Lift And Cruise: The Ins & Outs Of eVTOL Design Architecture

Separating vertical lift from forward flight thrust generation.


Photo: Archer Aviation

Apart from light transport applications, aircraft manufacturers aim to extend urban and regional mobility programs to passenger and cargo transport using electric vehicles. New and established manufacturers continue to build electrical vertical takeoff and landing (eVTOL) prototypes for proof of concept and preliminary testing. They envision the future of air mobility programs to be based on electric or hybrid-electric power and propulsion systems.

The eVTOL aircraft is designed to take off and land vertically but fly in a conventional aircraft wing mode during the cruise. Controlling pitch and yaw movements can either be performed through control surfaces or by varying the rotor RPM using fly-by-wire inputs.

Lift and Cruise VTOL design

The Lift and Cruise VTOL design distinguishes the vertical lift thrust generation from the forward flight thrust generation. In other words, some rotors are responsible for generating lifting thrust, while others are used for forward thrust. The design increases the number of rotors, motors, and inverters to establish a coherent propulsion system. However, the design is simplified by streamlining the transition between vertical and horizontal (forward) flight.

The lift and cruise design is considered to be ideal for urban and regional mobility programs. The design allows propulsion units to be operated in combination (independently or coherently) during lift, hover, and forward cruise. For example, Airbus’ CityAirbus NewGen has eight identical propulsion units where only two rear units provide power during hover. To better understand the Lift and Cruise architecture, vertical and forward flight must be described separately

Vertical flight

Vertical thrust systems have fixed pitch rotors since the speed range in the vertical direction is limited. The rotor disc area is not compromised by the conflict between vertical and forward air mass flow and overspeed requirements. This means the vertical part can have low disc loading. Vertical thrust can be divided over multiple rotors of equal or varying sizes.

For example, the Beta Technologies Alia-250 features four large rotors that provide vertical thrust. On the other hand, the Wisk Cora design has 12 smaller rotors, all providing vertical thrust. The Embraer EVE lies in between with eight medium-sized rotors for lift. Multiple rotors are employed to provide redundancy to the system. The aircraft must be able to sustain vertical flight with some rotors or other parts of its lift system failing.

Forward flight

All eVTOL designs use pusher propellors for forward flight. As a result, the prop wash scrubbing drag and wing lift distribution influence for the VTOL is eliminated. Both fixed and variable pitch propellors can be used for forward flight configuration. An efficient propellor can increase the speed range of the aircraft. Similarly, forward thrust can be optimized to increase the operational range of the eVTOL vehicle.

EVE eVTOL has an updated main wing and tail design with twin forward-flight ducted pusher propellers. The propellors are installed at the end of the inner lift thrust beams. With both the fixed and variable pitch configurations, the separation of lift and cruise power will likely allow a more straightforward design.