Bjorn’s corner: Sustainable air transport. Part 42. eVTOL range.
October 21, 2022, ©. Leeham News: This is a summary of the article Part 42P, eVTOL range. It discusses the range of a typical eVTOL performing a supply mission from a city center to an airport.
Article 42P details the energy consumption for each stage of the mission and the distance traveled. We summarize the results here.
How range is calculated for an eVTOL
eVTOLs, like all battery-powered aerial vehicles, are highly energy limited. You therefore calculate the range of these vehicles by first listing all the mandatory energy consumption zones. Then you calculate the range of the vehicle with the remaining energy.
First, we need to understand how much energy we can fit into our eVTOL. We defined a mass fraction for energy in Part 39, Figure 2.
We have 743 kg to spend on our battery system. If we assume that 700 kg are available for the battery modules (the rest is battery management with cabling and liquid cooling system for the 56 modules), we obtain 144 kWh of battery energy for a system delivered in 2025 (energy density at system level 206 Wh/kg).
We have defined that the battery system has been used for a period of time and that a full charge reaches a SOC (state of charge) of 90%. Our hover landing requires approximately 750 kW of battery power, and the battery can only provide such power levels above 10% SOC.
The minimum SOC may be higher for an end-of-life battery as the internal resistance increases and the maximum current capacity decreases. It takes a long time test to establish what is beyond our scope for items. We assume that a minimum of 10% SOC is acceptable.
We now have 114kWh of useful energy to expend on our mission (80% of 144kWh), Figure 2. Not all power can go to propulsion; we need power for all eVTOL systems and passenger comfort. For our mission, 2.8 kWh are blocked for such use.
Before starting the mission calculations, you block the regulatory reserves, in our case 35.5 kWh for 20 minutes of waiting on the outward flight before transition.
We now have 76.8 kWh to use for our flights. Our vertical takeoff and transition takes 6.5 kWh, followed by the climb 31KWh. We climb to 8,000 feet, which gives a range a few nm longer than if we climb to 5,000 feet.
Our descent, our approach and our vertical landing consume 20.8 kWh. There are 11.7 kWh left for cruising, which is not a lot. Our total distance traveled is 53 nm. That’s when you count all the distances. As airports practice arrival and departure procedures, we should only count 50nm or less as a useful range.
What is the use of a range of 50 nm?
It works to transport passengers from a city center to a nearby airport IF the weather is good. However, we have not introduced any margin for bad weather or flying in icing conditions.
It is not useful for general air taxi services. It is rare for travel distances to be less than 50 nm; in such cases, it is faster to stay in the car and cover the distance. In the case of downtown to airport, city traffic speed is a motivating factor for eVTOL services, not air taxi.
In the next Corner, we’ll introduce more variables like non-ideal weather, normal landing instead of hover landing, etc., and see what happens.