In This Issue
Summer Bridge Issue on Aeronautics
June 26, 2020 Volume 50 Issue 2
The articles in this issue present the scope of progress and possibility in modern aviation. Challenges are being addressed through innovative developments that will support and enhance air travel in the decades to come.

Electrified Aircraft Propulsion

Thursday, June 25, 2020

Author: John S. Langford and David K. Hall

The past decade has seen the number of electric and hybrid electric cars sold increase from about zero to over 2 million per year at an annualized growth rate of over 60 percent (Hertzke et al. 2019). Is a similar revolution in store for aircraft? The rise of the flygskam (flight shaming) movement has created societal pressure to reduce flights that produce carbon emissions and raised both popular and regulatory interest in electric propulsion for aircraft.

This article reviews the prospects for widespread use of electrified aircraft propulsion (EAP) and concludes that a large-scale conversion as is currently seen in terrestrial use is unlikely in aviation. EAP is not a “drop-in” technology that can be easily retrofitted into existing designs. Indeed, its fundamental benefit is that it adds new dimensions to the aircraft design space. By blurring the lines between airframe and propulsion systems, EAP allows highly integrated designs that can be tailored to specific missions and performance objectives, and these designs are unlikely to resemble current aircraft.

Case Study: Electric Retrofit of a Commuter-Class Aircraft

Figure 1

Figure 1 shows what happens when a conventional commuter-class passenger aircraft is converted to operate on batteries. The results, based on a de Havilland Canada DHC-6 Twin Otter, are broadly applicable to this class of turboprop-powered aircraft (e.g., PC-12, Caravan, Q-400, ATR-72). Propeller power is generated by a combination of fuel-burning gas turbines and battery-powered motors. The degree of hybridization, µ, is the weight of the battery divided by the combined battery + fuel weight. When µ = 0 the aircraft is a conventional turboprop, and when µ = 1 the aircraft is entirely electric.

In the context of payload and range capability, electrification always makes the performance worse. This is because the energy density in current batteries, no better than 250 W-hr/kg (watt-hour per kilogram), is a small fraction of that for liquid hydrocarbon fuels (figure 2). Major increases in battery energy density—for example, a doubling to 500 W-hr/kg—improve the results but do not change the fundamental trend of reduced range capability relative to energy-dense hydrocarbon fuel.

Figure 2 

The situation improves if one considers energy efficiency, as in the lower graph in figure 1, showing the ratio of total energy required to the product of range and payload weight, a measure of energy usage normalized for every mission. Battery-electric propulsion can achieve only a fraction of the range of the baseline turbo­prop, but it is considerably more efficient at those ranges. The achievable range can be increased with the decreasing µ, but energy efficiency decreases.

The fundamental trade is between the power-­conversion efficiency of an electric system, which can be a factor of 2 higher than for a gas turbine engine, and the energy density of hydrocarbon fuel, which is an order of magnitude higher than for batteries. The implication is that, for very short ranges, electric propulsion might have an energy efficiency advantage.

Energy efficiency is also an excellent proxy for CO2 emissions, which basically scale with the mass of fuel burned. For a battery aircraft, emissions come from the power grid rather than the onboard turbine, and hence the source of power for the grid matters. Emissions in the Northwestern United States, where the grid has a high fraction of hydro power, are lower than in the Midwest, where grid power comes mostly from coal-fired generators.

Energy efficiency is not the sole determinant of aircraft operating costs. Direct operating costs (referred to as cash aircraft-related operating costs, or CAROC) are measured in dollars per available seat mile and include fuel, maintenance, crews, insurance, and airport fees. Direct operating costs account for between one-third and two-thirds of aircraft-related operating costs (AROC), which include costs related to acquiring and owning or leasing the aircraft. These vary with the acquisition cost of the aircraft and its use; commercial airlines use their assets many more hours per year than corporate, charter, or private operations.

It is generally accepted that to successfully launch a new aircraft type, the CAROC must be at least 15 percent below that of the aircraft it is replacing. Since fuel is typically 10–20 percent of AROC, figure 1 suggests that this will be possible for EAP only at extremely short ranges; hence the interest in trainers or electric vertical takeoff and landing (eVTOL) air taxis with short ranges.

