Hybrid-Electric Engines: The Advantages to BizAv

Two potential major advantages would potentially arise in using a hybrid-electric propulsion in business aircraft.

In Rolls-Royce’s view, hybrid-electric engines could be used to power every class of aircraft from an urban air mobility vehicle, all the way up to a single-aisle mainline airliner or a Mid-Size business jet.

More important still is that by using electrical power to provide some of the thrust required during peak-power phases of flight, the gas-turbine engines providing the aircraft’s primary propulsion source could be made smaller and less powerful than they would otherwise be, thus burning less jet fuel (whether SAF or fossil-derived).

Additionally, because electrical motors would be providing some of the peak-power requirement, less maintenance is likely to be required for the main gas-turbine engines, because they wouldn’t be working as hard throughout the flight. Electrical motors have far fewer moving parts and operate at much lower temperatures than gas-turbine engines, keeping maintenance costs low.

Did you miss Part 1 to this article? Read it here.

Hybrid-electric propulsion could also be used to support optimal engine sizing, such as for helicopters, suggests Jean Thomassin, Executive Director for New Product and Service Introduction, Pratt & Whitney Canada.

In terms of maximum power output, all aero engines are sized for the most extreme case in the operational envelope — which for a twin-engined helicopter would be the maximum power required from each single engine to keep the helicopter airborne should its other powerplant fail.

If the rotorcraft had a supplementary power and energy source producing additional electric propulsive power when required but for a small duration of time, each of the two main engines could be less powerful, thus burning less fuel, or the passenger count could be increased with the same engines, thus reducing the energy consumption per passenger.

One other major operational benefit is likely to arise from applying hybrid-electric propulsion architecture to gas turbine-powered business aircraft designs in the future.

If the hybrid-electric propulsion system — whether powered by a turbo-generator or anything else — can provide enough propulsive power for taxiing, take-off and initial climb purely from the aircraft’s batteries, then the aircraft’s primary gas-turbine engines needn’t be powered up to anything beyond flight idle for the initial phase of the flight.

Using only the aircraft’s electric motors or powertrain to the propellers to provide all the thrust for take-off and first-stage climb would make the take-off much quieter, says Alberstadt — potentially a huge advantage in seeking permissions for late-evening or early-morning operations at noise-restricted airports in densely populated areas.

Turbo-Generator Developments

Modifying an airliner APU or small turbine engine for use as a turbo-generator to provide supplementary electrical propulsive power for a larger aircraft is not as fanciful as it might sound. Both Rolls-Royce and Honeywell have already done so. 

Rolls-Royce has used a modified AE2100 turboprop engine as a turbo-generator driving a 2MW electric motor in its E-Fan X hybrid-electric power experiment, and an M250 helicopter engine in another project, say 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.

And Taylor Alberstadt, Global Sales and Marketing Lead for Unmanned Aerial Systems and Urban Air Mobility, Honeywell says his company has modified a 1,700shp HGT 1700 APU, normally used in the Airbus A350 XWB, by removing the load compressor it uses to provide bleed air for the A350 cabin.

Instead the gas turbine APU generates 1MW of electrical power to serve as primary propulsion for an eVTOL urban air mobility craft, or even a small business jet on a short-range mission. Like Honeywell and Rolls-Royce.

Pratt & Whitney Canada and GE Aerospace are deeply involved in collaborative, megawatt-class, integrated hybrid-electric propulsion R&D projects; P&WC with Collins Aerospace and de Havilland Canada, and GE in NASA’s Electrified Powertrain Flight Demonstration program.

And Rolls-Royce Electrical is now going a step further. In June, Rolls-Royce announced that its teams throughout Europe are developing a brand-new small turbo-generator (pictured left), sized to produce 500KW-1MW of electrical power. 

This entirely clean-sheet design (the systems of which will be completely integrated in each aircraft application with those of the primary gas-turbine engines producing most of the propulsive power) will offer smart power distribution.

This will enable aircraft to switch between power sources in flight and allow the turbo-generator to charge the aircraft’s batteries when it is not directly powering the engines’ propellers.

The new turbo-generator will provide enough primary propulsive force to fly an eVTOL aircraft between cities or a small commuter aircraft on island-hopping routes in locations such as Norway and among Scotland’s many islands, Rolls-Royce notes.

More Electric Aircraft

Beyond hybrid-electric power is what the aviation industry terms ‘more electric’ aircraft. This architecture will rely on gas-turbine engines to provide all, or almost all, of the propulsive power for the aircraft (as is the case today), and because of the high energy density of jet fuel it will continue to provide enough propulsive energy to propel the largest aircraft on the longest sectors.

So, in the future, the largest and longest-range airliners and the biggest, longest-range and fastest-cruise business jets are likely to fall into the ‘more electric’ class — which theoretically could remain the highest emitters of carbon gases and hydrocarbon particles.

