Lilium Jet, battery powered plane, do I smell bull$h1t? - Page 1
Forum section: electronics / general

author: niels007
author_rank: Contributor
timestamp: December 18, 2019, 06:17:57 pm
content: I did a quick forum search for Lilium but it didn't bring up any results.Saw this today:A small electric plane thingy that will fly for 1 hour at 300km/h they say. I sniffed it and it kinda reeks pretty badly.Surely you can't possibly take the energy required with you without getting stuck in a downwards spiral (no pun intended) of weights getting completely out of control?Or am I missing something here?

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 18, 2019, 06:36:44 pm
content: Nothing in the video shows us it can really fly at 300 km/h. And even less so for one hour. All we see is a short flight probably at around 70 km/h (if that) max for a couple minutes. And apparently, the project has been ongoing for several years and all they show is still this.Yes the numbers just don't add up. And even if they could, I don't see any selling point in this as far as usability goes, apart for just leisure.Just another typical start-up bullshit.

author: StillTrying
author_rank: Super Contributor
timestamp: December 18, 2019, 07:01:06 pm
content: I think close to 100 miles by electric is possible now, but never by their vertical take off one.https://www.nytimes.com/2019/11/05/technology/flying-taxis-lilium.html

author: jogri
author_rank: Frequent Contributor
timestamp: December 18, 2019, 07:10:51 pm
content: I did a back-of-the-envelope calculation based on the fuel consumption of a Kitfox (seems quite a bit bigger than this XXL drone, but it should be good enough for a rough estimate) and their numbers just don't add up: A Kitfox will burn around 22 liters of avgas per hour at 200 kph (since the drag relates to the square of the velocity this number would be much higher for the claimed 300 kph). If we convert the liters per hour to the energy, we get around 700 MJ/h.Li-Ions have an energy density of roughly 1 MJ/kg, so you'd need around 700 kg of batterys for one hour of flight @200kph.Yeah, i call BS on that number.Btw: They claim that it is quiter than a heli, but they somehow don't include the sound of this thing in their videos... Could it be because three dozen electric engines are just a bit noisy? Who knows...

author: ogden
author_rank: Super Contributor
timestamp: December 18, 2019, 07:35:25 pm
content: Vertical takeoff is under question here. The rest is already happening and some even aiming at FAA cert @2020. Pilot+passenger, max 250km/h cruise speed, max 3.5h endurance (not necessarily at max speed):https://en.wikipedia.org/wiki/Bye_Aerospace_eFlyer_2. There are few more "sports" e-planes,some reach> 300km/h, dunno about endurance figures.

author: fcb
author_rank: Super Contributor
timestamp: December 18, 2019, 08:38:41 pm
content: Quote from: jogri on December 18, 2019, 07:10:51 pmI'm not so sure. A KitFox is probably less aerodynamic than the Lilium, and I don't think you have taken into account the conversion efficiency of a Rotax engine or similar.100LL (low lead 100 octane) is 43.50MJ/kg, 22L would be 957MJ. Not sure what a Rotax efficiency is like, but probably not more than 35% (0.35*957)=335MJLi-Ion 'state-of-the-art' is perhaps 400Wh.kg = 1.44MJ/kg. Assuming they have 90% efficiency in conversion from the battery to the motor (i've seen figures of 96.5% before), they are at (1.44*0.90)=1.296MJ/kgSo perhaps 335/1.296=258Kg of battery per hour of flight. So much less than the 700Kg. We are comparing apples and pears though.

author: jogri
author_rank: Frequent Contributor
timestamp: December 18, 2019, 09:19:33 pm
content: Quote from: fcb on December 18, 2019, 08:38:41 pmWhoops, forgot to calculate the efficiency. Btw, a new Rotax has a rather high efficiency (i calculated 54% for the new 915iS, it needs 12 kg/h or 536 MJ/h to produce 81kW or 290 MJ/h). They use avgas (higher octane number than petrol), but according to Wiki they don't differ much in their energy density. With those 54%, we get 517 MJ/h or 400kg of batteries.And yes, a Kitfox is significantly larger than that plane and has more drag, but with the higher speed (300 instaed of 200 kph), we get an 2,25 times increase in drag and that should more than account for the better aerodynamics of the e-plane. So i think we can still assume that you would need more than 400-500kg of batteries for the claimed 1hr flight time @300kph. But i don't think that the +/- 100kg even matter as you are probably not going to get more than 200kg of batteries into that plane as it is rather heavy even without batteries (it has 36 engines after all, that's a sh*tload of copper and aluminum).

