Electrically-powered aircraft convert electrical energy from batteries and deliver this energy to drive an electric motor connected to a propeller (or a fan), producing propulsive thrust for flight. Consequently, such aircraft can potentially have a higher net energy efficiency than fossil-fueled propulsion systems, i.e., an internal combustion engine (ICE), such as a piston-prop, turboprop, or turbofan. Indeed, the reductions in net energy consumption can be desirable, with electrically-powered aircraft potentially using less than half of the energy per power delivery unit than possible with an ICE. However, for an aircraft, what matters is not the power or energy required for flight per se but the power or energy required per unit of flight weight, i.e., power-to-weight ratio or energy-to-weight ratio.
Electrically-powered aircraft have, at least, for now, been limited to various types of Unoccupied Aerial Vehicles (UAVs) or drones and, more recently, small light-sport aircraft, the latter being shown in the photograph below. However, the upward scalability of electrically-powered aircraft is unattractive, mainly regarding the weight growth and associated high volume of the needed batteries. Nevertheless, this situation will likely change as the required technologies mature in the coming decades. Electrically-powered aircraft can improve energy efficiency and reduce greenhouse gas (GHG) emissions because of their commensurately lower “carbon footprint.” GHG emissions contribute significantly to the depletion of the ozone layer in the atmosphere and so to concerns about global warming.
Consequently, some industry leaders have annunciated that electric propulsion will be the future paradigm of the aerospace industry, if not the apotheosis of aviation technology. But, of course, the first rule of success is to have a vision! This vision, however, is unlikely to be achieved in the shorter term, and not just for technical reasons. While improvements in battery technology have begun to decarbonize terrestrial transport, electrifying the world of aviation poses many significant technical, operational, economic, and educational challenges.
What is a “carbon footprint”?
A carbon footprint refers to the amount of greenhouse gas emissions, primarily CO2, released into the atmosphere as a consequence of a specific human activity. Everyone has a carbon footprint – even breathing creates a carbon footprint. In general, it can be used to measure the impact that individuals, organizations, vehicles, aircraft, etc., have on climate change. Various activities contribute to carbon footprints, including burning fossil fuels, industrial processes, deforestation, and producing (and disposing) of goods and services. By assessing carbon footprints, such as from aircraft, it becomes possible to identify areas where emissions can be reduced or mitigated to lessen the longer-term environmental impact.
Nevertheless, as shown in the image below, ambitious proposals are being fielded by NASA and other organizations to do the needed research and achieve electric propulsion goals for aviation. These goals include replacing piston-prop and turboprop engines with electric motors or using ducted fans driven by electric motors instead of turbofans to power commercial airliners. However, the aviation industry has a history of long innovation cycles, at least a decade, primarily because of the significant investments, the time needed for research and development, and the lengthy certification processes for using new technologies on aircraft. Therefore, it is unlikely for the foreseeable future that passengers will be flying on an electrically-powered airliner.
Another challenge for the electrification of aviation, particularly in the commercial sector, is the relatively long time needed to recharge batteries. Refueling an aircraft using Avgas (aviation gasoline) or Jet A-1 (a form of kerosine formulated for aircraft use) is relatively quick, taking 10 to 30 minutes for even the biggest airliners flying on transoceanic routes. However, recharging a battery-powered airliner would take significantly longer, perhaps of the order of hours, and an airliner sitting on the ground for that long makes no economic sense for an airline.
Continued research and development efforts by engineers are necessary to push the boundaries of electric motor and battery technologies to realize the higher energy efficiencies possible with electrically-powered aircraft and prove their economic viability. An attractive byproduct of this technical effort is undoubtedly the reduction and eventual elimination of GHG emissions. However, in the shorter term, a combination of approaches, including sustainable aviation fuels (or SAFs), improved and perhaps new novel aircraft designs, hybrid-electric propulsion, or hydrogen fuel cells, will likely be necessary. In the meantime, engineers need to be well-versed in the fundamentals of this emerging and rapidly growing field of electric propulsion for aircraft.
- Understand the benefits and challenges of developing electrically powered aircraft.
- Be aware of the significance of greenhouse gas emissions and “carbon footprint.”
- Know the meaning of the “energy density” of batteries.
- Appreciate some of the performance and weight tradeoffs of electrically-powered aircraft.
- Understand some of the capabilities and limitations of battery technology.
- Appreciate the significance of the energy-to-weight ratio for an electric aircraft.
- Know what an eVTOL aircraft is and why they are being pursued for urban aerial mobility.
Why Electric Propulsion?
As with all forms of electrified transportation, the foundational technologies for engineers to understand are the characteristics of electrical energy storage systems (i.e., batteries) and the means of converting electrical energy into mechanical work for propulsion (i.e., motors). In addition, engineers need to be aware of the necessary infrastructure to supply the required electrical power as well as the economic and other trades involved. Electrification efforts can also focus on new engineering solutions to replace traditional pneumatic and hydraulic systems on aircraft with electrically-driven alternatives.
The energy required for an electrically-powered aircraft comes from batteries through an electrochemical energy conversion process to create a current to drive an electric motor. The motor then creates mechanical energy to drive a propulsor in the form of a propeller (or fan), as shown in the figure below, although other solutions may be possible. Batteries and electric motors are certainly familiar technologies, having been around for centuries. However, during the last decade, sufficient technical progress has been made to make these essential elements viable enough to power aircraft in terms of the higher energy density in the batteries (i.e., more energy storage per unit weight) and by using better motors (i.e., higher power output per unit weight). A further consideration is the need for a cooling system, which can increase the system weight by 20%.
The upside is that this approach to aircraft propulsion is significantly more energy efficient than aircraft powered by an ICE, especially during the cruise phase of the flight. One reason is that electric motors have higher efficiency than an ICE in converting the available energy into mechanical energy in the form of shaft torque and hence into usable power. Another reason is that electric motors have excellent power-to-weight ratios compared to an ICE. The downside is that the energy source, the batteries, which by any standard are heavy, have much less energy density than is contained in fossil fuel. Therefore, there must be a lot of heavy batteries on the aircraft to supply the energy needed for flight.
However, the net energy balance must always be considered, including the trades between higher electrical efficiency, net weight, flight requirements, and overall operating costs. Regarding costs, the relatively low price of electric power in terms of $ per kWh is attractive. Yet, a significant consideration is also the high battery replacement cost because of the relatively limited recharge cycles with a Li-ion battery (less than 3,000 recharge cycles).
What are greenhouse gases (GHGs)?
- CO2: Carbon dioxide is one of the primary greenhouse gases emitted through human activities, such as burning fossil fuels, deforestation, and industrial processes.
- CH4: Methane is a potent greenhouse gas released from various sources, including natural gas and oil production, livestock farming, and the decomposition of organic waste.
- N2O: Nitrous oxide is a greenhouse gas emitted from agricultural and industrial activities and the combustion of fossil fuels and solid waste.
- F-gases: Fluorinated-gases refer to a group of chlorofluorohydrocarbons (CFC), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6). These gases have very detrimental effects on the ozone layer, contributing to global warming.
Battery Energy Density
When engineers discuss electrically powered aircraft, the term “energy density ” inevitably comes up. In energy storage applications, the energy density is the ratio of the energy stored compared to the volume or weight of the storage medium. To this end, energy densities are often quoted in terms of gravimetric energy density and volumetric energy density, which are often referred to, in general, as specific energies. For an aircraft of any type, the weight of the energy storage medium is critically important because the power and energy required for flight increase with the square of its weight. It costs energy to carry energy, especially in the case of batteries, which represent such a high fraction of the total weight.
