Saleem Zaheer, XTI Aircraft Company

As developments in the evolving Advanced Air Mobility (AAM) space gradually continue, the market for passenger Vertical Takeoff and Landing (VTOL) air mobility is bifurcating into:

  • All-electric “e-VTOL” aircraft. Electric motors powered by batteries, providing lower noise and zero direct emissions, with accompanying range limitations.
  • Hybrid-electric “he-VTOL” aircraft. Electric motors powered by a combination of generators driven by a turbine or piston engine, and batteries, enabling long-range, high-speed, all-weather operation.

While there are a few hydrogen fuel cell air vehicles under development which promise longer flight times and range, the ecosystem required for wide deployment of these vehicles will result in limited use for some years.

Most media attention has been focused on Urban Air Mobility (UAM), the short-range hops relevant to solving city/urban congestion. This is largely due to the commendable efforts of Uber Technologies in developing the vision for all-electric air taxis in its original white paper in October 2016 which served as a catalyst for development of those aircraft to address the worsening traffic congestion in cities.  Since cities are Uber’s domain, its solution is all-electric VTOL vehicles to reduce congestion, pollution, and noise.   Battery-only electric propulsion (despite its range limitations) can meet the very short (10-60 mile/15-100 km) flight segments projected to be served in metro areas.

However, the vision of thousands of these e-VTOLs flying over dense urban areas seamlessly blending into an all-encompassing mobility ecosystem (scooter, taxi, bus, train, air taxi) is an end state that is likely to be over 20 years away. The reasons for this are many:

  1. Battery energy-density limitations will restrict useability to very short segments, especially if regulations require flight time reserves to be greater. For example, if FAA regulations for UAM require 30-minute flight time reserve by day, as they do for day VFR (Visual Flight Rules) aircraft flights (14 CFR § 135.209), then most e-VTOL flights will be severely restricted.
  2. Operations in bad weather will pose challenges. Based on scattered information available, it is likely that battery-only e-VTOL flights will be restricted to VFR which means flights in good weather conditions only, at least initially. This will be especially true if icing conditions are present, as significant electrical power will need to be extracted for de/anti-icing. Given slim reserves of electrical power to begin with, this could significantly reduce useability in many large urban areas prone to bad weather, possibly restricting e-VTOL deployment to cities with fair weather conditions. This is why Uber has chosen Los Angeles, Dallas, and Melbourne, Australia as test cities. Operations of these very light e-VTOL aircraft in dense cities with high-rises, under poor visibility, low clouds, thunderstorms, ice, rain, and erratic wind patterns between tall structures are likely to pose significant challenges.
  3. Lack of Scale. As Uber has stated at their Uber Elevate conferences, their business model makes money only at scale - with the deployment of thousands of these e-VTOLs, preferably with autonomous control, so that the pilot seat can be used for revenue generation. Other factors working against scale include the following:
    1. Certification. Regulators in the US and Europe (the FAA and EASA respectively) are pro-active and helpful in working with manufacturers to usher in this new era in aviation. However, they will not compromise on the safety standards required for certification of these vehicles, especially since these vehicles will operate over densely populated cities where an accident is likely to have serious ramifications. We expect them to take a cautious approach, even under the new performance-based regulations now in place. And when this is extended to certifying pilotless autonomous flight carrying paying passengers over dense cities, expect the bar to be very high.
    2. Public Acceptance. While limited deployment of e-VTOLs over some urban areas will occur in the next five years or so, scaling this up to profitability (e.g., 1000 vehicles over an urban area) may lead to public resistance, especially if the noise is annoying to people on the ground and/or if there are accidents. Also, some cities may embrace e-VTOLs overhead, while others resist. If e-VTOL use is largely restricted to good weather days only, such services may be deemed unreliable, making widespread deployment unlikely.
    3. Infrastructure. At scale, wide deployment of e-VTOLs will require setting up vertiports in different parts of cities, ideally in locations where inter-modal connections are easy (e.g. on the roof of a metro/train station). These will have to include battery charging infrastructure, passenger waiting areas, aircraft landing and parking stands, etc. This will require new building codes for high energy charging, fire suppression and containment for lithium battery fires, crowd egress from a rooftop in case of a fire, etc. Moreover, some cities may embrace and amend zoning regulations to comply, while others may be slow or may not adapt. This will slow deployment.
    4. Air Traffic Management. Large-scale deployment of e-VTOLs will require new regulations and new technology for managing the volume of low-altitude traffic envisaged. Work has commenced on this, though it is unclear if this will evolve at the pace required to lead to large-scale profitable deployment.

