Aviation accounts for 2% of global carbon emissions, with more than half of that contributed by international flights.International aviation produced more than 492 million tonnes of CO2 in 2014, making its output larger than that of the United Kingdom. And that figure is projected to skyrocket in the coming years, with more than 56,000 new aircraft projected to hit the skies by 2040, according to the Environmental Defense Fund (EDF), an environmental group based in New York City. The EDF says that CO2 emissions from air travel and transport could triple or even quadruple over that period. The NRDC reports that when a plane is used instead of a cargo ship, 4.5 times more particulate matter and 25 times more nitrous oxides are added to the environment.
The need to optimize aircraft performance, decrease operating and maintenance costs, and reduce gas emissions is pushing aircraft industry to progress towards more electric aircraft (MEA), and ultimately an All Electric Aircraft.Electric propulsion can be powered by rechargeable batteries, fuel cells, or solar energy.
The first step is more electric aircraft that uses electrical energy instead of hydraulic, pneumatic, and mechanical means to power virtually all its subsystems, including flight control actuation, environmental control system, and utility function. The idea is to significantly reduce fuel consumption by improving overall energy efficiency. In the “No-Bleed” architecture, the pneumatic aircraft systems, namely engine starting, wing de-icing, landing gear deployment system, secondary flight control actuators, cabin pressurization, environmental control, and braking systems will be replaced by electrically powered machines.
Researchers at NASA’s Glenn Research Center are actively researching the next generation of efficient aircraft. One of their current objectives is to help shift the industry from solely using gas turbines to start implementing hybrid or turboelectric propulsion. The benefits of this are reduced energy consumption, emissions, and noise.
Research is on-going to obtain the weight savings benefits of a full more electric architecture implementation. Joint Strike Fighter (JSF), C-141 transport aircraft, and UAV’s like Condor, Global Hawk and Dark Star are some examples where the MEA concept has been introduced. With MEA technology, the weight in an aircraft can be shifted from hydraulic systems and plumbing to passengers, fuel or mission payloads. MEA technology could dramatically reduce per passenger costs and ticket prices for commercial aircraft, while giving military planes more maneuverability and survivability due to less vulnerability to enemy fire, says Honeywell.
The report “More Electric Aircraft Market by Technology (Power Electronics, Thermal Management, Energy Storage & Others), Application (Power Generation, Passenger Comfort, Air Pressurization & Others) Platform (Fixed Wing, Rotary Wing), and Region – Global Forecast to 2021”, The more electric aircraft market is projected to grow from USD 7.68 Billion in 2016 to USD 10.94 Billion by 2021, at a CAGR of 7.33% from 2016 to 2021.
Using electrical power to actually propel planes, however, is a more complicated challenge. In a conventional jet airplane, the engine sucks air in through its front, a compressor squeezes it, and fuel is sprayed in and lit, creating burning gases and forward thrust. Electric plane power is much simpler — batteries power an electric motor that spins a propeller. It’s more efficient, but involves far less thrust, which is why electric planes tend to be slow.
In July 2015, the E-Fan, an Airbus design with batteries, crossed the English Channel, however Airbus’ two-seat electric plane could only go a maximum speed of about 136 miles per hour. A solar-powered plane that completed an around-the-world journey this summer had an average airspeed of 47 miles per hour. The plane, called Solar Impulse 2, had more than 17,000 solar cells that powered four electric motors.
“But solar power, while an interesting technical challenge, is not a particularly realistic option for mass transit of passengers. As can be seen from the Solar Impulse aircraft, the power output from the Solar Panels on a very wide wingspan is able to transport only the aircraft and the pilot for any significant distance,” writes Dr Peter Wilson.
