VTOL On Demand Mobility

[Viper2000]

It looks as though the aft fans will swallow the slipstream of the front fans, which doesn't seem very sensible.

I agree with the noise comments; low blade number & activity factor imply fairly high tip MN, which isn't good for noise; not convinced by the selection of 6 stators either, though there isn't really enough duct for cut-off. I am also surprised by the apparent lack of liner in the duct.

I suppose they may just want to build something which flies for PR & will then do a load of trouble-shooting to get acceptable behaviour down the line, but it doesn't strike me as a sensible configuration because it's got such a low wetted aspect ratio, so cruise L/D will be poor (probably significantly less than 6, with my intuition being nearer to 4), whilst the higher disc loading compared with a helicopter will hurt hover performance, as will the transmission losses inherent in all these hybrid-electric schemes.

Indeed, the flight displays in the exhibit imply a cruise speed of (edit) less than 100 KTAS (86 KIAS is 100 KTAS at FL100) (/edit), which is really disappointing, & extra load on the aft rotors (presumably due to the aforementioned slipstream ingestion problem. This requires 100% torque from the GT, which adds to the coefficient of disappointment. The Verge claims that the payload capability is only 600 lbm (272 kg) (https://www.theverge.com/2019/1/7/18168814/bell-air-taxi-nexus-uber-flying-car-hybrid-ces-2019) which means that some of the 5 seats are presumably for show. I note that The Verge also claims 150 mph top speed; this would obviously require assistance from the battery. It's not obvious to me that they can really do this because the average torque in the screenshot is about 70%, so we might reasonably expect the top speed to be 100 KTAS * (1/0.7)^(1/3 = 112 KTAS, or 128 mph, assuming fixed propeller efficiency & drag coefficient (both of which are likely to be optimistic assumptions).

Interestingly 112 KCAS at FL100 is about 150 mph TAS, so it may be that somebody has double-accounted the air density effect, which would be rather embarrassing if true...

At this level of performance, surely you'd be better off in a helicopter? An R66 will do everything that the Bell concept will do (apart from not be a helicopter): https://robinsonheli.com/r66-specifications/
; 50% more payload 10 knots faster in a smaller package without all the electrickery.

From a safety perspective, it doesn't look great either. The lack of any sort of variable area system will force the ducted propellers to be variable pitch, & so the critical failure mode is likely to be blade pitch actuator failure; it's an interesting debate as to what the worst case might be, but I'd want to pay particular attention to failure of either a front or outboard system at maximum dynamic pressure, as I can see that being very exciting & potentially breaking the vehicle or imposing some sort of placard (this may be the driver behind the biplane vertical tail), though I suppose the low cruise speed capability will tend to mitigate this risk to a significant extent, though the fact that departure doesn't break the aircraft doesn't mean that it's recoverable.

Duct misalignment during transition would also probably be extremely exciting, & I note with interest that there doesn't appear to be any way for the pilot to actually see what the ducts are doing (the rear ducts being a particular concern) so that sensor failure could easily lead to an XC-142 type accident.

I hope that great care is taken to make sure that these vehicle are genuinely safe before they start flying over urban areas.

 

[zebedee]

Interestingly Bell seem to be touting the lack of any form a parachute as an 'advantage' over its competitors...

"You'll see some of our competitors out there using parachutes and so forth, but Bell will not be doing that in the urban environment that we are talking about being in, We believe in controlled descent to the ground under power which would be provided by the battery system."

Via Flight

https://www.flightglobal.com/news/articles/bell-unveils-design-and-nexus-name-for-urban-air-tax-454735/

Zeb

 

[AeroFranz]

It looks as though the aft fans will swallow the slipstream of the front fans, which doesn't seem very sensible.

Yes. Vibration and noise can't possibly be good and you cannot tweak that in future versions; it's there as long as the ducts overlap in the front view. My guess is there's no way to handle engine out reasonably without n >6 rotors.

