By Bjorn Fehrm.
October 7, 2022, ©. Leeham News: Last week, we discussed the reality of mass fractions for certified aircraft. There is an abundance of statistics on projects that have gone through the arduous development and certification phase, which always turns out heavier than projected.
Using such statistics, we have a base from which to fly a typical hover and cruise eVTOL design and see what we get in terms of energy consumption and range.
Figure 1. The Vertical Aerospace VX4 in an early rendering with similar looks to the eVTOL we discuss. Source: Vertical Aerospace.
eVTOL mission parameters
To capture the energy consumption of our typical four-passenger eVTOL, we need to look at how much energy the different phases of a mission consume. For this, we need to define a typical mission.
We operate our eVTOL in an air-taxi style feeder role to a larger airport, where regional airlines take over and fly the passenger to his destination or a major hub. We don’t set a defined range or mission time; instead, we discuss a feasible operational profile given the eVTOL design and whether we fly VFR or IFR due to weather. Then we discuss the resulting range and if it’s operationally suitable for this type of mission.
Let’s start with the easy parts first, the beginning of the mission:
- We assume we make a vertical start with a full ship, pilot, and four passengers with bags at 100kg each. Our hovering time is assumed to be 20 seconds, then a transition phase follows, followed by a climb to cruise altitude.
- Our cruise altitude is between 5,000ft and 8,000ft, depending on the mission range and weather. Projects assume 10,000ft as the cruise altitude to get the lowest drag and, thus, the longest range, but this is unrealistic. It’s a limit for flight without using oxygen in an unpressurized cabin, but not every passenger can handle 10,000ft. For this reason, airliner cabins never go above 8,000ft, and the trend is toward 6,000ft for better comfort. Anyone that has flown at 10,000ft in a private plane knows you better not have Sauerkraut for lunch!
Then we come to the more troublesome part of the mission, the descent and landing:
- The mechanics of the descent, the transition to hover, and the hover to touch-down are not the problem. We assume cruise speed for the descent, followed by a transition and a 45-second hover to land. The weather is. It’s straightforward to take off and climb in good and bad weather (as long as there is no icing) and land in nice weather. Landing in less-than-perfect weather is where it gets dicey. The problem is the increased reserve energy allocation needed that takes up an important part of an eVTOLs battery capacity.
- If we have unambiguous VFR weather, we have VFR regulatory reserves. These come in handy if there is, e.g., a delay in departing heliport eVTOLs or helicopters, i.e., our landing patch is not free. FAR fixed-wing rules say 30 minutes of flight time reserves, and Helicopter rules say 20 minutes. Which one will be applied to lift and cruise eVTOLs is not clear. Also, will these reserves be for forward flight mode or hover (there is a big difference in energy consumption per minute)? We don’t know; it’s not clear yet. We check what these mean in terms of reserves and range.
- If the weather forecast says we must plan for an IFR approach with an alternate landing spot, the situation is tricky. FAA fixed wing operations rules say we need flight energy to the alternate and after that for 45 minutes cruise flight, which reduces to 30 minutes for a helicopter. The EASA rules are similar. Neither regulator have said what the IFR rules for VTOLs are.
- Experienced pilots know that regulatory reserves are a starting point for your reserve planning. It’s the bare minimum. You pad these reserves in many cases as your experience tells you there is nothing so unpredictable as the weather, and you don’t risk your and the passenger’s lives. We discuss what margins there are for padding the reserves in an eVTOL.
We now have the mission parameters we need for our energy consumption calculation next week. Funnily, the vertical part is the simple one. The more difficult calculation is the forward flight phase.