G2 down in Switzerland - 15 June 2022

[TimdeBoer]

Just came across this video on a Dutch website:
https://www.dumpert.nl/item/100034058_461b7738

Likely linked to this accident?
https://aviation-safety.net/wikibase/279275
https://www.polizei-schweiz.ch/epagny-fr-bei-vergnuegungsflug-mit-helikopter-abgestuerzt/

Looks like a close call initially... :ooh:
Great save and survivable landing apparently.

[ShyTorque]

Well that one didn't want to go quietly, did it? :eek:

Hopefully no serious injuries caused.

[Reely340]

G2s crashworthiness is truly amazing 👍

[Hughes500]

I bet there was a change of underwear required there

[Ovc000]

Wonder if there's some similarity to the UK crash..

[twinstar_ca]

I bet there was a change of underwear required there

Any crash requires new underwear!!!

[Prizmatic]

again, not enough right pedal!

seems to be a theme for Cabris.
You have no idea what happened in this incident yet you knock the G2? I thought this crap was over.... yawn.

[MightyGem]

Good effort. :ok:

[Robbiee]

You have no idea what happened in this incident yet you knock the G2? I thought this crap was over.... yawn.

Seems more like a knock on the pilot.

[Robbo Jock]

How on Earth did he/she save that? Tremendous outcome.

[Agile]

Feels good to see a light helicopter with its tail boom still attached after that tiny bit of aerobatic.

[[email protected]]

Don't think a Robinson would have survived those gyrations intact even before the ground intervened.

[Robbiee]

Don't think a Robinson would have survived those gyrations intact even before the ground intervened.
​​​​​​
I knew a Robinson that had a tail rotor failure (bird strike) shortly after takeoff (as described here). It survived long enough to get them down, and they both walked away.

[Rotorbee]

Since Robisons were certified a long time ago, when crash worthiness and general strength wasn't so high on the list, no probably not. Neither would have a JetRanger or a H300 and many other helicopters. Composite materials have their advantage. It is absolutely impressive, how the Cabri holds together. And how the pilot saved it.
On the other hand, Robinson helicopters are know for their good tail rotor authority. Probably the same accident wouldn't have happened in the same way with an R22.

In the days, I had situations in the R22 where it started to spin with full opposite pedal, because the customer wanted me to fly very slowly with a (light) tail wind (Notice to self: learn to say NO when you are still young and alive). A bit of forward speed and everything was fine again. But I learned two methods to stop the spinning when I felt it coming. Just wiggle the cyclic for a second and it disappears. The second is a quick down and up movement of the collective. Don't like that one, but it works, too.

But that it the second accident with a spinning Cabri in a few days. In this accident, after the plunge toward the earth, the pilot recovered and flew for a very short while straight and level and the spinning started again. This might point to a mechanical failure.
On the other hand, if the Cabri can get away from you and you can't to nothing against it (if that is the case), I wonder if it is such a good training tool. Students tend to be a bit slower or wrong in their reaction and I feel, that a good training helicopter should be more forgiving. Strong it is already, no doubt about that.

[Two's in]

Is it just me thinking chopping the throttle might have led to a less exciting arrival? That or make sure your right boot is touching the perspex...

[Reely340]

In the days, I had situations in the R22 where it started to spin with full opposite pedal, because the customer wanted me to fly very slowly with a (light) tail wind (Notice to self: learn to say NO when you are still young and alive). A bit of forward speed and everything was fine again.
Weird. S-300C AFM states something like hovering with wind from all directions has been demonstrated for up to 17 kts. :E

Being the curious guy that I am I tested that on a 10-15 kts day, right on the apron, 20 ft below the tower and 25 yards from emerg. vehicles garage.
After all the LTE stories and wind azimut caveats I was very very cautious. Of course the smack at the fuselage when wind direction swung past 180° is impressive, but it was clearly hoverable in a controlled fashion.
Up to now I not sure what is wrong with tail wind or wind from 5 or 7 'o clock below say ~12 kts when hover taxiing. :8

I admit that looong ago I reversed for ~ half a mile from the training square to the gas station when nasty wind picked up to 20ish kts.
Felt stupid and looked inept for sure, but we didn't want to do a full circuit on low fuel.

[B2N2]

I bet there was a change of underwear required there

Well, pilot was 65 and his passenger 70….that in itself requires it.

[Hughes500]

do we actually know if was a mechanical failure or a pilot failure ?

[Ascend Charlie]

After all the LTE stories and wind azimut caveats I was very very cautious. Of course the smack at the fuselage when wind direction swung past 180° is impressive, but it was clearly hoverable in a controlled fashion.

