AFAIK the "jet" in classical jet engines is a jet of air. Unlike rocket engine, where hot combustion gases are your reaction mass, jet engines in aircraft use fuel primarily to spin turbines that compress and move air from the front of the engine to the back of it. You don't need combustible fuel to spin turbines - you can do that with electricity too.
Wow, the 50s were so awesome! This is the kind of thing I wanted to do when I studied engineering. Would anybody else rather make nuclear-powered jets than web sites?
Pretty sure everyone - including the people who worked on it - is happy that Project Pluto [0] / SLAM [1] never flew. Despite how awesome the projects were.
IANAAE (I Am Not An Aerospace Engineer), but I think the key difference here is the compression that happens with turbines as opposed to propellers which only move the air.
Modern "high bypass" jet engines are actually ducted fans, with the shaft powered by a "gas turbine" whose role is mostly to provide torque. All propulsion units compress the air to some extent, even propellers - if they didn't, they would be silent. However, in a gas turbine, the function of the "compressor stage" (which is NOT the "turbine" part) is to get a lot of oxygen in one place, where it can be combined with a lot of fuel, which makes it very hot, which causes it to adiabatically expand a lot, driving the turbine at the back (this is what a 'turbine' does - extract power). If the air weren't compressed, you wouldn't be able to burn much fuel with it, and it wouldn't get so hot and expand so much. (By the way, this is the same reason for superchargers and turbochargers in piston engines. More air = more fuel = more heat and expansion)
This process isn't actually very efficient at moving air though, which is why gas turbines are now mostly used to power ducted fans, propellors, and helicopter rotors.
In a propeller aircraft, basically you waste the energy left in your exhaust gas. But in a turbojet you make some use of it.
Also there is the distinction between just propeller, a turboprop, a turbojet and a turbofan. What we call jet airliners are typically high bypass turbofans. Turbojets (without bypass) have low efficiency at subsonic speeds, but they were used on Concorde and on fighter aircraft.
A propellor turns shaft torque into air momentum. You need some kind of motor to drive it.
A ducted fan also does this, though its aerodynamics are a bit different. Like a propellor, there is little compression.
A 'turbine' turns flow into torque.
A traditional jet engine turns fuel into torque with a compressor and a turbine. Confusingly it's sometimes called a 'gas turbine'. It also incidentally moves air, but not very efficiently.
A turboprop drives a propellor with a 'gas turbine'.
A modern 'high bypass' jet engine (or "turbofan") drives a ducted fan with a 'gas turbine'.
An 'electric jet' drives a ducted fan with an electric motor. No turbine is involved.
Nitpick: a turbine extracts power from a moving fluid. A fan (or impeller) imparts power to a fluid. A traditional jet engine contains both components, which is why they are sometimes referred to as "gas turbines". In an electric jet, there are no turbines and the proper term is "ducted fan" or "electric ducted fan (EDF)".
In jet engines the thrust comes form shoving material out of the end - the jet itself.
In props the primary thrust comes form the force due to the pressure differential on the prop blades basically it’s like someone is pulling the aircraft.
> The “Lilium Jet” uses 36 electric-powered ducted fans. Inside each, a small rotor ingests air from the front and pushes it out of the rear at higher speeds. They’re not technically jet engines (so the aircraft isn’t a “jet”). The lack of spinning blades improves efficiency, reduces noise, and eliminates the risk of turning passing birds into chop suey.
> The lack of spinning blades improves efficiency,...
A ducted fan is, in fact, a bunch of blades spinning in a duct. A well-designed ducted fan can be more efficient than a propeller of the same diameter. That efficiency is also more dependent on airspeed, as any given duct shape must be designed for a particular flow [1]. (it looks as though this design may have a variable nozzle, which could broaden the efficiency hump over a wider speed range.)
>...reduces noise...
To some extent, though the ducted fans on airliners still demonstrate a fair amount of noise from the fan itself. The video continues the practice of not giving a clear indication of how much noise is being generated.
>...eliminates the risk of turning passing birds into chop suey.
As there are rotating blades, this does not follow. It is not going to be healthy for either bird or fan when they collide. At least this airplane has a lot of fans.
They say "20% as loud as a helicopter" and I found a Bell J2-A at 100 Ft is 100dB, so maybe 93ish dB at 100 feet? That's about as loud as a motorcycle at 25 feet, so significantly louder still than any road traffic.
