Sustainability and BizAv: Electric Engine Update

How is the future landscape of electric propulsion for business aircraft shaping up, and what advantages are promised to owners, operators, and the environment, generally? Chris Kjelgaard speaks to those spearheading the development programs.

Chris Kjelgaard  |  26th October 2022
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    Chris Kjelgaard
    Chris Kjelgaard

    Chris Kjelgaard has been an aviation journalist for more than 40 years and has written on multiple topics...

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    eVTOL in flight over a cityscape


    As the aviation industry makes progress toward its environmental target of Net Zero carbon emissions by 2050, new forms of propulsion will be vital for Business Aviation. Including electric or hybrid-electric powerplants.

    One of the most enduring pop music hits of 1984 is the memorable song ‘Together in Electric Dreams’, written as the theme tune for the romantic comedy ‘Electric Dreams’. Completely unintentionally, almost 40 years later, the lyrics are curiously relevant in describing the near-term importance of electrical power as a new form of propulsion for business aircraft.

    Specifically, “We’ll always be together” and “No matter where I go I'll never find a better prize” succinctly describes the enormous importance electrical power will assume for aviation, both as a supporting and a primary method of propulsion for aircraft of almost all sizes.

    Perhaps the only less-than-prophetic line in the song’s lyrics as it applies to electrical propulsion and Business Aviation is the title itself, ‘Together in electric dreams’, because it is now clear that electrical propulsion will be no dream for Bizav, but reality.

    Senior Executives of Honeywell, Pratt & Whitney Canada and Rolls-Royce are firmly of the opinion that various forms of electrical propulsion will be applicable to Business Aviation just as much as they are to any other sector of the aviation and aerospace industry.

    Electrical power will be available to propel aircraft from a variety of onboard sources. Together with the mission requirements for and the size of the aircraft in question, the characteristics of each different source will determine whether that source will allow the aircraft to be propelled entirely by electrical power, or a combination of electrical power and gas-turbine propulsion.

    The Future Electric-Propulsion Landscape

    A broad consensus exists among the major engine OEMs — all of which are working hard to develop more-electric forms of propulsion, as a foundation stone in their efforts to achieve Net Zero carbon emissions by 2050 — on the basic shape of the aviation electric-propulsion landscape.

    The most basic characteristic of the landscape, according to Honeywell Aerospace’s Taylor Alberstadt, is that electric power can be provided as a supporting or primary form of aircraft propulsion in a variety of ways, including combinations with existing gas-turbine engines.

    Now Honeywell’s Global Sales and Marketing Lead for Unmanned Aerial Systems and Urban Air Mobility, Alberstadt’s previous role was as Honeywell’s Business Development Director for Electric and Hybrid-Electric Propulsion, and he notes there may be as many as seven different ways to use electrical power to propel aircraft.

    The best method in each case will depend on a few basic variables, including:

    • Size and weight of aircraft
    • Mission profile (range, passenger capacity, cruise speed, etc.)
    • Physical design of aircraft, and to what extent that’s fundamentally integrated with the propulsion system(s) to optimize aerodynamic performance, and
    • Scale of the operational benefit sought in incorporating electric power as a supporting or primary component of the aircraft’s propulsion system.

    GE Aviation and NASA Hybrid-Electric Testing

     Benefits of Electrical Propulsion

    In every case, the basic benefit desired in using electrical power as a propulsion method is to reduce the amount of energy required per passenger or pound of payload when performing the mission, according to Pratt & Whitney Canada’s Jean Thomassin.

    As Executive Director for New Product and Service Introduction for the company, Thomassin oversees P&WC’s efforts to develop enterprise-level requirements for future sustainable propulsion systems, technologies and business models.

    From the reduction of energy per pound of payload, all the other performance benefits that may be possible with electrical propulsion will flow. The aircraft will use less fuel; its net emissions of carbon will be reduced or even eliminated if the aircraft is entirely electric; and its overall operating cost will fall.

    Depending on how well the electrical-power component of the propulsion system is integrated into the aircraft’s overall design and function, and to what extent electrical power contributes to the total propulsive force available to the aircraft, using electrical power to propel the aircraft may also confer important reliability, safety, system redundancy and maintenance-cost benefits, says Alberstadt.

    The total amount of energy required by aircraft of different sizes for different missions will determine in each case the sources of electrical propulsion it can effectively use, and to what extent those sources can contribute toward the total propulsive energy required by the aircraft.