An aircraft’s power and energy demands are much more significant than a terrestrial vehicle. Indeed, the power and energy required for a small aircraft is about one order of magnitude higher than a road vehicle of the same gross weight, such as an average-sized family saloon. Therefore, to make an aircraft practically and economically successful in carrying a payload over any required distance, a high energy density in the fuel and low specific fuel (or energy) consumption of the engine or motor are crucial. For conventional aircraft propulsion using fossil fuel, such as Avgas or Jet A-1, the energy is stored in liquid form inside the fuel tanks; this energy is then liberated by combustion inside the engine. For electric propulsion, the energy is stored in batteries, which is liberated through an electrochemical process to supply a current to drive the shaft of an electric motor.
Gravimetric Specific Energy Density
The mass or weight or “gravimetric” specific energy density of a battery is defined as
where is the stored energy (i.e., the ability to do work), and is the mass of the battery containing the stored energy. Remember that weight is mass times acceleration under gravity, so one has to be careful with units!
VOLUMETRIC SPECIFIC Energy Density
The volumetric specific energy density of a battery is defined as
where is the battery volume occupied by the stored energy.
Units of Energy Density
For all energy storage devices, the available energy is usually quoted in terms of energy available for a certain unit of time (usually one hour), i.e., in units of Watt-hours (Wh) or kilowatt hours (kWh) in SI units. Notice that energy has units of work (i.e., force times distance) or base units of M L2T-2, which is equivalent to a Joule (N m) in SI units or foot-pounds (ft-lb) in USC units. One Watt is a Joule per second, so one Watt-hour equals 3,600 Joules of energy. In USC units, energy output can be quoted in horsepower-hour (hp-hr). However, this latter unit is rarely used, the SI system being the international preference for electrical energy and power, even in the U.S.
Comparison with Fossil Fuels
So, how do these values stack up with fossil fuel? On the one hand, the energy density of jet fuel is approximately 11.9 to 12.5 kilowatt-hours per kilogram (kWh/kg), depending on the formulation and grade. The corresponding volumetric energy density is about 9.69kWh/Liter (or kWh/L). The energy density of Avgas can also vary depending on the specific formulation and grade and is approximately 12.2 to 12.8 kilowatt-hours per kilogram (kWh/kg). On the other hand, Li-ion batteries have a gravimetric energy density of only about 0.22 to 0.25 kWh/kg, which is 50 times less than jet fuel, and a volumetric energy density of about 0.69 kWh/L, which is 14 times less than jet fuel. The figure below makes the point that the mass (and weight) of the batteries is a more critical issue for an aircraft than the battery volume. However, both weight and volume are apparent concerns in the design process.
Therefore, one of the most significant challenges in the electrification of aviation now becomes apparent: the relatively low amount of energy contained in storage batteries compared to the energy contained in fossil fuels, as well as the higher weight penalty in storing that energy. For an aircraft, weight means everything and is one of the so-called “killers” of performance, the other being aerodynamic drag. However, these concerns are also offset by the higher energy conversion energy rate of an electric motor compared to an ICE, although this benefit must still be addressed on a system level in conjunction with the type of propulsor, e.g., propeller or a fan.
What is a liter or a litre?
A liter (in the US) or litre (elsewhere) is a unit of volume used in the SI system. A liter is equivalent to 1,000 cubic centimeters (cm3) or one cubic decimeter (dm3), which is a cube with sides measuring 10 centimeters (0.1 meters) in length. The symbol for a liter is “L” or sometimes “l.” A liter is equivalent to 1.057 quarts or 0.264 gallons in USC units. The unit of a liter is widely used for measuring the volume of liquids, but remember, it is not a base engineering unit; the base unit of volume is meters cubed (m3).
Considerations for Electric Propulsion
The uptick in the interest and development of electric aircraft is driven by the general goals of improving energy efficiency and reducing GHG emissions. To the flying public, however, the cost of a ticket to fly from one place to another matters to them, not so much whether they care that they generate a substantial carbon footprint in the process. Furthermore, the core technical issue for an airline is improving aircraft efficiency and reducing seat-mile costs, i.e., transporting one passenger by one mile. The airlines will quickly align if this goal requires electric propulsion as the next step.
In the meantime, the International Civil Aviation Organization (ICAO) continue to push for tighter regulations that will lead to reduced GHG emission from commercial aircraft. Regulations can help drive technical developments in the needed directions. Yet, at the end of the day, it still very much remains an economic as well as a regulatory problem. But in reaching these worthy goals, engineers must continue to address several key issues and technologies in developing electrically-powered aircraft.
Batteries are a crucial component of an electric aircraft, which provides the source of the electrical energy required for flight. Li-ion batteries are commonly used, but all types of batteries are heavy, and battery technology still has a long way to go before they can have the energy densities needed for aircraft. Other battery formats and chemistries are also being developed by engineers to improve energy storage capabilities.
Electric motors convert electrical energy into mechanical torque at a shaft and then into propulsion using a propeller or a fan. They can be of various types, such as brushed or brushless, synchronous, or induction motors. Electric motors can be 90% or more efficient, the 10% loss being mostly thermal losses, so further efficiency improvements may be difficult to realize. However, advances in the power-to-weight ratio are still possible, which is the metric that matters for an aircraft.
Thermal management systems are essential to regulate the temperature of the batteries, motors, and other electrical components. Electrical current draws are high for aircraft, and the batteries can get hot. Proper cooling or thermal mitigation measures must be implemented to ensure optimal performance and prevent overheating or other temperature-related issues. A worst-case scenario is that overheating could cause failure or a fire, which is inevitably catastrophic for an aircraft.
Electrically powered aircraft will require a suitable charging infrastructure at airports for recharging their batteries. These charging stations must accommodate an aircraft’s high power and rapid charging demands. Merely extending the existing infrastructure for fast-charging electrically-powered terrestrial vehicles, which are increasingly found at gas (petrol) stations worldwide, cannot be considered a viable solution for aviation purposes.
Li-ion batteries are expensive in terms of specific energy and replacement costs, which are also tied to the number of usable recharge cycles, which may only be between 2,000 and 3,000 cycles, even for the best batteries. In 2021, the cost of Li-ion batteries for non-aviation applications was about $250/kg, and for aviation applications (requiring additional certification) was about $500 per kWh. Additionally, it is essential to consider that the total cost of implementing Li-ion batteries in aviation involves not only the cost of the battery itself but also the cost of system integration, battery monitoring systems, safety measures, and numerous paperwork processes. Reducing aviation battery costs will require a combination of technological advancements, economies of scale, and industry investments. Indeed, continued basic research into battery technology will be crucial for achieving significant cost reductions in the future.
An electric motor is a relatively simple electro-mechanical machine, at least in principle, which converts electrical energy into shaft torque and rotational mechanical energy. Electric motors operate through the interaction between the magnetic field and the electric current in wire windings to generate torque at the motor’s shaft, as shown in the animation below. Motors used for aviation must be designed with a high power-to-weight ratio, just like any other propulsion system.
One significant benefit of the electric motor over other methods of producing rotational torque, such as from an ICE, is that they are much more energy efficient. Typically, electric motors are over 90% efficient, while ICEs using fossil fuels are between 30% and 50% efficient. Electrical motors are also lightweight, compact, mechanically simple, and can provide almost instant increases in power when required.
The power or strength of electric motors is quantified in terms of their specific power density (measured in kW/kg), which is equivalent to a power-to-weight ratio. Today’s state-of-the-art electrical motors can achieve power-to-weight ratios from 5 kW/kg to as much as 7 or 8 kW/kg, and further improvements are still possible. Such power-to-weight ratios are significantly higher than those achievable with an ICE.
Specialized types of electric motors, such as the brushless and radial-flux designs, provide various advantages over the primary motor design. One design of particular importance for aviation use is the axial-flux motor; its main advantage is higher power density and electrical efficiency. Axial-flux motors are also compact, so they can be used in applications where space is limited, which is typical of an aircraft. In addition, it is possible to stack multiple motor units together to achieve the desired level of net power output.