I'm an outsider, but it's probably about failure tolerance. A twin engine has to be able to fly with one engine in case the other fails.
36 engines means with three engine failures you still have over 90% of power.
My lay person take is they're using what is basically an EDF, you spin the impeller at high speeds sucking air in and ramming it out the back that is a lower diameter than the front. Surely many smaller engines would be safer than a couple of larger ones at those speeds. Also, maybe they found out it was more efficient to use many smaller ones.
300km/h and 300km range means 1hr endurance. That means it probably cannot be used at all for paid flights in the USA. Reserves required are greater than that (enough fuel to fly to destination, to alternate, and then 45 mins).
Isn't the worst case wait for a runway to open much longer than the wait for a flat spot to be clear? It doesn't make a ton of sense to use the same duration for both.
The conventional reserve amounts account for diversions such as an airport being closed, or weather conditions at such as crosswinds exceeding the aircraft's capability to land at that airport. Another case is unexpected headwinds using up fuel, losing an engine, or going out over water and needing to turn back half way.
If you're going to make VTOL exceptions, you still can't just punt and say put it down anywhere. What if you're over LA and there's no clear spot for miles? Over water? In instrument conditions (necessary for scheduled flights, because weather) and you can't see below you?
Not saying we shouldn't amend the FARs for new aircraft types, but there's some thinking that needs to go into them.
Airliners, yes. Airtaxis? Not sure about that. Also no idea what the requirements for helicopters is which are much closer to the Lilium jet than conventional passenger aircraft.
The gist: max. 5 PAX, max 2,000 kg VTOl craft requires an own set of certification criteria as neither aircraft nor rotorcraft requirements are feasible.
No idea about the current small rotorcraft certification requirements so.
Sure, in a world where replacing a DIN norm by the successor EN one on the drawings can take 6 months, sure. But things are moving, which is good thing.
And at least regulations are worked on in advance. That Airbus is also working on a similar project might help to speed things up with EASA as well. Just what that means for Lilium remains to be seen.
The reserves are just as important for electrical VTOLs as they are for gas, when the power runs out the thing drops out of the sky. If anything the electric VTOLs need stricter requirements, not lower standards.
The point being that it doesn't need a runway. There might be opposition to changing the rules, but rules intended for planes that needs to land at an airport makes little sense for something that can land on pretty much any small flat-ish surface.
Please tell me. You're over center of LA. Where's that mythical flat area you'll land on? Oh, and how will you find it through the fog bank below you for miles...
Plenty of roads, rooftops, parking lots and other flat areas. Looking at a satellite map I see mile after mile after mile of sufficiently large plat areas for an emergency. We're talking about a need for a circle smaller than a helicopter pad.
And for a VTOL plane that can do a descent as slow as they've shown with their two seater, getting low enough to find them is far less of an issue.
But the other thing I'd forgotten is that if it were to run out mid air or you can't find anywhere else: the two seater at least had a whole vehicle parachute, and it's built to be water resistant.
Alternate + 45 minutes is a rule designed for airplanes that need a runway to land. Helicopters typically have 10m fuel reserve rules because they can land just about anywhere.
I'd expect VTOL electric rules to be similar.
I honestly do not see anything impressive in the latest video. To me this is just an empty done staying afloat for a few seconds. The design looks nice, but that's it. I have been far more impressed by these things:
Not often, but I don't enjoy it when it happens; they're very loud. "Less loud than a helicopter" isn't much consolation. Noise pollution will definitely need to be taken into consideration before people start taking mini-choppers in place of Uber/Lyft.
It doesn't really make sense to call it a case of NIMBY because noise pollution is a real issue. This isn't a matter of them looking ugly; we do have to be able to sleep. If it's significantly louder than a train, that might be a bit too much.
Growing up on a street where the neighbors landed a helicopter at their house a few times per month I can tell you it’s extremely disruptive. As a kid you think it’s fun, I’m sure my dad was annoyed when he was trying to hear the tv though.
Thank you for that link, it is more in depth than the OP and talks about noise pollution:
"He said that the ambition for an app-based on demand service would not mean “landing in every garden... You’d be working with regulation around the world, integrating with public transport systems where they have them. We’re coming at a respectful way of thinking how people live, how we create corridors and not just fill the skies with these things.”"
