Advances in Aircraft Propulsion

In recent months, spectacular advances have taken place in the technology-intensive arena of aircraft propulsion. The world’s largest aero engine ever, the GE9X, completed its flight tests on a specially modified Boeing 747 and is now being geared up to be test flown on Boeing 777-9 for which it is intended. The GE9X has a fan diameter of 134 inches. To put it into perspective, its diameter is more than twice that of the CFM56 family that powers a majority of the world’s single-aisle, narrow-body commercial aircraft families of Airbus A320 and Boeing 737. The other remarkable event was the test flight of Ampaire 337, the highest capacity hybrid electrical aircraft ever flown. Originally a twin-engine Cessna 337 Skymaster, the aircraft was retrofitted with the replacement of one of the internal combustion engines with an electric motor, resulting in a parallel-hybrid propulsion system. While the GE9X is a momentum-driven progression of internal combustion engines that guzzle up Aviation Turbine Fuel (ATF) in proportion to their size/thrust, the Ampaire 337 is part of a technological campaign to find an alternative to ATF.

AS FOSSIL FUEL RESERVES GET DEPLETED AT A DISQUIETING PACE, THERE IS PRESSURE ON ENGINE ORIGINAL EQUIPMENT MANUFACTURERS TO INNOVATE

Two motivations drive the electric aero engine quest – the need to conserve finite fossil fuel reserves and the pressure of reducing the effect of aviation-related emissions on climatic change. All internal combustion engines, jet, turboprop or piston, burn fossil fuels. However, there are now programmes and projects in place to produce equivalent fuels from biological sources, but commercially viable bio-fuels are still some distance away in time while fossil fuels are being depleted at an alarming pace. As far as the correlation between aviation fuel consumption and emissions is concerned, an Intergovernmental Panel on Climate Change (IPCC) working under the auspices of the United Nations Environment Programme (UNEP), monitors climate change and has computed that air transportation contributes to the 4.9 per cent of human-caused climate change, including emissions of CO₂ and other Greenhouse Gases. ATF is the largest contributor as it is used by jet engines which are the majority of aircraft engines and also because it gives out a lot of CO₂ during its combustion inside the engine. Thus, there exists a need to seek advances in aircraft propulsion from the point of view of both – fossil fuel conservation and the adverse impact on the environment due to its combustion.

INTERNAL COMBUSTION LEGACY

As fossil fuel reserves get depleted at a disquieting pace, there is pressure on engine Original Equipment Manufacturers (OEMs) to innovate for engines with lower Specific Fuel Consumption (SFC) which is the ratio of fuel consumption per unit time (in kg/hr) to power produced by engine (in kWh). However, engine design is dictated by the prospective aircraft or aircraft family that it is meant for. Approximately two-thirds of all commercial airliners lies in the single-aisle, narrow-body segment and this is where significant engine advances have occurred over the past decade or so when the antagonistic competition between Airbus and Boeing in this segment has been intense.

The existing aircraft families, A320 and Boeing 737 respectively, have been stretched and fitted with more and more powerful engines until a decision point was reached. The choice was between designing a new aircraft to bridge the gap between narrow-body and wide-body, against enhancing existing designs of the two families. This choice became necessary as the incremental changes to the two families were becoming smaller and smaller and both the contenders were in search of substantial improvements to their aircraft.

In 2010, Airbus announced its decision to re-engine the existing A320 family, introduce some design features to reduce fuel consumption and reduce weight through use of composites. The enhanced version of the aircraft was called the A320neo (new engine option). Boeing was less enthusiastic about the concept and had been thinking of a new aircraft to fill the gap between the narrow-body and the wide-body space since 2006, but was forced to follow suit with a decision in 2011 to introduce its own family of 737 MAX aircraft. The favouring of the decision to re-engine over the risk of developing a new, expensive aircraft was termed by some analysts as the ‘Age of Incrementalism’ and was largely driven by technology promising reduced fuel consumption. The lure of cost savings, as can be expected, enticed airlines all over the world to order neos and MAXs.

POWER PLANT OEMS ARE CONSTANTLY RESEARCHING NEW ARCHITECTURES FOR GREATER EFFICIENCY AND FUEL OPTIMISATION

Variants of CFM’s Leap engines power the family, Boeing 737 MAX versions and the Chinese Comac C919. The Leap engine architecture was selected after a deep study of eighteen different engine architectures including geared fans, counterrotating and unducted fans, and a direct-drive single-stage design such as the one used in CFM56 which is possibly the world’s best selling engine. CFM’s competitor Pratt & Whitney (P&W) has been studying new versions of its Geared Turbo Fan (GTF) which it calls “the future architecture of choice for commercial aviation” and which already powers A220s, A320neos, Embraer E-Jets E2 and in-development Mitsubishi Aircraft MRJs and Irkut MC-21s.