Electrically-powered aircraft and their operation set unique and stringent requirements for the design of batteries. The batteries must be light, compact, quickly rechargeable, and able to provide the high current and power delivery required for all phases of flight, especially during takeoff and climb. They must also have protections to ensure safe operation and thermal control.
Rechargeable lithium-ion (Li-ion) batteries are the dominant type of battery being used today. They have become ubiquitous, powering everything from smartphones to laptop computers to electric cars to airplanes, mainly because they have the highest specific energies of any other type of battery. As previously discussed, specific energies are measured in terms of gravimetric energy densities of kWh/kg or volumetric energy densities of kWh/L, both metrics being important for aviation applications.
Over the last two decades, advances in understanding battery chemistry have more than tripled the energy density of Li-ion batteries at the cell level. The specific gravimetric energies of Li-ion batteries have increased from 0.08 kWh/kg (circa 2001) to 0.256 kWh/kg (circa 2022), with corresponding volumetric energy densities increasing from 0.2 kWh/L to 0.7 kWh/L. However, as previously discussed, these values are still very low in comparison to Avgas or Jet A-1 fuel, which have a specific energy that is nearly 50 times higher at about 12 kWh/kg and an energy density that is about 14 times higher at about 9.69 kWh/L.
What is lithium?
Lithium is a chemical element with the symbol Li and atomic number 3. It is the lightest metal and belongs to the periodic table’s alkali metal group of elements. Lithium is highly reactive and has a silvery-white appearance. It is commonly found in small amounts in the Earth’s crust and primarily obtained from spodumene, petalite, and lepidolite. Lithium has various industrial and commercial applications, one of which is in rechargeable lithium-ion batteries.
Battery Concerns for Aviation
There are several concerns that are important when it comes to the use of batteries on aircraft, including installation losses, thermal effects, longevity, safety concerns, and costs.
The electrical metrics quoted for Li-ion batteries are also better than “pack-level” metrics, which must consider installation losses. These losses include the “hidden” weight of the wiring from the batteries to the motor, insulation, structural materials, and the cooling systems required. Still, they will not otherwise contribute to its energy storage capacity. Such considerations can add at least 20% to the cell-level battery weights. Therefore, it is essential for aircraft design that allowances for such factors be included in the weight estimates and performance evaluations of electrically powered aircraft.
High Discharge Rates
For electrically-powered aircraft, the battery discharge rates are much higher than they would be for an electric road vehicle. One problem is that they also get hotter when the batteries discharge quickly. Heat generation shortens the battery’s life and requires specialized cooling systems to mitigate a temperature rise. Aircraft batteries will likely go through multiple deep discharges and thermal cycles per day. The high power demands of takeoff and climb-out will result in exceptionally high battery discharge rates and thermal management issues, which, if not appropriately controlled, can seriously reduce the number of duty cycles a battery achieves.
Li-ion batteries can be dangerous if not carefully monitored and protected with temperature, voltage, and current sensors monitoring the condition of each battery cell using a battery management system. Because a typical battery pack to power an aircraft may have tens or hundreds of thousands of individual cells, just as many protection sensors and circuits are needed. The reported incidents involving Li-ion batteries in aviation have been attributed to faulty cells exceeding their safety limits, often leading to a phenomenon called thermal runaway. Thermal runaway is a chain reaction within a battery pack, where exothermic reactions occur in one cell and then spread to neighboring cells, potentially causing a fire or explosion of the entire battery. This situation can pose significant risks to an aircraft and its occupants if not being catastrophic.
Hidden Carbon Footprint
The carbon footprint of the Li-ion battery production process is an important consideration when assessing the overall emissions from electric aircraft. The production of batteries causes its own environmental and climate issues. Most of the global lithium supply comes from China and Chile, where extraction methods have been heavily criticized for causing environmental damage. Lithium, a silvery metal, is not easily obtained from mining and can take 500,000 gallons of water to extract one ton of lithium from the earth, water that becomes contaminated and cannot be released back without treatment.
The carbon footprint created by manufacturing Li-ion batteries is difficult to assess and could be considerable, although it is likely to be offset by carbon savings in the long run. GHG emissions from battery production are typically 60 kg CO2/kWh. However, it is worth noting that the emissions from battery production are a one-time upfront cost, whereas fossil-fueled aircraft emit GHGs throughout their operational life. The decarbonization of the electric grid system will also play an essential role in reducing the overall carbon footprint of electric aviation.
Battery Replacement Cycles
The frequency of battery replacements for an electrically powered aircraft still needs to be determined. Most research on battery aging has focused on passenger cars, for which many have now reached the end of their useful lives. Battery usage in an electric aircraft will differ from electric road vehicles, partly because of the higher energy, current draw rates, and accompanying thermal factors.
Without specific battery age modeling for aircraft, a Li-ion battery has a typical life span of 3,000 duty cycles. This roughly equates to an aircraft performing four daily flights for two years. Once this lifespan is reached, the battery must be discarded and replaced. It is likely that future certification requirements, such as under FARs or EASA rules, will require that the batteries be replaced more frequently than 3,000 cycles to ensure a safe operational life without the possibility of failure, i.e., a standard regulatory safe-life policy used for many other aircraft components.
Future Battery Technology
Further developments in battery technology have goals of up to 1 kWh by 2050. In the shorter term, commercially produced high-energy density Li-ion batteries that use cathodes made of lithium-nickel-manganese-cobalt-oxide or lithium-nickel-cobalt-aluminum-oxide can achieve energy densities up to 250 Wh/kg. Further research and commercial development of Li-ion batteries could potentially reach up to 0.5 kWh/kg at the cell level by 2030.
However, achieving Li-ion battery energy densities over 0.5 kWh/kg will require much more significant technical advancements in battery technology and may even need the development of different battery chemistry. Based on the pace of current research on batteries and the time it takes for this research to be realized in production batteries, energy densities greater than 0.5 kWh/kg are still several decades away.
Where does the electric power come from if I plug my electrically-powered airplane into an outlet?
The majority of electricity in the U.S., about 80% of the total, is still generated from traditional fuel sources such as coal, natural gas, and oil, which have a considerable carbon footprint. Wind energy has seen significant growth in the U.S. and now accounts for around 10%. Solar power represents about 3% of total electricity generation. Hydroelectric power accounts for roughly 7%. Biomass (such as wood, agricultural residues, and dedicated energy crops) and geothermal energy contribute less than 1%. Therefore, when you plug your airplane in for a recharge in the U.S., you still generate a substantial carbon footprint. Germany is the global leader in renewable energy adoption, and only about 52% of its electrical energy use comes from traditional sources.
Power Chain Efficiency & Weights
It is essential to understand better how an electrically-powered aircraft’s power and the associated weight compare to what might now be called a “traditional” fossil fuel-powered aircraft. A modern turboprop or turboshaft engine has a net mechanical and thermodynamic efficiency of about 50%, i.e., 50% of the fuel energy can be converted to useful work to produce power at the engine shaft. A piston engine (sometimes called a piston-prop or piston aero-engine) has a somewhat lower net efficiency of about 35%. However, an electric motor’s net mechanical and thermal efficiency is much better, approximately 90%; this means that the motor converts 90% of the available energy into usable power. Therefore, one may be tempted to replace an ICE and its fossil fuel supply with an electric motor and a battery “because it is more efficient.” However, this decision requires a deeper justification through analysis.
Motor Efficiency & Weight
A turboshaft or turboprop has a shaft brake power-to-weight ratio, on average, of about 1.32 kW/kg (0.8 hp/lb) to 1.97 kW/kg (1.2 hp/lb). For piston-prop engines, their brake power-to-weight ratios are lower, on average being about 0.33 kW/kg (0.2 hp/lb) to 0.66 kW/kg (0.4 hp/lb). Yet these values are still substantially lower than for an electric motor, which can achieve, on average, brake power-to-weight ratios of 5 to 8 kW/kg. A nominal value of 5 kW/kg represents 2020 motor technology. While the values for an electric motor are desirable, a propulsion system on an aircraft also needs to be compared relative to the weight of the energy source carried onboard, which is contained in the batteries.
