The Minnesota Department of Transportation (MnDOT) Office of Aeronautics is exploring how Advanced Air Mobility (AAM) aircraft can support the Minnesota Aviation System and the potential benefits of operating these novel kinds of aircraft. AAM can be briefly defined as new and novel aircraft featuring either environmentally sustainable propulsion systems or with highly automated piloting systems. Many of the AAM aircraft feature fully electric propulsion systems with onboard batteries and electric motors that produce vertical and horizontal thrust through several distributed propellers.
There are a couple of key terms that need to be defined to understand electric propulsion systems and the batteries that provide energy to help make sense of the differences in piloting an electric aircraft.
- Specific Energy: This is the most used battery performance parameter and is used in estimating electric vehicle endurance. It describes the amount of electrical energy stored per unit of battery mass (Wh/kg).
- Specific power: This is the amount of power delivered per unit mass of the battery. This parameter can be used to describe how fast a battery can take in energy and release it (kW/kg).
- State of charge: The measurement of how much energy is left in a battery (%).
- Battery cycle life: The number of complete charge/discharge cycles that the battery can support before losing performance (# of cycles).
For electric aircraft, specific energy is the predominant driving force for any performance limitations and capabilities of the aircraft. The specific power is important to ensure the battery can meet the demands of the mission, an example being if the aircraft needs to take off and land vertically, significantly more power is needed than for conventional takeoff and landing.
For a pilot with a gasoline or kerosene-based fuel system, performance calculations are relatively straightforward with easy calculations for range and endurance based on estimated, or actual, fuel burn during phases of flight. The engine’s full capabilities are available during that time until nearly the last drop of fuel. For electric aircraft, pilots must consider a few more variables to ensure they have sufficient performance for the specific maneuver or phase of flight they wish to enter.
As batteries state of charge is reduced throughout the flight, the battery voltage drops until the state of charge hits zero. In addition, excessive power demands for takeoff, landing, or vertical lift further reduce the batteries usable energy (what is available to the pilot). This can be visualized in FIGURE #. Among other things, battery cycle life and the battery temperature also play a role in usable energy.
The good news is that the complexities discussed do not necessarily mean the piloting job will become more difficult, just different. With battery performance models provided by the manufacturer, pilots can safely plan flights with contingencies accounted for. The General Aviation Manufacturers Association (GAMA) is working with industry and the Federal Aviation Administration (FAA) to further develop a performance-based reserve concept to replace the prescriptive time-based reserve standards in place today for this next generation of aircraft technology.
For more information on AAM technologies and aircraft electrification, please reach out to MnDOT Office of Aeronautics’ Advanced Air Mobility Program Manager Joseph Block, joseph.block@state.mn.us.