Electric motors offer other advantages too compared to combustion engines, they don’t need air to produce power, so they can maintain their full rated punch even at high altitudes, where the air is thin. The use of electric power in airframe systems saves fuel and helps in cost reduction. Electric motors can be made quite light and small and still develop considerable power with high reliability—they scale well. Air-breathing engines, both reciprocating and turbine, have many more moving parts, are more complex. The more electric aircraft architecture also helps in reducing maintenance costs as it requires fewer moving or non-moving parts, as compared to the conventional aircraft systems.
Brian German, associate professor at the Daniel Guggenheim School of Aerospace Engineering at Georgia Tech, said there are no distinct lines in the sand yet for aircraft categories, but generally researchers are looking at all-electric, or battery only, systems for smaller aircraft and various hybrid or turbo-electric systems to power the larger ones.
German said the trend in the military is for “more electric,” writes Stew Magnuson in NDIA. That is, jets need more power to run its suites of energy-hungry electronics, radars, and so on. It wants less fuel being diverted to these systems. “Another flight profile might be an unmanned aerial surveillance aircraft that uses the gas engine for the flight to a targeted area, then switches over to the much quieter battery in order to be more stealthy, or extend its range,” writes Richard “Pat” Anderson, professor of aerospace engineering and director of the Eagle Flight Research Center at Embry-Riddle Aeronautical University’s Daytona Beach, Florida, campus.
One of the ways the technology can be applied for larger aircraft is distributed power systems, German said. For example, the larger a gas-turbine engine is the more efficiency can be squeezed out of it. That’s why there tends to be only a few massive engines hanging off aircraft such as the C-17. Researchers have known for many years that putting many smaller propellers or engine fans, distributed at key areas would be even more efficient. Every aircraft has a boundary layer, an area of dead air above the wing that builds up and creates drag as the plane flies. By placing several smaller fans along the aircraft, the boundary layer is “ingested” and almost disappears, making the aircraft faster or more energy efficient, German explained.
Engineers look at this one of two ways. It’s either making the engine fans more efficient, or creating less drag on the wing. Both effects are at play, German said. This is the principle behind DARPA’s LightningStrike vertical takeoff and landing X-plane that it is developing with Aurora Flight Sciences. An artist’s rendering of the plane shows 26 hybrid-electric propulsion fans distributed on the aircraft. The program has flown a 325-pound scale model and expects to build a full-scale version within the next two years, the company said in a statement. Operating from austere landing zones is the requirement the program is seeking to fill, it said.
NASA’s 18-Engine Electric Propulsion-Powered Experimental Aircraft Underway
NASA is researching ideas that could lead to developing an electric propulsion-powered aircraft that would be quieter, more efficient and environmentally friendly than today’s commuter aircraft.
The proposed piloted experimental airplane is called Sceptor, short for the Scalable Convergent Electric Propulsion Technology and Operations Research. The concept involves removing the wing from an Italian-built Tecnam P2006T aircraft and replacing it with an experimental wing integrated with electric motors.
An advantage of modifying an existing aircraft is engineers will be able to compare the performance of the proposed experimental airplane with the original configuration, said Sean Clarke, Sceptor co-principal investigator at NASA’s Armstrong Flight Research Center in California. The Tecnam, currently under construction, is expected to be at Armstrong in about a year for integration of the wing with the fuselage. Armstrong flew a different Tecnam P2006T in September to gather performance data on the original configuration.
NASA researchers ultimately envision a nine-passenger aircraft with a 500-kilowatt power system in 2019. To put that in perspective, 500 kilowatts (nearly 700 horsepower) is about five times as powerful as an average modern passenger car engine.
However, to reach that goal NASA researchers intend to fly the Aeronautics Research Mission Directorate-funded Sceptor in about two years. Progress in three areas is happening now to enable that timeline, Clarke said.