Battery Efficiency & Weight
One kilogram of Avgas or Jet A-1 fuel stores approximately 12 kWh of energy, which is 50 times much better than a Li-ion battery pack, which holds only about 0.25 kWh of energy. Therefore, using these values gives the useful power available for a turboprop or turboshaft as about 6 kW per hour per kilogram when running on Jet-A, which is 6 kWh/kg (in terms of kiloWatt-hours) or 8 hp/lb. For a piston-prop engine running on Avgas, it is about 4.2 kWh/kg or 5.8 hp/lb. However, even with a 90% efficiency, the power available at the shaft of an electric motor running on a Li-ion battery is only about 0.225 kWh/kg or 0.3 hp/lb at the system level, which is obviously a lot less than for an ICE.
Net System Efficiency
Now, it is possible to have better engineering clarity about the net system efficiency of an ICE propulsion system versus an electric propulsion system, at least in electrifying an existing aircraft with an ICE. The forgoing values are all summarized in the table below, showing an electric propulsion system’s high efficiency but poor overall net system efficiency in brake power per unit weight. The main reason for this outcome, of course, is that the high weight of the batteries pulls down the net weight efficiency of any electric propulsion system.
|Type||Efficiency||Power-to-weight (kW/kg)||Fuel/energy type||Fuel/energy brand||Energy density (kWh/kg)||Average available brake power (kWh/kg)|
|Piston engine||32 – 35%||0.30 – 0.33||Gasoline/petrol||Avgas||11.2 – 12.8||4.2|
|Turboprop||45 – 50%||1.22 – 1.32||Jet fuel||Jet A or Jet A-1||11.9 – 12.5||6.0|
|Electric motor||90–95%||5.0 – 8.0||Batteries||Li-ion||0.22 – 0.25||0.225|
For example, consider replacing a 150 kW (200 hp) piston-prop engine on a relatively small airplane with a payload of 400 kg (882 lb) when carrying a full amount of Avgas. If the engine has a mass of 136 kg (weight of 300 lb), then an equivalent electric motor would have a mass of only about 20 kg (44 lbs), as shown in the figure below. Running the piston-prop engine for one hour requires 150 kWh of energy, equivalent to the energy contained in 13 kg (28 lb) of Avgas. But to run a comparable electric motor with the same shaft power for one hour would require at least 667 kg (1,471 lb) of Li-ion batteries, i.e., 50 times more weight than a tank of Avgas. Similar arguments can be made for turboprops or turboshafts, in general.
Again, while an electrical motor’s low weight and high efficiency are extremely attractive, one now begins to see the challenges of simply retrofitting an existing aircraft with an electric propulsion system because it seems like a good idea on the surface. It is the power-to-weight ratio or, more specifically, the energy-to-weight ratio of the propulsion system, including the weight of the energy source, that matters for an aircraft.
In this regard, the weight of the batteries needed for an electric propulsion system, even for a short flight, would quickly erode the aircraft’s payload capacity. It would not be a matter of maintaining the payload and increasing the aircraft’s gross weight. The certified weight of the aircraft is just that, and for a good reason, so flying an overweight aircraft is not only foolish but illegal. In the process of aircraft design, which are very much point designs, it is rarely enough to change out a propulsion system without it leading to a significant redesign of the entire aircraft.
This analysis applies to a 746 kW turboprop engine with a mass of 227 kg (weight of 500 lb). On the one hand, a turboprop has a power-to-weight ratio of about 1.32 kW/kg (0.8 hp/lb) to 1.97 kW/kg (1.2 hp/lb) and an efficiency of about 50%. On the other hand, an electric motor has a power-to-weight ratio of 5 to 8 kW/kg and an efficiency of 90%. Assuming the turboprop has the lowest average value of the power-to-weight ratio of 1.32 kW/kg and the electric motor has the highest average power-to-weight ratio of 8.0 kW/kg, then the equivalent electric motor would have a mass of
which is significantly lighter than the turboprop engine! But to run this electric motor for one hour at the rated shaft power of the turboprop would require 746 kWh. With a battery gravimetric energy density of 0.256 kWh/kg, the needed battery weight will be
which is a lot of heavy batteries! For comparison, the turboprop engine would need only about 210 kg (463 lb) of jet fuel. Clearly, it is difficult to justify replacing the turboprop other than for experimental flight testing purposes and perhaps for research and development.
Aircraft Weight Considerations
The power and energy demands for the flight of any aircraft depend on its airspeed, rate of climb, operational altitude, thrust or power required, and engine characteristics. For a fossil fuel-powered aircraft, the weight of fuel carried is generally a significant fraction of the takeoff weight. It costs fuel to carry fuel, but the aircraft becomes lighter and more efficient as fuel is burned off. The thrust and energy demands for flight decrease with the square of the reduction in fuel weight, so the improvements in efficiency as fuel is burned off are significant. However, an electrically powered aircraft’s weight stays constant throughout the flight because all the energy is stored in fixed-weight batteries.
At the most basic level, the gross takeoff weight, , of a conventional (i.e., non-electric) aircraft can be expressed as the sum of its empty weight, , plus the useful load, , i.e.,
The aircraft’s empty weight comprises its structure, the engines, internal fixtures, oils, hydraulic fluids, etc., and everything else it needs to be ready to fly, but without loading any payload or fuel. The useful weight, , is the sum of the payload weight, , and the fuel weight, , i.e.,
Payload is the weight the aircraft carries that pays the bills, such as passengers and their baggage and cargo. But fuel is not payload.
The takeoff weight must always be less than or equal to the maximum allowable (officially certified) maximum gross weight of the aircraft, . Therefore, the gross takeoff weight of the aircraft, , will be
The empty weight, , is
where is the airframe weight and is the weight of the powerplants (motors or engines). Therefore,
Another way of expressing this latter sum is in terms of weight ratios, i.e., a component weight as a fraction of the gross takeoff weight. Therefore, in this case
where the values are called weight fractions.
For a traditional (i.e., fossil-fueled) aircraft, the values of the empty weight fraction, , can vary from about 0.6 to as low as 0.5 for general aviation airplanes and from about 0.55 to 0.45 for commercial jet airliners. This means that useful load fractions, , average between 0.55 to 0.4 for conventional aircraft. This value must be split between the “wet” fuel fraction, , which is needed to carry a specific payload over a certain flight range, which depends on the aircraft and engine efficiencies. The payload fraction, , comprises the number of passengers and cargo or whatever else pays the bills. Trades between the allowable payload and the achievable range apply to all traditional fossil-fueled aircraft.
For a battery-powered aircraft, however, the weight of the aircraft remains the same at landing as it was at takeoff. The same weight fraction sum still applies, but now the weight fraction of batteries, , becomes a permanent (fixed) part of the empty weight fraction of the aircraft (rather than a diminishing fuel weight), i.e.,
The question is what the battery weight fraction is and what is left as a payload fraction. The answer is that with existing technology, the battery weight fraction dominates over the payload fraction. Indeed, the payload fraction for an electrically powered aircraft may be no more than 0.1. Such low payload fractions are not economical for an airline.
Flight Profiles & Missions
The types of flight profiles or missions that could be satisfied by electrically-powered aircraft are similar to existing aircraft. However, at least with current battery technology, the flight ranges will inevitably be shorter, and the payloads that can be carried will be much lower. Furthermore, it is not realistic to retrofit any existing aircraft powered by ICEs with electric motors and expect it to have the same level of performance.
