Why am I imagining Clarkson and Co with their 'electric car'?
"How hard can it be?" etc
A German startup claiming that an electric vertical takeoff and landing aircraft powered by 36 electric fans is the future of personal transport has somehow scored $90m from investors. Lilium, which describes itself as “developing the world’s first all-electric jet aircraft capable of vertical takeoff and landing (eVTOL)”, has …
Wouldn't it be possible to vary the strength of each jet to act as a virtual rudder instead of a physical vertical tailplane?
While it doesn't seem like a problem they should have needed to solve in the first place, having the thrust be distributed as widely among a large number of engines as it is will allow changing thrust between engines to cause yaw.
Aircraft lacking vertical stabilizers aren't new and there are several ways to achieve this either actively or passively. Certainly differential thrust is active as would differential braking where the elevator and/or aileron splits (see deceleron) to add drag on the side that begins to advance. Finally, if the wing sweep is sufficient it might be done passively such that as the aircraft yaws the leading wing presents greater surface area and more drag providing a restoring torque in the same way the wings dihedral results in a differential lift when the plane begins to roll.
Given the look of Lilium's aircraft it's a good bet that nearly everything is done actively and why not given the power of modern computing. I recall reading recently that there is a good chance that there will be few if any future military aircraft designed with vertical stabilizers because they represent a substantial design challenge when desiring a stealthy aircraft.
@ArrZarr: "Wouldn't it be possible to vary the strength of each jet to act as a virtual rudder instead of a physical vertical tailplane?"
One big problem with using differential thrust for control is that its effectiveness is linked to the power settings; at low power settings you have very little control authority, which is not a Good Thing, especially when trying to land.
@Eddy Ito: "Finally, if the wing sweep is sufficient it might be done passively such that as the aircraft yaws the leading wing presents greater surface area and more drag providing a restoring torque in the same way the wings dihedral results in a differential lift when the plane begins to roll."
The increase in drag on the 'leading' or advancing wing is due to an increase of its effective span (which increases the presented cross-sectional area, not the surface area) but this effective increase in span (and decrease in sweep) results in differential lift, which in turn results in a roll. Because the wing produces more lift than drag, which is pretty crucial if you actually want to fly, a passive solution isn't really viable - the rolling factor will be greater than the retarding factor and you'll soon be inverted. And still in yaw.
There was some thinking, a decade or two ago, that future fighter-class aircraft might be able to do away with all flight control surfaces; not just the rudder but the ailerons and elevators etc. too. The advantage of this idea is that the wing design can be both more simple and stronger for the same mass (weight) due to not having to 'waste' some of its mass on control stuff. But, as I mentioned above, your control authority then becomes linked to the power settings and even trimming the aircraft for different regimes becomes problematic. An adaptable wing would get around this but then you're back to trading structural wing mass for control wing mass.
The increase in drag on the 'leading' or advancing wing is due to an increase of its effective span (which increases the presented cross-sectional area, not the surface area) but this effective increase in span (and decrease in sweep) results in differential lift, which in turn results in a roll. Because the wing produces more lift than drag, which is pretty crucial if you actually want to fly, a passive solution isn't really viable - the rolling factor will be greater than the retarding factor and you'll soon be inverted.
There are several tricks that can mitigate this to a large degree. Some include adding twist, a variable airfoil shape, or bending the tips down that get dirty faster than the increase in lift. It is particularly troublesome on the flying wing type of craft since even a standard vertical stabilizer doesn't have much of a moment arm to create a restoring torque and the same goes for pitch control as flying wings have no horizontal stabilizers either. Note that I said it was possible, not simple and that likely explains why there are so few surviving attempts at some flavor of stabilizer-less aircraft dating back before computerized flight controls.
@Eddy Ito: Whilst it's true that some washout, along with vertical surfaces, on the wing tips can aid yaw stability on swept wings, I'm still not convinced that they'd help restore yaw stability once it had been lost. Variable aerofoils are a nice idea but fiendishly difficult to implement, and I can't really see it helping much with yaw.
You're absolutely right about the yaw issues with flying wings: one of the problems with the X/YB-35s was that they took far too long to line up and stabilise for the bomb-run due to lack of positive yaw control and some aircrew actually reported suffering from motion-sickness while trying to get it flying straight. The addition of vertical control surfaces to the YB-49 only compensated for the removal of the prop-shaft fairings, which acted as vertical stabilisers on the XB-35, and didn't actually improve yaw control beyond that of the XB-35.
