This article originally appeared on LinkedIn, authored by Mehmet Emre YAZICI.
* The word “shape” is freely used to describe the external configuration of the aircraft.
Why Airframe Shape is Important?
For the first time in known history, humankind is this close to make the “flying-car” a reality. Today, 70+ manufacturers worldwide, (including; Airbus, Bell, Boeing, Embraer, Rolls-Royce, Toyota, Volvo, etc.) are seriously involved in developing some form of an electric vertical take-off and landing (eVTOL) vehicle. As of September 2018, total global investment volume has exceeded $1 billion. According to The Electric VTOL News™, since 2011 some 140 different eVTOL projects based on four basic configurations, have been announced. While wingless multi-copterconfiguration is the most favored (45%), electric helicopters are not-so-popular (3%).
There obviously are major difficulties ahead. Although, the prioritization differs from person to person, I believe that the top three are; regulatory issues, environmental effects (mostly noise) and public acceptance (due to security and privacy concerns). So, configuration of the aircraft is not among the top three challenges that UAM (Urban Air Mobility) pioneers are facing today. Why do I bring it forward, then?
Simply because, throughout the life-cycle of the aircraft, major changes on the airframe are avoided, not only due to cost or performance concerns, but also because of huge re-certification efforts required. Some minor modifications, however, such as adding winglets or external domes for sensors which are covered under supplementary type certificates (STC) are more common. Beyond that, you can easily change or upgrade almost any other component (avionics, engines, furnishing, etc.) that comes on the aircraft. So, the airframe shape or form (housing all the other components) becomes single fundamental element that will affect all; regulatory, environmental or public concerns.
Who is Doing What?
According to polls, the public tends to favor eVTOL products by well-known manufacturers vs the start-ups. So, it will be worthwhile to see which configuration the aerospace and automotive giants are preferring:
- Airbus: A³ Vahana and CityAirbus
Back in 2015, Airbus had been the first (among the traditional aerospace companies) to announce its plans to secure a place in the commercial eVTOL market. “A-cubed Vahana” is a tilt-wing (i.e. vectored thrust) aircraft project being handled by an Airbus subsidiary in the Silicon Valley. Whereas, CityAirbus is a four passenger, autonomous wingless multi-copter program ran by the Airbus Helicopters.
- Bell: Nexus and APT
Nexus is a six-passenger vectored thrust aircraft announced in 2017 Uber Elevate Summit. On the other hand, APT (Autonomous Pod Transport) is 20 to 32 kg payload tail-sitter that we can classify as a vectored thrust vehicle.
- Boeing/ Aurora: PAV and CAV
Creatively named as the Passenger Air Vehicle (PAV) developed together with Aurora Flight Sciences is part of the Boeing NeXt urban air mobility project. It is a two-seat lift+push design which made its maiden flight in January 2019. Also a part of Boing NeXt, CAV (Cargo Air Vehicle) is a 225 kg payload wingless multi-copter.
- Embrarer: DreamMaker
Also announced during the 2017 Uber Elevate Summit, the DreamMaker is a piloted four passenger lift+push aircraft.
- Rolls-Royce: EVTOL
During 2018 Farnborough Airshow, Rolls-Royce announced the EVTOL concept. Rolls-Royce’s aircraft is a hybrid tilt-wing (i.e. vectored thrust) carrying 4-5 passengers.
- Lockheed Martin/ Sikorsky: VERT and ARES (with Piasecki)
Lockheed Martin and its new subsidiary Sikorsky are working on both military and commercial VTOL concepts. While Lockheed Martin is indulged in development of a vectored thrust configuration for military use together with AVX and Piasecki, Sikorsky is working on a project named VERT. And that is all we know, for the time being…
- Toyota: Cartivator SkyDrive and Joby S4 (co-investor)
In 2017 Toyota agreed to invest in Cartivator -a Japanese start-up, to develop the world’s smallest flying car. SkyDrive concept is a single-seater wingless multi-copter. Additionally, Toyota is among the investors in the Joby Aviation’s S4 program. S4 is a four-seat vectored thrust aircraft.
- Volvo/ Lotus: Terrafugia TF-2
Chinese company Zhejiang Geely, which owns the Volvo and Lotus car brands, has acquired Terrafugia in 2017. In its current form, TF-2 is rather a modal transportation “system” than a simple “air vehicle”. The flying component of the well thought-out design is a lift+push solution.
Out of the 12 (excluding Sikorsky’s VERT) vehicles above, 50% have preferred vectored thrust, while lift+push and wingless multi-copter have 25% preferences each. The choice of giants are not in-line with the alternatives preferred in the eVTOL community. So, we can easily say that the best configuration is yet to be agreed.
Basic Disadvantages of Popular Configurations
The wingless (multi-copter) designs (preferred by 45% of the whole community and 25% of aerospace and automotive giants), although very popular and made the “drone revolution” possible, are doomed due to their slow top speeds. For commercial eVTOLs, the economics of the vehicle would dictate to perform as many cycles as possible in a given time. In addition to that, large un-protected propellers placed close to the ground is a major safety concern for passengers and crew alike. Therefore, although simple and well understood, multi-copter designs will unlikely be the choice for a successful flying-car design. (Best illustrative example: EHang 184/ 216)
Despite the simplicity and weight advantages offered by lift+cruise designs (preferred by 12% of the whole community and 25% of aerospace and automotive giants), they inherently possess serious safety challenges. Un-protected (i.e. un-ducted) horizontal propellers, operating very close to ground (usually due to the size limitations of small vehicles) constitute a safety risk just like the multi-copters. Therefore, I see a bleak future for the designs utilizing unprotected props located close to ground. (Best illustrative example: Boeing/ Aurora PAV)
Vectored thrust designs (preferred by 40% of the whole community and 50% of aerospace and automotive giants), provide the best compromise between versatility and speed. But, when utilize un-protected propellers (tilt-wings mostly), they have the same disadvantages of lift+cruise designs (Best illustrative example: Airbus A³ Vahana). On the other hand, ducts employed to eliminate risks associated with props, bring in weight disadvantages and some additional drag during the cruise (Best illustrative example: Bell Nexus).
So, what is the solution? Which design will prevail? Frankly speaking, I don’t know! But, I believe that the applications employing Distributed Electric Propulsion (DEP) concept through small units and in large quantities are closer to an optimum solution. Such configurations give way to enclose unnecessary units during cruise in other components (i.e. wings) to reduce drag. Additionally, large amount of redundancy in propulsion system relaxes safety margins. Aurora Flight Science’s VTOL X-Plane Program XV-24A LightningStrike and Lilium’s Electric Jet are best available examples of this concept, both of which have successfully demonstrated the feasibility of the concept.
The LightningStrike program had aimed to develop a vertical take-off and landing 12,000 lb (5.4 t) demonstrator aircraft with 18 motors in the wings and 6 motors in canards, that will achieve a top sustained flight speed of 300-400 kt (555-740 km/h). A 20%-scale demonstrator aircraft weighing 325 lb (147 kg) with a flight model of the full-scale demonstrator was flown in March 2016. Surprisingly, in April 2018 it was decided that the subscale model demonstrated the program’s major objectives and DARPA cancelled the project.
Lilium’s two-seat “Eagle” prototype completed a series of unmanned test flights in April 2017. It had a total of 36 electric fans: 12 on the upper surface of each wing with tilting flaps and two rows of three fans on each side of the canard, which was designed to be retracted in cruise flight.
Until someone comes up with anti-gravity technology, I believe this is the best shape for flying-cars of urban air mobility initiative…