I have mixed feelings about their "electric jet engines". I'm curious if those are more efficient than a quadcopter/plane hybrid. Though seems like the benefits is that they are using them for flight controls. If that is the case, they can change the airplane wing characteristics when they want which can be a game changer.
Hmm, I would probably put this in the class of planes with VTOL functionality, not a quad-copter/plane hybrid like Amazon and Google X's delivery drone. For instance, a V22 is considered a plane with VTOL but not a quad-copter/plane hybrid.
This is so cool. This is the one company I was really trying to get into as an electronics engineer. Must be fucking fantastic for everybody who worked on it.
> I love how in the video it shows 100+ people cheering and hugging each other in blissful victory for essentially making a drone fly.
Agree. The plane looks cool, and there definitely were a lot of people happy it flew, but the reactions on video look ridiculously staged. It reminds me of videos of rocket launches, except in those there's a high risk the rocket (and last X months of work) will just blow up on the pad, and you don't get to test-fly it beforehand. I don't think there were any outcomes possible for Lilium other than "it works" and "it almost works".
I feel like this is a bit gratuitously negative. This short flight is the product of months or years of work beginning to come to fruition. Let them celebrate.
Look at the videos of the two seater prototype. It has executed a lot more complicated maneuvers. They have some tweaking to do on their bigger model, that is all.
What are the tech specs of this thing? What altitude does it fly at? Does it use the ground effect for efficiency? If not, the claimed range seems a little too futuristic with current battery technology. 150 KW for flight ("it will require less than ten per cent of its maximum 2000 horsepower during cruise flight") x 1 hour range = 150 KWh and that's going to weigh more than the maximum takeoff weight of 640 Kg according to https://en.wikipedia.org/wiki/Lilium_Jet ...
300km range, so inter-city a lot of places. I live in London - with 300km range I could reach Manchester in an hour instead of spending an hour getting to the right train station to get onto a 3 hour train journey. Or I could reach Calais, France in less time. Not quite enough range to reach Paris, but still fast enough that you could pop over to Calais, go through passport control/customs, and take another one in to Paris in maybe 90 minutes - about the at which I could expect to have gotten onto a Eurostar train.
If they'll be able to land them enough places, close enough, and be cheap enough, this is just about enough range to cause massive changes in transportation patterns. I currently have one international airport within 20 minutes of my nearest transport hub. If I could get into one of these there, I'd be able to go direct to a lot of nearby locations, or reach 6 international airports within 20 minutes and probably 20+ within an hour. I'd be able to commute to the majority of England in the same time I can commute to the centre of London today.... It both opens fantastic possibilities and is a bit terrifying at the same time (in terms of driving energy demand..)
101 comments
[ 3.2 ms ] story [ 130 ms ] threadEdit: Apparently the rotors that compress the air are electrically powered, so sort of.
https://en.wikipedia.org/wiki/Aircraft_Nuclear_Propulsion
0: https://en.wikipedia.org/wiki/Project_Pluto
1: https://en.wikipedia.org/wiki/Supersonic_Low_Altitude_Missil...
This process isn't actually very efficient at moving air though, which is why gas turbines are now mostly used to power ducted fans, propellors, and helicopter rotors.
Also there is the distinction between just propeller, a turboprop, a turbojet and a turbofan. What we call jet airliners are typically high bypass turbofans. Turbojets (without bypass) have low efficiency at subsonic speeds, but they were used on Concorde and on fighter aircraft.
A ducted fan also does this, though its aerodynamics are a bit different. Like a propellor, there is little compression.
A 'turbine' turns flow into torque.
A traditional jet engine turns fuel into torque with a compressor and a turbine. Confusingly it's sometimes called a 'gas turbine'. It also incidentally moves air, but not very efficiently.
A turboprop drives a propellor with a 'gas turbine'.
A modern 'high bypass' jet engine (or "turbofan") drives a ducted fan with a 'gas turbine'.
An 'electric jet' drives a ducted fan with an electric motor. No turbine is involved.
[1] https://en.wikipedia.org/wiki/Motorjet
In props the primary thrust comes form the force due to the pressure differential on the prop blades basically it’s like someone is pulling the aircraft.
> The “Lilium Jet” uses 36 electric-powered ducted fans. Inside each, a small rotor ingests air from the front and pushes it out of the rear at higher speeds. They’re not technically jet engines (so the aircraft isn’t a “jet”). The lack of spinning blades improves efficiency, reduces noise, and eliminates the risk of turning passing birds into chop suey.
