201 feet (24,445 meters). In 2001, Helios set a record of 29,524 meters (96,863 feet) but crashed in 2003 during a flight over Hawaii.

In 2005, AC Propulsion tested an unmanned airplane called SoLong, which flew for 48 hours using only solar energy stored in batteries. This was the first non-stop flight of its kind.

The QinetiQ Zephyr is a lightweight solar-powered drone. In 2010, it flew for 336 hours (over two weeks), setting a world endurance record. It uses lithium-sulfur batteries to power itself at night. It also set an altitude record of 70,742 feet (21,562 meters).

For tethered devices, such as air observation platforms, power can be sent up a cable. In 1917, the Austro-Hungarian Petroczy-Kármán-Žurovec PKZ-1 electric helicopter used a ground-based generator connected by a cable. However, the motor failed after only a few flights.

In 1964, William C. Brown at Raytheon tested a model helicopter that received power through microwave transmission.

The first large-scale electric tilt-rotor aircraft was the AgustaWestland Project Zero, which flew in 2011. The first free-flying electric helicopter, the Solution F/Chretien Helicopter, was developed by Pascal Chretien in France and flew in 2011. In 2016, Martine Rothblatt and Tier1 Engineering tested an electric helicopter that reached 400 feet (120 meters) and later flew 30 nautical miles (56 kilometers).

In 2017, Airbus introduced the CityAirbus, an electric vertical takeoff and landing (VTOL) aircraft designed to carry four passengers. It planned to begin testing in 2018 and aim for commercial use by 2023.

NASA’s Ingenuity helicopter, which flew on Mars in 2021, was the first aircraft to operate on another planet. The Dragonfly rotorcraft, planned to operate on Saturn’s moon Titan starting around 2034, will use vertical takeoff and landing (VTOL) to move across the surface.

On October 21, 1973, the Militky MB-E1, a converted glider powered by nickel-cadmium batteries, flew for nine minutes with a pilot on board. Nickel-cadmium batteries have higher energy density than lead-acid batteries, making them suitable for heavier-than-air aircraft.

After human-powered flight was achieved, a renewed Kremer Prize allowed pilots to store energy before takeoff. In the 1980s, designs like the MIT Monarch and Aerovironment Bionic Bat used electricity generated by pedaling.

The Boeing-led FCD project tested a hydrogen fuel cell-powered glider, the Diamond HK-36 Super Dimona, in 2008.

The European Commission has supported many projects to develop electric or hybrid aircraft with low technology readiness levels (TRL). The ENFICA-FC project is one such initiative.

Environmental effects of aviation

The environmental impact of flying on climate change is a key reason for creating electric airplanes. Some teams are working to build electric engines that produce no emissions. Aviation contributes 2.4% of all carbon dioxide emissions from burning fossil fuels. Between 2013 and 2018, emissions from air travel increased by 32%. It is difficult to estimate how much aviation affects the climate beyond carbon dioxide, but factors like nitrogen oxides and airplane contrails may raise this impact to 3.5%. Reducing noise is another benefit, as the aviation industry faces serious noise pollution and the need to reduce it.

Offboard power supply

Mechanisms for providing electricity without storing it onboard include:

• Solar cells use special materials to change sunlight directly into electricity.
• Microwave energy sent from a distant transmitter.
• Power cables connected to an electrical supply on the ground.

Solar cells change sunlight into electricity, which can be used right away or stored temporarily. They produce a small amount of electricity, so many are needed together, which limits their use. Typical solar panels, which convert about 15–20% of sunlight into electricity, create around 150–200 watts per square meter in direct sunlight. Their usable area is limited because if one panel performs poorly, it affects the performance of all panels connected to it. All panels must be at the same angle to the sun and not blocked by shadows.

Between 2010 and 2020, the cost of solar power modules dropped by 90%, and it continues to decrease by 13–15% each year. Solar cell efficiency has also improved, rising from 2% in 1955 to 20% in 1985. Some experimental systems now exceed 44% efficiency, but most of these technologies have only been tested in labs, not in large-scale production.

Sunlight is free, making solar power useful for high-altitude, long-endurance uses. At higher altitudes, the air is colder and less dense, which increases solar panel efficiency. As altitude increases, the temperature drops by about 6.49°C per kilometer. At typical airplane cruising altitudes of around 35,000 feet, the temperature is much lower than at ground level.

Flights at night, such as those requiring 24-hour coverage, need backup storage systems. These systems are charged during the day with extra electricity and provide power during the night.

Power beaming, such as sending microwave energy, uses a ground-based power source. Compared to power cables, this method allows aircraft to move freely and adds less weight, especially at higher altitudes. However, this technology has only been tested on small models and needs further development for larger use.

For vehicles that replace tethered balloons, power cables can connect to a ground-based supply, like an electric generator or local power grid. At low altitudes, this avoids carrying heavy batteries, as seen in the 1917 Petróczy-Kármán-Žurovec PKZ-1 observation vehicle. However, as altitude increases, the weight of the cable becomes heavier.

Power storage

Storing electricity for electric aircraft requires several methods:

• Batteries produce electricity through chemical reactions. When recharged, these reactions reverse to store energy again.
• Fuel cells generate electricity by combining fuel (like hydrogen) with an oxidizer (like oxygen). They require refueling, often with hydrogen.

Batteries are the most common way to store energy in electric aircraft because they can hold a large amount of energy. Early batteries, such as lead-acid, were too heavy for use in aircraft. Later, batteries like nickel-cadmium (NiCd) became lighter and more practical for heavier-than-air aircraft. Today, most batteries used in aircraft are rechargeable lithium-based types.