A typical flight profile for an electrically-powered aircraft is one where the aircraft flies from A to B and will include a takeoff, climb, cruise to the destination, descent, and landing, as shown in the figure below. The cruise condition is predominantly in straight-and-level, unaccelerated flight at a relatively constant airspeed. A contingency plan for a possible diversion preventing a landing at the destination must be anticipated by regulation, which will require significant reserve battery energy to be accounted for in the planned flight, thus shortening the feasible range on one battery charge. Recharging at intermediate stops along the route could also be needed and accounted for in flight planning.
Other missions for an electrically-powered aircraft may involve an out-and-return flight profile, i.e., fly to a destination to conduct some operation and then return to base without a landing, which is typical of what is routinely done using drones. UAVs and drones have been around for the last two decades, satisfying numerous military and civilian requirements. These aircraft typically have shorter range and endurance goals, lower net energy requirements, and smaller, lighter battery packs. Such drone missions may involve photography or some surveyance. The task may include reconnaissance or weapon delivery for military drones, which have proved effective in recent conflicts.
Short flights of less than an hour in an electrically-powered aircraft make the significant weight penalty of the batteries feasible. In general, however, the energy required for longer flights means that the battery weight would quickly become prohibitive, eroding the aircraft’s payload and range. A commercial airliner’s payload is fare-paying passengers and/or cargo, so it is easy to see how payload could be quickly eroded to uneconomic thresholds. The volume of batteries also becomes another consideration in where to put them inside the airframe. To maintain the correct center of gravity of the aircraft, the batteries, and associated heavy wiring must be carefully positioned.
Payload & Range of Electric Aircraft
While an aircraft burning fossil fuel will be considerably lighter when it lands than when it takes off, a battery-powered aircraft will have the same weight throughout the flight. The range of a battery-powered aircraft can be expressed as
where is the takeoff weight, is the weight of the batteries, and where is the stored energy per unit weight of the battery pack, i.e., the gravimetric energy density. Therefore,
The efficiencies , , and are the efficiency of the battery, motor, and propeller, respectively; the net or total efficiency will typically be about 0.7 to 0.8.
Therefore, to maximize the flight range of an electrically-powered airplane, which is not much different from a fossil-fueled aircraft, it must be flown in such a way that:
- The motor and propeller must operate at or near their respective maximum efficiencies.
- The battery must have as high an energy density as possible.
- The airplane flies at or near its best lift-to-drag ratio .
- The battery pack, controller, power cables, etc., must all be as light as possible.
As previously discussed, the net battery weight, , on an electric aircraft (which includes cabling and other electronic components to regulate and protect the batteries) is a significant fraction of the gross aircraft takeoff weight, . Based on current technology, the value of is typically 0.4 to 0.5 for an electrically-powered aircraft. For the same energy storage as aircraft using an ICE, for which the fuel fraction is 0.3 or less, it is not difficult to understand why an electric aircraft will have a limited payload and flight range.
The characteristics of any aircraft’s payload-range performance, as shown in the figure below, are such that an aircraft can only carry a specific payload over a particular range (distance). When a conventional (fossil-fuel) aircraft operates at maximum payload, the fuel tanks generally cannot be filled to their total capacity, a characteristic of all aircraft. Longer flight ranges can be flown by reducing payload (i.e., limiting the number of passengers and/or cargo) in exchange for more fuel, which is the boundary between points A and B in the figure below. Along the boundary B to C, the payload must be reduced significantly to obtain the needed range. Point C corresponds to a ferry flight where the aircraft is flown over an extended distance with maximum fuel but without payload.
The payload-range situation changes markedly for an electrically powered aircraft. Not only is the maximum payload considerably lower, all other things being equal, because of the high weight of batteries, but the achievable flight range is also relatively short. Reducing the payload reduces the energy requirements, so extending the range somewhat. However, because of the low payload weight fraction, the gains in the range are still relatively small.
But in the end, the weight and size of all components are critical factors for any aircraft because they directly impact its overall efficiency, range, and payload capacity. In the case of an electrically-powered aircraft, the lighter and more compact the batteries will allow the aircraft to carry more payload and/or have greater flight range. The other arguments that can be made here based on environmental factors are always worthy of deeper consideration.
First, the weight of the battery pack is needed, i.e.,
Next, the available energy contained in the batteries needs to be determined, i.e.,
The energy available at the electric motor will be reduced because of the system efficiency, so the actual available energy is
If the motor is running at = 250 kW, then the time to use 50% of the available energy in the battery pack for the cruise segment will be
During this time, the aircraft will have covered a distance of 150 0.357 = 53.55 nautical miles. Note: The other 50% of the energy in the battery packs would need to be used for the takeoff, climb, and descent, plus a reserve. Again, the challenges with batteries become apparent in that they comprise a significant weight fraction but do not allow an aircraft to have much flight time or range.
Electric aviation can encompass “all-electric” aircraft and hybrid designs that pair ICEs with electric motors. Hybrid power systems can offer certain advantages while mitigating the limitations of “pure” or fully electric propulsion. By utilizing hybrid architectures, aircraft designers can leverage fossil fuel’s higher energy density and longer-range capabilities while benefiting from the efficiency and environmental advantages of electric propulsion. Therefore, hybrids can be considered a viable near-term solution to bridge the gap between traditional aviation and the eventual adoption of all-electric aircraft.
Hybrid propulsion systems can employ different configurations. For example, they can use an ICE to generate electricity, which is also used to power electric motors for propulsion, as shown in the schematic below. This combination allows the ICE to operate at its most efficient power output while reducing emissions compared to using it directly for propulsion. Additionally, hybrid systems can utilize regenerative braking and energy recapture systems to improve efficiency further and extend flight range. Of course, these hybrid power systems have been used with automobiles for over a decade.
The two main types of hybrid-electric propulsion systems for aircraft are serial and parallel hybrid. In the serial hybrid, an ICE generates electrical energy that charges a battery and/or runs the electric motor that spins the propeller. In the parallel combination, an ICE turns the propeller directly, however, its work is supported by an electric motor during take-off and climb.
While there are clear performance and weight trades with all hybrid systems, the main benefit is that hybrid propulsion systems may give many of the advantages of electric propulsion systems, as suggested in the figure below, while compensating for their drawbacks. As such, hybrids are an excellent near-term solution before all-electric aircraft become feasible. Hybrid propulsion systems may also reduce the certification challenges for a pure electric aircraft, but this remains to be seen.
Electrifying a Turbofan?
According to some claims, a turbofan engine can be retrofitted by an electric motor driving the same fan stage. This all sounds easy, but this claim needs a deeper analysis. Remember that an electric motor would only replace the engine core, as shown in the figure below, so one immediate issue is the loss of jet thrust from the core of a turbofan engine, which is about 30% of the total thrust. Another issue is the accessory components, such as pumps for hydraulics and pneumatics, which would still be required with an electric drive.
Therefore, if an electric motor were to drive the fan used in any particular turbofan engine with the same torque, rotational speed, and blade section Mach numbers (which affect efficiency), the resulting thrust would be lower. This “lost” thrust could be recovered using a larger fan or other design modifications. In the case of a bigger fan, if the fan is uniformly loaded, then the new “electric” diameter, , can be estimated in terms of the original diameter, , by using
where is the required thrust and is the lost thrust from the engine core. For a 30% loss of thrust, the new fan diameter will be about 20% larger.
The bottom line is that to “retrofit” a turbofan with an electrically driven fan, the entire engine must be substantially redesigned. Another issue to remember is that with bigger fans on existing airframes, the ground clearance between the engine and the runway is already tight for turbofans on wings. Nevertheless, an electrically powered fan system will be considerably lighter than a turbofan, which is a significant advantage. The weight of the batteries, however, comprises the same issues that have been raised before.
