By Bjorn Fehrm
October 28, 2022, ©. Leeham News: This is a summary of the article Part 43P, eVTOL IFR range. It discusses the range of a typical eVTOL flying a feeder mission from a city center to an airport during IFR conditions.
IFR conditions mean we have a dicey weather forecast for our airport destination and must plan with an alternate landing site where the weather forecast is better.
Figure 1. The Vertical Aerospace VX4 in an early rendering with similar looks to the eVTOL we discuss. Source: Vertical Aerospace.
Mission Energy
eVTOLs, like all battery-based air vehicles, are severely energy constrained. We defined last week we have 144 kWh of battery energy for a typical eVTOL delivered in 2025.
We defined that the battery system had been in use for a while and that a full charge achieves a 90% SOC (State of Charge). Our hover landing requires about 750kW of power, and the battery can only deliver such power levels when above 10% SOC.
It leaves us with 115kWh (80% of 144kWh) of useful energy to spend on our IFR mission, Figure 2.
Figure 2. The mission with an alternate as we have bad weather at the destination. Source; Leeham Co.
As last time we start by blocking off the invariable parts of the regulatory reserves. These take 81 kWh of the available 115kWh for the mission, leaving 32kWh. And this doesn’t include any alternate cruise distance, just the climb from Decision Height (DH) and the descent plus transition/landing at the alternate.
So our regulatory helicopter reserves invariable parts consume 75% of the energy we have for flying to the destination, keep the eVTOL supplied with system energy + cabin comfort, and divert to our alternate.
This is if we use the FAA helicopter IFR rules. Switching to EASA, we increase the reserves as it adds destination contingency fuel/energy reserves to the above.
Our invariable parts for our destination flight (takeoff, transition, climb, descent, and approach to DH) consume 29 kWh of the remaining 32kWh, and the eVTOL own system consumption takes the rest. So there is no energy to cruise to the destination; we only have the climb and descent distance of 19nm, and the alternate must be within 13nm (the alternate climb and descent distances).
Does this change if the eVTOL changes to a normal landing at the alternate? It adds energy for a 5nm destination or alternate cruise (the energy consumption per nm is about the same at 5kft and 3kft cruise altitudes), still inadequate for destinations beyond 20nm with alternates outside 13nm (the distances covered at climb and descent).
Margins
So, could an IFR flight work at 20nm (the climb and descent destination distance) with an alternate at 13nm (the alternate climb and descent distance)? No. We have no wind effects, which can cut up to 20% of the range, no tail number factory performance variation ( delivered vehicles vary in performance up and down from a norm spec.), no operator’s additional margins, and no pilot added margins.
As I said before, the regulatory margins are the starting point for margins for real operations. You almost always add on top, dependent on the situation.
Conclusion
The eVTOL OEMs present business plans with up to 30 missions per day, 350 days a year. It’s only valid in the few spots in the world where there are persistent VFR conditions. Forget the typical US or European/Asian weather.
eVTOL OEMs have not claimed their products can fly IFR missions, and we now know why; it doesn’t work. And it will not change soon. We need battery improvement above 50% for it to work, which will not be available this decade.
The eVTOL industry will start in parts of the world with guaranteed VFR conditions and fly the city center to airport missions. It will take time before eVTOLs can fly other mission types.
