Bjorn’s Corner: Sustainable Air Transport. Part 26. VTOLs.

By Bjorn Fehrm

July 1, 2022, ©. Leeham News: Last week, we finished our discussions around Fuel Cell-based airliners using hydrogen as fuel.

We could see the technology has true zero emissions, but the maturity of the many parts needed (hydrogen tank and fuel system, multi-MegaWatt class aeronautical fuel cells, motors, and controls) are not there. We are in the crawling before walk stage with sub-MegaWatt systems to make their first flights over the next years.

Another area claiming Green credentials is the VTOL space. Because these are based on electric technology, VTOLs are claimed as environmentally friendly and a good way to transport people.

We will analyze this industry and its claims of being an efficient, environmentally friendly way of transportation.

Figure 1. Joby S4, the VTOL project that has come the furthest. Source: Joby Aviation.

Are VTOLs part of Sustainable Aviation?

To understand whether VTOL is an intelligent way of getting us from A to B, we need to know how it operates and why it should be a better way for short air hops than helicopters or ground transports like cars or trains.

We look at the fundamentals of the technology behind these vehicles and how it affects their capabilities, performance, and environmental credentials.

What’s the difference between VTOL and a helicopter?

Helicopters have been transporting us in the same way as VTOLs since WW2. The big main rotor lifts the helicopter vertically, obliviating the need for a takeoff and landing strip. A parking slot the size of the helicopter rotor with some margin is all that’s needed.

The hovering flight of a helicopter is the easy part, just as it is for the VTOLs. That is not to say it’s easy; it’s technically just easier than flying forward.

Aircraft are, properly designed, stable (they don’t want to crash all the time), the helicopter is not. It requires constant corrective input from the pilot during hover, or it will slide out of control and crash.

To enable this control, the helicopter can vary the angle of the rotor blades as they rotate, Figure 2.

Figure 2. The principle for the main rotor pitch control from the Helicopter Flying Handbook. Source; FAA.

The graph in Figure 2 shows the principle for the pitch control of the main rotor blades. In reality, it’s even more complicated as the rotor head is free to tilt in any direction (within limits) to keep the rotor disc close to horizontal during flight.

Figure 3. A main rotor head implementation with a swash plate and the different bearings. Source: FAA.

The change in blade pitch is continuous during the rotation of the main rotor, Figure 4. This makes the control of the helicopter hover possible. By using the blade pitch to tilt the main rotor head, a forward flight is enabled.

Figure 4. The constant change of blade pitch, from the Helicopter Flying Handbook. Source: FAA.

To keep the helicopter from spinning around due to the rotor spinning in one direction, there is a tail rotor generating a variable counter moment (you control the pitch of the tail rotor with the pedals), Figure 5.

Figure 5. Tail rotor illustration from the Helicopter Flying Handbook. Source; FAA.

To enable the continuously variable helicopter rotor requires an expensive and complicated main rotor gearbox that transmits the rotor power but also carries the helicopter weight through the main rotor shaft. The gearbox and the rotor head control parts working on main long rotor blades are expensive and maintenance-intensive technology (ref Figure 3).

The major innovation of the VTOL is that it does away with the helicopter gearbox, rotor, and rotor head. It replaces these with the simple multirotor system we know from the drone industry, where the control of the vehicle is performed by controlling the RPM of the electric motors driving the rotors, rather than by changing the pitch of the rotors. By spinning the rotors in different directions and applying more torque to some, we can also control the yaw of the drone/VTOL.

The VTOL variants

In its simplest forms, the VTOL uses the same method to fly forward as the helicopter; it tilts the VTOL to deliver a combination of vertical and forward thrust. This method has the beauty of simplicity and avoids a troublesome transition between hovering and forward flight. Its drawback is a limited forward speed and, therefore, range.

Alternative VTOL schemes combine multirotor vertical flight, like the drones, with a transition to wing-borne forward flight, like aircraft. A wing-born flight is more energy-efficient and allows a higher speed and, therefore, range. But it presents new problems. The transition between rotor-born and wing-borne flight is tricky, so tricky that some projects haven’t mastered it in their seventh year.

We will use the series to go through the different VTOL schemes and look at their advantages, disadvantages, and how energy efficient they are. We start the look at “Multicopter,” “Vectored Thrust,” and “Lift and Cruise” VTOLs next week.

