Electric aircraft

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In 2016, Solar Impulse 2 was the first solar-powered aircraft to complete circumnavigation of the world.

An electric aircraft is an aircraft powered by electric motors. Electricity may be supplied by a variety of methods including; batteries, ground power cables, solar cells, ultracapacitors, fuel cells and even power beaming.[1]

Electrically powered model aircraft have been flown since the 1970s, with one unconfirmed report as early as 1957.[2][3] They have since developed into small battery-powered unmanned aerial vehicles or drones, which in the twenty-first century have become widely used for many purposes.

Although manned flights in a tethered helicopter go back to 1917 and in airships to the previous century, the first manned free flight by an electrically powered aeroplane was not made until 1973 and most manned electric aircraft today are still only experimental demonstrators.[4] Between 2015 and 2016, Solar Impulse 2 completed a circumnavigation of the Earth.[5]

Design[edit]

All electric aircraft to date have been powered by electric motors driving thrust-generating propellers or lift-generating rotors. Some of the propeller-driven types have been airships.

Mechanisms for storing and supplying the necessary electricity vary considerably, and each has distinct advantages and disadvantages. Mechanisms used include:

  • Batteries can retain a significant electrical charge, although their weight still limits the range achievable.
  • Power cables connect to a ground-based supply.
  • Solar cells convert sunlight directly into electricity.
  • Ultracapacitors can store a limited amount of energy for short bursts of high-power use
  • Fuel cells are similar to batteries but draw their reactants from an external source.
  • Microwave energy has been beamed from a ground-based source.

Batteries[edit]

Batteries are the most common Energy carrier component of electric aircraft, due to their relatively high capacity. Batteries were the earliest source of electricity, first powering airships in the nineteenth century. These early batteries were very heavy and it was not until the arrival of technologies such as nickel-cadmium (NiCad) rechargeable types in the second half of the twentieth century, that batteries became a practicable power source. Modern battery types include Lithium-based and a number of other less widely used technologies. Such batteries remain a popular power source today, although they still have limited life between charges and hence limited range.

Batteries are also often used for temporary storage of electricity generated by another source.

Power cables[edit]

An electrical power cable may be connected to a ground-based supply, such as an electric generator. At low altitudes this can avoid carrying heavy batteries and was used by the experimental Petróczy-Kármán-Žurovec PKZ-1 observation helicopter of 1917. However such a craft must remain tethered to a ground facility, and the higher it flies, the heavier the weight of cable it must lift with it.

Solar cells[edit]

A Solar cell converts sunlight directly into electricity, either for direct power or temporary storage. The power output of solar cells is small, even when many are connected together, which limits their use and is also expensive. However their use of freely available sunlight makes them attractive for high-altitude, long-endurance applications.

For endurance flights, keeping the craft in the air all night typically requires a backup storage system, which supplies power during the hours of darkness and recharges during the day.

Ultracapacitors[edit]

An Ultracapacitor can store a limited amount of energy for short bursts of high-power use, such as when taking off, but due to its relatively small storage ability it is not suitable as a primary power source. Its advantage over a small battery is the ability to charge and discharge much faster with higher peak currents.

Fuel cells[edit]

A Fuel cell reacts two fluids such as hydrogen and oxygen to create electricity. Unlike a battery, the fluids are not stored in the battery but are drawn in from outside. This offers the prospect of much greater range than batteries and experimental examples have flown, but the technology has yet to reach production.

Two types of cell have been considered for aircraft, SPFC (polymer) and AFC (alkaline). SPFC is the lightest and simplest air-breathing technology, but is inefficient and requires precious metal electrodes. AFC provides higher output using cheaper materials, but requires either pure oxygen or additional equipment for carbon dioxide (CO2) treatment.[citation needed][clarification needed]

Microwaves[edit]

Power beaming of electromagnetic energy such as microwaves, like a power cable, requires a ground-based power source. However, compared to a power cable, power beaming carries much less weight penalty as altitude increases. The technology has been demonstrated on small models but awaits practical development.[1]

History[edit]

Pioneers[edit]

The use of electricity for aircraft propulsion was first experimented with during the development of the airship which took place in the latter part of the nineteenth century. In 1883, Gaston Tissandier flew the first electrically-powered airship.[6] The following year, Charles Renard and Arthur Krebs flew La France with a more powerful motor.[6] Even with the lifting capacity of an airship, the heavy accumulators needed to store the electricity severely limited the speed and range of such early airships.

