Helicopter crash New York City

[The Sultan]

Originally Posted by wrench1

the same "vertical hop" and tail wag are possible with Bell blades as well..

It is my understanding that the “vertical hop” with Van Horn blades is asynchronous with main rotor 1/rev which makes it completely different from the “hop” with Bell blades. With Bell blades the “hop” at certain torque settings (ground and flight) is at 1/rev and is aggravated by a swashplate with freeplay when the sum of the forces on the swashplate oscillates through zero. With Bell blades this “feature” is never at a level that would be considered violent. The VH “hop” sounds more like it is sourced from a rotor mode coincident with an airframe/pylon mode.

[wrench1]

Originally Posted by The Sultan

It is my understanding that the “vertical hop” with Van Horn blades is asynchronous with main rotor 1/rev which makes it completely different from the “hop” with Bell blades. With Bell blades the “hop” at certain torque settings (ground and flight) is at 1/rev and is aggravated by a swashplate with freeplay when the sum of the forces on the swashplate oscillates through zero. With Bell blades this “feature” is never at a level that would be considered violent. The VH “hop” sounds more like it is sourced from a rotor mode coincident with an airframe/pylon mode.

Bell calls it collective bounce which I believe is the same term VH uses in its bulletin on the matter which goes into more detail. I just used the same terminology as the previous poster. I've always been instructed its main cause was pilot vs collective interaction that was magnified due to improper collective system friction which was addressed in the Bell OSN. There are also a number of other docs on the issue and its my understanding any 2-blade teetering system is susceptible to it regardless of blade type. As I noted, there was one L-model accident that was attributed to collective bounce back in the late 2000s with Bell blades.

[The Sultan]

Originally Posted by wrench1

Bell calls it collective bounce which I believe is the same term VH uses in its bulletin on the matter which goes into more detail. I just used the same terminology as the previous poster. I've always been instructed its main cause was pilot vs collective interaction that was magnified due to improper collective system friction which was addressed in the Bell OSN. There are also a number of other docs on the issue and it’s my understanding any 2-blade teetering system is susceptible to it regardless of blade type.

What you are describing is PIO (pilot induced oscillation) and it is not limited to two bladed helicopters. What I am alluding to is actual “slop” in the swashplate assembly due to worn components. If these are present the swashplate has a degree of freedom to pump the rotor independent of actual control position. This occurs when the sum of the pitch link loads oscillates through zero which is around the 40% torque range. If the pitch link loads between blades are equal this zone can entered without a perceived change in vibration. If the mean pitch link load between blades are not equal (generally due to sloppy track and balance) this imbalance pumps the rotor without crew input.

[212man]

Originally Posted by The Sultan

What you are describing is PIO (pilot induced oscillation) and it is not limited to two bladed helicopters. What I am alluding to is actual “slop” in the swashplate assembly due to worn components. If these are present the swashplate has a degree of freedom to pump the rotor independent of actual control position. This occurs when the sum of the pitch link loads oscillates through zero which is around the 40% torque range. If the pitch link loads between blades are equal this zone can entered without a perceived change in vibration. If the mean pitch link load between blades are not equal (generally due to sloppy track and balance) this imbalance pumps the rotor without crew input.

Indeed. Commonly referred to as biomechanical feedback. The first Bristow EC225 had an interesting experience on its initial ferry from Marignane, after stopping in Norwich.

[wrench1]

Originally Posted by The Sultan

What you are describing is PIO (pilot induced oscillation) and it is not limited to two bladed helicopters. What I am alluding to is actual “slop” in the swashplate assembly due to worn components. If these are present the swashplate has a degree of freedom to pump the rotor independent of actual control position.

Except the issue with VH blades has been documented and discussed with being due to collective bounce and not swashplate play. And while I've seen a number of interesting results when the swashplate friction is not properly set on the 206 series, if the friction is at its required limits and you had enough play in it to affect blade track that sw/plate was well past its useful wear limits and would have been considered unairworthy. I doubt seriously this operator would have allowed that level of neglect.