HEIST’s first experiment was called the Leading Edge Asynchronous Propeller Technology, or Leaptech. The experiment began in May at Armstrong and consisted of 18 electric motors integrated into the carbon composite wing with lithium iron phosphate batteries.The wing is made of carbon composite, while the electric engines are powered by lithium iron phosphate batteries. Tests so far show the distribution of power among the 18 motors creates more than double the lift at lower speeds than traditional systems, he said. Leaptech is a collaboration of Armstrong and NASA Langley Research Center in Hampton, Virginia, and California companies Empirical Systems Aerospace of Pismo Beach and Joby Aviation of Santa Cruz. Key potential benefits of Leading Edge Asynchronous Propeller Technology (LEAPTech) include decreased reliance on fossil fuels, improved aircraft performance and ride quality, and aircraft noise reduction.
For example, researchers are integrating Sceptor aircraft systems with an Armstrong flight simulator for pilots to evaluate handling qualities. Researchers also will be able to study balancing the power demands of the motors with batteries and then a turbine, Clarke explained. Researchers are interested if a hybrid of distributed electric motors and gas-powered turbines could provide power to extend the aircraft’s range and enable the envisioned 9-place concept aircraft, Clarke explained.
Sceptor could be a solution to greater fuel efficiency, improved performance and ride quality and aircraft noise reduction. NASA will be key in developing those technologies for the future that will be with people when they fly.
Airbus Group Innovations has flown its E-fan
Airbus Group Innovations has flown its E-fan proof-of-concept electric aircraft with hybrid propulsion, adding an internal-combustion engine as a range extender.
The 600-kg (1,320-lb.) aircraft has an endurance on batteries only of 30 min. This is increased to 2 hr. 15 min (plus recharging the batteries to 80%) using the range extender, a two-cylinder, two-stroke piston engine with electronic fuel injection providing 50 kw (68 hp) of power on aviation gasoline. The Solo 2625 02i engine, certified for use in ultralights, weighs 24 kg dry.
The combustion engine sits behind the cockpit, and is only used to charge the plane’s lithium-ion batteries which are embedded in the wings. The combustion engine doesn’t provide any of the plane’s thrust. The two ducted fan engines that do give the plane its thrust are mounted on each side of the fuselage. The plane takes off and lands on electric power only — to make it as quiet as possible. The combustion engine will charge the batteries only during the cruise portion of each flight.
In its previous all-electric configuration, as the E-Fan 1.1, the aircraft crossed the English Channel on electric propulsion in July 2015, a flight lasting 37 min. Airbus is confident that it can also reduce the carbon emissions for all new aircraft by 75 percent.
This small two-seater experimental aircraft called ‘E-Fan’ measures 19 feet from nose to tail, has all-composite construction and manufactured by Toulouse-based Airbus. It has pair of electric engines generating a combined power of 60 KiloWatts powered by 120 lithium-ion polymer batteries, enough to sustain one hour of flight.
Airbus is working with general aviation manufacturer Daher and electric motor supplier Siemens to put the two-seat, all-electric E-Fan 2.0 into production as a training aircraft. First flight is planned for 2017.
Hybrid electric engine
A small experimental airplane recently took to the skies near Sywell Aerodrome near Northampton, United Kingdom, for an important test flight. But unlike the other airplanes at the airport, this single-seat airplane was powered by a hybrid electric engine propulsion system that uses a combination of traditional fuel and electricity. An electric motor and petrol engine work together to drive the propeller. The demonstrator aircraft uses up to 30% less fuel than a comparable plane with a petrol-only engine. The aircraft is also able to recharge its batteries in flight, the first time this has been achieved.
During take-off and climb, when maximum power is required, the engine and motor work together to power the plane, but once cruising height is reached, the electric motor can be switched into generator mode to recharge the batteries or used in motor assist mode to minimise fuel consumption. The team also developed an advanced battery management and state-of-health monitoring system.
“Until recently, batteries have been too heavy and didn’t have enough energy capacity. But with the advent of improved lithium-polymer batteries, similar to what you’d find in a laptop computer, hybrid aircraft – albeit at a small scale – are now starting to become viable,” said Dr Paul Robertson of Cambridge’s Department of Engineering, who led the project.