All-Electric Aircraft

Urban Air Mobility

The potential efficiency and cost benefits of electric propulsion at short range have led to a rapid rise in interest and investment in eVTOL for urban air mobility (UAM). For example, the ride-hailing company Uber has developed plans to fly riders to their destina­tion via eVTOL. Building on NASA research, the company envisions four-passenger vehicles with a range of 60 miles, cruise speeds of at least 150 miles per hour, and a battery-electric propulsion architecture (Uber 2016). The latter not only speaks to the CAROC benefits but also aims to address issues of carbon footprint, local emissions, and noise.

Figure 3

To address the UAM market, Aurora Flight ­Sciences developed a passenger air vehicle (PAV) prototype (figure 3) to demonstrate the feasibility of an electric propulsion system and autonomous operations. The PAV is a separate-lift-and-cruise configuration: vertical takeoff and landing are achieved with multiple lift rotors, which are then shut off for efficient, wing-borne, propeller-driven forward flight in cruise. This design allows for VTOL operations in an urban environment while maximizing range with a battery energy storage system. The feasibility of UAM missions depends not only on electric propulsion technologies but also on novel vehicle configurations like the PAV to meet challenging new efficiency, emissions, and community noise requirements.

Commercial Aircraft

At present, it appears that EAP makes economic sense only for aircraft that fly extremely short ranges (50–200 miles), such as general aviation aircraft, especially training aircraft, and eVTOL air taxis. Is there any potential for large aircraft?

For commercial transports, the large amount of ­energy required to move hundreds of passengers hundreds or thousands of miles poses a challenge for battery energy storage. The difference in energy density between batteries and hydrocarbon fuels means the range of an all-electric transport will be significantly reduced relative to an equivalent gas-burning aircraft, as in figure 1 for smaller aircraft. The high efficiency of current large engines—in many cases emitting less CO2 per unit power produced than the grid from which the competing batteries would be charged—further complicates the value proposition of an all-electric airliner (Epstein and O’Flarity 2019).

Figure 4 

The technical challenge of battery-powered transports is illustrated in figure 4, which shows contours of the battery-specific energy (BSE) required to enable an all-electric aircraft with a given payload fraction—the ratio of payload weight to aircraft maximum takeoff weight—and range. Data points for the payload and range capability of existing aircraft are included.

Even with generous assumptions about aero­dynamic and propulsive efficiency, structural weight, and required reserves, a specific energy over 300 W-hr/kg is required to enable the capability of the 19-passenger DHC-6-400 Twin Otter. Taking into account that this value of BSE includes extra weight of packaging and thermal and safety protections, which can discount the cell-level performance shown in figure 2 by up to half, it becomes clear that battery-powered transports are more than 20 or 30 years away without breakthroughs in battery technology inconsistent with the historical trend.

Hybrid Electric Aircraft

Rather than attempting to displace aircraft fuel with batteries, there has been growing interest in hybrid electric concepts, i.e., propulsion systems that can draw from energy stored both in batteries and in hydro­carbon fuel. Hybrid electric vehicles in the automotive industry enabled step-change improvements in fuel efficiency and paved the way for the current generation of all-­electric vehicles. Could the same model apply to electrified aviation?

In answering this question, it is important to point out the differences between automobile and aircraft propulsion. First, terrestrial vehicle energy requirements are much less sensitive to weight than aircraft. Second, and perhaps more important, hybrid and all-electric cars enjoy energy savings due to regenerative braking. For aircraft, there is little opportunity for analogous regenera­tive deceleration: the energy recovery potential of an aircraft cruising at high altitude is much smaller than the energy used to overcome irreversible drag over the course of a commercial transport mission. Further, aircraft already practice regeneration without an electric system: during descent, the engine power is reduced and the glide slope extends the range by using the aircraft potential energy to generate extra thrust.

