Bjorn’s Corner: Sustainable Air Transport. Part 40. VTOL mission.

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.

17 Comments on “Bjorn’s Corner: Sustainable Air Transport. Part 40. VTOL mission.

  1. 20-30 Minutes flight reserves for aircraft that can hardly make their foreseen flights even with tomorrow’s hoped for battery capacity..

    Apparantly the eVTOL industry is hoping for major exemptions and wavers from flight safety and ATC requirements?

    • Or lots of alternative heliports with an UAM industry standard autoland systems close to your intended destination. If the destination is an airport you don’t transition to vertical mode just make a rolling landing. Similar for T-O from an airport to use an available UAM short runway. In the cities you need vertiports and you need reserves, still you can always turn back to the city airport and make a rolling landing if battery juice is enough. If might be a US certification requirement to be able to make a rolling landing on baseball diamonds or football stadiums. Like interrupting a Yankees game during bad weather for a couple of UAM’s landings to be pushed off field by Linebackers after landing.

  2. Since all these flights are bound to be short, there is very little risk of landing weather becoming a lot worse than what it was at the time of takeoff. Because of that, it may make sense for the lower (helicopter) times to be required, although it seems logical that it be in hover mode.

  3. Initial operations are meant to be short-duration and short-range. I highly doubt that there will be much consideration for IFR in the beginning. With short range/duration, localised weather prediction becomes less of a problem (still a problem, just maybe not as big as implied in the article).

    However, limiting ops to VFR will definitely detract from the appeal of such a service, in terms of convenience and availability.

    • The heights they say they want to fly at indicate it will often be in cloud , in say Europe and US Northeast.
      It doesn’t have to be extreme weather for
      beyond VFR , just in cloud will cover that. And yet for the range they want to be at the extreme altitude for non pressurised cabins, now you suggest they can be VFR as well.
      It all doesn’t add up

      • Yes, at many locations the weather can switch very fast. I can think of Japan, Tasmania, Iceland, Newfoundland, Fairbanks Alaska, Denver as called in the US “weather whiplash”. It can be hard to notice as when moist air freezes to clear ice and would effect UAM’s quickly as few will be certified for flight into known icing conditions. After a few accients Rain and Ice protection systems ATA30 will be mandatory..

  4. Maybe the electric vehicle can pick people up in front of their house/ office, it can be super silent, spacey, go in real bad weather & cost 10 times less to build and maintain. And you don’t need next gen ATC and pilot linceses. Name it EQ, Tesla or iX.. order thousands today for delivery within a year.

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