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Comment by Simon_O_Rourke | Hacker News Reader

I'm guessing that if it loses power during flight then it's curtains for the airframe and anyone unlucky enough to be in it.

There's lots more work to do to push it out to the mass market, but I could definitely see a niche for it.

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If they aren't stupid (And I bet they aren't), they made several independent redundant power system. I think distributed power systems (i.e. lots of small engines) is actually on of Lilium's main strength, can be made very reliable.

Lost 2 batteries and 7 rotors, fine, there's still 6 batteries and 25 rotors left. Or something like that

Check out their other video posted around the same time as this one, especially the part about changing the wheel design to allow for rolling landings. Seems like regulators had the same concern.

https://youtu.be/qZ73PftBfFg

Why not add a parachute? Some small planes actually build one in.

I like those a lot but they only help above some altitude (I think it’s something like 300 or 600 ft). Ie they won’t save you during VTOL

Airplanes glide. Some better than others, but they do glide.

That's the whole point of GP's comment. Planes glide with the control afforded by a rudder and a vertical stabilizer. Without that, it ain't pretty.

The Lilium plane has 12 sets of 3 electric motors. Each pair of sets is powered by its own battery pack, and each set is individually actuateable. So it basically has 12 control surfaces. Separate aerodynamic controls aren't needed.

Writing control software that can handle arbitrary failures isn't easy, but there is plenty of redundancy with the three separate flight computers.

I think this “plane” has a glide ratio that’s only slightly better than a thrown rock.

Powered lift aircraft are not controllable in a glide, nor are they capable of autorotation like a helicopter. Any complete loss of power will result in an unrecoverable spin. For safety these aircraft rely on redundant systems, plus a parachute.

That's the same if a jet engine losses power

If a [civilian] jet aircraft loses an engine, it's still a strong favorite to end that flight upright, intact, and on a runway. There are a couple dozen jet engine failures per year; the overwhelming majority end up in a safe conclusion.

https://www.cbsnews.com/news/jet-engine-failures-rare-usuall...

Passenger jets have very good glide ratios. A 747 gliding with no power (all four engines somehow broken) from 10,000ft will get even further than a small Cessna 172 doing the same.

This is counter intuitive to even most pilots, but it's how efficient the wing design on a passenger jet is. Their lift to drag ratio is better than small planes.

Is this achievable by the size of the craft itself? Or the fact that these bigger planes have 1000s of engineers behind them optimizing every possible factor to squeak out the best possible lift to drag ratio?

It's the overal effort that goes into the design, combined with a different design goal. Airliners are specifically optimised for low drag because it saves fuel. A Cessna 172 is a design from the 1960s that wasn't much optimised after that, and the design goal was stability and benign flying not the lowest possible drag. Later similarly sized small aircraft like the DA40 and SR20 are better at low drag because they're computer designed and fuel cost became more of a thing in modern times.

For airliners the retractable gear saves tons of drag, but they also have things like winglets, and lots of smaller things like optimisation on the wing-root (airliners have smooth transitions here, trainers are sort of just square) that all reduce drag.

An airliner is also much less friendly to handle in a stall condition, while a trainer almost recovers on it's own. And similar for yaw stability, if you get the rudder a bit wrong in a trainer nothing much happens, while an airliner will typically start to oscillate. Both of these are solved on the airliner by electronics, a stick shaker or pusher to avoid the stall and a yaw damper to enhance stability. The Cessna 172 doesn't have any electronics involved in flying, only for navigation and those are optional.

Nope.

Look at the tiny wings, and at what speed it still depended on vertical thrust. It's going to have horrible glide ratio, which means unpowered landings might be, well, not a thing, which leaves you only with emergency full-plane parachute as an option.

Pretty much all jet engine planes can do unpowered landings in glide, though admittedly some have pretty high speeds involved - but then they operate by default only on airports that have facilities for such speeds.

See this famous example, where a jet airliner ran out of fuel in flight and glided to a successful landing:

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

Also Air Transat Flight 236. https://en.m.wikipedia.org/wiki/Air_Transat_Flight_236

The movie Sully [0] opened my eyes to the amazing glide ratio of a commercial passenger plane like the A320. (I'm assuming that the movie was pretty accurate in that regard.)

[0] https://en.wikipedia.org/wiki/Sully_(film)

Yes, the movie was accurate in that regard.

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