    This is a matter of basic physics and the energy density available from the source producing the electrical power. Because of the available energy density from each source, Frik-Jan Kruger and Frank Moesta, Chief Engineer for Future Programmes for Rolls-Royce Electrical and SVP Strategy and Future Programmes for Rolls-Royce Civil Aerospace, respectively, say each different electrical-power source is only useful in propelling certain classes and sizes of aircraft, as a primary or supporting propulsion method.

    However, the sizes and classes of aircraft which can be propelled by different electrical-energy sources overlap, in part, so that more than one source could theoretically be used to help power a given aircraft size or class.

    On the other hand, say Kruger and Moesta, non-electrical gas turbine propulsion (using Sustainable Aviation Fuels or traditional jet fuel) can be used to power everything from a small private aviation aircraft flying on short sectors to the largest widebody airliners, because of the high energy density inherent in jet fuel per unit of volume.

    This is why alongside Rolls-Royce's development in electric systems, the company is also looking at a combination of different solutions, maturing over different timelines including SAFs, electric, hybrid, and more efficient gas turbines, powering different missions and complementing one another, to help reach aviation’s decarbonization goals.

    Different Types of Electrical Propulsion

    Battery Technology: As a result of the relatively low energy density available from today’s lithium-ion battery technology, batteries are only “marginally useful” as a source of electrical propulsion power, says Thomassin.

    So, in Rolls-Royce’s continuum of electrical-propulsion methods, today’s batteries can only be used to provide primary electrical power for small, short-range aircraft, the many eVTOL urban air mobility craft now being developed all over the world, and potentially small commuter transports.

    For batteries ever to be considered as sources of power for primary electrical propulsion of any aircraft larger than these, battery technology will need to develop to allow energy storage densities far greater than those available today with lithium-ion batteries, Thomassin says.

    Battery technology is continuing to evolve fairly rapidly, so in the future it’s certainly possible that game-changing battery chemistry might become available that could be used to provide primary electrical propulsion for somewhat larger aircraft.

    Batteries Paired with Turboprops/Turbofans: Another electric-propulsion architecture envisaged by Honeywell for eVTOL craft and more conventionally shaped aircraft up to commuter-aircraft size would pair batteries with the primary turboprop or turbofan engines.

    The batteries would provide power via electric motors to the engines’ propellers or fans, either in the form of supplementary power during flight phases when peak power is required or as primary power in lesser-power phases such as cruise.

    Hydrogen Fuel Cell: Another type of energy storage system (ESS) for electrical power which is becoming available for aircraft propulsion is the hydrogen fuel cell.

    Still in its infancy for aviation use, hydrogen fuel cell technology should be able to provide enough electrical power to propel aircraft of up to regional-airliner size on their typical short-range missions, Rolls-Royce reckons.

    And it is not a stretch to imagine the same energy source could be used to power turboprops on missions of similar range.

    This is why Rolls-Royce is partnering with the Hyundai Motor Group on the development of electric propulsion systems based upon hydrogen fuel cells as an energy source for Hyundai’s Regional Air Mobility platforms.

    Hybrid-Electric Propulsion: The next class up of electrical-energy propulsion is hybrid-electric propulsion, a generic term used for a potentially flexible range of propulsion architectures.

    This type of propulsion represents a combination of primary gas-turbine propulsion and supporting electrical propulsion. The electrical power is produced either by hydrogen fuel cells, or by one or more supplementary turbo-generators, which will be small gas-turbine engines producing approximately the same power output as the auxiliary power unit in a large commercial airliner.

    But while the APUs in airliners are mainly used to power load compressors which provide bleed air to airliner cabins, as turbo-generator designs (new-design or modified from existing APU designs) such units would do away with their load compressors and be used entirely to generate electrical power.

    This would then be used to power electrical motors driving fans that would provide the aircraft with additional thrust during periods of peak power requirement, such as take-off, climb and final approach.

    During phases of flight requiring less propulsive power, the electrical power generated by the turbo-generator(s) would be used to recharge the aircraft’s batteries, so maximum electrical power would be available for when it’s needed.

    Next, read about the advantages of hybrid-electric engines to business aircraft.

    More information from:

    GE Aviation: www.geaviation.com
    Honeywell Aerospace: https://aerospace.honeywell.com
    Pratt & Whitney Canada: www.pwc.ca
    Rolls-Royce: www.rolls-royce.com

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