UAM requires creation of a new, currently non-existent market. Once the UAM market is sufficiently developed, these eVTOLs will be an excellent contribution to reducing traffic congestion.  However, as they will be limited to short-range (less than 60 miles/100km), low-speed flights, they will be unable to compete in most current helicopter and airplane markets, other than for niche areas such as short distance air taxis, Emergency Medical Services (EMS) and some cargo or military applications. Reaching scale and profitability in the UAM market may take decades. There is one OEM, Lilium, projecting 186 mile/300km range for its e-VTOL aircraft on a single charge, with plans to compete in the longer-range regional inter-city market. This would be an impressive accomplishment if Lilium is able to achieve that performance.

While work is ongoing to solve all these issues, the team at XTI Aircraft Company has pursued a different approach – a fixed-wing VTOL aircraft with a hybrid-electric propulsion system.  Our aircraft and strategy will lead to faster market entry and wider overall market penetration for the following reasons:

  1. Hybrids are not limited by battery energy density. A Hybrid-Electric VTOL aircraft uses a smaller fuel burning engine to power generators, thereby supplying electricity to motors that turn the propellers or fans, depending on the design. Batteries add supplemental power during vertical takeoff and landing. This allows the aircraft to cruise on a smaller, much more fuel-efficient engine than would be required for engine-only propulsion. Once in wing-borne flight, the batteries’ charge is topped off by the generators during flight, so they are fully charged and available for the high-power landing phase. This eliminates the need for any ground battery charging infrastructure, with vast implications for widespread deployment, as hybrids will be able to operate from any existing helipads, airports, paved or grassy areas.
  2. A radical drop in CO2 emissions. Hybrid-electric propulsion offers a radical improvement over pure internal combustion propulsion systems. We anticipate a 40% reduction in CO2 emissions and ~ 50% lower Direct Operating Costs (fuel and maintenance) over competing turbine helicopters and fixed-wing jets and turboprops. Further, once either battery or hydrogen fuel cell technology improves to a point where these long-range, high-speed missions are possible, a switch to those can be made, leading to zero emissions. For now, we believe that starting with a hybrid electric system is rational, as automobiles have done over the past few decades. To aim for zero emissions at this stage would significantly sacrifice mission capabilities, as no technology currently exists to do so.
  3. Compete in existing markets. Hybrids can directly offer radical improvements over existing helicopters and business aircraft. You could take off from the Manhattan/Wall Street Heliport and land at the Chicago Vertiport non-stop in just over two hours. This unique capability opens up a vast range of potential applications. For example, London Heliport to Paris Heliport (230 miles/370 km) in 55 minutes, saving three hours trip time compared to airline travel.
  4. Open new markets and democratize travel. Hybrid VTOLs can be designed with greater seat capacity, even matching medium to large helicopters. This results in Cost per Available Seat Mile (CASM, a useful metric for comparing efficiencies of operators), similar to that of a passenger car. This makes possible a host of profitable use cases for the operator, while being very price competitive against aircraft or other modes of surface travel. For example, Manhattan Heliport to Washington DC in an hour for $149, the same cost as an economy airline ticket (November 2020) while saving 2 hours and 15 minutes in travel time. Or compare this to the Acela high speed train between Manhattan and Washington DC which takes three hours and costs $215 (November 2020).

    building topXTI TriFan 600

  5. Capable of all-weather operations. Hybrids can be certified for all-weather IFR (Instrument Flight Rules) operations and dispatch into known icing conditions. This is a distinct advantage over e-VTOLs. This will result in greater deployment and useability worldwide, and far greater dispatch reliability year-round as they will be able to operate under foggy, rainy or snowy conditions.
  6. Require no additional ground infrastructure or new Air Traffic Management procedures, as they require no battery charging and can operate out of existing infrastructure. This will allow rapid entry into service.
  7. Do not require autonomous operation to be profitable. With lower direct operating costs and lower CASM due to increased seating capacity, hybrid VTOL operations are profitable without the need to remove the pilot so as to use that seat for revenue. While we do see unpiloted autonomous operation occurring in the future, we anticipate such certification to be at least 10 - 15 years away.

In conclusion, this is an exciting phase of aviation that we have embarked upon!  The field is energized and on the verge of radical improvements in aerial mobility.  We at XTI Aircraft believe that, given the state of the industry today, hybrid-electric VTOL aircraft will find greater deployment in all use cases other than the very short hops possible by all-electric e-VTOLs.

I’ll end with a statement by Eric Bartsch, CEO of VerdeGo Aero (developers of hybrid and electric powertrains for AAM), “Why have so many urban air mobility startups emphasized battery-only solutions? I think, unfortunately, it’s because a lot of people aren’t doing the math and don’t understand the physics… When you want to do these commercial missions and get a vehicle that’s capable of more than a demo hop, you really have to know how much energy you’re going to have on the aircraft.  The fact is, the math doesn’t work for a lot of these market segments for batteries right now. The math will work in 20 years, but we don’t want to wait.”


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