For a tethered device such as an air observation platform, it is possible to run the power up the tether. In an attempt to create a more practical solution than the clumsy balloons then in use, the Austro-Hungarian Petróczy-Kármán-Žurovec PKZ-1 electric-powered helicopter was flown in 1917. It had a specially-designed 190 horsepower (140 kW) continuous-rated electric motor made by Austro-Daimler and received its power up a cable from a ground-based DC generator. However electric motors were not yet powerful enough for such applications and the motor burned out after only a few flights.[7]

The problems of power-to-weight ratio for electric flight were formidable even for small models. A claim made in 1909 has since been challenged, while in more modern times success has been achieved at least since 1957.[8]

Microwaves present an alternative way of transmitting electrical power from the ground. In 1964, William C. Brown flew a model helicopter that received all of the power needed for flight from a microwave beam.[9]

NiCad batteries and solar power[edit]

Success in a full-sized aeroplane would not be achieved until Nickel-cadmium (NiCad) batteries were developed, having a much higher storage-to-weight ratio than older technologies. In 1973, Fred Militky and Heino Brditschka converted a Brditschka HB-3 motor glider to an electric aircraft, the Militky MB-E1. It flew for just 14 minutes to become the first manned electric aircraft to fly under its own power.[4][10]

Developed almost in parallel with NiCad technology, solar cells were also slowly becoming a practicable power source. Following a successful model test in 1974, the world’s first official flight in a solar-powered, man-carrying aircraft took place on April 29, 1979. The Mauro Solar Riser used photovoltaic cells to deliver 350 watts at 30 volts. These charged a small battery, which in turn powered the motor. The battery alone was capable of powering the motor for 3 to 5 minutes, following a 1.5-hour charge, enabling it to reach a gliding altitude.[11]

Under the direction of Freddie To, an architect and member of the Kremer prize committee, the Solar One was designed by David Williams and produced by Solar-Powered Aircraft Developments. A motor-glider type aircraft originally built as a pedal-powered airplane to attempt the Channel crossing, the airplane proved too heavy to be successfully powered by human power and was then converted to solar power,[12] using an electric motor driven by batteries that were charged before flight by a solar cell array on the wing.[13] The maiden flight of Solar One took place at Lasham Airfield; Hampshire on June 13, 1979.[14]

Human-powered hybrids[edit]

Following successful human-powered flight, a relaunched Kremer prize allowed the crew to store energy before takeoff.[15] In the 1980s several such designs stored electricity generated by pedalling, including the Massachusetts Institute of Technology Monarch and the Aerovironment Bionic Bat.[16]

Advancing technology[edit]

The human piloted Solair 1, developed by Günther Rochelt, flew in 1983 with notably improved performance.[2][17] It employed 2499 wing-mounted solar cells.[2]

The German solar-powered aircraft "Icaré II" was designed and built by the institute of aircraft design (Institut für Flugzeugbau) of the University of Stuttgart in 1996. The leader of the project and often pilot of the aircraft is Rudolf Voit-Nitschmann the head of the institute. The design won the Berblinger prize in 1996, the EAA Special Achievement Award in Oshkosh, the Golden Daidalos Medal of the German Aeroclub and the OSTIV-Prize in France in 1997.[18]

UAVs arrive[edit]

Pathfinder Plus (left) and Helios Prototype (right) on the Dryden ramp.
The electric-powered NASA Helios, an unmanned aerial vehicle.

NASA's Pathfinder, Pathfinder Plus, Centurion, and Helios were a series of solar and fuel cell system–powered unmanned aerial vehicles (UAVs) developed by AeroVironment, Inc. from 1983 until 2003 under NASA's Environmental Research Aircraft and Sensor Technology program.[19][20] On September 11, 1995, Pathfinder set an unofficial altitude record for solar-powered aircraft of 50,000 feet (15,000 m) during a 12-hour flight from NASA Dryden.[19] After further modifications, the aircraft was moved to the U.S. Navy's Pacific Missile Range Facility (PMRF) on the Hawaiian island of Kauai. On July 7, 1997, Pathfinder raised the altitude record for solar–powered aircraft to 71,530 feet (21,800 m), which was also the record for propeller–driven aircraft.[19]

On August 6, 1998, Pathfinder Plus raised the national altitude record to 80,201 feet (24,445 m) for solar-powered and propeller-driven aircraft.[19][21]

On August 14, 2001 Helios set an altitude record of 96,863 feet (29,524 m) – the record for FAI class U (Experimental / New Technologies), and FAI class U-1.d (Remotely controlled UAV: Mass 500 kg to less than 2,500 kg) as well as the altitude record for propeller–driven aircraft.[22] On June 26, 2003, the Helios Prototype broke up and fell into the Pacific Ocean off Hawaii after the aircraft encountered turbulence, ending the program.

Coming of age[edit]

The first commercially available production electric aircraft, the Alisport Silent Club self-launching sailplane, flew in 1997. It is optionally driven by a 13 kW (17 hp) DC electric motor running on 40 kg (88 lb) of batteries that store 1.4 kWh of energy.[23][clarification needed]

The first certificate of airworthiness for an electric powered aircraft was granted to the Lange Antares 20E in 2003. An electric, self-launching 20-meter sailplane with a 42-kilowatt DC/DC brushless motor and lithium-ion batteries, it can climb up to 3,000 meters with fully charged cells.[24] The first flight was in 2003. In 2011 the aircraft won the 2011 Berblinger competition.[25][clarification needed]

In 2005, Alan Cocconi of AC Propulsion flew, with the assistance of several other pilots, an unmanned airplane named "SoLong" for 48 hours non-stop, propelled entirely by solar energy. This was the first such around-the-clock flight, on energy stored in the batteries mounted on the plane.[26][27]