Boeing has completed a series of fuel cell demonstrator flight tests, and recently wrapped up a NASA study that identified hybrid electric propulsion technology as having good potential to meet future environmental goals for air vehicles in the 2030 to 2050 timeline.
DARPA’s LightningStrike vertical takeoff and landing X-plane
The US Defense Advanced Research Projects Agency’s (DARPA’s) LightningStrike vertical take-off and landing (VTOL) X-Plane programme has completed an initial flight demonstration phase with a scale model. DARPA said Wednesday the subscale X-Plane model demonstrated auto takeoff, sustained hover, directional and translational control, waypoint navigation and auto landing functions during flight tests that began in March 2016. That test run is now over and officials will next focus on a full-scale system demonstration.
Officially known as the XV-24A LightningStrike Vertical Take-off and Landing (VTOL) XPlane, the 20% scale craft completed its planned flight test program in early March at Webster Outlying Field in Southern Maryland. Funded by the Defense Advanced Research Projects Agency (DARPA), the SVD successfully demonstrated key technical features the full-scale XV-24A will perform, including outbound and inbound transition flight. The SVD aircraft is a 325 pound, Lithium battery powered scale model of the 12,000 pound, 61 foot wingspan XV-24A. It will remain in flight status to supplement the full-scale XV-24A flight test program, currently scheduled to begin in late 2018.
Aurora’s LightningStrike VTOL X-Plane was recently designated the XV-24A by DARPA and the U.S. Air Force.
The fully sized XV-24, a 24,000-pound craft will have a 61 foot tiltwing wingspan, and is soon to be tested —it is hoped the craft will be able to fly twice as fast as a traditional helicopter (approximately 300 to 400 knots) and with 15 percent more hovering efficiency.
The fans on the craft are spread across the wings and canard (tailpiece) and will eventually be powered by a what the team calls an electric distributed propulsion system—each fan has an electric motor powered by a Rolls-Royce AE 1107C turboshaft engine, which currently powers the military’s V-22 Osprey twin prop tiltrotor aircraft—it will turn three generators from Honeywell.
The mockup was powered by a lithium ion battery. Aurora Flight Sciences said the subscale version proved the radical theory behind the craft. The subscale aircraft weighs 325 pounds and is a 20% scale flight model of the full scale demonstrator Aurora will build for Darpa in the next 24 months.
Virginia-based Aurora Flight Sciences built a prototype of its unmanned, hybrid-propulsion ‘LightningStrike’ aircraft after beating two competitors for the coveted Darpa project.
More Electronic technologies
Key components and functions include engine starting, electrical power generation, power conditioning and routing, air cycle environment control, avionics, fuel/oil cooling, ventilation, flight control actuation, and overall vehicle & propulsion system thermal management – especially waste heat recovery and/or rejection.
The limiting factor for an electric-powered aircraft is the battery challenge is to like develop lightweight batteries that can store enough electricity for long distance flights. Batteries are heavy, and their energy storage capacity increases only at an average of eight percent or so a year.
“Typical lithium-ion batteries in use today have a maximum energy density of around 1,000,000 joules of energy per kilogram, and while newer research promises the possibility of higher densities, these are not available commercially, says Dr Peter Wilson, professor of electronics and systems engineering at University of Bath. A million joules sounds like a lot. However, compare this with 43m joules per kilogram for aviation fuel. Swapping the fuel tanks for a battery weighing 43 times as much isn’t a viable option: clearly there’s a significant storage problem to be solved before electricity can power large aircraft over long distances.
Power electronics help improve both the generator’s (mechanical to electric) and actuator’s (electric to mechanical) energy conversions and also enable better energy regeneration. Power electronics also helps in reducing weight, is easier to maintain, and provides more controllability and intelligence which includes fault detection and diagnosis.
Power generation and management plays an important role in the more electric aircraft architecture. With the growing aircraft electrical system power levels, the diversity of the power generation types is increasing as well.