What does it cost to charge up and run an electrically powered aircraft? To answer this question, it is first necessary to compare the cost of electric power from the grid with what it costs for the equivalent energy from jet fuel. Industrial consumers of electricity in the U.S., on average, pay about $0.075 (7.5 cents) per kWh. The price of Jet A-1 fuel fluctuates daily, but in 2022, it was priced at approximately $820 per metric tonne. A metric tonne is 1,000 kg or 2,204 lb, which equates to about $0.82 per kg or $0.37 per lb. However, airlines usually pay less for their fuel by using a process of “fuel hedging” to fix fuel prices for a period to mitigate the risk of price volatility.
Remember that at a system level, the usable power available at the shaft of a turbofan or turboprop will be about 6 kW per hour per kilogram of Jet A-1. The corresponding power available from an electric motor is 0.225 kW per hour per kilogram of battery pack. Therefore, the cost of powering a jet engine is about $7.5 per kW per hour, and the cost of an electrically-powered engine will be about $3 per kW per hour, so 2.5 times less cost. This is obviously an attractive outcome!
However, another significant cost consideration is the maintenance cost of the engines versus the replacement cost of batteries. Li-ion batteries typically reach 3,000 duty cycles before they need to be replaced, but FAA or EASA certification requirements may limit the batteries to 1,500 cycles or less for safety reasons. Turbofans and turboprops are reliable and may only need a major overhaul once every 20,000 flight cycles. Nevertheless, such overhauls can cost hundreds of thousands of dollars. Typically, this works out to be about $200 per flight cycle for a turboprop to $500 per flight cycle for a turbofan.
For now, assume the battery replacement frequency is 1,500 cycles (flights). Consider a turboprop engine in the 820 kW shaft power (1,000 shp) range. This means the maintenance cost for 1,500 one-hour flight cycles will be about $300,000 per engine or $200 per kWh. The cost of replacing the equivalent Li-ion battery pack for aviation use is about $500 per kWh, so 2.5 times the comparable maintenance cost of the turboprop per hour. While the energy cost per kW for electric propulsion is 2.5 lower than for a turboprop, this advantage is negated by the high battery replacement cost! The result is a zero net sum gain, which makes it difficult to justify electric propulsion, at least based on this cost argument. But, in time, battery costs may be reduced, resulting in a positive financial net sum gain in terms of cost per flight hour.
Electrically-Powered VTOL Aircraft
Electrically-powered VTOL (Vertical Takeoff and Landing) or “eVTOL” refers to an aircraft capable of vertical takeoff and landing and forward flight using one or more electric propulsion systems. eVTOL aircraft typically rely on multiple electric motors, each with its own propeller or rotor, to provide vertical lift and/or forward propulsive thrust. These aircraft are designed to combine the benefits of helicopters and fixed-wing aircraft, potentially offering the same benefits as other electrically-powered aircraft but also with VTOL capability. The FAA refers to eVTOL aircraft as a form of “powered lift” and so subject to all of the requirements and regulations of this category of aircraft. The ICAO does not recognize powered lift aircraft at present, which poses an interesting dilemma in the development and eventual certification of eVTOL aircraft.
The world’s first eVTOL aircraft, which was flown in 2009, was the Agusta-Westland Project Zero, as shown in the photograph below. This aircraft was an all-electrically-powered, fly-by-wire tiltrotor concept, which represented the state-of-the-art in the use of electrical motors and battery technology at that time. It was a demonstrator for technologies never flown previously on a rotorcraft but never designed to go into production. Yet, it was to set the stage for future eVTOL developments.
The design and development of other eVTOL aircraft have gained significant attention in the last decade, driven by continued advancements in battery technology and electric propulsion systems. These eVTOL aircraft are seen as potential solutions to enable Urban Aerial Mobility (UAM), which is the vision of providing on-demand transportation services in congested urban areas or serving as “air taxis.” The popular mantra of the eVTOL community is that such aircraft have the potential to revolutionize urban transportation by offering faster and more efficient travel, reducing congestion on roadways, and providing access to areas that are otherwise difficult to reach. However, like all other electrically-powered aircraft, many challenges must be addressed before eVTOL aircraft become a valuable part of the aviation spectrum.
If the eVTOL has 12 rotors, then it can be assumed that each rotor carries 1/12th of the weight, , of the aircraft, i.e., each rotor produces a thrust of
The power required for each rotor, according to simple rotor theory, is
where is the disk loading, , and = 0.7. Inserting the known values gives
where the density at MSL ISA is 0.002378 slugs/ft3. Therefore, the total power required to hover will be
If the gravimetric energy density of a Li-ion battery is 0.256 kWh/kg, the needed battery weight to hover for 10 minutes (one-sixth of an hour) will be
Adding up the weights leaves 2,000 – (377.9 + 300) = 1,422 lb for the airframe, motors, rotors, wiring, and everything else, which seems reasonable based on the claims. However, 10 minutes is not much flight time to be very useful.
Today, over a hundred established companies and startups are actively working on developing eVTOL aircraft that could satisfy the needs of UAM, one example being shown in the figure below. These companies are conducting research, building prototypes, and performing flight testing to one day bring these eVTOL aircraft to the marketplace. Many of these aircraft are autonomous and designed to fly without a pilot on board. While eVTOL aircraft have shown the world there is promising potential, the widespread adoption and integration of these aircraft into the national airspace system will require further advancements, regulatory approvals, and significant infrastructure development, not to mention public acceptance. It will likely take much work to convince the general public to step onto an aircraft without a pilot!
Some believe eVTOLs can function like terrestrial buses or rail services running over specific routes at certain times. However, the convenience of eVTOL vehicles that can take off and land directly at the customer’s location, other than a designated eVTOL port or heliport, will likely be another primary mission. eVTOLs will have to function similarly to existing taxis or, like Uber, providing easy, efficient, and inexpensive point-to-point transportation for the general public.
While there are a vast number of proposed eVTOL configurations, however, as shown in the figure below, they can be broadly categorized into the following:
1. Oversized Drones: These are configurations with multiple propellers reminiscent of existing drones, such as quadrotors. While some have a small enough footprint to fit in a front yard or on a city street, it is essential to consider the regulations and infrastructure required to accommodate them. If regulations permit, the smallest versions could pick up and drop off a few passengers at their location.
2. Modified Helicopters: Some proposed eVTOL designs resemble existing helicopters but with many more rotors or propellers. These vehicles often have a footprint similar to that of traditional helicopters, which may make it challenging for them to fit conveniently on city streets. Their long-term prospects for use in UAM are doubtful, especially considering potential regulatory and infrastructure limitations.
3. Rotor/Fixed-Wing Hybrids: Several eVTOL designs incorporate tilting wings and/or rotors and multiple propellers. These vehicles offer the advantage of requiring less energy in cruise flight than flying on lifting rotors like a helicopter. However, their larger footprint limits their suitability for UAM, most likely to designated landing areas such as at local airports.
4. Retractable Wings and Rotors: Some eVTOL solutions include vehicles with retractable or foldable wings and/or rotors. While they may sometimes be called flying cars, they are more accurately described as drivable airplanes. It is unlikely such configurations would be technically or economically feasible for UAM because of their mechanical complexity, high empty weight and battery fractions, and limited payload fraction
Autonomous flight refers to the ability of an aircraft to operate, navigate, and make decisions without direct human intervention. Autonomy may be needed for an eVTOL to make it useful and practical. Autonomy may involve using technologies such as artificial intelligence and machine learning combined with sensors and control systems to enable the aircraft to perform various tasks independently. Autonomous flight systems are designed to perceive and interpret the environment, plan and execute flight trajectories, and make real-time decisions based on sensor inputs and pre-programmed algorithms. These systems should also be able to handle tasks such as takeoff, landing, navigation, obstacle avoidance, and even responding to emergencies.