Nick Lappos, long-time member of this forum and chief test pilot for Sikorsky (and later Bell) would have something like this to say:

When the US Army had bought a lot of early model B 206 / Kiowa, they had a lot of accidents where the pilot lost yaw control. Army asks Bell to investigate, and rather than admit that the tail rotor was too small to do the job, they came up with LTE.

Their spin merchants were VERY good at their job, even convincing FAA and a lot of others that LTE was real, and applied to every helicopter, and was the cause of the loss-of-yaw-control accidents. Pilot error for letting the wind get into the wrong quadrant, rather than an inadequate tail rotor. LTE appeared in every text book, pilots then said "It was LTE" for any and every case where the pilot didn't control the aircraft, or put it into a situation where the piddly tail rotor could not perform.

A bigger tail rotor subsequently appeared, but the LTE Urban Myth still persists. And mysteriously, it has never been proven to exist on any other helo type, especially NOT the Robinson line, because Frank was a tail rotor specialist and designed it correctly from the start.

[Rotorbee]

Just for the record, I never said I had LTE. What I experienced in the R22 was a rather gentle affair. Nothing like the wild spinning of a Cabri. And it wasn't LTE. I suspect that it was just an interaction of the main blade tip vortex and the tail rotor.
Anyway, if you live in a legal system - the US legal system -, where admitting a mistake can bring a company down due to stupid lawsuits that make at least the lawyers rich, the culture is bound to move towards spin doctoring, creatively finding explanations that are natural causes that could not be seen because physics wasn't advanced enough.

[megan]

it has never been proven to exist on any other helo typeThe UH-1H manual has a section on LTE with an diagram of the warning areas mapped out. That being the case you would expect to see the same warning/diagram in 212, 412 and AH-1 manuals, but don't

[Hot and Hi]

Well, pilot was 65 and his passenger 70….that in itself requires it.
I believe you think you are funny. However, I am unsure whether your post is in-line with this network’s policy prohibiting discriminatory posts.

[joe_bloggs]

Anyway, if you live in a legal system - the US legal system -, where admitting a mistake can bring a company down due to stupid lawsuits that make at least the lawyers rich.

This immediately brought back Frank Robinson’s words when I was fortunate enough to attend a Factory Safety course quite some years ago. He asked for questions and I enquired about the doors off speed limit… oh… he said…pausing… thats’s the lawyers!
You see, if the door is off and you stick your arm out you might get injured.
So it was decided, to minimise claim risk, speed would be limited for door/s off.
(No aerodynamic or structural reasons)

[Robbiee]

This immediately brought back Frank Robinson’s words when I was fortunate enough to attend a Factory Safety course quite some years ago. He asked for questions and I enquired about the doors off speed limit… oh… he said…pausing… thats’s the lawyers!
You see, if the door is off and you stick your arm out you might get injured.
So it was decided, to minimise claim risk, speed would be limited for door/s off.
(No aerodynamic or structural reasons)

,....but if you stick your arm out at 99 kts (or at 100 kts in the R22) you'll be just fine! :hmm:​​​​

[Rotorbee]

,....but if you stick your arm out at 99 kts (or at 100 kts in the R22) you'll be just fine! https://www.pprune.org/images/smilies/yeees.gif​​​​
Perhaps not you, but Robinson. They told you so. It is always hard to admit one made a mistake, much easier to blame somebody else. Especially in a society that is fixated on success and always being the best. At least pilots should be familiar with situations where we mess up and hopefully catch our mistakes before anything happens. That is the idea of the regs we follow ... more or less, but there are many people out there, who never, ever make any mistakes. Therefore if something goes wrong, it must be the fault of somebody else and they find a lawyer that is only too happy to sue for ridiculous amounts of money on the basis of psychical distress because somebody got an outchy on their picky. Therefore companies must protect themselves from anything a stupid person might do, despite common sense. There is this saga that when Dornier certified the 228 in the US, they had to add, that the plane has to be turned in the direction of the runway for takeoff. I truly hope it isn't true, but it sounds like the loop hole a ambulance chaser would love to see.