The earliest flying wing designs of which I'm aware are those of J. W. Dunne; apparently his 1912 D.8 was rejected by the War Office, funnily enough, because it overemphasised stability at the expense of controllability. It has to be said though, that Dunne's primary objective with his swept-wing tailless biplane designs was stability. I suspect that up-sized Dunne designs might have made good bombers (for their time) but as far as I'm aware no consideration was given to this.
Well, once upon a time most things were impossible, so its always worth letting inventors have the chance to demonstrate their ideas.
OTOH, looking at the energy density of aviation fuels, the power/weight ratio and empty weight/payload ratios of a helicopter, I remain highly sceptical that a battery powered aircraft will be able to deliver the claimed payload and performance on any known battery chemistry. And I suspect the real killer of (full sized, decent endurance) battery powered aircraft is that the aircraft is always flying at its full take off weight. Sure, a large UAV can be battery powered, eg by lithium sulphur batteries. But the energy mass density of a lithium sulphur cell is about one 300th that of kerosene. If research trebles the battery energy density, it's still a mere 1% of the energy in the same mass of aviation fuel.
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the energy mass density of a lithium sulphur cell is about one 300th that of kerosene
What he said.
Maybe they have a great design concept for a flying car. However, it seems to me there's very little intrinsic benefit in choosing electric propulsion over conventional propulsion, and plenty of downside. (Perhaps the speed control of an electric motor is a bit simpler than having a throttle actuator, but that's about it).
The main value of choosing electric propulsion seems to be in getting gullible investors to part with their cash. Anything electric sounds much more 21st century.
I would posit that, in terms of aircraft, jet would be understood to be some form of gas turbine propulsion. A description that doesn't apply when said propulsion has no compressor / turbine combination and has the recognised description of "ducted fan" - at best it's disingenuous, at worst it's an outright lie.
Actually, "ducted fan" =/= "turbojet" or "turbofan". But "jet" meaning stream is common usage (see, e.g. Jet Skis, jet stream, jet of cold water on an otherwise good snark).
So their tech is actually not dishonestly named... but that doesn't solve the range problem (and no, I don't buy their numbers).
Certain big motor corps had already built and then scrapped a fleet of decent electric cars by 15 years ago. People loved them. GM refused to sell any, and recalled the leases in 2002.
Electric engines where an also ran to the internal combustion engine for motor vehicles mass adoption. Hell, most internal combustion engines require an electric one to start :)
There have been, and continue to be, high usage EVs in industrial settings. I've only encountered milkfloats and massive forklifts, but I'm sure there are other examples. So at least some heavy industrial manufacturers can manufacture the parts needed when it's required.
There is a big difference between taking a series of established and demonstrated principles and combining them to make something much better than the current market price. Tesla and SpaceX both do this, albeit for very different definitions of cargo and cost.
This is not to downplay the hard work and engineering in the doing of this, but Musk didn't dramatically improve the electric motor or a rocket engine, or make a ten-fold increase in battery capacity. No dramatic breakthroughs in the fundamental technologies. Just hard work and a bit of luck making the theoretically possible actual.
It's also very possible that the investors know full well that it'll never do what it says on the tin, but that the parts and process created will be worth more than what goes in.
That is a bit of a misty eye'd view of the EV1. Here is a bit of a more hard nosed assessment of the past and future of electric cars at Ieee Spectrum by Matthew N. Eisler:
"Toyota developed a plug-in hybrid, too. But sales of all plug-ins pale in comparison to the conventional Prius. Consumers, it turned out, preferred a relatively affordable hybrid that ran on a gasoline engine most of the time and produced low emissions to a more expensive hybrid that ran on an electric motor most of the time and produced very low emissions. This technological-economic calculus also explains the record of Nissan’s Leaf. It is the best-selling all-electric, but, like all compliance cars, has struggled to make money. Chevy’s all-electric Bolt is unlikely to change this record."
