By Bjorn Fehrm
July 28, 2022, ©. Leeham News: This week, we analyze the Lilium Jet VTOL.
It’s a vectored thrust design, but it’s different enough in its characteristics from the vectored thrust VTOLs we looked at in Part 28 (Joby S4 ..) to motivate a separate article.
Figure 1. The final Lilium Jet configuration transports six passengers plus a pilot. Note the changed number of wing jets (from 24 to 20). Source: Lilium.
Lilium vectored thrust eVTOL
The Lilium eVTOL has a very different design from other eVTOLs. It uses (in the final configuration, Figure 1) 30 electric ducted fans to produce 30 jets that can be vectored from vertical to horizontal thrust, Figure 2.
Figure 2. Lilium propulsion concept with swiveling jets. Note the variable nozzle area. Source: Lilium.
The vectored jets are placed inside main wing elevons and canard elevators, Figures 1 and 3.
Figure 3. The main wing elevon with its built-in electric jets. Source: Lilium.
The propulsion concept of the Lilium Jet has advantages and disadvantages. Let’s start with the advantages:
- The canard design and the placement of the propulsive jets on the canard and wing avoid jet wash scrubbing drag and wing lift distribution effects (ref. the discussion from Part 28).
- The low mass flow and high overspeed of the jets give them a low thrust lapse rate (decline of thrust with speed). The design cruise speed at 10,000ft is, consequently, 150kts, about 20kts higher than other vectored thrust designs.
- The fans are housed in nacelle-type ducts. With careful fan/duct design and a variable nozzle area, the fan efficiency can be kept high for hover, climb, and cruise. With a perforated duct with sound-absorbing linings, fan noise is reduced.
But there are also disadvantages:
- The low mass flow/high overspeed jet design gives a very low hover efficiency. In Helicopter/VTOL speak, we talk about high disc loading. The hover power demand from the batteries is 2.2MW, unheard of in the eVTOL space.
- A canard design has a narrower center of gravity range than a tailed design. A canard can only operate with a positive lift, whereas a horizontal tail operates with both negative and positive lift to control the pitch of a vehicle.
- The integration of the jets into canard elevators and wing elevons (we assume the canard elevators operate in unison as the vehicle pitch elevator and the elevons in roll, with a secondary pitch/flap function) voids the design of additional pitch control by movables separate from thrust vectoring. This forces the heavy elevators (around 200kg each) and elevons (about 300kg each) to counter fast pitch and roll movements. It will require sizable actuators and high power flows to actuators and motors.
- Redundancy in pitch and roll control relies on the elevators/elevons to move in all scenarios. Their actuators will have high demands for power and redundancy.
The Lilium jet’s struggle with the transition
The Lilium project is in its seventh year, yet it has not transitioned from hover to forward flight during this period. The widely proclaimed transition in early June was a main wing transition, not a transition for the vehicle (canard + main wing). Such a transition is yet to be made.
Figure 4 to 7 is taken from a Lilium video where the main wing transition is shown. Observe that the canard at all time was in a vectored trust, stalled condition, Figure 7.
Figure 4. The Lilium wing in hover phase. The thrust is vectored about 30° down, and the flow over the elevon is still stalled (tufts in all directions) at a speed of 66kts. Source: Lilium.
Figure 5. The Lilium wing starts the transition. The thrust is vectored about 10°-15° down; the elevon is still stalled at the far end (tufts in all directions). Speed is 70kts. Source: Lilium.
Figure 6. The Lilium wing has transitioned. The elevon with its thrust is vectored aft. The elevon has attached flow (tufts are aligned with the flow). Speed 70kts. Source: Lilium.
Figure 7. During the whole test flight, the canard elevator worked in vectored thrust mode at angles around 30°. The flow over the elevator was never attached. Source: Lilium.
The fact that a VTOL developer makes such noise about a transition from hover to forward flight of a part of the vehicle tells you a lot. Other VTOL OEMs transition to forward flight within months of the first hover flight. Why is it such an issue for Lilium?
The use of jets voids Lilium of help from a rotor wash to attach the flow around wings and movables. The integration of jet thrust and movable also voids Lilium of a fast movable control surface to counter the pitch disturbances that are part of a transition. The canard design augments the pitch control problem by forcing a nose-heavy design.
Figure 8 shows the transition control problem. As the jets are vectored to forward thrust, their vertical authority shrinks. With increased forward speed, the wing and canard move from deep stall to top of lift force (red arrows). It creates a sudden additional lift force. The problem is when canard and wing transition at different instances and speeds.
If the main wing transitions before the canard, the canard must compensate with a nose-up force to counter the lifting of the tail from jet thrust and lift. But the RPM-controlled jet lift authority is weak; at 30° degrees, it’s half, and at 20° one-third. Moving the elevator to a higher jet vector thrust drives it further into the stall, reducing its lift. If the canard transitions before the wing, the chain reverses.
Figure 8. The Lilium jet problem with diminished jet thrust lift authority and lift that moves in and out of stall. Source: Leeham Co.
The bottom line is the transition is a major technological challenge for the Lilium design. It will be interesting to see how it solves transitions with failed components and different loading of passengers and cargo, as required by certification requirements.
The hover power requirement
The jet thrust design creates a need for four times higher lift power than for other VTOLs. It requires larger/heavier motors and inverters, but the biggest effect is on the batteries. Batteries are characterized by their power delivery capability in kW and energy content in kWh.
Typical VTOLs have power and energy needs of around 500kW and 100kWh. Lilium ups this to 2,200kW and 300kWh. The extreme power demand at all battery SOCs (State Of Charge, see below) has forced Lilium to engage with a special cell developer, ZENLABS, to produce a battery that can deliver the power.
While we agree with Lilium’s assertion that the power-hungry vertical takeoff phase is short (Figure 9), the real problem will be landing in bad weather and the resulting reserve requirements for vehicles like the Lilium jet.
Figure 9. The MTOW power and energy profile as presented by Lilium on Lilium’s Battery blog. Source: Lilium.
Here is where we don’t agree with Figure 9:
- You don’t assume a battery system with 100% SOC (State Of Charge). It means a brand new battery system. You shall specify the SOC at the end of the battery’s life (the worst operational case). Typically maximum SOC is then 85% to 90%.
- The calculation assumes that 20% of SOC is OK as a reserve. It gives us ~54kWh (300*0.9*0.2) reserve energy. If you have to search for a landing place in bad weather, you very likely will do this while hovering. Hovering power consumption is 37kWh per minute, which gives us 1.5 minutes of reserve time to search for a landing spot.
- The above means that the reserve must take a much larger part of battery capacity and that the practical operating range of the Lilium jet will be well below 175km. It will all be decided by the reserve requirements for VTOL by the regulators. Today’s helicopter requirements call for 20 minutes VFR or alternate + 30 minutes IFR reserves at cruise condition (where the Lilium consumes 3kWh per minute). An approach and landing shall be possible before and after the consumption of the reserve energy. There is a 2-minute VTOL reserve in discussion, but this applies to flights along fixed pre-recced routes (with landing spots at 2-minute intervals), a far cry from the air-taxi aspirations of the Lilium jet.
Lilium, like all VTOL OEMs that are investor financed, makes inflated claims about operational capabilities such as range. Where Lilium will land with its claims will largely depend on how Certification requirements will affect a design with a difficult transition phase and very high demands on battery power when hovering.