35 Comments on “Bjorn’s Corner: Sustainable Air Transport. Part 26. VTOLs.

  1. Regarding the standard helicopter issues most of them were solved with simpler solutions by Charlie Karman in his range of helicopters. While the Karman Seasprite was more standard design with tail rotor it still used servo flaps mid rotor or so to twist the blade for advance and retreat. The other choppers used twin intermeshing rotors to dispense with a tail rotor.
    The culmination was the last design the K max single engined logging helicopter, which because of it’s inherent stability and load carrying by under sling was used by US forces for fully remote control cargo delivery in Afghanistan
    Very comprehensive story here
    https://www.heli-archive.ch/en/helicopters/in-depth-articles/kaman-k-1200-k-max

    • The intermeshing blade “synchropter” is attributable to Dr Anton Flettner. The Germans manufactured 2 dozen Fi 282 “Kolbri” during WW2 and had planed 1000. Final version of the Fi 282 with a duel seat enclosed cockpit look quite modern. The configuration is known for it enormous and delightful stability and controllability in the hover and is thus used when flying rescue or flying crane is needed such as the successful Kaman HH-43 Huskie which served from 1953 to 1973. Anton Flettner invented the ‘servo tab’ which are often referred to as flettner tabs in Europe. These trailing edge devices are seen on Kaman rotors as well. Flettner also built the interesting Magnus effect ship. Flettner joined Kaman in 1947. They have some downsides such as a need to approach the sides with care.

      • Bit of NIH. Flettner is mostly unknown as the originator
        of Kaman’s product line.

        In a range of cases Hometeam tops Better mousetrap.
        There is a streak in US society to go for destructively competing. apply base ball bat and see: it doesn’t work.

  2. Thanks, haven’t read about this configuration before. Interesting variaton.

    • I have read about Kaman for lo many years.

      It is a variation that has its own challenges.

      If it was better (like ailerons vs wing warping) it would have changed the industry.

      Yes you can do it and yes it works but I don’t see it changing anything other than an interests aspect like Wankel Rotary engines. A lot of variations on IEC and we still use basic IEC.

        • Duke:

          That is not changing the world. Do you think people are so stupid as not to put a Turbine in a Helicopter no mater what the rotor system?

          they may have been first but when a 5 million animal heard of buffalo is right behind you?

          Nanner nanner rights but ……

  3. “Are VTOLs part of Sustainable Aviation?”

    I’m not so sure what is so Sustainable about this way of Air Transport. An (electrical) cab or underground to go to e.g. an airport seems cheaper and more environmental friendly to me.

    I’m afraid only the growing defense budgets will be able to operate this VTOL technology, e.g. to move supplies, take out tanks or move unmanned ground vehicles into enemy territory.

    Since someone got the brilliant idea to rename flying cars into flying taxis, dumb capital has been flowing again.

    So many visible initiatives, many start to reconsider it must be a good idea after all. Well, discuss with your wife & invest your old day savings.

    • keesje:

      We are on the same page on that one!

      Up here we call it buying into an airline, gold mining or a fishing boat.

      I picked up a contract job to fix up a Tug boat. The buyer and his partner has sold out a downtown piece of land and his accounted told them they needed to find someplace to loose money so they bought the Tug! (which needed some remedial work before they sailed it across the gulf of Alaska)

      Now those guys were going to make money long term, but the money drain to do so, great tax breaks for years! (think of it as Program accounting)

      • Like Crypto, it has a huge environmental impact over and above what we have now.

        Why?

    • The LilumJet website provided some estimates of CO2 emissions estimates.
      Very Fast Train: 15 grams per kilometre per passenger.
      LiliumJet eVTOL: 18 grams per kilometre per passenger.
      Estimates of VFT emissions came from Fraunhofer Institute.
      eVTOL obviously won’t need roads, rail or expensive fixed wing runways and infrastructure.
      Lilum sees itself in “Regional Air Mobility” and intends to avoid 20km flights or less. Cities like Zurich, Milan, Munich, Verona, Dijon can all be interconnected. In the US the centre of New York could connect to Harrisburg, Providence, Baltimore etc.
      Batteries and Electric Drives are very efficient. 90% of the electricity generated in a wind turbine and then stored in a battery will come out of the electric motor shaft again. So long as the battery mass fraction of the aircraft stays under 35% or so and ranges are short (250kmh or so).