Aviation has different levels of autonomy, ranging from limited automation to full autonomy. These are some commonly recognized control levels:
- Pilot-in-Command (PIC): The aircraft is entirely under human control, with no autonomous features.
- Autopilot Systems: The aircraft has essential automated functions, such as maintaining altitude and heading, but a human pilot is still responsible for overall control and decision-making.
- Flight Management Systems (FMS): The aircraft has more advanced automation, including automated navigation, performance calculations, and guidance. However, a human pilot must monitor and intervene when necessary.
- Highly Automated Systems: The aircraft can perform most flight operations autonomously but requires human oversight and intervention in certain situations.
- Fully Autonomous Systems: The aircraft can operate and navigate without direct human control. Without human intervention, it can perform all flight operations, including takeoff, landing, and navigation.
Autonomous flight systems can offer benefits such as increased safety, improved operational efficiency, reduced human error, and the ability to operate in challenging or dangerous environments. However, the vulnerability of autonomous flight vehicles to cyber-attacks must also be considered because they rely on wireless communications. Attackers can use jamming devices or generate other wireless interference to disrupt the control signals, rendering the vehicle ineffective or causing it to crash. Various measures can be taken to mitigate vulnerabilities, including encryption and authentication mechanisms, implementing secure protocols, regularly updating software using secure coding practices, and employing intrusion detection systems.
The widespread adoption of fully autonomous flight for eVTOLs, or in any aspect of commercial aviation, is still some way off. Ensuring the safety, reliability, and integration of autonomous systems into existing airspace and infrastructure remains a complex challenge that requires extensive testing and validation. As previously mentioned, the more significant issue may be perception and how to convince the flying public that it is safe to step onto pilotless aircraft.
Urban Aerial Mobility
What eVTOL can contribute to the transportation needs of the general public needs a more critical evaluation. Such an evaluation should be considered on technical grounds but also in terms of accessibility, convenience, and cost. Besides the question of technical feasibility, the most obvious question would be: Will UAM be affordable to the public at large, or is it just another elitist proposition for the wealthy?
As shown in the image below, some think this will be a new aviation utopia, where everyone flies around doing their business and commuting to their homes in an eVTOL aircraft. But to be fair, there has also been a perception that electric cars are elitist; the evolving market dynamics and efforts to improve accessibility and reduce costs are gradually disproving this notion. Therefore, in time, the same may happen with eVTOL and UAM.
Assume that the technical issues with eVTOL can be overcome, and they are then in widespread use in large U.S. cities such as Orlando or Washington D.C. Consider now the following scenario. Each lane of an interstate highway, such as I-4 in Orlando or I-270 in Washington D.C., has a flow rate of about 2,000 cars per hour. Assuming two occupants per car, this flow rate is 4,000 people per hour per lane. A three-lane highway can allow 12,000 people per hour to go about their business, assuming no accidents or delays. Transit rail (e.g., MetroRail in Washington D.C.) can carry even more, with a flow rate of up to 6,000 people per hour per track. Therefore, the question arises: how many eVTOLs would it take to offload the carrying capacity of one highway lane, or what could be achieved with MetroRail?
The answer is simple arithmetic. Assuming every eVTOL flight carried three passengers, there would have to be more than 2,000 eVTOLs in the air simultaneously. Based on an average of 10 to 20-minute journey time, there would be up to 12,000 take-offs and landings per hour and potentially over 500,000 eVTOL movements a day, the FAA considering a “movement” as either a takeoff or a landing. At the largest airports in the U.S., there may be as many as 2,000 movements a day. Therefore, it is hard to imagine any large metropolitan city accommodating 500,000 air traffic movements a day, even if it was an affordable form of transportation for the public.
The issue of weather is also a concern, and just like many other aircraft, eVTOLs will not be able to fly in adverse conditions, e.g., low visibility conditions, heavy rain, thunderstorms, freezing conditions, etc. The proximity of major airports is also a concern because of the potential for air traffic conflicts with commercial airliners. Air traffic control (ATC) decisions about who receives airspace and terminal access during times of congestion will undoubtedly contribute to delays and emergency diversions, and the batteries of eVTOLs will be quickly depleted. Popup ATC airspace restrictions, which are all too frequent in the Washington D.C. area, could ground all air traffic for hours at a time. UAM could then degenerate into urban aerial chaos.
Noise pollution is a significant concern when it comes to eVTOL aircraft. While many are relatively quiet by themselves, it is the cumulative noise footprint of many of these aircraft flying over densely populated areas that matter, which can result in a high level of public annoyance. Having hundreds of eVTOLs zooming over your house in the suburbs every hour will likely not work without a massive public backlash. Therefore, addressing the issue of noise to realize UAM will be crucial for public acceptance and successful implementation.
Even if eVTOL could provide an affordable service equivalent to a lane of interstate traffic or a line of MetroRail, it would only have a minimal measurable impact on the mobility of the general public. What will happen in the coming decades remains to be seen, but there are reasons to continue pursuing eVTOL concepts and electric propulsion methods.
In forward flight, the lift on the aircraft is equal to its weight, and the thrust needed is equal to its drag. For a propeller-driven aircraft, the best range efficiency will be when is the maximum. To find this condition, then we can write
We want to find the maximum or the minimum . Differentiating the previous expression gives
So solving for gives
with the corresponding value at this value of given by
Therefore, the best (or ) for this aircraft is
The lift on the wing is equal to weight , so
and rearranging gives the airspeed corresponding to this value of as
where . Substituting the numerical values gives
To find the power required for flight, we need the total drag force, , i.e.,
In level unaccelerated flight, , so that the net brake propulsive power required is
The energy available from the battery pack is
Therefore, based on the available battery energy, then the maximum time of the cruising part of the flight will be
and the distance (range) traveled by the eVTOL aircraft during this time will be
Supporting Electrical Infrastructure
Developing an electrical charging infrastructure is crucial to enabling the adoption of any electrically-powered aircraft, particularly for commercial purposes. In particular, eVTOL aircraft will need many charging stations, and in a large city, one of the immediate questions is where to put them. The answer may be on the rooftops of buildings or in parking lots.
All airports, including the smaller regional airports, would also need to invest in charging infrastructure to provide the necessary power and rapid turnaround times for various electrically powered aircraft. To make any economic sense, this aircraft charging infrastructure would need to accommodate the charging needs of different aircraft sizes and types, from general aviation to commercial airliners to eVTOL. It would be chaos if 1,000 eVTOLs arrived at the same airport simultaneously for a battery top-off!
Another consideration is that charging electric aircraft requires more powerful and specialized charging solutions compared to charging electric road vehicles. The power and electrical current demands of aircraft charging are significantly higher than for road vehicles, and the infrastructure also needs to be able to support rapid battery charging to minimize turnaround times. In the meantime, efforts to electrify airport ground operations, such as ground support equipment, tugs, baggage carts and loaders, and maintenance vehicles, can provide a starting point for the development of the significantly upgraded electrical charging infrastructure needed to supply the power levels required to recharge electric aircraft.
Quantifying the load on the electrical grid, determining costs, and identifying the specific types and locations of charging stations required for electric aircraft will be critically important to the success of eVTOL and other electrically powered aircraft. Understanding the technical specifications, power requirements, and cost implications of different charging solutions will also be crucial for airport operators and aircraft manufacturers. Ultimately, addressing the infrastructure challenges, including the electrical charging infrastructure, will become an essential part of a holistic approach to the electrification of aviation.
Regulation & Certification of Electrically-Powered Aircraft
The FAA and EASA are the two central regulatory authorities responsible for ensuring the safety and regulation of aviation. These authorities are crucial in establishing and enforcing standards for aircraft design, production, pilot licensing, and maintenance and operating requirements. Their main goal is to ensure that the design of all aircraft meets accepted aeronautical standards and that the aircraft is safe to fly.