Strategies for hybrid electric aircraft are thus centered on improving gas turbine performance through integration of a supplemental electrical power source. Gas turbines have long been preferred for transport aircraft propulsion because of their high efficiency, power-to-weight ratio, high-altitude capability, and low emissions. One drawback is that they are most efficient at their highest power, and designing the engine to meet peak power requirements during climb reduces the maximum achievable efficiency during cruise. Hybrid electric systems have the potential to remove this physical constraint by augmenting the power of the turbine during high-power conditions, enabling the engine to be optimized for peak performance during cruise, where most of the fuel is burned, at the cost of a minimal battery electric system. The inclusion of a high-power electric system for propulsion also opens up possibilities for new technologies like electric taxiing and vehicle system-level energy management, based on close integration between propulsion and aircraft electrical systems.

A recent study by United Technologies suggests that such a hybrid system for a future single-aisle transport could reduce fuel burn by 4.2 percent and energy consumption by 0.3 percent relative to an advanced-technology turbofan (Lents and Hardin 2019). Another claims a hybrid reengine of a regional turboprop could reduce cruise fuel consumption by 25 percent, albeit at reduced range (Bertrand et al. 2019). The difference in results highlights the importance of both mission (electrification may have a greater benefit at short range) and the baseline for comparison; when considering new aircraft designs, one must be careful to compare equivalent levels of technology and equivalent mission requirements between electrified and conventionally powered aircraft.

Distributed Electric Propulsion

Discussion to this point has focused on EAP concepts with some level of battery energy storage. ­Another potential benefit of electrification, independent of energy storage medium, is the decoupling of mechanical power generation and thrust generation processes. Doing so would continue a decades-long trend in aircraft engine design: the shift from turbojets to turbofans improved efficiency by using a larger mass of lower-velocity bypass flow to generate thrust, and the recent introduction of geared turbofans relaxed the speed constraint on the fan and turbine, allowing both to be designed at their most efficient speeds.

The concept of distributed electric propulsion (DEP) takes this decoupling one step further, by introducing flexibility in the number and arrangements of propulsors (propellers or fans). This benefits the propulsors, which follow a cube-squared scaling: the weight of a propulsor is approximately proportional to its diameter cubed and, for fixed jet velocity, the thrust is approximately proportional to the diameter squared. A distributed system with many propulsors will thus weigh less than a single propulsor producing the same thrust at the same jet velocity.

Alternatively, distributed propulsors provide for a larger total fan area and thus lower jet velocity and higher efficiency than a single propulsor with the same total weight, although in this case the benefit also trades against an increase in nacelle drag, which scales with fan area. Electrification allows distributed propulsors to be driven by a single gas turbine core, which, experience has shown, will be more efficient than multiple smaller cores providing the same net power (Lord et al. 2015). These benefits must be traded against the added weight, transmission losses, and complexity of an electric power distribution system, which have to be evaluated at the overall vehicle performance level.

A Turboelectric VTOL Model

A notable recent example of a DEP design is the DARPA XV-24A concept developed by Aurora Flight Sciences. The novel tilt-wing/tilt-canard vehicle configuration (figure 5) arose in response to challenging requirements for efficient vertical lift capability and high-speed cruise at 400 knots, which are not achievable with conventional rotorcraft or tilt-rotor designs. DEP was a key enabling technology that allowed large propulsor disk area for efficient vertical takeoff without a large-diameter rotor and associated aerodynamic disadvantage at high speed.

Figure 5 

The envisioned propulsion system had a turboelectric powerplant: electrical power for motor-driven fans was developed by electric generators coupled to a gas turbine engine with no batteries. This architecture leverages the configuration benefits of DEP while achieving the payload and range potential with energy-dense hydrocarbon fuel. The concept was revolutionary, but it was unable to achieve the needed full power capability because of component limitations and was cancelled by DARPA before it could demonstrate the benefits of DEP at full scale.

Approaches to Propulsion-Airframe Integration

Beyond improving the efficiency or weight of the propulsion system, DEP opens up potential benefits in overall vehicle performance through propulsion-airframe integration. Rather than designing the engine as an isolated, thrust-producing system, integrated design of the combined propulsion-aircraft system may unlock aerodynamic efficiency improvements for both, because DEP provides the designer with the flexibility to dis­tribute and integrate propulsors in the vehicle to an extent not possible with conventional propulsion.