> The lack of spinning blades improves efficiency,...
A ducted fan is, in fact, a bunch of blades spinning in a duct. A well-designed ducted fan can be more efficient than a propeller of the same diameter. That efficiency is also more dependent on airspeed, as any given duct shape must be designed for a particular flow [1]. (it looks as though this design may have a variable nozzle, which could broaden the efficiency hump over a wider speed range.)
>...reduces noise...
To some extent, though the ducted fans on airliners still demonstrate a fair amount of noise from the fan itself. The video continues the practice of not giving a clear indication of how much noise is being generated.
>...eliminates the risk of turning passing birds into chop suey.
As there are rotating blades, this does not follow. It is not going to be healthy for either bird or fan when they collide. At least this airplane has a lot of fans.
[1] http://massflow.archivale.com/ductbook.htm
To be fair, it's a lot easier to put a debris (including birds) filter in front of a ducted fan than a prop.
If you're going to make VTOL exceptions, you still can't just punt and say put it down anywhere. What if you're over LA and there's no clear spot for miles? Over water? In instrument conditions (necessary for scheduled flights, because weather) and you can't see below you?
Not saying we shouldn't amend the FARs for new aircraft types, but there's some thinking that needs to go into them.
IMC seems to mean Instrument meteorological conditions
https://www.easa.europa.eu/document-library/product-certific...
The gist: max. 5 PAX, max 2,000 kg VTOl craft requires an own set of certification criteria as neither aircraft nor rotorcraft requirements are feasible.
No idea about the current small rotorcraft certification requirements so.
And at least regulations are worked on in advance. That Airbus is also working on a similar project might help to speed things up with EASA as well. Just what that means for Lilium remains to be seen.
[1] https://en.wikipedia.org/wiki/Powered_lift
And for a VTOL plane that can do a descent as slow as they've shown with their two seater, getting low enough to find them is far less of an issue.
But the other thing I'd forgotten is that if it were to run out mid air or you can't find anywhere else: the two seater at least had a whole vehicle parachute, and it's built to be water resistant.
https://www.youtube.com/watch?v=-kB-BGMXxZc https://www.youtube.com/watch?v=RMeEh5OUaDs
https://youtu.be/shlZySkGq6g
https://news.ycombinator.com/item?id=17515820
But how often do helicopters land near your house? Sounds more like a case of NIMBY.
It doesn't really make sense to call it a case of NIMBY because noise pollution is a real issue. This isn't a matter of them looking ugly; we do have to be able to sleep. If it's significantly louder than a train, that might be a bit too much.
https://www.theguardian.com/science/2019/may/16/electric-fly...
Edit: Also this article says they expect to be integrated with existing transport hubs with last mile done with conventional ground transport.
"He said that the ambition for an app-based on demand service would not mean “landing in every garden... You’d be working with regulation around the world, integrating with public transport systems where they have them. We’re coming at a respectful way of thinking how people live, how we create corridors and not just fill the skies with these things.”"
It looks extremely unstable and dangerous.
Agree. The plane looks cool, and there definitely were a lot of people happy it flew, but the reactions on video look ridiculously staged. It reminds me of videos of rocket launches, except in those there's a high risk the rocket (and last X months of work) will just blow up on the pad, and you don't get to test-fly it beforehand. I don't think there were any outcomes possible for Lilium other than "it works" and "it almost works".
The MIT battery primer [1] states a specific energy of 150Wh/kg for Li-Ion cells. 150kWh would therefore weigh 1000kg.
Even at double the specific energy, the battery would still weigh in at 500kg, almost 80% of the maximum takeoff weight.
If there is a future for electric airplanes, it is going to be in fixed-wing applications.
[1] http://web.mit.edu/2.009/www/resources/mediaAndArticles/batt...
If they'll be able to land them enough places, close enough, and be cheap enough, this is just about enough range to cause massive changes in transportation patterns. I currently have one international airport within 20 minutes of my nearest transport hub. If I could get into one of these there, I'd be able to go direct to a lot of nearby locations, or reach 6 international airports within 20 minutes and probably 20+ within an hour. I'd be able to commute to the majority of England in the same time I can commute to the centre of London today.... It both opens fantastic possibilities and is a bit terrifying at the same time (in terms of driving energy demand..)