August 12, 2022, ©. Leeham News: Last week, we looked at two eVTOLs that don’t fit the terminology we use; Multicopters, Vectored thrust, or Lift and Cruise, the Vertical VX4, and the Archer Maker. We look at Airbus’ CityAirbus NG this week, a Lift and Cruise design like no other (Figure 1).
Figure 1. The Airbus CityAirbus NG. Source: Airbus.
A market far from given
Airbus is the World’s largest airliner and helicopter manufacturer. It had an early start in the eVTOL space with two projects, the Vahanna (Figure 2), a thrust vectoring concept, and CityAirbus (Figure 3), a large multicopter design.
Figure 2. Airbus Vahana VTOL during tests in California. Source: Airbus.
Figure 3. The CityAirbus during tests in Donauworth, Germany. Source: Airbus.
I chatted with the design team of the CityAirbus NG a year ago in Toulouse and talked with Sarihan Balkiz, Airbus Head of UAM Strategy Execution & Partnerships, last week.
Here is my takeaway from these discussions, the strategy part first and then the technology:
- Airbus is in the eVTOL space with a long-term view. It has declared at briefings that any forecasts of a large VTOL market are high-risk. Airbus is no wiser than anyone else regarding whether there is a sizeable market. VTOL noise will be critical for the acceptability of the category. To help acceptance, Airbus focuses on medical transport and mobility to remote areas as initial services.
- The first generation of CityAirbus NG is deliberately conservative in its design, claims what it can do, and its market prospects. The NG project doesn’t have to pitch to an investment community for its budget, where it must promise great performance, a large market, and a bright immediate future for its VTOL.
- The team around CityAirbus NG had access to all the data from the Vahanna and CityAirbus prototypes. It learned a lot. From Vahanna about vectored thrust, from CityAirbus, the hover problems with a large VTOL. A lot of research post these prototypes were done on rotor noise and stability. The large VTOL rotors are prone to instability, such as flutter, when their load changes.
- The decision was to make the first generation CityAirbus NG as simple as possible to ease Certification and create operational reliability. The rather different architecture shall be understood in this context.
The CityAirbus NG Architecture
As can be seen from Figures 1 to 3, the NG architecture is very different from both Vahana and CityAirbus. In fact, it’s different from all other VTOLs I have studied.
In its base setup, it’s a multicopter with eight identical propulsion units. But the two rear rotors sit at about a 35 degrees angle from the horizontal and work as lift units during hover and combined lift and forward thrust units during cruise flight.
The six wing propulsion units sit on booms where the rear units eat into the wing trailing edge, Figure 4.
Figure 4. CityAirbus NG from above. Source: Airbus.
In a normal case, this would impact the function of the wing ahead of the cut-outs. But as all rotors rotate during forward flight, these rotors generate a low pressure that will suck the rear boundary layer off the wing into the rotor stream. Therefore, we expect that the wing has attached flow throughout the span at the low cruise speed of 65kts/120km/h.
The wing is large but cannot carry the aircraft’s weight due to the low cruise speed. It seems it can contribute about half of the lift at cruise with the lift rotors to generate the rest.
The control concept of the VTOL is simple as it’s 100% through RPM changes of the eight rotors by the VTOL’s Fly-By-Wire system. There are no movable thrusters or control surfaces.
For hover, all rotors work together, and the VTOL is tilted slightly backward to compensate for the forward thrust of the rear rotors. The transition is easy, tilt the VTOL to horizontal or slightly forwards, and the rear rotors go from 35° to 45° tilt, where they contribute to forward speed with 70% of their thrust.
If the control system and buildup of the propulsion are minimalistic, the NG structure is not. The wing area is larger than all other VTOLs we looked at except the VX4, and the twin-boom tail presents a doubled wetted area.
Advantages and Disadvantages of the architecture
Here is a summary of the advantages of the concept:
- The NG has the simplicity of a multicopter, yet it’s a lift and cruise design with more speed and range than a multicopter.
- The concept has a trouble-free transition, as the only change is the tilt of the VTOL in the forward direction. Rotors don’t start or stop, nor do their loads change significantly during flight.
- A simple RPM control of the rotors gives the flight control authority needed for a wing and tail design.
- The rotors stay within a reasonable RPM and speed range during the flight; therefore, these can be of the simple fixed pitch type.
- The wing contributes about half the lift in forward flight, lowering the energy consumption of the VTOL.
But there are also disadvantages;
- The cruise speed of 65kts is low compared with competing designs’ 120kts, and the range is also low at 43nm versus the typical 85nm. Part of this is the design; part is conservative marketing where these values contain margins. The figures also reflect a refreshing realism of what batteries can do by the introduction to service 2026 (batteries are from Airbus internal battery competence center).
- The CityAirbus NG has a larger airframe than competing four passenger thrust vector designs and about the same as faster, longer ranged lift and cruise designs. A large structure generates more drag due to size (parasitic drag) and weight (induced drag). The low cruise speed masks the parasitic drag, but a mass disadvantage will show up in higher hover energy and induced drag, eating into the VTOL range.
The Airbus CityAirbus NG project is a rare breed. Not only for choosing an architecture different from all others but also because of the non-inflated projections about the potential market and for the speeds and feeds conservatism around what the NG can do.
As we will see in a later Corner, few VTOL projects can realize their speed and range claims. The CityAirbus NG might be one of the few that can.