The GTF’s gear decouples the fan and turbine so each rotates at optimal speeds, with the fan turning about one-third the speed of the turbine. According to P&W, the design is 16 per cent more efficient than previous generation engines. While the GTF’s gear improves the engine’s propulsive efficiency which is a measure of how well it converts mechanical energy into aircraft kinetic energy, P&W is now focusing on thermodynamic efficiency which indicates how well an engine converts chemical energy in fuel into mechanical energy. P&W is studying better cooling, improved coatings and increased use of advanced materials such as Ceramic-Matrix Composites (CMCs). Rolls-Royce, with a very successful family of ‘Trent’ engines and heavy ongoing investment in a geared architecture called ‘UltraFan’, is absolutely certain that the gas turbine engine has a long way to go before it can be replaced with something better although it admits that continuation of gas turbine engines will not permit achievement of the ambitious emission reduction targets set for aviation.

Power plant OEMs are constantly researching new architectures for greater efficiency and fuel optimisation. Unducted fans, also called open rotor engines or prop-fans, have been toyed around with by several manufacturers including GE and Boeing in the past but have recently started attracting more attention as technology enables more efficient design. The unducted fan design eliminates nacelles altogether, allowing fans to spin freely in air, which enables a higher bypass ratio without the penalty incurred by increasing the size of an engine’s fan which entails the weight and drag of the nacelle and the limitations due to underwing design of current aircraft. There is a huge problem with noise with the unducted fan but undoubtedly, this will be solved in the near future.

According to Bill Brown, Director of Commercial Engines Marketing at GE Aviation, part of the joint venture with Safran that forms CFM, the advancing state of technology enables around one per cent gain in aircraft fuel efficiency annually, with engines driving about 75 per cent of efficiency gains and airframes accounting for the rest. That is good news for the NMA if it starts flying around 2030, as projected because the engine it could have may be 10 to 15 per cent more efficient than the Leap variants. Interestingly, unlike the Boeing 737 MAX family which comes with only one engine option (CFM), three engine OEMs (CFM, Pratt & Whitney and Rolls-Royce) are in contention for producing the most fuel-efficient engine option for NMA albeit Rolls-Royce has shown steadily reducing interest. GE is also focusing on how to extract more thermodynamic efficiency from its designs, partly through expanded use of Ceramic Matrix Composites (CMCs) which permit reduction of the quantum of air that needs to be diverted from the compressor to cool the turbine. Additionally, GE intends to use more additive manufactured parts and more carbon fibre composites in cold sections of engines for further reduction of weight. However, all ongoing developments in the field of internal combustion engines pale into comparative insignificance when compared to the promise of electric propulsion.

ELECTRIC AND HYBRID PROPULSION

Rather unobtrusively, electric propulsion has permeated into life around us by way of bicycles, scooters, cars, golf carts, utility vans and ecofriendly vehicles. Aviation applications have been slower in coming as the weight of the batteries that must be carried onboard and gravity plays a spoilsport. However, climate change pressures and fossil fuel fears are providing enormous impetus to R&D in the electrical propulsion arena, the only near term viable alternative to internal combustion engines. The developments in the electric propulsion area are varied and copious. Some of them are discussed in the succeeding paragraphs.

In May this year, NASA announced a programme called Centre for Cryogenic High-Efficiency Electrical Technologies for Aircraft (CHEETA) at the University of Illinois in the US. It will work on the development of fuel cell and cryogenic liquid hydrogen energy storage technologies for a completely electric aircraft. It will include a consortium of researchers with the knowledge and experience to make disruptive advancements in cryogenic and superconducting technologies, as well as novel methods for electric propulsion integration, as applied to aircraft systems. In May 2019, Embraer and WEG, a Brazilian company operating worldwide in the electric engineering, power and automation technology areas, announced a scientific and technological cooperation agreement to jointly develop new technologies and solutions to enable electric propulsion in aircraft using an EMB-203 Ipanema single-engine agricultural aircraft modified into an electric propulsion test bed.