[helispotter]

Originally Posted by The Sultan

It is my understanding that the “vertical hop” with Van Horn blades is asynchronous with main rotor 1/rev which makes it completely different from the “hop” with Bell blades. With Bell blades the “hop” at certain torque settings (ground and flight) is at 1/rev and is aggravated by a swashplate with freeplay when the sum of the forces on the swashplate oscillates through zero. With Bell blades this “feature” is never at a level that would be considered violent. The VH “hop” sounds more like it is sourced from a rotor mode coincident with an airframe/pylon mode.

Sultan: when you say "asynchronous with main rotor 1/rev" are you saying the "hop" is at 0.5/rev or perhaps instead 2/rev? I am no aviation engineer, but I would associate a 1/rev with a mass imbalance in main rotor, or inconsistent incidence settings on (both) blades. The 2/rev corresponds to blade passing frequency of a 2-blade main rotor and can easily be explained by unsteady aerodynamics of the rotor in forward flight, even with perfectly balanced and set-up rotor. The 2-rev is what the Nodamatic suspension on the Long Ranger (and a few other Bell types) is intended to smooth out. A 0.5/rev might be possible too. The equivalent on ships operating in a seaway is called "parametric rolling" (sometimes also called asynchronous rolling) as opposed to "synchronous rolling" and sometimes leads to loss of containers when very large but relatively slow roll motions build up on containerships.
I still have an uneasy feeling that blades with different mass and stiffness characteristics than OEM Bell blades might be adversely interacting with the Nodamatic system in a way that hasn't been fully understood by VH, Bell or regulators? Furthermore, I feel it might be possible any "hop" may not be doing much good to the securing arrangement for the inertia masses in the blade tips. So the need for the tap testing (and VH has a more recent reminder of the importance of this on their website) and the incidence of "hopping" could be linked. Speculation?... sure, I accept it is. The sooner NTSB has findings on New York accident and TSB has findings on latest Long Ranger loss, the better.

[The Sultan]

Helispotter,

I mean it is not related to any rotor harmonic or sub-harmonic, so no .5, 1, 2 etc /revs. These are generally at a natural frequency of the rotor, pylon, and/or airframe. For an asynchronous to become a problem the “and” is the operable term. It generally takes a gust exciting the rotor at a frequency close to a fixed system mode which results in a response in such a manner that it can feed back into the rotor to sustain or amplify the vibration.

[helispotter]

Originally Posted by The Sultan

Helispotter,

I mean it is not related to any rotor harmonic or sub-harmonic, so no .5, 1, 2 etc /revs. These are generally at a natural frequency of the rotor, pylon, and/or airframe. For an asynchronous to become a problem the “and” is the operable term. It generally takes a gust exciting the rotor at a frequency close to a fixed system mode which results in a response in such a manner that it can feed back into the rotor to sustain or amplify the vibration.

OK, so who has experienced the so-called "VH hop" and lived to tell the tale? I am still confused as to whether it is concentrated around a defined frequency or is vibrations all over the show (sort of a "white noise"). Would any pilot that has experienced this even be able to characterise it compared to the regular vibrations they are familiar with from their helicopter?

You earlier wrote: "...Bell blades the “hop” at certain torque settings (ground and flight) is at 1/rev and is aggravated...". So this must be different to the 2/rev vibration that Bell has intentionally tried to tune out with the Nodamatic suspension? Is the "Bell hop" mentioned here something that comes and goes?

You also earlier wrote : "The VH “hop” sounds more like it is sourced from a rotor mode coincident with an airframe/pylon mode". Do you mean the various natural frequencies for the rotor and airframe/pylon? Are you also suggesting an initial disturbance such as effect of a gust is perhaps setting off a vibration at such natural frequencies that isn't being naturally damped out, along the lines of flutter?

[wrench1]

Originally Posted by helispotter

I am still confused as to whether it is concentrated around a defined frequency or is vibrations all over the show (sort of a "white noise").

Some references on the issue:
Van Horn Info letter
FAA ACS
Incident mentioned in ACS
TSB Report of accident with Bell blades installed

[Coastal2]

Recent 206L crash in Canada due to VH main rotor blade separation.