Lithium polymer batteries (LiPo), a type of lithium-ion battery (LIB), are widely used in unmanned aircraft because they are lightweight and can be recharged. However, their energy density limits their use mainly to small drones. Increasing flight time by simply making larger aircraft with bigger batteries is not efficient because adding more battery weight reduces the aircraft’s ability to carry passengers or cargo. For example, a small electric aircraft with an average weight of 1,500 kg and an energy density of 150 Wh/kg could fly about 80 miles with one passenger, 60 miles with two, and less than 30 miles with three.

In 2017, batteries provided about 170 Wh/kg of energy, while gas turbines extracted 6,545 Wh/kg from fuel. By 2018, lithium-ion batteries offered 160 Wh/kg, compared to 12,500 Wh/kg from aviation fuel. At that time, battery energy storage was only 2% as efficient as aviation fuel. This large difference makes electric propulsion unsuitable for long-distance flights. For example, a 500 nautical mile (930 km) flight for a 12-passenger aircraft would require batteries with six times more energy density than current ones. However, battery-electric motors are more efficient (~90%) than jet engines (~50%), and improvements in battery technology may help.

For electric aircraft to be practical, energy storage must improve. Energy density is the biggest challenge for zero-emission aircraft. Another issue is the speed at which batteries can release energy, as aircraft need high power during takeoff and landing. Electric aircraft also require new ways to manage heat and ensure battery safety.

By 2019, the best lithium-ion batteries reached 250–300 Wh/kg, enough for small aircraft. A regional airliner would need batteries with 500 Wh/kg, and a larger aircraft like the Airbus A320 would require 2 kWh/kg. Electric power is currently only suitable for small aircraft. For larger planes, battery energy density would need to improve by 20 times compared to lithium-ion batteries.

Using batteries can lower operating costs for short flights. For example, the Harbour Air Beavers use electricity costing about $0.10 per kWh, compared to $2.00 per liter for gasoline. Jet fuel is cheaper and gas turbines are more efficient overall. In 2021, new battery technologies like solid-state batteries and lithium-air batteries showed promise for better performance.

In 2018, the SAE International AE-7D committee was formed to create standards for electric aircraft charging and energy storage. One standard, AS6968, focuses on sub-megawatt charging systems. Another, AIR7357, addresses megawatt-level charging. Some airports already have electric car charging stations that can also charge aircraft.

An ultracapacitor is a hybrid energy storage system that combines features of batteries and capacitors. It can charge and discharge quickly with high current and lasts longer than batteries. However, its energy density (about 5 Wh/kg) is much lower than batteries, and it is more expensive, even with a longer lifespan.

A fuel cell generates electricity by combining chemicals like hydrogen and oxygen in a controlled chemical reaction, similar to how a rocket motor works but without producing thrust. Aircraft must carry hydrogen, which has risks, but oxygen can be taken from the air.

Propulsion

Most electric aircraft developed so far use electric motors to power propellers that create thrust or rotors that create lift.

Although batteries are heavier than the same amount of fuel, electric motors are lighter than traditional piston engines. In small aircraft used for short flights, this difference in weight can help balance the lower energy storage of batteries compared to gasoline. Electric motors also do not lose power at high altitudes, unlike internal-combustion engines. This avoids the need for complicated and expensive systems, such as turbochargers, that are used to maintain power at altitude.

The experimental Extra 330 LE uses a 260 kW (350 hp) Siemens SP260D motor that weighs 50 kg and has a 37.2 kWh battery pack. The total aircraft weight is 1,000 kg. This replaces a 235 kW (315 hp) Lycoming AEIO-580 piston engine that weighs 202 kg. The piston-engine version of the Extra 330 weighs 677 kg empty, or 474 kg without the engine. The Lycoming engine uses 141 lb (64 kg) of fuel per hour when producing 315 hp (235 kW), or 0.27 kg of fuel for every 1 kWh of energy. To produce the same 37.2 kWh, it would require 10 kg of fuel.

In addition to the motor, an aircraft’s weight is affected by the energy reserves it must carry. For example, a 19-seat aircraft must carry 5% extra fuel for emergencies, plus fuel for flying to an alternate airport and holding for 30 minutes before landing. This requires 308 kg of fuel for a turboprop engine or 4,300 kg of batteries with an energy density of 250 Wh/kg. Electric systems also include a power inverter, while fuel-powered systems have their own fuel systems.

The 750 hp (560 kW) experimental magniX magni500 electric motor weighs 297 lb (135 kg). This is the same weight as the 729 hp (544 kW) certified Pratt & Whitney Canada PT6 A-114 piston engine, which also powers the Cessna 208 Caravan.

Increasing the power of an aircraft, along with modifications approved by aviation authorities, can help balance the weight of batteries by allowing the aircraft to carry more weight overall, including during landing. Aircraft that use fossil fuels are lighter when landing, which allows for lighter structural designs. In battery-powered aircraft, the weight remains constant, which may require stronger materials.

A hybrid electric aircraft uses a power system that combines electric and traditional fuel-based power. These aircraft typically use electric power for takeoff and landing, and traditional piston or jet engines for long-distance cruising. This design allows for longer flights while reducing carbon emissions. By May 2018, over 30 hybrid-electric aircraft projects were underway, with plans for short-haul hybrid-electric airliners by 2032. Examples include the Zunum Aero 10-seater, the Airbus E-Fan X demonstrator, the VoltAero Cassio, a modified Bombardier Dash 8 by UTC, and the Ampaire Electric EEL prototype, which first flew on June 6, 2019.

In November 2018, engineers at MIT achieved the first free flight of a model aircraft with no moving parts, called the EAD Airframe Version 2. The aircraft is powered by creating an ion wind using magnetohydrodynamics (MHD). MHD has been used for vertical lift before, but only when connected to an external power source.