In Europe, where the aviation emissions reduction pressures are more than in the US, the French aerospace major Airbus has signed a Memorandum of Understanding (MoU) with SAS Scandinavian Airlines to undertake joint research on operational and infrastructural opportunities and challenges involved with the large-scale introduction of hybrid and full electric aircraft into airlines. In 2015, Airbus reached a milestone when its two-seater, battery-powered E-Fan crossed the British channel in just 40 minutes in a purely electric powered trainer aircraft. Visitors to AERO 2017 in Germany were witness to companies including Siemens AG, Pipistrel, Geiger, MGM Compro, Evolaris and Engiro, as also some universities that presented various electrically powered aircraft and some under-development hybrid propulsion systems. Similar exhibits were seen at the last Paris Air Show.

Electric flight or E-flight has been approached by Airbus and Boeing differently. Airbus started this E-flight journey a decade ago with single-seat aircraft and is investing strongly in urban air mobility while Boeing has chosen a different approach by investing its venture capital arm HorizonX in finding promising start-ups to invest, often along with partners. One such partner is Zunum Aero, a Seattle-based start-up backed by Boeing and JetBlue Technology Ventures that announced in October 2017, plans for its first hybrid-electric plane - a 19-seater with a range of 700nm. The prototype is expected to fly by 2020, and if that happens, its entry would be highly disruptive for the regional airliner market worldwide.

ELECTRIC PROPULSION HAS PERMEATED INTO LIFE AROUND US BY WAY OF BICYCLES, SCOOTERS, CARS, GOLF CARTS, UTILITY VANS AND ECO-FRIENDLY VEHICLES

However, the current state of battery technology renders their weight and cost too high for commercial flight. Battery-supported road transportation that would probably carry four passengers over 500 km, would be inadequate to carry two passengers for 10 km in flight. The key metric used in this context is ‘energy density’ defined as the amount of energy stored in a given system or region of space per unit volume. A rough estimate of the current battery technology state is that a measure of jet fuel gives us about 43 times more energy than a battery with the same volume. According to estimates, even for small-scale aviation needs, batteries will have to be five times denser than they are today and, at the current pace of progress in battery technology, it probably will not be until 2030, that even hybrid electric technology is used in commercial aviation. There is also the problem of batteries catching fire in flight - a recipe for mid-air disaster. The challenge lies in eliminating fires or at least rendering them manageable in flight without turning into fatal catastrophes.

Full scale jet propulsion using electric power is currently not in the grasp of technology as electric energy capabilities of batteries are not dense enough. Hence the term ‘hybrid’ in the context of electric propulsion of aircraft to indicate the supplemental addition of electric power to existing or modified power plants in aircraft. Until the ratio of battery weight to its energy improves, with the help of emerging technologies, of course, electrical power may be used in cruise phase while internal combustion engines are used during take-off and landing. That is not to say that there are no electric propulsion aircraft flying already. The Ampaire 337 was mentioned earlier. Slovenian manufacturer Pipistrel exhibited it in 2017, and has been offering for sale since then, is an all electric two-seat Alpha Trainer. Zunum Aero, the electric jet startup backed by Boeing HorizonX and JetBlue Technology.

Ventures, is aiming to get its hybrid electric jet off the ground by 2022. Airbus E-Fan X is being developed with Rolls-Royce and Siemens as a hybrid electric airline demonstrator, while Kitty Hawk, the electric VTOL start-up founded by Google’s Larry Page, has started selling its short-range, one-seat Flyer which looks like a bobsled mounted on a couple of pontoons surrounded by a bank of rotors. Faradair, a UK start-up has declared its intention to fly by 2022 an 18-seater hybrid electric aircraft which has a turboprop engine generating electricity to drive a ducted, contra-rotating pusher prop fan with vectored thrust and a high-lift, triple-box wing for use in passenger and freight roles. Meanwhile, Rolls-Royce has conducted ground tests of a hybrid electric propulsion system based on an M250 turbo-shaft engine and plans to start flight tests in 2021.

A real windfall in the area of electric propulsion is ‘distributed propulsion’, the concept that the energy source i.e. the batteries, can be located far away from the electric motor(s) they drive. All that is required is electric wiring to carry electric power from the batteries to the motors. In this way, the batteries which are very heavy, can be located close to the aircraft centre of gravity and inside the fuselage while the electric motors can be located where the designer feels they may give maximum benefit/efficiency. Two huge benefits are available from this happenstance; the number of motors/propellers can be unlimited. Small, easily manageable motors can be used instead of huge, more complicated ones and these can be located almost anywhere on the aircraft wings, tail or fuselage in contrast to the internal combustion engine which has limited options.

