5 ms·

Moving to an electric ducted fan instead of an internal combustion engine with a variable pitch propeller definitely simplifies things, though replacing that IC

Moving to an electric ducted fan instead of an internal combustion engine with a variable pitch propeller definitely simplifies things, though replacing that ICE and prop with 36 ducted fans mounted to elevons seems like you're just swapping one set of complexities for a different set, though gaining complexity through redundancy isn't really the worst thing. My biggest concern with this system is what kind of glide ratio this has in a power loss scenario.

null0ranje 5y ago
My biggest concern is how you maintain directional control in a power loss scenario.
- GravitasFailure 5y ago
  That too. As far as I can tell, this thing has all the control and glide capabilities of a brick. At least the brick has the inherent safety feature of not being mistaken for a passenger aircraft.
- aidenn0 5y ago
  ballistic parachute?
  - GravitasFailure 5y ago
    Maybe. Not sure I've seen one that will work on a 7 seater, especially with the extra mass of batteries.
    - bewaretheirs 5y ago
      Design for hot-swap batteries, and eject the batteries (with their own smaller chute or chutes)?
      - GravitasFailure 5y ago
        Forgive me if I don't consider dropping lithium batteries from several hundred meters up a safety feature. A parachute on this system is a lot of extra mass to compensate for a problem solved when the Wright brothers were around.
    - Tuxer 5y ago
      There is a ballistic chute on the cirrus SF50 (their single-engine jet). It's a 7 seater IIRC (6 minimum), and roughly the same weight (2.7T for the cirrus, 3.1 for the lilium).
- aidenn0 5y ago
  I mean will a 7-seater helicopter autorotate sufficient to provide control in a power-loss situation?
  - krisoft 5y ago
    Yes of course, why wouldn't it? Here is a video of a Mi-26T performing an autorotation: https://www.youtube.com/watch?v=O27T07bIQB8 https://www.youtube.com/watch?v=O27T07bIQB8 That beast can fit 90 troops or 20t cargo. Autorotation can scale to large helicopters.
Centmo 5y ago
A way around this risk is to come up with a design for which a total power loss is extremely unlikely (i.e. it would take multiple simultaneous non-correlated failures in a short span of time). This could be achieved through a distributed battery system where each motor has its own battery physically and electrically separated from the others, and critical flight electronics have their own backup batteries. A ballistic parachute is always nice as a last-resort hail mary.
- GravitasFailure 5y ago
  Redundancy does go a long way, but that also introduces a lot of extra weight and complexity into an already heavy aircraft and can screw with your weight distribution. The parachute can work since there are ones for air dropping tanks, but at the same time I'm not sure I've seen anyone try to use a parachute as safety equipment on an aircraft this heavy.
  - Centmo 5y ago
    Agreed, the distributed battery system does not seem practical for the Lilium design but you could get at least part way there by segmenting the central battery into parallel modules with separate safety disconnects. Not sure about the feasibility of the parachute system, it may be impractical due to the aircraft weight as you mention.
  - faeyanpiraat 5y ago
    Would it be enough for the safety batteries at the motors to be only as big as to provide power for the aircraft for an emergency landing? Like for a 30 sec full thrust? That might not bother the weight distribution that much.
    - GravitasFailure 5y ago
      Maybe? Honestly my weight distribution comment is a tad silly since complexity and weight are far bigger concerns, and two that just don't seem to have any avenue to go away.
    - 5y ago
skykooler 5y ago
It's fly-by-wire. Uncontrollable in a power loss scenario.
- jillesvangurp 5y ago
  Like most modern jets. Fly by wire is quite common. Also, in many recent cars, the steering column is no longer mechanically coupled to the wheels. Power loss scenarios are bad, which is why people mitigate against such things with all sorts of redundancies. And of course mechanical connections can fail too.
  - nradov 5y ago
    Most modern airliners have batteries and ram air turbines in order to power the flight control systems even if the engines are out and fuel is exhausted.

null0ranje 5y ago

My biggest concern is how you maintain directional control in a power loss scenario.

GravitasFailure 5y ago
That too. As far as I can tell, this thing has all the control and glide capabilities of a brick. At least the brick has the inherent safety feature of not being mistaken for a passenger aircraft.
aidenn0 5y ago
ballistic parachute?
- GravitasFailure 5y ago
  Maybe. Not sure I've seen one that will work on a 7 seater, especially with the extra mass of batteries.
  - bewaretheirs 5y ago
    Design for hot-swap batteries, and eject the batteries (with their own smaller chute or chutes)?
    - GravitasFailure 5y ago
      Forgive me if I don't consider dropping lithium batteries from several hundred meters up a safety feature. A parachute on this system is a lot of extra mass to compensate for a problem solved when the Wright brothers were around.
  - Tuxer 5y ago
    There is a ballistic chute on the cirrus SF50 (their single-engine jet). It's a 7 seater IIRC (6 minimum), and roughly the same weight (2.7T for the cirrus, 3.1 for the lilium).
aidenn0 5y ago
I mean will a 7-seater helicopter autorotate sufficient to provide control in a power-loss situation?
- krisoft 5y ago
  Yes of course, why wouldn't it? Here is a video of a Mi-26T performing an autorotation: https://www.youtube.com/watch?v=O27T07bIQB8 https://www.youtube.com/watch?v=O27T07bIQB8 That beast can fit 90 troops or 20t cargo. Autorotation can scale to large helicopters.

Centmo 5y ago

A way around this risk is to come up with a design for which a total power loss is extremely unlikely (i.e. it would take multiple simultaneous non-correlated failures in a short span of time). This could be achieved through a distributed battery system where each motor has its own battery physically and electrically separated from the others, and critical flight electronics have their own backup batteries. A ballistic parachute is always nice as a last-resort hail mary.

GravitasFailure 5y ago
Redundancy does go a long way, but that also introduces a lot of extra weight and complexity into an already heavy aircraft and can screw with your weight distribution. The parachute can work since there are ones for air dropping tanks, but at the same time I'm not sure I've seen anyone try to use a parachute as safety equipment on an aircraft this heavy.
- Centmo 5y ago
  Agreed, the distributed battery system does not seem practical for the Lilium design but you could get at least part way there by segmenting the central battery into parallel modules with separate safety disconnects. Not sure about the feasibility of the parachute system, it may be impractical due to the aircraft weight as you mention.
- faeyanpiraat 5y ago
  Would it be enough for the safety batteries at the motors to be only as big as to provide power for the aircraft for an emergency landing? Like for a 30 sec full thrust? That might not bother the weight distribution that much.
  - GravitasFailure 5y ago
    Maybe? Honestly my weight distribution comment is a tad silly since complexity and weight are far bigger concerns, and two that just don't seem to have any avenue to go away.
  - 5y ago

skykooler 5y ago

It's fly-by-wire. Uncontrollable in a power loss scenario.

jillesvangurp 5y ago
Like most modern jets. Fly by wire is quite common. Also, in many recent cars, the steering column is no longer mechanically coupled to the wheels. Power loss scenarios are bad, which is why people mitigate against such things with all sorts of redundancies. And of course mechanical connections can fail too.
- nradov 5y ago
  Most modern airliners have batteries and ram air turbines in order to power the flight control systems even if the engines are out and fuel is exhausted.