"At any rate, the Bloomberg analysis is based largely on extrapolating trends in battery cell cost unfolding since 2010, omitting mention of battery pack lifetime and the nettlesome question of pack replacement costs over the average lifespan of an electric vehicle. Indeed, it is extremely difficult to prove a battery’s longevity, and although strides are being made in this neglected area of research, the focus on cell cost alone is highly misleading. Actual battery costs are a virtual trade secret and much disputed, and have been further obscured by federal and state subsidies, which will not be around forever."
A better zero emission solution than all electric cars might be to have a hybrid with an IC engine that runs on ammonia. Although that needs a source of ammonia that is cheaper than petrol, which will probably only come about if high temperature 4th generation nuclear molten salt or gas cooled reactors spread.
All electric cars are probably going to remain stuck in the luxury segment, although I'd be happy if this turned out not to be the case.
All electric cars are probably going to remain stuck in the luxury segment,
In terms of modern personal transport, you have to consider that EVs are in their infancy. For simple reasons of manufacturing economics, EVs are considerably more expensive than any equivalent ICEV, and that means that even with government subsidies these are low profit or loss making (see Tesla's accounts for the real red ink). As a result, EVs need to be sold to wealthy, aspirational customers, able and willing to accomodate the current limitations of the technology. The cost is coming down as volume builds and if those volumes build far enough, the cost balance will quite suddenly tip over, and ICEVs will become the lower volume, higher cost vehicles, only bought for specific applications. Similarly the technology restraints (like painfully slow charging, limited range) are progressively being improved by larger battery capacities. Nissan Leaf started off with a 24 kWh battery, became available with a 30 kWh, this year's new model has a 40 kWh unit, and there's expectation that next year there will be a 60 kWh battery pack - that implies a maximum range of perhaps 300 miles if driven carefully. If those range improvement continue then by 2020, the range anxiety for EVs should be a fairly minimal problem.
However, the extent to which EVs become the norm will be decided by the ability of the electricity sector to generate and distribute power, and the emergence of an affordable charging solution for people who can't do overnight charging on their own drive. Neither of those have quick or cheap fixes, so through the 2020s there may be a decade where market demand would prefer to buy EVs, but the charging issues make it impossible to support the volume of charging. In that case, prices will rise, they percolate down to affordable vehicles much more slowly, and to avoid electricity system problems, EV suers may well need something like a "charging licence" issued by your electricity supplier (or the DNO). Each grid-connected charging point would only get a licence if there's the capacity to safely charge based on its expected use, when the network runs into constraints the next person wanting to buy an EV may have to wait until network improvements have taken place.
In terms of modern personal transport, you have to consider that EVs are in their infancy.
Are they? EVs used to be the preferred method of transportation before internal combustion engines became reasonably stable when they competed with external combustion engined vehicles like the Stanley Steamer. There was no clear winner back then but IC engines won the day with help in part from electric motors which made them easier to start. Today we've largely segregated different power production forms to a particular usage based on ease of use. Perhaps that time is changing for some uses but it remains a stretch particularly for the high power, long endurance, and mobile segments of industry.
"In terms of modern personal transport, you have to consider that EVs are in their infancy."....
Of course. The clue in my post was that phrase "in terms of modern transport". The early EVs used technologies that have little application in modern EVs other than the very basic principles. They died out, and all development work for the next hundred years went into ICEVs. It is only with the current moral panic about climate change (and to a much lesser extent at the moment) air quality that EVs have become a credible proposition, but there's still some way to go before the EV really supplants the ICEV. And in some parts of the world without a widespread electricity grid and plenty of generating assets, it is possible that EVs will never be practicable for a century or so.
If the early car makers had continued with EVs, then by now we'd have had a hundred years more traction battery research. We possibly wouldn't be so reliant on lithium technology that is arguably better suited to small mobile devices. And when the electricity grid became universal in developed countries (1950s-1960s), at the same time that car ownership was becoming the norm, then the grid would have been built with the capacity to charge cars, and generating assets would have been built to deliver the required power. And I suggest that if we had that alternate past, then by now EVs would be a lot more practical to own, cheaper, and have a 500 mile range.
EVs will probably achieve several of those things by the late 2020s, although at that time I'm sure that the electricity system will be more of a constraint than an enabler to a universal EV future.
Leaving out how unlikely it is to actually land on you personally, or your property, assuming it is blaring some sort of warning siren as it falls you'd have plenty of time to look up and get out of the way.