author: Domagoj T
author_rank: Frequent Contributor
timestamp: December 18, 2019, 09:24:01 pm
content: A comparable plane would be Long-EZ, a two seater, canard layout, 300 km/h max speed (230 km/h cruise), 600 kg max take-off mass (200 l fuel).Only this one has a range of 3000 km, not 300.Of course, it's not VTOL, which requires TWR > 1; something that is not easy to achieve and is almost entirely unnecessary for civilian flight. I'll side with Burt Rutan on this topic.Yeah, it would be nice if you could have an unmanned aircraft land on a nearest opening and whisk you away, but realistically, if that thing has an hour of flight time in its batteries, just how far can it realistically fly and still have enough juice to come pick you up, go to where you want it, and head back to recharge? Those 300 km would quickly run out, especially with three take offs and landings that require about 5 times as much power as level flight.

author: BrianHG
author_rank: Super Contributor
timestamp: December 18, 2019, 09:45:22 pm
content: My guess is that they are banking on battery storage density to improve between 50% to 100% by the time they are ready to release a product. Looking at the current investment trends in battery technology, it is a viable business gamble.This isn't in the realm of free energy. They have a legitimate shot of winning the bet in the long term instead of those who wait for the new battery tech, then decide to engineer their own VTOL vehicle later and be behind in the game as well as not engineer a product to be as efficient as possible as one would do in the face of not knowing the exact scale of the improvement in battery tech. The ones beginning here and able to keep a continuous investment flow until that time comes has a shot of being ahead of the rest.

author: jogri
author_rank: Frequent Contributor
timestamp: December 18, 2019, 09:54:47 pm
content: Quote from: BrianHG on December 18, 2019, 09:45:22 pmThere is a big difference between "new technology is available" and "technology gets approved for use in commercial aircrafts"... And given how allergic the FAA reacted to the 787/737 MAX lithium (LiCoO) batteries (they had to pack them in a sealed steel container with a vent line to the outside), i don't think they are quick to approve such a massive battery bank that can't be shielded.

author: thm_w
author_rank: Super Contributor
timestamp: December 18, 2019, 10:00:31 pm
content: The regulations take forever, so maybe it makes sense as Brian says to develop upfront of the tech actually being available. That way the certifications are in progress, and you end up earlier to market.These guys are trying to retrofit an existing plane:https://www.theverge.com/2019/12/12/21012891/electric-airplane-first-flight-magnix-harbor-airand it still is going to take years to get it certified. In this case though its 100% viable as these planes fly very short distances, they've also gone with a low Wh/kg battery for safety.

author: m98
author_rank: Frequent Contributor
timestamp: December 18, 2019, 10:47:22 pm
content: Quote from: jogri on December 18, 2019, 09:54:47 pmElectric aircraft with Li-Ion batteries are already an established product. Also, in the realm of small aircraft, a battery pack isn't any more dangerous than having a armed rocket right behind the cabin.

author: jogri
author_rank: Frequent Contributor
timestamp: December 18, 2019, 11:33:20 pm
content: Quote from: m98 on December 18, 2019, 10:47:22 pmSure, but they are almost certainly flying as experimentals. They either get the experimental airworthiness certificate (that gives you a lot more freedom in chaning the plane but permits commercial use) or get a permit for that single plane (which is hella expensive).

