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VTOL On Demand Mobility

fredymac

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Bell's Nexus 4EX design change is based on customer feedback indicating inner city transport is more in demand.

https://www.aviationtoday.com/2020/...tal-infrastructure-and-an-all-electric-nexus/

“[Customers] wanted range, so we felt, let’s start at range and we can work our way back in,” Bell CEO Mitch Snyder told journalists during a December press visit to the company's headquarters. “And from the discussions, we felt there were really becoming two distinct requirements, one for inner-city movement and one for more range.”

With demand growing for inner-city aerial transit, Bell is pivoting toward that mission with the Nexus 4EX. Designed to maximize efficiency in forward flight, which is crucial now that the aircraft is fully electric, the 4EX has two fewer rotors and a longer back wing to provide more lift.

Bell is targeting a 60-mile range for 4-5 passengers and luggage, including reserves and multiple takeoffs and landings. Innovation lead Scott Drennan expects that by the time the Nexus 4EX is operation in the “mid-to-late 2020s,” energy storage density will have improved roughly 10-15 percent, allowing the design to hit these targets.
 

TomcatViP

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And many things more here. The approach taken by Bell looks infinitivly more realistic than other competitors and is widely explained there.
 

stealthflanker

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Do we have any regulations for propwash/downwash speed ? As smaller rotor may means faster downwash speed... This could pose potential hazard namely it can fly small things, kicking up dust and blow structures away such as tent.

It might not be a problem for lightweight system but as things grow heavier.. means more power and with smaller ducted fans or propellers means more propwash.

This is an example :

 

VTOLicious

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I recently came across the VRT 500 helicopter and was wondering how all those Airtaxi Concepts compare to a modern single engine helicopter... The VRT 500 is not certified yet. However, I appears to perfectly fit in the Airtaxi niche. Actually it is promoted for Airtaxi applications as well
 

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VTOLicious

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Bell X-22A revisited 54 years later... Unfortunately the drawbacks inherent to ducted propellers cannot be solved by distributed electric propulsion.
What kind of drawbacks? I'd think they'd be safer than tilt-rotors.
It's not about safety. That is mainly a certification aspect and dependent on what is considered as acceptable... Btw, are four rotors "safe"?

The X-22A is without a doubt one of the most sucessfull experimental aircraft ever tested in the golden age of VTOL. The NASA tech report server has plenty of reports available... As far as I remember the main issues of ducted propellers are the following:

  • A duct can either be designed for hover or cruise conditions => Variable inlet / outlet geometry adds weight / complexity.
  • Flow seperation at the inlet, especially during transition, is another issue. That brings me back to the first item => "bell mouth" required for transition / hover => Lots of drag in cruise.
  • Ducts generate large moments in cross flow condtions (cross wind, transition,...).
  • Additionally, it may not be as quiet as one would expect ;)

BR Michael
 

sferrin

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I was thinking safety as in no exposed rotors (but then I guess with a tilt-rotor they're up in the air anyway. . .)
 

yasotay

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Well the enclosed rotors are much better for operating in urban areas I would assume. I don't know that they are significantly quieter than a TR, but would imagine the impulse is less since the blade tips are enclosed. I recall reading/hearing that the X-22A made a loud buzzing noise, but cannot confirm that. Biggest drawback that comes to mind is a similar (possibly more intense?) downwash/outwash.
 

sferrin

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Well the enclosed rotors are much better for operating in urban areas I would assume. I don't know that they are significantly quieter than a TR, but would imagine the impulse is less since the blade tips are enclosed. I recall reading/hearing that the X-22A made a loud buzzing noise, but cannot confirm that. Biggest drawback that comes to mind is a similar (possibly more intense?) downwash/outwash.
Yeah, more like an F-35 lift fan than a rotor. (Though closer to the latter than the former given the differences in HP.)
 

shedofdread

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The drag of ducted props / fans is enormous. They're one of the technologies that we all hear about that promise so much ....until you speak to anyone that's ever been involved in their use ;)
 

Sundog

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The drag of ducted props / fans is enormous. They're one of the technologies that we all hear about that promise so much ....until you speak to anyone that's ever been involved in their use ;)
They're very efficient in the hover, so if you're just going do a lot of hovering or slow speed flight they're great. Of course, one of the reasons they like them for vehicles like this is safety. There aren't tips passengers or people walking around them can't see. Everything is enclosed within the duct.

They also become problematic if you try to use the duct to generate lift, because one side can be stalling while the other side is generating a lot of lift, which can cause huge moments and control issues.
 