>>I agree with the noise comments; low blade number & activity factor imply fairly high tip MN, which isn't good for noise; not convinced by the selection of 6 stators either, though there isn't really enough duct for cut-off. I am also surprised by the apparent lack of liner in the duct.

I was actually surprised the stators are radial. They could have tried some offset geometry, kind of like Fenestron stators for example. But who knows, noise is black magic as far as i'm concerned...

>>The lack of any sort of variable area system will force the ducted propellers to be variable pitch, & so the critical failure mode is likely to be blade pitch actuator failure

Eh, there's a chance they can design a duct/fan they can live with in both static and cruise conditions. It won't be great in either...but maybe it's a lesser evil than the liability of all the actuators?

 

[sferrin]

The more I look at it, 6 looks pretty clunky. With today's modern FCSs you'd think they'd still be able to get down with a a failure or two, even with only four fans. The X-22 had a flap in the airstream of each nacelle. I'd think between those flaps, and nacelle tilt, one could create enough lift to accomplish a rolling landing even with one or two prop motors out. ???

 

[Zoo Tycoon]

Parachutes are a common feature in e-vtol because the proposers don’t understand how to meet a safety case that won’t need it. It’s a lack of understanding about what they’re doing rather than an impossible task. Furthermore I believe they don’t appreciate that you can’t take credit for a parachute within a safety case;- it’s classified as survival equipment ie only of use after a catastrophic event..... in common with ejection seats, crash helmets, Nomex suits etc.etc

EASA-SC-VTOL-01 will clearly define the the minimum safety requirements for transiting over populated area and if you achieve this with good systems redundancy, the parachute will never be used;- it’s just dead weight which is expensive to maintain.

Bell have a much better understanding of redundancy in safe design, hence no parachute.

 

[Viper2000]

Yes. Vibration and noise can't possibly be good and you cannot tweak that in future versions; it's there as long as the ducts overlap in the front view.

Looking at the PFD screenshot in my previous post, things are made worse by the fact that aircraft is at almost -10 degrees AOA in cruise, so the rear set of rotors may be eating a partial wake, which is really horrible (unsteady loading).

My guess is there's no way to handle engine out reasonably without n >6 rotors.

Yes. The problem is things like bird ingestion mean that simply providing multiple redundancy of motors isn't a panacea.

I was actually surprised the stators are radial. They could have tried some offset geometry, kind of like Fenestron stators for example. But who knows, noise is black magic as far as i'm concerned...

The big levers are blade count (more is better) & tip MN (less is better; nose is proportional to something like the 6th power of tip MN...). Angling the stators or using weird gap / stagger combinations is probably only worth a few dB (say < 6 dB).

See e.g.

https://vtol.org/files/dmfile/20-TVF5-2018-Brentner-PSU-Jan191.pdf

The above slides are from the 2nd presentation in this video:

https://www.youtube.com/watch?v=LmyJBgKjo5M&t=13m0s

Eh, there's a chance they can design a duct/fan they can live with in both static and cruise conditions. It won't be great in either...but maybe it's a lesser evil than the liability of all the actuators?

According to this article they are using variable RPM for control in the hover,

http://gramsluftfartsblogg.blogspot.com/2019/01/uam-urban-air-mobility-bell-pusser-stv.html

so it's possible that they've got fixed pitch blades. I'm surprised by this decision, because it's hard to get this sort of system to scale up, & probably requires the motors to be quite significantly under-sized in the hover. However, it would go a long way towards explaining the low blade count & activity factor, as they'll be desperate to keep the moment of inertia as low as possible.

This will also contribute to the disappointing cruise speed.

I still think it's a strange decision.

 

[fredymac]

It looks like Bell has placed design priority on safety and liability. A crash onto a crowded city street would probably generate serious political pressure to restrict air taxi operations as well as boost insurance costs. I’m guessing safety considerations drove the architecture away from exposed blade helicopters to shrouded rotor designs. At that point, choosing 6 rotors to enhance safety margins to compensate engine loss is no surprise.

