Register here

Author Topic: VTOL On Demand Mobility  (Read 18395 times)

Offline Viper2000

  • CLEARANCE: Restricted
  • Posts: 10
  • I really should change my personal text
Re: VTOL On Demand Mobility
« Reply #120 on: January 08, 2019, 12:59:49 pm »
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.


« Last Edit: January 08, 2019, 01:52:00 pm by Viper2000 »

Offline zebedee

  • Senior Member
  • CLEARANCE: Confidential
  • **
  • Posts: 132
    • Mumbling
Re: VTOL On Demand Mobility
« Reply #121 on: January 08, 2019, 01:07:41 pm »
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

"I have often had the impression that, to penguins, man is just another penguin - different, less predictable, occasionally violent, but tolerable company when he sits still and minds his own business" Bernard Stonehouse

Offline AeroFranz

  • Aerospace Engineer
  • Top Contributor
  • CLEARANCE: Top Secret
  • ***
  • Posts: 2115
Re: VTOL On Demand Mobility
« Reply #122 on: January 08, 2019, 01:50:45 pm »
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?

All modern aircraft have four dimensions: span, length, height and politics.   TSR.2 got the first three right - Sir Sydney Camm

Offline sferrin

  • Senior Member
  • CLEARANCE: Top Secret
  • **
  • Posts: 11257
Re: VTOL On Demand Mobility
« Reply #123 on: January 08, 2019, 02:26:51 pm »
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.  ???
"DARPA Hard"  It ain't what it use to be.

Offline Zootycoon

  • CLEARANCE: Confidential
  • *
  • Posts: 93
Re: VTOL On Demand Mobility
« Reply #124 on: January 08, 2019, 02:31:07 pm »
Parachutes are a common feature in e-vtol because the proposers donít understand how to meet a safety case that wonít need it. Itís a lack of understanding about what theyíre doing rather than an impossible task. Furthermore I believe they donít appreciate that you canít take credit for a parachute within a safety case;- itís 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;- itís just dead weight which is expensive to maintain.

Bell have a much better understanding of redundancy in safe design, hence no parachute.
« Last Edit: January 09, 2019, 09:46:08 am by Zootycoon »

Offline Viper2000

  • CLEARANCE: Restricted
  • Posts: 10
  • I really should change my personal text
Re: VTOL On Demand Mobility
« Reply #125 on: January 08, 2019, 02:37:27 pm »
Quote
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).

Quote
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.

Quote
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:




Quote
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.

Offline fredymac

  • CLEARANCE: Top Secret
  • ***
  • Posts: 1297
Re: VTOL On Demand Mobility
« Reply #126 on: January 09, 2019, 05:05:54 am »
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.  Iím 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 donít know if a shrouded rotor will significantly cut down the noise.

Offline AeroFranz

  • Aerospace Engineer
  • Top Contributor
  • CLEARANCE: Top Secret
  • ***
  • Posts: 2115
Re: VTOL On Demand Mobility
« Reply #127 on: January 09, 2019, 07:55:49 am »
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.
All modern aircraft have four dimensions: span, length, height and politics.   TSR.2 got the first three right - Sir Sydney Camm

Offline Viper2000

  • CLEARANCE: Restricted
  • Posts: 10
  • I really should change my personal text
Re: VTOL On Demand Mobility
« Reply #128 on: January 11, 2019, 05:09:31 pm »
Quote
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.

Quote
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.

Offline AeroFranz

  • Aerospace Engineer
  • Top Contributor
  • CLEARANCE: Top Secret
  • ***
  • Posts: 2115
Re: VTOL On Demand Mobility
« Reply #129 on: January 11, 2019, 07:06:15 pm »
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.
All modern aircraft have four dimensions: span, length, height and politics.   TSR.2 got the first three right - Sir Sydney Camm

Offline jmspeedfreak

  • CLEARANCE: Restricted
  • Posts: 1
Re: VTOL On Demand Mobilit
« Reply #130 on: January 12, 2019, 01:40:30 pm »
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? Iím recalling a NASA project related to Vietnam era tree top flight surveillance called ĎThe Quiet Oneí.

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.

Offline Viper2000

  • CLEARANCE: Restricted
  • Posts: 10
  • I really should change my personal text
Re: VTOL On Demand Mobility
« Reply #131 on: January 13, 2019, 06:14:31 am »
DOC = Direct Operating Cost;
LRU = Line Replaceable Unit

Offline AeroFranz

  • Aerospace Engineer
  • Top Contributor
  • CLEARANCE: Top Secret
  • ***
  • Posts: 2115
Re: VTOL On Demand Mobility
« Reply #132 on: January 13, 2019, 07:14:47 am »
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.
All modern aircraft have four dimensions: span, length, height and politics.   TSR.2 got the first three right - Sir Sydney Camm

Offline yasotay

  • Senior Member
  • CLEARANCE: Top Secret
  • **
  • Posts: 1873
Re: VTOL On Demand Mobility
« Reply #133 on: January 13, 2019, 02:00:07 pm »
Just a quick thanks to all recent contributions to this thread.  Very interesting reading indeed.

Offline fredymac

  • CLEARANCE: Top Secret
  • ***
  • Posts: 1297
Re: VTOL On Demand Mobility
« Reply #134 on: January 14, 2019, 03:40:47 am »
Some explanations on design trades.  At 1:06 mark, fans and noise are discussed.