A WINDFALL IN ELECTRIC PROPULSION IS ‘DISTRIBUTED PROPULSION’, WHEREIN THE ENERGY SOURCE IS LOCATED FAR AWAY FROM THE ELECTRIC MOTORS THEY DRIVE

According to NASA, the large number of small propellers that ‘distributed propulsion’ facilitates may translate into a huge saving of energy needed for flight. For example, in its project called X-57 Maxwell which has fourteen electric motors of which twelve are on the leading edge for takeoff and landing and one larger motor on each wingtip for use during cruise at altitude.

Electric propulsion would save on fossil fuel and avoid environmental pollution but carries the risk of batteries running out in the air over terrain the aircraft cannot land on safely. In an interesting side story, Harbour Air, an operator with 42 seaplanes operating in Vancouver, British Columbia and Seattle, has announced plans to convert its entire fleet to electric power using technology from MagniX, a Washington-based innovator company which builds its own dedicated aircraft-quality electric motors. According to MagniX, it uses state-of-the-art batteries that have 200 watt-hours per kilogramme and can deliver 30 minutes of flight time and 30 minutes of reserve power. The fact that Harbour Air’s flying is mainly over the sea provides the additional safety that in case of battery power running out, a reasonably safe landing over the sea can still be the alternative plan.

Suffice it to say that electric power on aircraft is now not a question of if, but when. An electric airliner however, is some time away. Scaling up the current technology to a development of a family of airliners in the 10 to 19 seat range is the next challenge that is expected to be overcome by late 2020s. Airbus and Boeing have already started to dream about several projects with a long term target of airliners with up to 100 passenger capacity. On the other hand, short, point-to-point passenger delivery is already a reality.

URBAN MOBILITY AS AN IMPETUS

The business-driven need to transport business executives from airports to offices and back in large metros has provided stimulus to flying cars and similar projects. At least 20 companies, including legacy aircraft manufacturers like Airbus and Boeing, are developing aerial taxi plans; the objective being to build aircraft that are electricpowered to eliminate the noise and pollution typically associated with helicopters and jetliners. Uber is predicting test flights of its electric-powered vertical takeoff and landing aircraft by 2020. It has hired Tesla’s in-house battery expert, Celina Mikolajczak, to speed up its effort to develop a battery that is powerful yet light enough to get its plane-helicopter hybrids in the air. In 2018, Safran completed a first ground test of a hybrid electric propulsion system designed for urban mobility vertical take-off and landing based on ‘distributed propulsion’ system.

In the rotary-wing space, small, remote controlled toys and drones have slowly grown in size and offer the promise of autonomous or remotely piloted personal transportation. Projects like the Volocopter 2X, with nine electric engines and eighteen rotors, is already flying. It can carry two passengers and is expected to contribute hugely to urban air mobility in the near future.

CONCLUSION

Bio-fuel technologies, driven by the dual motivations of fossil fuel depletion and climate change imperatives, are progressing at a rapid pace. The technological feasibility of using bio-fuels in aircraft engines has already been demonstrated amply. it is only a matter of time before the cost of bio-fuels, which is around three to four times that of equivalent fossil fuels, is brought down by technological innovations to lower than that of fossil fuel. Changes to existing internal combustion jet engines to switch over to bio-fuel, would be minimal and easily achievable.

Meanwhile, according to German consultancy firm Roland Berger, there are around 170 electrically powered aircraft programmes already under development and this number is likely to go up to 200 by end-2019. Understandably, half of all these projects are related to urban air taxi segment. Europe, which has the most robust aviation emissions regulatory guidelines in place, has the highest concentration of these programmes, with the US following close behind. Urban air taxis and general aviation dominate the projects as they are smaller and the current electrical systems technology supports only low power, short distance flights.

Air transportation has indubitably been one of the finest gifts of technology to the world. The convenience of international air travel has made the world a better place to live in. However, environmental pollution directly attributable to aircraft engines burning fossil fuels is continual collateral damage, the quantum of which is increasing steadily. According to industry estimates, aviation fuel demand is expected to increase by 1.9 per cent to 2.6 per cent each year until 2025. At this rate, the projected growth in the aviation industry is expected to increase its share of global emission increase to 22 per cent by 2050 from the present 4.9 per cent. The growth in aviation is inevitable as air travel has become a necessity. Thus there is a neck-on-neck race between aviation growth and aviation emission reduction through advances in aircraft propulsion. Hopefully, the latter will draw ahead in the near future.