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Is Bell testing a multi-fenestron?

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Is Bell testing a multi-fenestron?

Old 23rd Feb 2020, 12:58
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It's a proof of concept demonstrator

So let's not read too much in to it, frankly we'll done Bell forgetting some real size flight hardware to this stage. I suspect the 4 motor choice is a combination of redundancy, noise, weight, heat dissipation and response reasons. I know other manufacturers are looking at electric tail control but this is the first I have heard using more than 1 rotor, a very elegant solution in my opinion as the motor is likely to be the weak link in the chain reliability wise, cabling, generators and control systems have aircraft qualification already. Would be very interesting to see how it handles in crosswords above the minimum spec point (17kts). BTW what is the normal low speed envelope for the 429?

DM🐀
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Old 23rd Feb 2020, 15:28
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That doesn’t account for any eventual complexity required to certify the system. Keep in mind there are zero fly by wire civil certified helicopters in existence. Adding an FBW system like this would have flight critical software, redundancy requirements all along the routing of the tail boom and a requirement for no “common mode” failures. That generally means 4 identical fans and motors would not be easily qualified with in flight critical applications because they were not designed for such a purpose.

I bet it is heavier than the system it replaces by a large margin... increases susceptibility to common mode critical failures like software, lightning or EMI.... and substantially less efficient from a pure energy transfer perspective... lots of physics to combat to prove me wrong here.

im glad Bell is investing some money in commercial space though. This seems like a real project as opposed to the FCX1 thing they showed a few years ago... so that’s nice.
Well said bellblade2014 and captures my thoughts closely. Still cool they built and flew it. Taking IR&D to flight test is a sign of a healthy engineering department and good leadership.
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Old 23rd Feb 2020, 21:19
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Four layers of redundancy with four electric motors. Considerable weight saving at the rear, C of G moving forward. More streamlined than current options. Large reduction in maintenance requirements, current systems are complex and heavy.
10-15% Power saving that is now available for lift rather than yaw control.
Well worth more investment and testing.
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Old 23rd Feb 2020, 21:28
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Baby steps

I think of the Bell EDAT as a baby step for their Nexus air taxi. How will you ever FAA certify something like an all electric quad tilt rotor aircraft when you canít certify an electric tail rotor on a conventional helicopter?

Last edited by CTR; 23rd Feb 2020 at 21:58. Reason: Grammar
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Old 23rd Feb 2020, 21:35
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dangermouse -

Your comment regarding the number of motors and reliability/failure rates is correct. Below is a typical fault tree for an electro-mechanical flight control actuation system. The threshold for primary flight control applications (like a helicopter tail rotor) is 1x10^-9 failures/flight hour. As you can see the least reliable parts of the system are the motor power electronics and controllers. A system like this would probably require 3 or more independent and isolated power/control circuits, and fault tolerant multi-phase motors. Fixed pitch blades are less efficient, but eliminate the need for very high reliability (and complex) actuators.



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Old 23rd Feb 2020, 21:39
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Originally Posted by riff_raff View Post
dangermouse -

Your comment regarding the number of motors and reliability/failure rates is correct. Below is a typical fault tree for an electro-mechanical flight control actuation system. The threshold for primary flight control applications (like a helicopter tail rotor) is 1x10^-9 failures/flight hour. As you can see the least reliable parts of the system are the motor power electronics and controllers. A system like this would probably require 3 or more independent and isolated power/control circuits, and fault tolerant multi-phase motors. Fixed pitch blades are less efficient, but eliminate the need for very high reliability (and complex) actuators.


rate

AND [4 of each] ...
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Old 23rd Feb 2020, 23:11
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But that does not address common mode software or firmware failures.
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Old 23rd Feb 2020, 23:35
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The math is not that simple

Dangermouse and Riff Raff, the math is not that simple, or the whole requirements story.

First, loss of tail rotor control on most FAA certified helicopters is not classified as catastrophic, requiring 1E-9 probability of failure. Loss of tail rotor control is typically classified as hazardous, requiring only 1E-7 probability of failure. For Part 29 certification, the cyclic and collective controls however a catastrophic classification is correct.