Given these things will probably be flying just above skyscrapers, noise is going to be a big factor in public acceptance. I have watched bird sized RC quadcopters flying around parking lots and I am surprised how loud they are. They sound like a gigantic bee buzzing around. These are exposed rotor quadcopters so I don’t know if a shrouded rotor will significantly cut down the noise.

 

[AeroFranz]

Without elaborating further on the source, i heard from people with more knowledge of the relevant physics that the duct may not help unless it's long enough in relation to the diameter - something that has to do with the frequencies you're trying to block. Long ducts are a pain, both in terms of structural weight and drag in cruise.

 

[Viper2000]

Without elaborating further on the source, i heard from people with more knowledge of the relevant physics that the duct may not help unless it's long enough in relation to the diameter - something that has to do with the frequencies you're trying to block.

At a really simple level, the thing which matters is the size of the duct relative to the wavelength of the sound that you're trying to block; if the barrier (i.e. the duct) is small in comparison to the wavelength then the sound will simply diffract around it.

This thesis presents a nice general treatment of low noise design techniques applicable to turbofan airliners.

It's important to understand that civil turbofans are designed quite tightly around certification requirements, & therefore you will find references in the thesis above to "critical polar angles"; these fall out from the interaction between the directivity function, the microphone position, & the flight path of the aeroplane.

The eVTOL concepts in this thread don't have quite such tightly defined paths & microphone positions, & obviously the directivity function is going to be driven by duct vector, so the problem is much more general & therefore difficult.

On the other hand this paper would have you believe that ducted fans operating at subsonic tip Mach number should ideally be silent (see section 6.2 on page 32) because all the tones are cut off. Naturally, this isn't really true, especially if the duct is operated at incidence relative to freestream. However, if the incidence is low & the blades are subsonic, then ducts may reasonably be expected to at least knock a decent hole in the directivity plot close to 90 degrees.

Long ducts are a pain, both in terms of structural weight and drag in cruise.

Short ducts are also difficult because they're more likely to stall under off-design conditions; drag advantages rely upon the assumption that the drag is due to skin friction; this clearly becomes invalid if the duct is too short & stalls.

I think a lot of these vehicles are quite cynically designed to trade off performance against perceived novelty; as discussed, the published performance seems markedly inferior to an R66 so it might be simpler to take a simple helicopter & go for a more complex rotor system plus NOTAR to hit the noise target.

There may be some DOC advantage to distributed propulsion if propulsors are LRUs (as Bell claim), but this is offset by the capital cost of the spares inventory, & it's fundamentally hard to make this case at vehicle level if the underlying architecture still includes a GT; all these hybrid schemes are one propulsion system for the price of two.

 

[AeroFranz]

Thanks, those are interesting points. Yeah, i'm a bit wary of papers presenting theories not substantiated by experimental data. It's hard to capture all the physics involved in acoustics.

 

[jmspeedfreak]

Re: VTOL On Demand Mobilit

Sorry, can someone clarify for me the meanings of DOC and LRU.

In case anyone has feel for the answers...

If MN or tip velocity is under 180m/s is it generally accepted that the blade noise will be very low? I’m recalling a NASA project related to Vietnam era tree top flight surveillance called ‘The Quiet One’.

Anyone remember? If most of the time the objective is to keep tip speed very low then 6 props could be part of that justification, likewise an rpm based control strategy would be most of the time operating way down the inertia bucket, hence would only be a real pain if one or two of the props is out. Additionally I though operating two discs in series in the cruise actually gave a modicum of variable area like behaviour as the overall jet velocity is hiked by using two in series obvious counter rotating too. I can imagine the outer pair of ducts being feathered in high speed flight and maybe only these two having variable pitch for roll authority.

Just my take anyway.

