July 15, 2022, ©. Leeham News: We started the analysis of the market’s most prominent VTOLs with multicopters last week. Now we continue with vectored thrust VTOLs.
The most known exponent for vectored thrust VTOLs is Joby Aviation’s Joby S4 VTOL, Figure 1.
A vectored thrust VTOL uses a tiltable rotor to work both in vertical and horizontal flight. The idea is old and has been in operation for 15 years by the Bell Boeing V-22 Osprey tilt-rotor.
The concept has advantages and disadvantages.
It’s an advantage that the same motors and rotors can be used for both vertical hover and horizontal wingborn flight. By tilting the lift rotors forward after the vertical takeoff, they can generate forward thrust, thus enabling a wingborn flight. When transitioning to wingborn flight, the thrust needed is reduced by the Lift/Drag ratio of the vehicle. It makes possible higher forward speeds than a multicopter at a lower power setting, thus saving on battery energy.
Another advantage is that during the transition from vertical to wingborn flight, the rotors wash the lifting surfaces with their slipstream, thus cleaning away tired boundary layers and creating an attached stream earlier than the wing alone can achieve. This helps with the tricky transition from vertical to wingborn forward flight.
A disadvantage is the large difference in thrust need between vertical and wingborn flight. We have learned the thrust during vertical flight must equal or exceed the weight of the VTOL. For the Joby S4, we have a hover thrust need of 22kN/5,000lbf, whereas forward flight requires 1,5kN to 2kN/350lbf to 450lbf, depending on flight altitude and speed. The top speed of the S4 is 200mph/170kts, with an optimal cruise speed of around 150mph/130kts.
The thrust needed for wingborn forward flight is then up to 15 times lower. It’s a positive for energy consumption but poses several design challenges. A difference between top thrust and cruise thrust of 15 times leads to a difficult compromise for the motor/rotor combination.
All rotors must be variable pitch; fixed pitch rotors are not possible with thrust vector designs. But the too large rotor area for forward high-speed flight means you have a very high mass flow and very low overspeed. While this gives high theoretical efficiency, the practice adds blade drag that reduces the efficiency gains, and your thrust lapse with speed is high.
You can either lower the total disc area to gain forward flight performance at the cost of an inefficient hover, or you have a large total rotor area and accept that forward flight propulsion is compromised. The electric motors can only work efficiently inside a restricted torque/RPM range, and you must decide if this is the hover or cruise range?
Another problem with the large rotors of Joby, Archer, and other vectored trust VTOLs is extra airframe drag from the swirling rotorwash that hits all parts of the VTOL. Figure 2 shows a CFD simulation of the propwash of a C-130 Hercules.
The simulation clearly shows the corkscrew motion of the slipstream from the propellers and how this impinges on the nacelles, the wings, and for the inner engines, the horizontal tailplane.
It creates an extra drag called scrubbing drag, and it also changes the lift distribution of the wing from the ideal elliptical distribution to a more rollercoaster value created by the propeller streams.
The result is that a well-designed wing and tail get their carefully tailored lift distribution destroyed by propeller/rotor streams. VTOL wings thus have lower L/D than their designers believe in their basic lift/drag analysis.
In the case of Joby S4 and several others, the large rotors needed for the takeoff and landing engulf a major portion of the airframe with its slipstream, creating the above drag and lift inefficiencies, Figure 3.
The forward inner rotors also inject their propwash into the rear rotors. The long blades and the pulsating load on the rear rotors make blade flutter a critical problem for a vectored thrust design.
The challenges involved in vectored thrust have led many VTOL teams to separate the two thrust forms into a dedicated vertical system and an independent system for forward flight. We will examine such a system next week.