Second, the Bell 429 is a Part 27 certified helicopter. For Part 27 certification flight controls a FEMCA is not required. However it is required under Part 29. I admit for new and novel technologies this is a gray area.

Third, the analysis you are providing as a reference is for a screw EMA actuator. You can delete the screws from the probability analysis. Additionally bearing failures are easily mitigated by use of redundant bearings or bushings, along with a sensor to detect primary bearing failure. So the probability of a dual simultaneous bearing failure is not even worth considering. Finally, main reason screw actuators are not used in aircraft primary flight controls is the difficulty in mitigating the common mode jam failure of a single screw. Configure two or three actuators in parallel, one actuator jams the assembly. Configure two or three actuators in series you end up with a very long actuator.

This is a big reason the Bell EDAT makes sense. In the event of one jammed fan, the remaining fans performance are not impacted.

Last edited by CTR; 23rd Feb 2020 at 23:36. Reason: Spelling
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Old 24th Feb 2020, 02:07
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Originally Posted by CTR View Post
Dangermouse and Riff Raff, the math is not that simple, or the whole requirements story.

First, loss of tail rotor control on most FAA certified helicopters is not classified as catastrophic, requiring 1E-9 probability of failure. Loss of tail rotor control is typically classified as hazardous, requiring only 1E-7 probability of failure. For Part 29 certification, the cyclic and collective controls however a catastrophic classification is correct.

Second, the Bell 429 is a Part 27 certified helicopter. For Part 27 certification flight controls a FEMCA is not required. However it is required under Part 29. I admit for new and novel technologies this is a gray area.

Third, the analysis you are providing as a reference is for a screw EMA actuator. You can delete the screws from the probability analysis. Additionally bearing failures are easily mitigated by use of redundant bearings or bushings, along with a sensor to detect primary bearing failure. So the probability of a dual simultaneous bearing failure is not even worth considering. Finally, main reason screw actuators are not used in aircraft primary flight controls is the difficulty in mitigating the common mode jam failure of a single screw. Configure two or three actuators in parallel, one actuator jams the assembly. Configure two or three actuators in series you end up with a very long actuator.

This is a big reason the Bell EDAT makes sense. In the event of one jammed fan, the remaining fans performance are not impacted.
Common mode failures including software, EMI, HIRF, Lightning (IEL in particular), and latent manufacturing defect will all be tough to meet for this application.

The energy efficiency argument is tricky though... the engine>>>generator>>>distribution>>>motor is quite inefficient vs straight mechanical power transfer... losses are very small along a traditional series of bearings and gears.

cant wait to see those performance numbers.


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Old 24th Feb 2020, 11:58
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Originally Posted by bellblade2014 View Post
Common mode failures including software, EMI, HIRF, Lightning (IEL in particular), and latent manufacturing defect will all be tough to meet for this application.

The energy efficiency argument is tricky though...
Fully agree on all points. But as stated earlier, EDAT is a good baby step for electric distributed propulsion. Because if you canít do an electric tail rotor requiring only 1◊E-7, how will you ever do an all electric EVTOL requiring a probability of failure better than 1XE-9?



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Old 24th Feb 2020, 16:10
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Originally Posted by CTR View Post
Fully agree on all points. But as stated earlier, EDAT is a good baby step for electric distributed propulsion. Because if you canít do an electric tail rotor requiring only 1◊E-7, how will you ever do an all electric EVTOL requiring a probability of failure better than 1XE-9?
phenomenal question. I do not have faith in full electric EVTOL in the next 3 generations of occupants. I believe we will have at least one full generation of residents on Mars before we have full electric EVTOLís FAA certified for part 135 equivalent operation. Technical risks abound and the civil certification mountain is getting much worse not better. The 737MAX will take a horrendous process to new heights (or lows). Just one example, it has long been an embarrassment to take approved and in Service STCís and get them incorporated into an aircraft TC. Thatís literally an approved change getting incorporated in an approved aircraft. These processes get denied all the time for stuff like.... Lithium BATTERIES! Should be easy to get approval to power the whole ship with them!

