Bjorn’s Corner: Sustainable Air Transport. Part 30. Mixed architectures.

By Bjorn Fehrm

August 4, 2022, ©. Leeham News: This week, we look at two eVTOLs that don’t fit the terminology we use; Multicopters, Vectored thrust, or Lift and Cruise. The Vertical VX4 and Archer Adam are Lift and Cruise designs, but they use vectored thrusters for the cruise thrust, Figure 1.

Figure 1. The vertical VA-X4. Source: Vertical Aerospace.

Thrust vectoring Lift and Cruise
vertical aerospace

Vertical Aerospace, a Bristol UK-based upstart, has iterated its way to the VX4 architecture. Its pilot plus four passenger VX4 VTOL is designed to takeoff and land vertically but to fly in a classical aircraft wing mode with a V-tail controlling pitch and yaw and ailerons on the wings controlling roll.

The Vertical wing is large. It shares the 15m wingspan with Beta Technologie’s Alia-250, but the wingspan is 35% larger.

While the Beta design is a pure Lift and Cruise design, the VX4 is a hybrid with four vectored thrust propellers/rotors ahead of the wing and four lift rotors aft of the wing.

The vectored thrust units have variable pitch rotors, whereas the lift ones are fixed pitch units that stow. The unique feature of the VX4 is a four-blade lift rotor concept that stows the two blades on top of each other (Figure 1). It forces some kind of mechanism for the stow. The four blades increase the rotor solidity so that it can work with a higher power to match the five-blade forward units.

The Vertical project has gathered a seasoned team of aerospace people, and this might be the background to the impression of a large and generously sized VTOL when I saw the full-sized mockup at the Vertical stand at Farnborough.

Vertical focuses on battery and rotor technology as its core IP, with partnerships with GKN for structures, Rolls Royce for electric motors/inverters, and Honeywell for the Fly-By-Wire.

Using vectored thrust for 50% of the propulsive units saves a dedicated forward flight motor and propeller. The vectored thrust units must still go from 80% thrust and power while hovering (the remaining 20% ​​is for aircraft control with RPM changes and redundancy should one vertical lift unit fail) to around 15% thrust/power in forward flight.

The optimization problem we discussed in Part 28, Vectored thrust VTOLs, remains; it’s just less severe.

Archer Adam

If Vertical’s VX4 is generously sized, the Archer Adam is the opposite. It’s a two-seater functional prototype to test the concept and flight dynamics of a 12-rotor “six by six” concept, Figure 2.

Figure 2. The Archer functional prototype Adam. Source: Archer.

If the functional prototypes validate the concept, the production model will be a larger variant of the 80% scale Adam. One can assumes a four-passenger production model based on this information.

The Lift plus Cruise concept is the same as for the Vertical VTOL; it’s just divided over 12 smaller rotors than eight larger ones.

Here is a summary of the advantages and disadvantages of the concepts:

The advantages:

  • The fully winged plus tail concept allows for an efficient forward flight mode. The vectored thrust propwash can help the wing to attach flow during the tricky transition phase.
  • The dedicated lift units allow the rotors to be fixed pitch designs. Their angle of attack at their stowed forward flight position will be critical so as not to shed vortices from the stowed blades during flight. It generates a high drag, but the concept halves the number of forward flight stowed rotors compared with a full Lift and Cruise VTOL.

The disadvantages are;

  • The use of half the vertical lift propulsive units for forward flight thrust forces these to be variable pitch units with motors and inverters that need a decent efficiency both at 15% forward flight power and 80% hover power.
  • It also adds the weight and complexity of the vectoring joints and actuators. It seems the weight could be in the order of a forward flight motor and propeller.
  • The wings and tails for both VTOLs will be subject to propwash. It creates scrubbing drag and upsets the wing lift distribution (ref. Part 28).

It remains to see which choice will prevail, the Wisk, Beta, EVE pure Lift and Cruise principle that avoids the propwash problems (by using pusher forward flight thrusters) or the mixed vectored thrust Lift and Cruise designs of Vertical and Archer.

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