    • To explain the logic (and I don’t necessarily accept it, just putting this out there), VTOLs are optimal for intermediate distnaces, under 200 miles, which generally take 4-6 hours by car. This is too short for (time) efficient air travel, but annoying by car or bus. In europe this distance is well served by trains, but the US does not have efficient intercity train service outside the east coast (and barely has efficient service there).
      This distance is also the distance that helicoptors are used for, but helicoptors are a) dangerous, and b) have a high cost per flight hour (they’re mechanically complicated and drink fuel like water). Electric VTOLs are mechanically simple and relatively energy effiient, particularly if they can transition to level flight. They also produce less noise than helicoptors since they have completely subsonic props and no turbines.
      There’s a technical issue with how battery electric vehciles work that is under appreciated… while batteries have poor energy density (J/kg) Li ion has excellent power density (KW/kg), approaching that of turbines. Moreover, larger batteries generally also can have higher peak load (thermal issues however need to be dealt with). This is the reason why electric sports cars are a thing… a battery large enough for high range is also a battery large enough for sub 2 second 0-60. Electric motors also of course support extremely high torque.
      An implication of this for electric aircraft is that they should have smaller wings than conventional aircraft. That’s because they can produce larger thrust than you would expect. Shorter wings mean less drag and longer ranges (i.e. the wing only needs to function at high air speeds) VTOL is a way of getting away with very short wings.

  4. The UAM’s still require a certified pilot making for an expensive short trip for a few pax. The cost will change if UAM’s are allowed to fly autonomous with control from ATC and you only klick on the screen where you want to go and ATC controls the flight and can divert you to any approved alternate landing sites in case of need. The EASA are working on certification requirements and we don’t know how it is progressing and what capability they will certify.

    • Absolutely agree, pilotless eVTOL will be essential for the technology to become truly affordable. I would expect the technology to be ‘semiautomatic’ and so simple to use such that only modest levels of pilot training is required during the initial period and certainly not the elaborate procedures, check list pilots of passengers jets must remember. The automotive world will not be standing still and we may see self driving cars in the same time framepilotless eVTOL if not sooner. Self driving cars if made aware of the phasing of traffic lights in advance offer the possibility of door to door travel without ever stopping at a traffic light. Cars will simply slow, bunch up, and pass through traffic lights in compact trains that never stop.
      Secondary Batteries will likely reach 500Whr/kg but not beyond. This is twice current mass production levels (though Amprius’ Silicon Nanowire Lithium Ion Batteries are commercially selling at 450Whr right now). This will mean electric cars with twice the range of gasoline powered cars.
      The pint I’m making is that aviation may be challenged not by the train but self driving cars and trucks if it doesn’t adapt.

      • UAM’s will be popular eventually in cities with lots of money and traffic jams. Not only need the UAM’s, pilots, automatic ATC with secure communication but aslo skyports with high current charging equipment be certified, alot to new rules not yet released. If one catch fire in flight it need to land quickly and the combination of Li-ion or similar high power batteries and carbon/aluminum structures will make an interesting fire to contain by the fire dep. in any park/playground or round-about. The UAM certification trails might include have one catch fire and show that it can be contained.

        • -Hi Claes, I concur there are safety issues with eVTOL UAM, RAM vehicles and unpiloted drones but I believe they can be managed and that eVTOLs will be safer than fixed wing aviation. New rules are needed: For instance a 45 minute hold and 100nmi divert is not needed for eVTOL because, unlike fixed wing runways, dozens of alternate vertiports the size of tennis courts can be provided along the route. These routes will be well planed and certified in advance. Areas of pastureland, even Golf Courses can be used as an alternate in the extreme.
          -In terms of the battery thermal run away fire issue. The cells in a battery pack will be monitored for temperature, current and maybe pressure. Any incipient thermal runaway can then be managed by shutting down the battery pack. Should the cell catch fire anyway the battery shall be certified to limit the speed of the spread of the fire from cell to cell and the escape of the fire from a battery firewall will be certified to contain the fire for example say 7.5 minutes. This would mean that for a 120kmh capable UAM alternate emergency landing points will need to be about 10 km apart allowing 5 minutes diversion and 2.5 minutes landing and evacuation. eVTOL’s will be equipped better than the A320 US Airways Flight 1549 which because of loss of engines from bird strike ditched in the Hudson River. (Surprised there wasn’t background background software running in the A320 ECAMS providing glide to airport solutions continuously to the crew).
          -There are worse risks in conventional helicopters and fixed wing aircraft. Those that use no advanced technology at all are known as hunter gatherers and that has many risks.

          • The pilot cost is a tiny part of the system cost that adds onto a cost and reducers no costs environmentally or any other way.

            A solution to a problem that does not exist.