author: vk6zgo
author_rank: Super Contributor
timestamp: December 19, 2019, 02:38:10 am
content: The term "jet" is bollocks ---- it uses ducted fan propulsion.Ducted fans had a big vogue in the 1930s, but notably not one of those designs ever reached production.https://en.wikipedia.org/wiki/Stipa-Capronihttps://en.wikipedia.org/wiki/Caproni_Campini_N.1Since WW2 they have popped up regularly, & disappeared just as regularly.The only widespread use I am aware of is in flying scale models of jet aircraft.I'm not an aerodynamics expert, but I have a passing knowledge of such things, & all those propulsion modules look to me, as spoiling the shape of the aerofoil in forward flight.

author: ogden
author_rank: Super Contributor
timestamp: December 19, 2019, 03:00:05 am
content: Quote from: vk6zgo on December 19, 2019, 02:38:10 amAllturbofanengines have ducted fans.

author: m98
author_rank: Frequent Contributor
timestamp: December 19, 2019, 03:33:06 am
content: Quote from: jogri on December 18, 2019, 11:33:20 pmThey are not. Beside the ultralights and light sport aircraft approved to lower standards, there are electric motor gliders and gliders with electric extender motors.All those classes are also eligible for commercial operations.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 19, 2019, 04:01:38 am
content: Quote from: ogden on December 19, 2019, 03:00:05 amIndeed, but they are driven by gas turbines, not toy electric motors.

author: niels007
author_rank: Contributor
timestamp: December 19, 2019, 08:26:48 am
content: I wanted to do some power requirement calculations and I often work with car aerodynamics / drag calculations.Trying to find the drag coefficient of planes is odd, you get like values of 0.03 where most cars are close to one order of magnitude more. I can see the shapes being better but this much?Then again, small prop planes don't have 1000hp engines, a Cessna 172 is about 1200kg in weight and flies 230km/h with its 180hp engine.However if the CD would be 0.03 and it does 180km/h, plenty of cars with 180hp get closeish to 200+km/h with a CD of nearly 10 times more.. Confused! :-)

author: Domagoj T
author_rank: Frequent Contributor
timestamp: December 19, 2019, 08:40:38 am
content: Ducted fans are used for airships, hovercrafts and other applications where high static pressure and low airspeed are needed. They are far from useless, and are better for VTOL than open propellers, but for cruise, open propellers tend to be preferred option since they are simpler, lighter and ultimately more efficient.Turbofans are a go to solution for airliners, but there is still the market for turboprop which are more efficient at lower speeds than turbofans. Main advantage of turbofans is their ability to fly at higher altitudes which leads to significant fuel savings.All in all, ducted fans are good for either very small aircraft that fly very slowly, or very big ones where you can install additional equipment for high altitude to compensate for their drawbacks compared to open propellers.In this particular case, the problematic design decision is not the ducted fan; it's electric motors which require heavy and low energy density batteries.

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 19, 2019, 02:54:31 pm
content: Quote from: Domagoj T on December 19, 2019, 08:40:38 amAgreed, although even here, the use of ducted fans is dubious, as they again claim a top speed of 300 km/h (and given the kind of aircraft, it won't fly at high altitudes either...)What's the market for this at the moment? Leisure (for the very wealthy) and training (you can see that the above cited aircraft is a training one.) But even so, the very limited flight time can clearly be a hazard in some situations. The plane doesn't look like it would fly very well on its own without engines on either... in case you must do an emergency landing with no more juice left.At this point, those companies just want to be "part of it" ("it" being electrifying everything we can touch), not making useful products. This is pretty much what many start-ups do. That still can be useful per se to work on some technology before it's really mature (that's sometimes how technology moves forward, note the "sometimes"), but here I don't see the point. There's nothing in this project that brings anything new to the table; every part of it are already well known, and the end-result itself is uninteresting.Yes we know we can build electric planes. The hard part now is to make them useful and safe, and just building yet another one won't get us that.

author: m98
author_rank: Frequent Contributor
timestamp: December 19, 2019, 05:15:18 pm
content: Quote from: SiliconWizard on December 19, 2019, 02:54:31 pmWho is "we"? If nobody is putting money into it, and actually develops and builds something, there isn't going to be any innovation. And there is no chance to make something "useful and safe" if it doesn't exist. If all companies who already build electric planes in series listened to your advice, there wouldn't be any.