Boundary Layer Ingestion

One such strategy is propulsion with boundary layer ingestion (BLI). The basic principle of BLI is for the engine to produce thrust by ingesting and accelerating air in the so-called boundary layer near the surface of the vehicle. Friction reduces the velocity of this flow in the frame of reference of the vehicle, which means the same thrust can be produced using less power.

Wind tunnel experiments carried out by MIT, Aurora Flight Sciences, and Pratt & Whitney at the NASA Langley Research Center showed a power savings of 10 percent by ingesting approximately 17 percent of the boundary layer of an advanced twin BLI-engine vehicle concept (Uranga et al. 2017). Analysis shows that the benefit increases with the amount of boundary layer ingested, and DEP provides a means to do this.

Figure 6 

Figure 6 shows two NASA concepts with BLI enabled by DEP. One (top image) is a conventional tube-and-wing aircraft configuration with a turboelectric propulsion system and a motor-driven BLI fan at the back of the fuselage. The other (middle image) is a hybrid wing-body concept with distributed BLI propulsors ingesting a large fraction of the vehicle’s upper surface boundary layer.

Blown Lift

Another DEP-enabled propulsion-airframe integration strategy is blown lift, which positions a wing and propulsor relative to each other such that the pressure field induced on the wing and the deflection of the propulsor jet increase the overall lift beyond that of the isolated airfoil.

The benefits of short landing and takeoff are well known and implemented in aircraft such as the DHC-6 Twin Otter, used in the hybrid payload-range analysis above, but these have been limited by the amount of blowing that can be achieved with only two or four propulsors. DEP opens the door for super-short takeoff and landing with smaller propulsors distributed along a larger segment of the wing’s span, enabling short field performance that begins to make it competitive with more technically complex eVTOL concepts.

Alternatively, the blown wing benefit can enable typical field lengths with less wing area and higher wing aspect ratio, leading to improved aerodynamic efficiency during cruise. This is the idea behind the NASA X-57 Maxwell concept (bottom image in figure 6), which claims reduction in cruise energy consumption rate by a factor of 4.8 relative to an unmodified, conventionally powered aircraft (Borer et al. 2016).

Conclusion and Outlook

The potential benefit of electrified aircraft propulsion is the flexibility it brings to the aircraft design space. This benefit comes at the cost of components with increased weight and transmission inefficiencies, but there appear to be a variety of aircraft missions and vehicles that can leverage electrification in different ways. The conclusions drawn here do not differ materially from those of a 2016 consensus study of the National Academies of Sciences, Engineering, and Medicine, which reported that battery-powered propulsion is well suited only to small vehicles with short range, and that distributed propulsion and boundary layer ingestion could yield significant performance benefits to commercial transport aircraft (NASEM 2016, pp. 51–70).

The potential for hybrid systems to improve the performance of gas turbine engines and propulsion-airframe integration effects such as blown lift also warrants further investigation. Other options for electrified propulsion, such as solar power or fuel cells, are beyond the scope of the discussion here, but they similarly offer benefits for unconventional vehicles or missions by introducing new options to the vehicle design space.

Challenges to implementing the vision for EAP remain. Urban air mobility may be feasible with current technology, but only just, and advances in the technology to improve the capability of UAM vehicles are necessary before transport-class hybrid concepts become competitive. Smaller concepts will require tens or hundreds of kilowatts, and larger transports could require a megawatt or more of electric power capability. Electric machines of these scales exist today for various ground-based applications, but new designs are needed to meet the stringent weight, efficiency, and reliability requirements for aviation.

All in all, there is reason to be cautiously optimistic about the future of EAP. The convergence of new technologies and new vehicles will allow new modes of mobility for the traveling public. The required innovations at the interface of aircraft, engine, and high-power electronics technologies will necessitate new interactions between established industries and generate opportunities for new entrants in an emerging industry. Finally, and not least, new technologies will inspire the next generation of students, ­researchers, scientists, and engineers in their drive to address the challenge of sustainability for aviation’s second century.


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About the Author:John Langford (NAE) was founder and CEO and David Hall leads the Propulsion Group, both at Aurora Flight Sciences.