          • @Dukeofurl, the Garmin ‘smart glide’ could presumably not only provide glide solutions to the nearest diversion airport but options to ditch in the nearest lake, beach, river or land in the nearest cornfield. I imagine there is even a cell phone app. The issue will be the legal ramifications and the need to maintain databases as to whether an airport has a viable runway or whether a river has obstacles such as bridges. It surely would have saved many lives over the last 50 years even in regular scheduled flights.

            @Transworld, A Lilumjet will have 6 passengers +1 pilot, eventually in unpiloted version 7 passengers while a stretched version will have 12 passengers. The pilot may not cost much if there is a full load of passengers and high frequency flights but it will cause cost problems if the flights are 40% full or if the frequency is low (pilot has to be paid whether flying or not). The Dream of UAM or RAM is costs comparable to a taxi or VFT train.

          • it seems the big issue with eVTOLS is survival with mechanicl or electrical failure. Airplanes of course can survive by gliding, and helicoptors can survive through autorotation (sometimes!). eVTOLS generally can’t do either. While redundancy is cheaper on a eVTOL, if redundancy fails, survival is impossible.

          • I agree they all can be certified by time. Still certification standards and test to show compliance with reglations need to be worked out for all parts of the system. EASA sounds working pretty systematic on the UAM including pilots certification, still the autoflight and communication standards need work. I think Police, M.D.’s and veterinarians in semi-remote areas will be ealy users arriving swiftly and quiet just when needed on pre-prepared landing spots.

          • @Mark, eVTOLs can in theory use ballistics recovery parachutes similar to the BRS-6 BRS Aerospace “Ballistic Recovery Systems” supply to Cirrus, Cessna and others. The parachutes are already extracted by rocket and if inflated by an explosive might have ‘zero zero’ capability like an ejection seat.
            LiliumJet recently announced it is upgrading its landing gear to cope with STOVL type operations. A big motivator was that if the batteries were depleted down to below 15% or 20% they would lack the power reserve to perform a vertical landing so the STVOL capability was added in to becalm customer concerns. It also will provide for genuinely short takeoff since many LiliumJet operations will likely originate from conventional airports but land in a vertiport.
            This beings up the whole issue of eSTOL. There are those that are arguing that an eSTOL equipped with a precision landing system needs no more foot print than a vertiport. It is possible to use electric motors to add boundary layer suction or wing flap blowing quite easily.

  5. A number of technological factors have combined to make eVTOL’s attractive.
    1 Electric Motors, Invertors and Batteries of very high energy or power density.
    2 The extreme ease with which high levels of redundancy can be achieved. Liliumjet has 36 electric ducted fan jets, 9 redundant batteries and I think three flaps on each wing/canard. These aircraft can tolerate multiple failures without interruption to their flight profile.
    3 The extreme simplicity of electric motors and actuators requiring little expert maintenance. Failures will be self diagnosed and dealt with by redundancy.
    4 MEMS “Micro Electro Mechanical Systems” which allow IMU (Inertial Measurement Units) complete with accelerometers, vibrating beam gyroscopes and tiltmeters to be integrated on one integrated circuit chip. These allow very robust and simple ‘fly by wire’ controls for personal quadcopter camera drones and they will make piloting eVTOL equally easy.
    5 There ease of piloting by remote control
    6 Feasibility of using a ballistics parachute in an emergency, which is not possible with a helicopter.
    7 A key is very low noise footprint.

    The main source of cost of the eVTOL may be battery replacement however that may be highly predictable, infrequent and once the battery is removed it can be recycled in a controlled environment.

    • To which I might add the extreme ease with which a vertiport (relative to a fixed wing runway) can be provided.

      • Users might want the vertifort to be fully equipeed with fire fighting, high voltage charging, raised above ground to aviod noise at ground, fully weater montoring to transmit local weather and warn if wind shear or heavy winds. Maybe some navigation help to connect to the UAM autoland system and lights. For winter operations deicing and maybe a small covered and heated parking lot to push your UAM into. A skybar would also be nice as when fully automatic flights you can take a drink or 2 before taking off.

  6. I’m fairly certain that a quad delivery of a pizza from the local supplier to my house(1 mile, 1/2 a mile as the crow or quadcopter flys) would be better for the environment and the finances of the business concerned than paying an employee to jump into a car to do the job. It’s amazing to me that no one is able to provide actual figures to indicate whether this is true or at what point does the weight or range make it unviable. Until then we are just wasting our time even talking about it.

    • Grubbie:

      Streets are simple, as is traffic management (done right with AI to change fixed timing to variable timing)

      You Pizza delivery is horribly expensive with major resources for no gain.