Google: Air transportation safety investigation A25W0084

I’m not allowed to post the URL cause I don’t have enough posts.

[TWT]

Originally Posted by Coastal2

Recent 206L crash in Canada due to VH main rotor blade separation.

Google: Air transportation safety investigation A25W0084

I’m not allowed to post the URL cause I don’t have enough posts.

URL: https://www.tsb.gc.ca/eng/enquetes-i.../a25w0084.html

[helispotter]

Originally Posted by Coastal2

Recent 206L crash in Canada due to VH main rotor blade separation.

Google: Air transportation safety investigation A25W0084

I’m not allowed to post the URL cause I don’t have enough posts.

The link shared by TWT is that also shared by Encyclo at post #458. The summary has: "...On approach to deliver an external load of saplings, the helicopter experienced a main rotor blade (blue) separation". However TSB has also provided a pair of low resolution photos. One an overall view showing disposition of wreckage with what is labelled "red" and "blue" blades as well as "fuselage" wreckage all widely separated, while other photo shows inboard parts of both blades still connected to rotor hub, mast and transmission but separated from fuselage. On latter photo, it seems there are red and blue colour coding bands around the blade grips which would explain the reference to "red" and "blue" blades. In short, this latest accident wasn't a case of an entire blade becoming detached as summary text might suggest.

[The Sultan]

Originally Posted by helispotter

this latest accident wasn't a case of an entire blade becoming detached as summary text might suggest.

The most disturbing thing to me is that the span location of the blade failure on the Canada accident is no where near the span location of the suspect failure on the New York accident (post 259, this thread). This indicates two separate root causes. It is interesting that both aircraft were operated in roles known for high GAG cycle rates.

[Salusa]

Originally Posted by The Sultan

The most disturbing thing to me is that the span location of the blade failure on the Canada accident is no where near the span location of the suspect failure on the New York accident (post 259, this thread). This indicates two separate root causes. It is interesting that both aircraft were operated in roles known for high GAG cycle rates.

That's the thing with composite material.

Hard to identify weak points unless they present, and when they do tends to be sudden.

Still leaning towards harmonic feedback into Nodal Beam system though.

Previous posts I have made will explain thinking.

[Encyclo]

And then, there's this one, with detailed analysis of the fractured blades

Air transportation safety investigation A19P0142 - Transportation Safety Board of Canada

Fly Safe, Always

[212man]

Originally Posted by Encyclo

And then, there's this one, with detailed analysis of the fractured blades

Air transportation safety investigation A19P0142 - Transportation Safety Board of Canada

Fly Safe, Always

At least the person making the bad decisions died, and not an innocent pilot.

[SansAnhedral]

Originally Posted by helispotter

OK, so who has experienced the so-called "VH hop" and lived to tell the tale? I am still confused as to whether it is concentrated around a defined frequency or is vibrations all over the show (sort of a "white noise"). Would any pilot that has experienced this even be able to characterise it compared to the regular vibrations they are familiar with from their helicopter?

https://demerarawaves.com/2022/12/07...pper-grounded/

[SansAnhedral]

Originally Posted by The Sultan

The most disturbing thing to me is that the span location of the blade failure on the Canada accident is no where near the span location of the suspect failure on the New York accident (post 259, this thread). This indicates two separate root causes. It is interesting that both aircraft were operated in roles known for high GAG cycle rates.

I suppose a blade could have lost a weight pocket initially, with subsequent failure causing the full separation of the remaining span (unlike NYC where the majority remained attached)

[helispotter]

Originally Posted by SansAnhedral

https://demerarawaves.com/2022/12/07...pper-grounded/

Also consistent with what I have read elsewhere: "...Mr Stoll said shortly after the blades were changed at Ogle and flown to other locations of Guyana, there was no vibration because he was transporting passengers and cargo, but the vibration occurred when there was one pilot, less than one tank of fuel and no cargo."

Oh, and I think Cassandra Forde should be taken up in that helicopter when it is equally lightly loaded to see if she also encounters what the pilot did!

[EESDL]

Rumour has it that bird remains found in cockpit…..