However, ‘more electric’ is a somewhat vaguely defined term, and in practice it almost certainly will include aircraft powered by gas-turbine engines powered entirely by hydrogen (thus not emitting hydrocarbons at all).

In addition to describing aircraft which do not use bleed air to power their onboard systems, but use electrical power instead (e.g. the Boeing 787, the first truly ‘more electric’ airliner), the term could also include aircraft which divert some of their gas-turbine propulsive power during lower-power phases of flight to charge batteries.

Those batteries could provide some of the propulsive force required for take-off climb and final approach. Whether this theoretical propulsion-system approach will be adopted for business aircraft remains to be seen.

Any Possibilities for Retrofit?

The experts interviewed for this article all think that, while retrofitting existing business aircraft for all-electric or hybrid-electric propulsion might be possible in some cases, the operational and cost advisability of doing so will depend on the size, mission optimization, and basic design of the aircraft type.

In some cases re-designing the aircraft’s wings could make it more suitable for an electric, or hybrid-electric propulsion retrofit.

Smaller aircraft with simpler systems and lower power requirements might make the best candidates for potential retrofit, Thomassin suggests. Retrofit options might also vary for a given aircraft type, depending on the particular mission profile for which it is optimized, Moesta adds.

However, Alberstadt and Thomassin accord with the view of Kruger and Moesta that in most cases retrofitting a hybrid-electric or all-electric propulsion system will not make sense from either a cost or operational viewpoint.

The systems of already-built, in-service aircraft are already optimized for the existing designs of those aircraft. So changing the design of the systems to accommodate a new, more complex propulsion architecture is virtually certain to create weight penalties and efficiency-reducing aerodynamic and performance trades, Thomassin says.

In most cases it will make much more sense to involve a hybrid-electric or all-electric propulsion architecture from the outset as a fundamental part of a new, clean-sheet aircraft design. In that way the electrical-power element of the propulsion system can be integrated as seamlessly as possible with the design of every other part of the aircraft’s propulsion system, structure, and control and environmental systems.

That will save weight, and allow design optimization of the aircraft’s aerodynamics, systems and performance capabilities.

In that regard, hybrid-electric propulsion architectures will provide particularly interesting possibilities for different line-fit propulsion configurations for a specific aircraft design, in order to optimize the basic design for various different missions, in terms of payload and range performance. 

In some cases similar mission-specific optimizations might be possible for retrofits of existing designs too.

“The most interesting aspect of hybrid-electric propulsion is that it enables us to optimize platforms for different missions,” by varying given aircraft types’ distributed propulsion systems to modify those platforms in different ways for each desired mission type, says Kruger.

Likewise, having more than one energy source available for propulsion on a given aircraft “allows us to save energy in the same mission, because it provides variables which give us more levers to pull in that mission,” thus reducing the energy required to perform the mission.

Timing of Availability

All four major engine manufacturers are making considerable progress in their R&D efforts to develop demonstrator hybrid-electric propulsion systems. After completing all the ground and flight tests required for the OEMs and their collaborative partners to assess the designs’ and systems’ performances fully, they can then use the results of those tests to help shape future production hybrid-electric propulsion systems sized and optimized for different classes and types of aircraft.

Most of these demonstrator R&D programs are still about three years away from completion. And it will take each OEM at least another three years to design and develop any production engine emerging from the finding of the initial R&D programs, Thomassin estimates.

It will then take at least another three years to integrate the hybrid-electric propulsion system into any production aircraft design. So the first availability of an aircraft designed from the outset to be powered by a hybrid-electric propulsion systems will be “a decade from now, more or less,” Thomassin reckons.

Alberstadt agrees. “We won’t see a switch overnight — it [hybrid-electric propulsion] will be phased in gradually, with new designs,” he says. “It’ll probably be ten years out before we see it take a real foothold.”

The first new-aircraft designs appearing with hybrid-electric propulsion systems are likely to be smaller aircraft, the executives believe.

Some of the all-electric eVTOL aircraft designs now being developed are probably less than five years from being certificated and entering service. As a result, it’s possible that all-electric propulsion systems on smaller aircraft — perhaps including some more traditional business aircraft designs — may appear in the 2028-2030 timeframe, along with a slew of new eVTOL aircraft designs entering service in that period, Kruger and Moesta predict.

But after that, owners and operators of business aircraft can expect a near-revolution in the designs of new aircraft types. They are very likely to look different, some boasting several electrically driven fans on their wings or fuselages — and also perform differently.

The differences in operating cost, noise performance, maintenance cost and dispatch reliability potentially will be particularly notable.

Missed Part 1? Read The Future of BizAv: Electric Propulsion

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