 

[Viper2000]

DOC = Direct Operating Cost;
LRU = Line Replaceable Unit

 

[AeroFranz]

Some numbers typically floated for quiet operations are Mtip < .45, but that's just one part of the equation unfortunately. Another major contributor is having a non-uniform inflow, so you really try to avoid having a fan behind the stators, for example.
Ideally all of the propulsive units would be simple (no variable pitch) and identical, to keep costs down. These vehicles have to be 'somewhat' affordable for the business case to close.

 

[yasotay]

Just a quick thanks to all recent contributions to this thread. Very interesting reading indeed.

 

[fredymac]

Some explanations on design trades. At 1:06 mark, fans and noise are discussed.

https://www.youtube.com/watch?v=9X3BNCJhflI

 

[VTOLicious]

It is interesting (and refreshing) to see, that Airbus (CityAirbus) and Bell (Nexus) came up with a completely different solution for the same problem / use case / business case, whatever you wanna call it.

...btw, still waiting for CityA to conduct first flight.

 

[yasotay]

It appears that the rotors are not "covered" by the... well whatever it is (still a duct?) SO does this mean that the noise propagation from those rotors will be completely unmasked?

 

[VTOLicious]

It appears that the rotors are not "covered" by the... well whatever it is (still a duct?) SO does this mean that the noise propagation from those rotors will be completely unmasked?

Only the lower rotors are shrouded. A configuration never seen before, I think.
The upper rotors are most likely a bit larger in diameter.

 

[VTOLicious]

In total 800 kW max continuous power

 

[Viper2000]

1500 Nm / 49 kg = 30 .6 Nm/kg.

This is even more impressive than the SP260D, which hits 20 Nm/kg.

Clearly we are now heading towards diminishing returns, because the motor mass fractions for practical vehicles will be small, but it's really impressive.

RE the City Airbus rotor shrouds, I think that they are intended to protect against collisions, much like a Fenestron helicopter tail rotor. I don't really understand how they're going to make it quiet, but it's possible that they have a higher noise target than we think, or that they are taking lots of credit for masking by background urban noise.

 

[AeroFranz]

1500 Nm / 49 kg = 30.6 Nm/kg.

This is even more impressive than the SP260D, which hits 20 Nm/kg.

indeed. One thing to note, though, is that I think Siemens gives their liquid-cooled motor weight exclusive of the cooling system, so it's not "as installed". So when they're claiming 5kW/kg specific power, it's a bit misleading comparison to air-cooled motors. Still, i have no reason to believe they're not among the best motors around, it's just hard to make an apples-to-apples comparison.

 

[Zoo Tycoon]

In my experience, a rule of thumb for highly optimised electric motors means a power density for a constant rated load of 4kw/kg for liquid cooled (cooling system not included) and 2kw/kg for air cooled. At these levels they’re running at high efficiency so it’s difficult to get any better. You can finesse these figures with higher rotor speed and trade with torque density but it’s about as good it gets within the available materials.

The basic motor performance is a function of magnetic flux density, resistivty, and thermal conductivity and these are limited by the best known materials to science.

I’ve seen motor developer fiddle thier figures by quoting from non constant load tests to get investment, headlines etc.

 

[TomcatViP]

Last but not least the rotors are fixed... While the supporting beam fully restricts the airflow. Something don't adds up. We might have here a very early configuration - what in Airbus world would be a "designer" work, something that has yet to be seen by a qualified engineer.

 

[VTOLicious]

Last but not least the rotors are fixed... While the supporting beam fully restricts the airflow. Something don't adds up. We might have here a very early configuration - what in Airbus world would be a "designer" work, something that has yet to be seen by a qualified engineer.

I think you are wrong. Look at the pics. They are building a prototype, not a mock up. Originally first flight was scheduled for late 2018.

 

[TomcatViP]

The frame suggests an iron bird: something build to test the ctrl laws and motors Then the propulsion units are not articulated on the mockup meaning that this thing could be called the Vomit-comet.

