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Aug 23, 2023

Bjorn’s Corner: Sustainable Air Transport. Part 30. Lilium Jet VTOL.

-Again I think folks are far too sceptical. -To achieve its performance

-Again I think folks are far too sceptical. -To achieve its performance objectives LilumJet requires battery cells that can provide 300W.Hrs/kg energy and 2500Watts/kg Power for less than 20 seconds during the vertical phases of flight at the take-off and landing stages. Yes this means the batteries will need to discharge at the 8 minute rate (C8 rate). Once in level flight consumption drops by a factor of 10. This shouldn't be viewed as a disadvantage. Being able to discharge at the C8 rate means it can be recharged at the C8 rate as well. The Uber Elevate White Paper concludes that there will be no time for charging and so battery swaps will be required. Swappable Battery Packs add considerably to weight but it looks like Lilium can transcend this. -Current state of the art commercial battery cells have an energy density of 450Watt.Hours/Kg. This is already 50% more than Lilium needs. -Lets have a look at the issue of an alternate landing area. Lilium will not be operating using anachronistic junk legacy standards of VFR rules. It will be able to land using a precision landing system in a pea soup fog in the Rhur Valley or similar. -Most airports have only 1 or 2 runways. A vertiport can easily have 8-20 tennis court sized landing pads with wells separated on opposite sides of a building so that an incident on one does not impinge on the other. An alternate vertiport can be a tennis court sized pad 100m away. Hence we don't need long divert or hold reserves. They’re simply not needed and pointless. -Now the issue of transition. Lilium started work 7 years ago with an extremely ambitious and idealistic goal as a startup. Its first 3 demonstrators (really test beds) didn't even have foreplane wings for the canards but simply ducted fans. I presume the power requirements were so ambitious. The 4th and 5th clearly have the full system. -Saying Lilum and its founders are fraudsters is I think a libel and a slander of the 3 young men that started Lilium. Lilium has 5 flying test beds. Lilium has achieved transition of the aft wing. Theronos had nothing. Its tests detected nothing. -The issues of transition are being over stated I think. At 45 degrees the foreplane flap jets will be producing 70.7% of its thrust as lift and 70.7% as forward thrust. It would thus require 141% thrust to hold level. At 30 degrees 50% of the thrust would resolved to lift and 86% to forward thrust requiring 200% thrust to hold level. To me it looks like attachment of airflow of the aft wing flap occurs at around 20 degrees at which point 34% of the thrust resolves as lift requiring 300% thrust. However the foreplan wing and its flap are producing lift and its wrong to assume that the is no aerodynamic lift at 30 or 20 degrees. It should be a considerable amount. The Cl is perhaps only 0.33 of Clmax but that should be enough. One simply increases the speed of the aircaft to twice minimum flyging speed. There are other ways such as borundary layer suction or splitting the flaps into 3 and transitioning one pair at a time. A way will be found. -LiliumJet is having its undercarriage improved and will be able to conduct STOVL operations. Obviously and airaft that can support itself can accelerate at 1G and its obvious that even a 0.5G acceleration will produce transition during the takeoff roll within about 160 meters.