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Comment by GravitasFailure | Hacker News Reader

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.

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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.

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.

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.

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.

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.

Most batteries don't have the power density to output their entire capacity in 30 (or 60, or 120) seconds though, if I'm remembering correctly.

Cirrus Airframe Parachute System

https://en.wikipedia.org/wiki/Cirrus_Airframe_Parachute_Syst...

Unless the eVTOL up for discussion is significantly heavier than the above, looks like a fair approach to last-resort failure recovery

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

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.

ballistic parachute?

Maybe. Not sure I've seen one that will work on a 7 seater, especially with the extra mass of batteries.

Design for hot-swap batteries, and eject the batteries (with their own smaller chute or chutes)?

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.

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).

I mean will a 7-seater helicopter autorotate sufficient to provide control in a power-loss situation?

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

That beast can fit 90 troops or 20t cargo. Autorotation can scale to large helicopters.

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

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.

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.

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