If you look at the design of the propulsion units, you'll see that it has only blade pitch actuation (just like a propeller on a plane). The high torque design converge also with this. It means that their intend is to fly like a quad rotor by varying unit power. If they do that with a fixed propulsion units, it means that the cabin will wobble constantly and speed will be a function of attitude. Hence the nick-name: Vomit-comet.

In all probability, Airbus is just bragging showing a concept hastily made out of an R&D project.

 

[galgot]

Yes it does look like the rotors are fixed, so it would work like a quad drone. But That way wouldn't make it "wobble contantly" more than a helo... Some quad drones don't need big change of attitude to change direction . What is interesting is the way there are 2 props per rotors, if that is a motor per prop, it's an interesting way of solving the "engine out" security for available space for the props, better than some other concepts I've seen with six ducted props on each side. Plus you can have bigger props.

Bragging... You mean like building a mockup or something like that ? : You know... It's funny how whenever a subject mention Airbus, your comments are so much predictable
Not that i am a big Airbus fan, We all have our fav, and hated makers, but wow it's so systematic from you.
You've been fired from Airbus or what ? Just curious.

 

[TomcatViP]

OMG

What is interesting is the way there are 2 props per rotors, if that is a motor per prop [...]

Each pod has Two engines (look at the diagonal bracing at the middle and engine description on the cardboard)

 

[galgot]

OMG

What is interesting is the way there are 2 props per rotors, if that is a motor per prop [...]

Each pod has Two engines (look at the diagonal bracing at the middle and engine description on the cardboard)

Thanks for the confirmation
And ?

 

[VTOLicious]

The frame suggests an iron bird: something build to test the ctrl laws and motors Then the propulsion units are not articulated on the mockup meaning that this thing could be called the Vomit-comet.

If you look at the design of the propulsion units, you'll see that it has only blade pitch actuation (just like a propeller on a plane). The high torque design converge also with this. It means that their intend is to fly like a quad rotor by varying unit power. If they do that with a fixed propulsion units, it means that the cabin will wobble constantly and speed will be a function of attitude. Hence the nick-name: Vomit-comet.

In all probability, Airbus is just bragging showing a concept hastily made out of an R&D project.

Oh boy

You should better inform yourself. It's definitely not a iron bird. It was announced long time ago that the CityAirbus demonstrator is scheduled to fly end of 2018.

 

[martinbayer]

The frame suggests an iron bird: something build to test the ctrl laws and motors Then the propulsion units are not articulated on the mockup meaning that this thing could be called the Vomit-comet.

If you look at the design of the propulsion units, you'll see that it has only blade pitch actuation (just like a propeller on a plane). The high torque design converge also with this. It means that their intend is to fly like a quad rotor by varying unit power. If they do that with a fixed propulsion units, it means that the cabin will wobble constantly and speed will be a function of attitude. Hence the nick-name: Vomit-comet.

In all probability, Airbus is just bragging showing a concept hastily made out of an R&D project.

Oh boy

You should better inform yourself. It's definitely not a iron bird. It was announced long time ago that the CityAirbus demonstrator is scheduled to fly end of 2018.

So, this being January 18, 2019, how did the maiden flight go?

 

[TomcatViP]

VTOLicious: It's not like something I comment should be a new design. It doesn't change a iota that we have seen this Airbus effort before I don't understand, neither the tone and the meaning. If then you'd had fall in sympathy with the despicable depiction of what should be me by our fellow Fr poster before (and above), I'd remind you that this is only hysteria.

Oh boy.

 

[Arjen]

So, this being January 18, 2019, how did the maiden flight go?

The latest news I have found is a tweet from Airbus Helicopter boss Bruno Even
https://twitter.com/BrunoEven/status/1055749844810321920
Power up of demonstrator on 26 Oct 2018 - not the iron bird, as testing of that started in late 2017. Nothing about a first flight yet.