If it lands on your roof then I guess that's your bad luck, but people have cars crash into their houses all the time and sometimes die or are seriously injured in their own bed. That would be a lot less likely with this craft parachuting onto your roof, especially if you live where there's enough winter that roofs are engineered for snow load.
If it can fly "300km at speeds up to 300kph on a single charge", then you can put the same batteries in a regular ground vehicle which doesn't need lift, and it should be able to do 1000km+ on a single charge.
There's your market right there. And if they can't do that, then this is just smoke and mirrors.
They claim it uses the about the same amount of electrical power cruising in flight as an electric car does while driving, though it doesn't say what speed that electric car is traveling at.
This probably doesn't have better batteries than you find in an electric car, just more of them.
No idea. But if it can glide, that increases distance massively. Most current electric heli/quad copter designs have just 15 mins flight time. But this one has full wings.
I would not be surprised if the "cheat" a little and add a small fuelled engine for the endurance, the electric just for city flight/landing.
I would say much more like the drones you can buy on alibaba starting at $20.
I would also say this is bullshit. We know battery powered drones can fly their own weight and a bit more for 20 minutes. The thing will have to be huge for that bit more to equal 5 passengers and it looks like it will still take a lot of power to maintain the speed needed to get enough lift from the small wings.
The prototype weighs as little as possible. The downwash on landing barely moves the grass - compare with this
We just had an article about a Chinese drone for deliveries with a 200kg payload and it needed a jet turbine and chemical fuel to provide enough power and range.
From the video it looks like 36 fans and each fan looks to be about 20 cm in diameter. Breaking out the back of an envelope, lets assume they can somehow achieve a differential pressure of 2.4 kPa, which is pretty good for such a small fan in a short duct, that gives us about 2.9 kN of lift. Now consider that a typical fan three times the diameter will do half that pressure at about 7.5 kW and 36 of those would run 270 kW and you're only half way there. If I had to guess the demonstrator isn't likely to exceed about 16 stone.
Actually, not really very like the F-35 fighter jet, because the F-35B has a lift fan distinct from the propulsion system.
But really quite like another VTOL aircraft made by a company called Hawker Siddeley, the Kestrel... and it's follow-ons, the Harrier, Harrier II / AV-8B, with it's Bristol Pegasus thrust-vectoring engine.
What is the backup plan for when the power goes off, other than tumbling out of control?
Edit: ah yes from the video "A parachute for the whole aircraft". The typical second resort of the Kerbal Space Program builder who realizes that a great plane design is not traditionally landable (the first resort is accepting that Kerbal pilots are expendable).
Flat batteries = 1) lose control, 2) deploy parachute, 3) "LOOK OUT BELOW!"
Yeah, just the thing for an air taxi over a congested landing zone.
I know what the safety certification authorities will say, and it ain't polite.
I'll bet those multiple small fans are claimed to provide some magic benefit too. Sorry, quite the opposite. Fewer, bigger thrusters are always better. This whole thing is total sh*t, reminds me of Luigi Colani.
Sad thing is that, with fewer, bigger fans and split ailerons with differential drag control under an independent and multiple-redundant power system and the VTOL joke dropped, it's actually feasible. But who would invest in boring?
From the Wired site article "Lilium adds that its plane is the first electric aircraft able to fly with both VTOL and jet-powered methods and, in terms of power, Lilium compared its jet's performance to that of a supercar, but with the energy only needed for 15 to 20 seconds to provide thrust. During flight mode, less than a tenth of the power is needed, dropping its energy consumption down to be comparable to that of an electric car."
Not even sure what they are trying to say; That horizontal flight needs 10% of the power that lifitng requires? Or what?
Looking at the video, there is insufficient wing surface or fan area to lift five people plus it's own weight, even if every cubic millimetre of the fuselage is battery, the energy density of the whole thing would be unlikely to move 5 people 1 kilometre let alone 300.
Also curious as to the envisaged operational height particularly considering the use of a parachute inlieu of gliding, climbing up a gravity well even if it's a few hundred metres is expensive in energy terms.
If anyone is interested I have plans for an AI robo-butler autonomous flying car that will serve you meals while flying to your destination, the first round of investment I am looking for £100 million, please send to..............