In 2008, The Boeing Fuel Cell Demonstrator

In 2007, the non-profit CAFE Foundation held the first Electric Aircraft Symposium in San Francisco.[28]

The Boeing-led FCD (Fuel Cell Demonstrator) project uses a Diamond HK-36 Super Dimona motor glider as a research test bed for a hydrogen fuel cell powered light airplane.[29] Successful flights took place in February and March 2008.[29][30]

The QinetiQ Zephyr is a lightweight solar-powered unmanned aerial vehicle (UAV). As of 23 July 2010 it holds the endurance record for an unmanned aerial vehicle of over 2 weeks (336 hours).[31] It is of carbon fiber-reinforced polymer construction, the 2010 version weighing 50 kg (110 lb)[32] (the 2008 version weighed 30 kg (66 lb)) with a span of 22.5 metres[32] (the 2008 version had 18 metres (59 feet)). During the day it uses sunlight to charge lithium-sulphur batteries, which power the aircraft at night.[33] In July 2010 a Zephyr made a world record UAV endurance flight of 336 hours, 22 minutes and 8 seconds (more than two weeks) and also set an altitude record of 70,742 feet (21,562 m) for FAI class U-1.c (Remotely controlled UAV: Weight 50 kg to less than 500 kg).[34][35][36]

The NASA Puffin was a concept, proposed in 2010, for an electric-powered, vertical takeoff and landing (VTOL), personal air vehicle.[37]

The first the NASA Green Flight Challenge took place in 2011 and was won by a Pipistrel Taurus G4 on 3 October 2011.[38][39][40]

The European Commission has financed many low TRL projects for innovative electric or hybrid propulsion aircraft. The ENFICA-FC is a project of the European Commission, to study and demonstrate an all-electric aircraft with fuel-cells as the main or auxiliary power system. During the three-year project, a fuel-cell based power system was designed and flown in a Rapid 200FC ultralight aircraft.[41]

In 2013 Chip Yates demonstrated that the world's fastest electric plane, a Long ESA, a modified Rutan Long-EZ, could outperform a gasoline-powered Cessna and other aircraft in a series of trials verified by the Fédération Aéronautique Internationale. The Long ESA was found to be less expensive, have a higher maximum speed, and higher rate of climb, partly due to the ability of the aircraft to maintain performance at altitude as no combustion takes place.[42][43]

Solar Impulse 2 flies around the world[edit]

Solar Impulse 2 is powered by four electric motors. Energy from solar cells on the wings and horizontal stabilizer is stored in lithium polymer batteries and used to drive propellers.[44][45]

In 2012 the first Solar Impulse made the first intercontinental flight by a solar plane, flying from Madrid, Spain to Rabat, Morocco.[46][47]

Completed in 2014, Solar Impulse 2 carried more solar cells and more powerful engines, among other improvements. In March 2015, the plane took off on the first stage of a planned round-the-world trip, flying Eastwards from Abu Dhabi, United Arab Emirates.[5] Due to battery damage, the craft halted at Hawaii until April 2016.[48] On 23 June 2016 the plane reached Seville, Spain on 23rd.[49] It has since returned to Abu Dhabi, completing its circumnavigation of the world.[50]

Electric rotorcraft[edit]

Although the Austro-Hungarian Petróczy-Kármán-Žurovec team flew an experimental tethered military observation helicopter in 1917, the use of electric power for rotor-borne flight was not exploited until modern times. Lightweight technologies have enabled the development of small, cheap radio-controlled UAVs, more commonly called drones, for recreational and other purposes. In particular the electric quadcopter has been manufactured in large numbers.


The Solution F/Chretien Helicopter the world's first man-carrying, free-flying electric helicopter was developed by Pascal Chretien. The concept was taken from the conceptual computer-aided design model on September 10, 2010 to the first testing at 30% power on March 1, 2011—less than six months. The aircraft first flew August 4 to 12, 2011. All development was conducted in Venelles, France.[51][52]

Solution F/Chretien helicopter

The electric Sikorsky S-300 of the Project Firefly was a project to flight test an electric rotorcraft, but the project was put on hold due to battery limitations.[53] The world's first large-scale all-electric tilt-rotor was the AgustaWestland Project Zero unmanned aerial vehicle technology demonstrator, which performed unmanned tethered fights on ground power in June 2011, less than six months after the company gave the official go-ahead.[54]

In September 2016, Martine Rothblatt and Tier1 Engineering successfully tested an electric-powered helicopter. The five minute flight reached an altitude of 400 feet with a peak speed of 80 knots. The Robinson R44 helicopter was modified with two three-phase permanent magnet synchronous motors, weighing 100 lb, plus 11 Brammo Lithium polymer batteries weighing 1100 lb and a digital cockpit display.[55][56][57] It later flew for 20 minutes in 2016.[58][59]

See also[edit]

References[edit]

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  59. ^ Video on YouTube

External links[edit]