VTOLicious

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fredymac

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Aerodynamically, what distinguishes a duct from a nacelle so it is different from an ultra large turbo fan on an airliner? A fan tilted 90 degrees so it faces the direction of travel should have constant blade speed relative to air at all "clock" positions so how do you get asymmetry in load? Is a "fenestron" a form of ducted fan? Just curious.
 

VTOLicious

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shedofdread

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Some development we were doing years ago involved ducted fan UAS. Before we flew the full, ducted versions, we flew an unducted test 'mule'. One day whilst conducting bad weather (high wind + turbulence) hovering tests with the ducted one, the pronounced 'nose-down' into wind attitude was observed. So the old mule was fired up and flown in similar conditions and could hover with a far less 'nose down' attitude even though it had no fairings etc covering it's structure etc. LOADS of parasitic drag. So, ducts can be such a drag, can't they? ;)

PS in tests the ducted one was found to be 78.62% cooler.....
 

Viper2000

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This sounds more like lift from the ducts than drag.
 

VTOLicious

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Some pics of it attached...
Imho that vehicle looks overly complex and pretty much oversized for a 5-seater.
  • 4 pairs of coaxial lift-rotors. Most propably fixed pitch.
  • 4 tilt rotors. Most propably variable pitch.
  • ...mounted on this super thin wing and empennage, obliged to transfer the loads o_O God bless CFRP!
However, I have to admit I like the visual design.
Btw, EmbraerX introduced a very similar design.
 

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Viper2000

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This sounds more like lift from the ducts than drag.
Nope
Surely aerodynamically, this flow turning amounts to an asymmetric lift force on the duct—even if the normal term of art is ram / intake momentum drag—because turning requires circulation & circulation is lift?

Another way to think about this is that if it was purely a drag force, it should scale with freestream dynamic pressure, and therefore be insensitive to the power setting.

Also, if this is genuinely driven by momentum rather than lift, it should be insensitive to specific thrust, but I suspect that the problem is worse for higher specific thrust systems, at least up to the point where shocks form around the the intake lip & induce separation...
 

Moose

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...said proposal of EmbraerX. It does not feature tilt-rotors at least.
Maybe just my own biases, but I'm more bearish on stop-rotor designs over the long term than I am tilt-rotors.
 

VTOLicious

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...said proposal of EmbraerX. It does not feature tilt-rotors at least.
Maybe just my own biases, but I'm more bearish on stop-rotor designs over the long term than I am tilt-rotors.
I don't have preferences. However, my concern is that tilt-rotors add another layer of complexity, due to the tilt mechanism, the variable pitch mechanism and the necessity to control both.
 

mboeller

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Toyota invests in Joby Aviation:


 

VTOLicious

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Toyota invests in Joby Aviation:
According to their website they were flight testing full scale production prototypes for 2 years and are in the process of type certification.
 

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Moose

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Those forward nacelles certainly have a different take on the issue of rotor tilt. Let's hear it for the wonders of relatively compact electric motors.
 

Zootycoon

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Although the vertical take off/landing disc loading look viable, it’s fiendishly complicated with all those variable pitch props, engine rotation, retractable gear and flying controls. I wonder what happens when you get ice on two blades of one of the rear props?

Crashworthiness also looks scary/marginal, a lot of mass/inertia above the cabin, I suspected the blade inertia is too low for auto rotation, wafer thin plexiglass is not known for its strength, ballistic parachutes take a good 150ft to deploy (not that they can be claimed in the certification) so the pax will be the filling in a sandwich.

I’m unconvinced by the range to speed to weight, seems to be the way these days, make outrageous claims to get outrageous money.
 
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VTOLicious

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"...Six aircraft intended for urban air mobility application are “well along” in pursuing type certification with the Federal Aviation Administration, said Jay Merkle, head of the FAA’s UAS integration office, at the Transportation Review Board’s annual meeting in Orlando, Fla..."

 

fredymac

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Found this CNET video on the Nexus which address timeline and reason for ducts (after 3:30 mark).

 

Moose

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Jeez, what did Cnet film that chunky video on?
 

VTOLicious

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With a number of companies in the process of certification I think we will see the wheat separated from the chaff soon
 

Zootycoon

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Here we go....

“But the dream could possibly end suddenly: there are increasing doubts as to whether Lilium - a showcase project, celebrated and admired by ministers and Chancellor Angela Merkel, funded by the European Space Agency Esa - is working seriously. The thesis stands in the room that the ambition of the founders is not compatible with the laws of physics.”

Interesting that the original technical report (thesis) has disappeared from every weby location..... seems to be a bit of a legal thing going on ..... but the laws of physics can’t be prosecuted.
 
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