With the emergence of electric aviation and related technical advancements, such as specialized electric motors, new types of batteries, fly-by-wire flight control systems, and autonomous flight, unique challenges need to be addressed within the existing regulatory framework, such as the FARs. The innovative technologies and components used in eVTOL and other electrically-powered aircraft generally differ from the certification standards for conventional aircraft. Therefore, adapting or creating new regulations becomes necessary.
Issues have arisen with the certification of Leonardo’s AW609 civil tiltrotor, a hybrid between a helicopter and an airplane. The program was launched in 1996, but as of 2023, the aircraft still needed to be certified under EASA rules. The only other civil fly-by-wire rotorcraft is the Bell 525 Relentless, which started development in 2012 and still needs certification from the FAA. Such lengthy certification processes for aircraft, not just those using new technologies, seem typical, not exceptional. Indeed, the FAA has made its position very clear: “In terms of AAM [advanced air mobility], we are viewing [those aircraft] as the same level of safety as any other passenger aircraft or any other piloted aviation.” “We believe the societal expectations for these aircraft is that they will operate like any other Part 21 or 23 aircraft with Part 91 operations.”
The good news is that the FAA and EASA seem well aware of the need to continuously evolve their airworthiness and operational regulations to keep pace with emerging technologies. They have engaged with industry leaders, research organizations, and manufacturers to understand the implications of electric aviation and autonomous flight. However, updating aviation regulations remains complicated and lengthy, and the certification process for an eVTOL aircraft is likely to take many years. Nevertheless, the authorities understand that to advance the aviation field; they must continue to strike a balance between ensuring safety and fostering innovation, including the formal certification and the eventual issuance of a certificate of airworthiness for all types of electrically-powered aircraft.
Looking to the Future
But what about looking to the future? Can electrically powered aircraft be realized and used in commercial aviation? Technically, it is just engineering. But at what cost? Can these aircraft also satisfy the airworthiness regulations set down by the FAA or EASA? The current battery technology, with a pack-level battery-specific energy of around 0.2 to 0.25 kWh/kg, has many limitations, especially for use in an aircraft that might be designed to carry passengers over reasonable distances. However, if battery-specific energy can be doubled to 0.5 kWh/kg, it would substantially improve the performance and capabilities of electrically-powered aircraft.
The corresponding increase in range and payload capacity makes electric aviation more competitive and practical for a broader range of applications, including regional or short-haul flights. For instance, taking the aircraft in Worked Example #2, then doubling to 0.5 kWh/kg would give it a flight time of 40 minutes, over which it would cover a distance of about 100 nautical miles, which is a lot more useful. In the meantime, the industry and research organizations forge ahead, exploring new technologies and electrically-powered aircraft concepts, including NASA’s X-59, as shown in the image below. According to NASA: “The primary goal of the X-57 project is to share the aircraft’s electric-propulsion-focused design, airworthiness process, and technology with industry, standards bodies, and regulators to inform certification approaches for all-electric propulsion in emerging electric aircraft markets.” The X-59 project was canceled in June 2023 because the new technology was apparently too risky to field on a flying aircraft. It seems ironic that an X-plane suffers cancellation because of higher than average risk, which is why X-planes are designed in the first place.
Advancements in battery technology continue to be pursued, and there is ongoing research into various battery chemistries that could significantly improve energy density while reducing weight and maintaining safety and durability. Meantime, hydrogen-powered fuel cells are another way to leverage the efficiency improvements of electric propulsion systems. They sidestep the problem of the low specific energy of batteries, but now the challenges of hydrogen production, storage, and delivery must be confronted. Hydrogen is very energy-dense and light but is highly reactive. Hence, a concern in its use is that leaks of high-pressure hydrogen are highly susceptible to spontaneous combustion, which may cause a fire or explosion. Nevertheless, investigating the potential performance characteristics and the GHG mitigation of fuel cell-powered aircraft is an opportunity for the future.
Electrically-powered road vehicles have reached a tipping point because current battery capacity has given such vehicles ranges of 400 miles (644 km). In the meantime, is there a tipping point to look toward for batteries beating Avgas or jet fuel? At this point, the goal must be at least 800 Wh/kg, but even then, the need for a mandated reserve of energy will make this the absolute minimum to be practical. Higher life cycle batteries remain at 250 Wh/kg today, even less after installation, but they are likely to exceed 400 Wh/kg by 2030. High life cycle batteries in the 1 MWh/kg class are probably another decade away, which is closer to the tipping point needed for aviation applications.
Summary & Closure
Electrically-powered aircraft offer numerous potential benefits for modern aviation, including increased energy efficiency and reduced carbon emissions compared to conventional fossil fuel-powered aircraft. However, it is likely that passengers will only fly on electric aircraft for short distances, even in the longer term. It is essential to note that battery technology is one of many factors limiting the feasibility of electric aviation. Other aspects of electric aircraft need to be considered, such as infrastructure development, charging systems, regulations and certification, overall economics, and public acceptance.
A combination of technical approaches will likely be employed in the next few decades to overcome the challenges with “pure” electrically-powered aircraft. This more diversified approach will continue to improve fuel efficiency, perhaps through new aircraft designs incorporating sustainable aviation fuels (SAFs) and exploring hybrid-electric or hydrogen-electric propulsion systems. When collectively applied, these interim technologies could help reduce emissions from the aviation sector. Meanwhile, engineers must continue to innovate and develop new or improved solutions that can meet the unique requirements of electrically powered aircraft while focusing on the broader global concerns of reducing the environmental impact of fossil fuels.
- Why does the goal of decarbonizing terrestrial transport and electrifying aviation also pose an educational challenge?
- Electric car manufacturers have been having problems with vehicles experiencing fires. How can the flying public be confident in electrically powered aircraft, knowing that an in-flight fire will likely be catastrophic?
- List critical factors that influence the performance of electrically-powered airplanes compared to traditional internal combustion engine aircraft.
- Given that air transport of lithium-ion batteries as cargo is banned or strictly controlled, how can their use as the primary propulsive energy source be considered for propelling an airplane?
- Turboprops and turbofan engines are well-proven, refined, efficient, and reliable. Why would the aviation industry ever want to move away from using those?
- Are hybrid-electric systems any more environmentally friendly? Discuss.
- Battery technology has yet to achieve sufficient maturity to make commercial electric air transport viable. Discuss this statement.
- The future of electric aviation will be characterized by advancements in battery and electric motor technology, as well as more efficient aerodynamic design. Discuss.
- Electrically-powered commuter-class airliners could take to the sky in the next few years, maybe even before the decade’s end. But they probably won’t be able to take very many of us very far. Why?
- Besides the low energy density of batteries, the infrastructure and regulatory hurdles are likely more severe challenges than the technical ones. Discuss this viewpoint.
- The development of new production standards and safety and certification rules by regulators often needs to catch up to technological development. Why?
Will eVTOL air taxis function like regular taxis, such as picking you up at your home and dropping you off at the airport?
Take a deeper dive into the field of electrically-powered aircraft by following up with some of these online resources:
- Chat with ChatGTP about what it has to say about electric aviation.
- A great webpage by Airbus about hybrid and electric flight.
- This is a good video on the topic of hydrogen-powered versus electrically-powered aircraft.
- Take a flight on the world’s first certified electric airplane.
- Find out about the world’s fastest 345 mph electric airplane!
- Flying 150 miles with Joby’s Electric Vertical Take-Off and Landing Aircraft (eVTOL).
- Anderson Cooper reports on eVTOLs, flying vehicles that may one day be the answer to bumper-to-bumper traffic.
- A video reviewing the progress of eVTOL and electric aviation.
- A video on how eVTOL could disrupt the $49B helicopter industry.
- The insane engineering of the Joby S4 eVTOL aircraft.
- The False Promise of Green Energy – lecture by Prof. Andrew Morriss.