 

[AeroFranz]

side note on stacked rotors (two on top of each other, driven by separate motors), be it for open or ducted rotors.
The stacked rotors are undeniably more compact, but unfortunately they don't decrease discloading (thus power required to hover) like individual rotors do. That is, what matters is the projected swept area of the rotors as seen from the top.

Additionally, the propellers have to be optimized to work together (i.e., the lower one is operating in the exhaust of the top one). So when one of the two motors goes out, the remaining propeller is a) working with some sort of blockage from the stopped prop, regardless of whether it's upstream or downstream, and b) it no longer has the optimum twist since the flow conditions have changed drastically.

 

[LowObservable]

Clearly, I am a Luddite and moreover an oldthinker who unbellyfeels urban VTOL.

Equally clearly, the urban VTOL revolutionaries will be able to:

Invent electrically powered VTOL aircraft that can haul a useful payload ~100 km and return, either doing so several times or recharging in minutes
Make these things quieter than conventional rotorcraft
Make them far less costly than any helicopter or VTOL
Develop an infrastructure that allows thousands of vehicles to operate unmanned over densely populated areas
Make them an order/orders of magnitude safer than any other small aircraft, from Day One.

In short....

 

[TomcatViP]

The stacked rotors are undeniably more compact, but unfortunately they don't decrease discloading (thus power required to hover) like individual rotors do. That is, what matters is the projected swept area of the rotors as seen from the top.

If w is the downward stream speed component of a single rotor disc, 2w will be the one for a stacked rotors (in idealized conditions). Respecting the proper conditions will induce some benefices. In case of a failure, the failed rotor's prop can be feathered. But it is true that the carrying beam structure will induce some losses to say the least.

 

[Foo Fighter]

I am not an expert but I never did get the trend for stacked rotors without a separate rotation for each. After all contra props always (As far as I have seen) have a displacement for each prop.

 

[AeroFranz]

The stacked rotors are undeniably more compact, but unfortunately they don't decrease discloading (thus power required to hover) like individual rotors do. That is, what matters is the projected swept area of the rotors as seen from the top.

If w is the downward stream speed component of a single rotor disc, 2w will be the one for a stacked rotors (in idealized conditions). Respecting the proper conditions will induce some benefices. In case of a failure, the failed rotor's prop can be feathered. But it is true that the carrying beam structure will induce some losses to say the least.

if you must have a feathering device, you might as well have variable pitch. With eight (or more) rotors, that means eight additional devices that are maintenance and reliability items. One of the advantages of multirotors tout is the small number of moving parts and reliability, which this now goes against. I'm not saying it can't be done, but there's costs associated with it.

 

[galgot]

Thanks for these precisions. Maybe one way to avoid such problems in case of a motor out, would be to have a big excess of power diverted to the remaining turning motor for a time, to be at least effective the duration of a fast automatic emergency landing , even in case of disrupting flow due to the upper or bottom non-turning prop… ? Dunno.
Or eject the non-turning prop all together… wait, in town… forget it ;D
What is the reliability of that kind of motor anyways compare to a helo turbine ?

 

[AeroFranz]

I don't know about the reliability of the motors, although it should be pretty good with just one moving part.
Typically the vehicles carry a substantial amount of excess power. The early Vahana was, according to the open source code posted on Github, sized for an all-engine operating thrust to weight close to 1.7, although to be fair that seems like a lot.
By oversizing the motors, you can afford to lose one and still have enough thrust generation. The more motors you have, the less you need to oversize them because losing one is a smaller overall loss. One thing to keep in mind is that besides generating enough thrust, the remaining forces must also be balanced. Take the simple case of a quad rotor. If one rotor becomes inoperative and you just increase the thrust on the remaining three equally, the vehicle will flip. More in general, multirotors will speed up the rotors next to the one that failed and maybe throttle down the opposite ones.

 

[TomcatViP]

if you must have a feathering device, you might as well have variable pitch.

To me that's what they have. But I might be overly naive and optimistic.