As long as the regulatory environment continues itís profound misplaced emphasis on process over substance, EVTOLís will be only used in Kit or Experimental applications... and helicopters have a bright and vibrant future!





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Old 25th Feb 2020, 08:39
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I confess to being an old school helicopter guy and find the battery-electric drones swarming the sky quite irksome.

However we must give those enthusiastic drone-youth some credit .... here is a prototype battery-electric they built to carry a passenger .... they are not hindered by anything ... probably no one told them it wouldn't work so they just went ahead and did it. Filmed at an old Zeppelin hangar in Germany I think

It is an 18 minute video .... and at 15:00 flies outdoors with a lightweight dummy .

Will be interesting to see if it will actually carry a passenger , and of course battery life will likely be under 30 minutes and not very practical as far as helicopters go.

Have to give these fellows some admiration ... not only that ... they are already self-trained to fly it by remote control ... I think I am jealous .... haaaa !!! ... and I doubt they have a multi-million dollar helicopter engineering department backing them .... they are more like video-game enthusiasts.

Anyway , it gives some credence to using several small electric motors with plastic propellers like the Bell 429 experiment.

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Old 25th Feb 2020, 11:42
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Originally Posted by Arnie Madsen View Post
Will be interesting to see if it will actually carry a passenger .....
Here you go....


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Old 25th Feb 2020, 12:08
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Filmed at an old Zeppelin hangar in Germany I think
Well they say that in the video, but it certainly doesn't look like it - maybe for a small modern airship
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Old 25th Feb 2020, 12:38
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Originally Posted by 212man View Post
Well they say that in the video, but it certainly doesn't look like it - maybe for a small modern airship
There are hangars in Germany for modern airships in Friedrichshafen and Essen-MŁhlheim, the first one could fit...

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Old 29th Feb 2020, 01:32
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Presumably, without the fuss of a long drive shaft, a folding tail is a doddle! if so, a desirable feature?


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Old 1st Mar 2020, 18:09
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To clear up a misconception, the electrically powered fans still present a load to the engine.
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Old 1st Mar 2020, 19:10
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Originally Posted by MLH View Post
To clear up a misconception, the electrically powered fans still present a load to the engine.
Correct. In fact when hovering the electric fans will impose a higher engine load than a conventional shaft driven tail rotor due to the lower efficiency of electrics.

However as forward speed increases, the electric fan speeds can be reduced or even stopped. Therefore the electric fan load on the engines can drop below that of a conventional shaft driven tail rotor.

Another factor to be considered is the impact of reduction in main rotor speed to reduce noise. Many helicopters today have the capability of reducing rotor speed to reduce noise. This results in reduced lift capacity and engine efficiency. The ability to reduce the tail rotor speed independent of the main rotor speed may allow for additional performance benefits.
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Old 1st Mar 2020, 22:00
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I suspect that a flat pitched conventional TR in cruise does not pull much power.
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Old 1st Mar 2020, 22:37
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Originally Posted by CTR View Post
Correct. In fact when hovering the electric fans will impose a higher engine load than a conventional shaft driven tail rotor due to the lower efficiency of electrics.

However as forward speed increases, the electric fan speeds can be reduced or even stopped. Therefore the electric fan load on the engines can drop below that of a conventional shaft driven tail rotor.

Another factor to be considered is the impact of reduction in main rotor speed to reduce noise. Many helicopters today have the capability of reducing rotor speed to reduce noise. This results in reduced lift capacity and engine efficiency. The ability to reduce the tail rotor speed independent of the main rotor speed may allow for additional performance benefits.
Just a question Re the above comment?

For doing what a lot of helicopters actually do, would a conventional tail rotor be better? A lot of helicopters are already maxed out when lifting heavy loads close to operating limits, and pushing a bit too much TR causes spikes that the computer loves to record for you. AS350 being an example. If the electric fans will draw more power than conventional TR, then surely they would be worse in these situations? As forward speed increases the draw on the conventional TR also decreases, but they aren't near the limits at this point.

So for operations such as Fire fighting, Utility, construction, Agricultural, lifting, logging, SAR if lots of winching, etc a conventional TR would probably be better??
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