And once more, one that might theoretically, given some not-totally-unimaginable advances in technology, have a chance to one day work. Continuing a proud tradition there, but I'll still not invest my money in it, nor hold my breath.
Might try and get the T-shirt if they have any.
There are lots of companies like this one. Usually their main selling point is that they will allow you to get around traffic jams... however...
The air might look like it has a huge capacity, but you have greater speeds and a less stable system which forces you to have higher safety margins... which means traffic jams again.
Any increase in capacity usually results in more traffic filling it.
The air might look like it has a huge capacity, but you have greater speeds and a less stable system which forces you to have higher safety margins... which means traffic jams again.
Not in the air you don't. The increased speeds are more that made up by the fact that you have an extremely wide "road" with an extra vertical dimension. A motorway, for example might have 4 lanes of traffic, while an "airway" can easily have 10 lanes horizontally and 5 lanes vertically, giving it effectively over 10 times the number of lanes than a motorway.
The congestion occurs at the destination, because there will be popular destinations where hundreds of vehicles converge and are attempting to land. Just as happens at airports. This can be compensated to an extent by fully automatic computer control, where your parking space is booked in advance and computer control allows close spacing during the final descent, but "rush hour" in a big city would still be impossibly congested, meaning that traffic-limiting methods would need to be applied.
This https://en.wikipedia.org/wiki/Heiligengeistfeld#/media/File:Hamburg_Medienbunker_01_KMJ.jpg is just one location which will be acquired and returned to its former glory, complete with 128-mm guns, and will defend der Vaterland from electric terrorfliegeren. Sie werden nicht bestanden!
(* 'Talleyho! Fat cars!' 1939-45 Luftwaffe-dayfighter-speak on sighting heavy bombers. Night fighter-speak was 'Pauke, pauke', 'kettledrum, kettledrum'. Der Luftwaffe, of course, ran the flak guns. Dicke Herman insisted.)
"During flight mode, less than a tenth of the power is needed, dropping its energy consumption down to be comparable to that of an electric car."
There's this thing called glide ratio. It's the ratio between altitude and distance you can travel horizontally without power. Sailplanes get up to 1:60 these days, i.e., one kilometer of altitude gets you 60 kilometers of distance. That's with sailplanes that have a wingspan of 18 meters, and are designed for the purpose: low weight and optimum wing surface. Large aircraft have a glide ratio of 1:20 to 1:30, still with a pretty big wing area. Fighter aircraft are around 1:10.
The glide ratio defines your power needs. If this gizmo with its stubby wings and no laminar flow because there's fans all over the wing should even get 1:20, you still need to gain 15km of altitude to go 300km. Sure, you don't go 15km up all at once, and you gain some distance while in powered flight, but you catch my drift.
So you'll need enough juice to lift 500 kilos by about 10 kilometers. Oh wait, that is just the payload? The airframe plus engines is another 500 kilos, easily. Plus the batteries themselves. Good luck with that.
They should just glue on some solar panels. Problem solved!
Ok, now I feel like I need to know. Let's say they can get the unicorn to only 1,000 kg. That means a hover requires 9,800 N of thrust. They claim 36 fans so 272 N each. Each fan has a diameter of 0.20 m and let's assume the center hub/motor is 0.07 m. That gives a fan swept area of .028 m2 and let's say the duct exhaust is 90% of the fan swept area giving .025 m2. Given an air density of 1.225 kg/m3 we can get the needed flow velocity. v = sqrt(272 / [1.225 * .025]) = 94.2 m/s; that's quite the unicorn fart. Now it's a simple matter of multiplying the need thrust by the flow velocity to get needed power. P = 9,800 N * 94.2 m/s = 923.576 kW = 1,240 hp. At this point I would like to point out that the above is the result of assuming that the fan and motors are operating at a unicorn friendly 100% efficiency and even with a 400 V battery pack like used in a Tesla it's pulling about 2,300 amps. With more realistic efficiencies of 78% for the fan and 90% for the motor it's about 1.2 MW and 3,300 A coming from the batteries or about 92 A per motor.
They could up the fan diameter to 1 meter each and that would reduce the power requirement to a mere 236 kW but then it gets rather large as at best you're looking at a 6 m by 6 m platform and a small helicopter has a rotor diameter of only 8 m so not saving much there.
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