July 22, 2022, ©. Leeham News: We analyzed vectored thrust VTOLs last week, like Joby Aviation’s Joby S4. Now we look at VTOLs that separate vertical and forward flight thrust generation into two different systems. Typical exponents are Wisk Aero Cora, Beta Technologies Alia-250, and Embraer’s EVE.
When we looked at Vectored Thrust VTOLs last week, we saw that the thrust requirements between vertical takeoff/landing and forward flight were very different.
If the forward flight part is well done (a good main wing and control concept), the difference between vertical and forward flight thrust is the Lift over Drag value of the VTOL, typically around 15 when the forward flight is at optimal cruise speed.
The 15 times difference necessitates variable pitch rotors for all motor/rotor units, and it means the motors and rotors work at 7% of full thrust when in cruise. It’s difficult to design efficient motor/rotors that span a 7% to 100% thrust range.
The Lift + Cruise VTOL principle separates the vertical lift thrust generation from the forward flight thrust generation. It increases the number of rotors, motors, and inverters necessary to make a complete propulsion system, but it simplifies the design of these parts. It also simplifies the tricky transition from vertical to forward flight.
Let’s go through the three leading projects and look at how they solved the problems for the two flight phases;
All three use vertical thrust systems with fixed pitch rotors as the vertical speed range is narrow. The rotor disc area is not compromised by the conflict between vertical and forward air mass flow and overspeed requirements; the vertical part can be designed to have a low disc loading.
How the vertical thrust is divided over rotors differs. One extreme is Wisk Cora with 12 smaller rotors, Figure 1. Beta Technologies Alia-250 is at the other extreme, with four large rotors, Figure 2.
The Embraer EVE lies in-between with an eight-rotor lift concept, Figure 3.
The numbers of rotors are dependent on the redundancy concept the VTOL is using. The vehicle must sustain vertical flight with different parts of its lift system failing. For Wisk and EVE, this is a rotor/motor/inverter combination not working.
Beta, with only four rotors, is probably designed with the mechanical rotor as a failsafe unit, whereas the motor, inverter, and battery system have built-in redundancies. It might also employ a forward flight landing ultimate redundancy, as it’s hard to control a VTOL with only three rotors.
The rotors of Wisk and Beta look funny because these put the motors and probably also inverters in the rotor hubs. The rotating shaft is then fixed to the rotor beams. As parts of the motor + inverter are spinning with the rotor, it gets efficient air cooling, saving liquid cooling for the lift system motors and inverters. EVE seems to opt for liquid cooling of motor and inverters as these are built into the beams and drive the rotors in a conventional way.
All three park the lift rotors for minimal drag when transitioned to forward flight. The drag of the parked rotors will be highly dependent on the angle of attack of the aircraft. We can expect these rotors to be angeled for a minimum of rotor blade edge vortices at the optimal cruise speed angle of attack.
All designs use pusher propellers for forward flight. This eliminates the propwash scrubbing drag and wing lift distribution influence for the VTOL.
The forward speed propellers could be fixed for these projects as they can be optimized for a narrower speed range. As the propellers are smaller than the lift rotors for all designs and a variable pitch design is not too hard to justify for a single propeller, Wisk and Beta use variable pitch forward flight propellers. The speed range with a good motor/propeller efficiency is thus enlarged.
EVE changed its design from a canard to a conventional main wing and tail design at this week’s Farnborough Airshow. The design keeps the twin forward flight ducted pusher propellers fitted at the end of the inner lift thrust beams. It’s not clear if EVE employs variable pitch propellers, but the separation of lift and cruise power would allow a simpler and lighter fixed design.
The three employ very different project approaches:
Wisk has built two-seat test VTOLs that have flown extensively in remotely piloted vertical, forward and transitional flights over the last year. The company, formed by Boeing and the Kitty Hawk owner Larry Page in 2018, has decided that the business model with a pilot flying four passengers (the production model has four seats) is not attractive.
Instead, the partners go to autonomous flights directly. It’s a large jump, but if successful, it will render piloted VTOL projects operationally expensive, as the Pilot pay is a major operational cost block together with battery costs. Additionally, the reduction of the carried payload by 20% helps with the mass carried.
The autonomous knowledge can be Boeing’s motivation for its substantial Wisk involvement. Autonomous flight has several applications in Boeing’s civil and military businesses.
Beta technologies has built a full-size prototype that it flies extensively in forward flight, Figure 4.
In May, it made a multihop 1,400 miles flight from Plattsburgh Airport, New York, to Bentonville, Arkansas. So the forward flight mode is well developed and tested. It has yet to install vertical flight motors and rotors, however. The vertical flight modes with the tricky transitional phase remain to develop.
EVE has extensive experience with forward flight vehicles using propellers (Ipanema, Bandeirante, Brasilia, and Tucano are all Embraer propeller aircraft); it, therefore, focused on validating the EVE vertical flight concept with an eight-rotor iron bird, Figure 5.
With this experience, it could go directly to a full-size certifiable prototype, combining vertical and forward flight. EVE is several years behind Wisk and Beta with its prototype, but in our view, it’s not the first to prototype flight that is important; it’s when a certified VTOL is operationally efficient that counts.
Embraer is the only of the three that has certified multiple aircraft designs. It’s also the only organization besides Airbus (with its CityAirbus NG, which we come to later) with a worldwide support and service organization to keep the VTOLs flying 24/7. It’s a major advantage, as a support and service organization takes time to build.