      A car uses the existing infrastructure with a low paid delivery guy in a car that runs cleaner than the air it travels through,

      What seems simple has a massive infrastructure behind it and the other factors are as listed.

      This has been the constant harping wonder about technology, how wondrous it is and how it will magically make our lives ones of ease and each time it occurs (or is possible) we find the visionaries were using LSD and little of it come to see the light of day and what little does, most is down the drain soon.

      With computers came the internet and cyber warfare and scams. No one has added the costs vs the gain there I know of either. My information has been stolen half a dozen times and that includes intense security background to work at an Airport (ramp access, gate access, custom clearance) – that means I had a Fed Check, a State check, a private security check (10 years)

      To this day I have no idea what could happen to that info, is China compiling a list to clean out the back account of every American?

    • Googles parent company Alphabet did 2 trials of delivery drones near Canberra Australia in rural areas several years ago. The 12 rotor drones delivered foods and services to about 100 homes etc by winching them down from a hover at what looks like 15 meters. They served mainly farm homesteads. The Australian regulator CASA gave exemptions. Some local residents objected complaining of noise and fears for bird life. Some were in tears over the issue and some threatened to shoot the drones down.
      -The reactions seem extremely histrionic given the very low population density (sheep grazing land) and the quietness of the drones and percentage of people prepared to go into a “tail spin” small but those folks exist and are very vocal. Certainly there are concerns but one can never underestimate the danger of them being exaggerated. I can not see them being usable in the standard residential suburbs except in post offices and commercial operations.
      -It reminds me of the British red flag act which required motor vehicles to be preceded by a man waving a red flag or Bertha Benz drive a motor vehicle 106km for the first time on public roads which may have only been possible for a woman to do since a man would be accosted with objections or the decade it took for my local town of 4000 to get its first small supermarket due to the endless objections of a cadre of the “progress association” there are complex psychological issues to deal with.

    • For pizza delivery and small 1-10lb cargo you don’t need an UAM that can take 2-7 pax but todays electric Chinese DJI and similar works fine and is a different market

  7. Recently I got a message on my phone for a guy on his aircraft parts, you are getting them whether you like it or not, you can not cancell this order!

    Normally I don’t return those calls (wrong number) but I know how much aircraft parts costs and Joe needs to know he is on the hook. So being the good citizen I am, I called back, gave them all the info I had so they could track Joe down and let him know.

    Nothing about aircraft is low cost. The only people that will be able to use air taxi are rich folks.

    The system to ensure safety and clearance is hugely expensive. I sure don’t want to pay for it.

    And elecric and hydrogen come from someplace ans batteries mean digging dirt and huge mines.

    So, like Boom who will use low emissions engines that cost 20X the carbon cost to move 20 X fewer passengers, this too will not fly.

    The Aviation system is worth it and works because it benefits a low smuck like me and we support it. We are not going to support transportation for the Rich so they can avoid the dirty masses.

    Anyone who thinks any of that pie in the sky stuff is going to be low cost is delusional.

  8. As was recently put, A coup in search of a legal justification

    In this case its a Tech involved in looking for a mission that is not needed and has huge costs and environmental impacts.

    We have this cool toy and we just have to do something with it and damn the cost to society, full speed ahead.

    • I’m afraid Defense (or Offense) iseally has the billions and utilzation need.

  9. I read this recent piece by an author at the German Center for Air and Space Flight. I gather that distributed propulsion per se doesn’t unlock a whole lot of efficiency gains at the system level, but that it allows for reduced wing size. Higher angle of attack during cruise typically yields improved lift-to-drag ratio, and a smaller wing has less wetted area hence reduced parasitic drag. A pretty interesting read for any one who is looking for some stimulating summer reading!

    https://link.springer.com/article/10.1007/s13272-021-00535-5

    • There would be several other advantages that could be achieved with distributed propulsion, particularly electrical.
      1 In eVTOL mote the wings and undercarriage can be smaller providing a considerable weight and drag saving.
      2 A smaller Rudder and tail is possible since in the case of an engine failure on take-off the asymmetric thrust is not so great in a single engine failure.
      3 Safe Take-off field is reduced as in the case of a single engine failure it has less effect.
      3 Boundary Layer Ingestion Propulsion can provide an 8.5% reduction of drag. Imagine a tail jet engine with an annular inlet that provides attaches the airflow to the fuselage in a laminar way. An electric fan is less vulnerable to the compressor stall issues that might develop.
      4 Blown flaps can reduce takeoff field length or allow higher wing loading.

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