[Robbiee]

Perhaps not you, but Robinson. They told you so. It is always hard to admit one made a mistake, much easier to blame somebody else. Especially in a society that is fixated on success and always being the best. At least pilots should be familiar with situations where we mess up and hopefully catch our mistakes before anything happens. That is the idea of the regs we follow ... more or less, but there are many people out there, who never, ever make any mistakes. Therefore if something goes wrong, it must be the fault of somebody else and they find a lawyer that is only too happy to sue for ridiculous amounts of money on the basis of psychical distress because somebody got an outchy on their picky. Therefore companies must protect themselves from anything a stupid person might do, despite common sense. There is this saga that when Dornier certified the 228 in the US, they had to add, that the plane has to be turned in the direction of the runway for takeoff. I truly hope it isn't true, but it sounds like the loop hole a ambulance chaser would love to see.

Well, Robby did eventually give us governors, hydraulic controls, and bladder tanks. Don't know if they admitted that not having these things was a mistake, but its still nice to have the improvements.

Maybe one day Bruno will just decide to make a version of the Cabri with a more "traditional" tail rotor,...just for the hell of it.

[Ascend Charlie]

The UH-1H manual has a section on LTE with a diagram of the warning areas mapped out. That being the case you would expect to see the same warning/diagram in 212, 412 and AH-1 manuals, but don't

Never Proven to exist on other types, and anyway, the tail rotors on the newer birds rotate upwards into the flow and are on the flipside of the tail boom.

[megan]

Forgot about that AC, even our 205's had them on the opposite side to the Hotel. The Hotel data plate said "Manufacturers Model 205" and "Customer Model UH-1H", were civil 205's originally with the left mounted tail rotor?

Edit, found a thread

https://www.pprune.org/rotorheads/453011-bell-205-tail-rotor.html

[Ascend Charlie]

And didn't the 205s have dual hydraulics?

[albatross]

And didn't the 205s have dual hydraulics?
205A single hyd. 205A-1 dual hyd. as I vaguely recall.
It was also advised that when flying with no collective hydraulics not lower the power below 35 PSI as raising it again would be difficult. …it was! ( I may be +- wrong about the 35 number.)
I luved flying the 205. Noisy, fun and forgiving…what more could a young boy want?

[megan]

Our 205A-1 were single hydraulic, one guy had a failure and being on fixed floats put it down in a sewerage farm pond/lake. Not a nice way to treat a lady. The 205A-1 maintenance manual says they may be single or dual systems.

[TimdeBoer]

Interesting to see where this thread is going...

So, back to the G2.
Structural wise I think the occupants were very lucky with the design!

Anyone with experience with regards to tail rotor authority?
A fenestron has it pros and cons I can imagine.

[Mutley1013]

Now there is a can of worms! We probably need a new subforum just for that topic. And another for Robinson vs the Rest

[Robbiee]

Interesting to see where this thread is going...

So, back to the G2.
Structural wise I think the occupants were very lucky with the design!

Anyone with experience with regards to tail rotor authority?
A fenestron has it pros and cons I can imagine.

This is all hear/say, as the G2 is out of my price range, but I've heard that though the tail rotor produces plenty of thrust, it takes a lot more pedal input than one might expect to get it.

So, if you're not Fred Astaire you might run into some issues? :eek:

[ShyTorque]

I instructed on RAF Gazelles in 1984/5. The fenestron tail was quirky but worked well enough.

I went back to fly it three years later and by then I was surprised to find that they had invented fenestron stall. It still flew the same, but it was far more restricted in cross winds.

[[email protected]]

I posted this on another thread
Like many fenstron equipped helicopters, the Cabri requires anticipation of the pedal requirement - in forward flight the fin is producing a lot of the anti-torque.

As you lose ETL, the fan has to work harder to replace that loss of fin-lift and so requires a great deal more power pedal.

Add in a crosswind where the aircraft wants to weathercock and it is quite easy to end up with undemanded yaw - not LTE - and often FULL right pedal is required to maintain or correct the heading.

So not much to do with the engine - until the very high rate of yaw has an effect - but mostly to do with lack of skill and possibly training.

Not saying this was the cause of the fatal accident in this thread - more a comment on the other accident linked above.

[[email protected]]

I posted this on another thread
Like many fenstron equipped helicopters, the Cabri requires anticipation of the pedal requirement - in forward flight the fin is producing a lot of the anti-torque.

As you lose ETL, the fan has to work harder to replace that loss of fin-lift and so requires a great deal more power pedal.

Add in a crosswind where the aircraft wants to weathercock and it is quite easy to end up with undemanded yaw - not LTE - and often FULL right pedal is required to maintain or correct the heading.


fenestrons have a different thrust to pedal position curve - a conventional TR is almost linear in its thrust, a fenestron is more S shaped. Training and anticipation are required but the fenestron still works fine.

[[email protected]]

SHY - Fenestron Stall was much misunderstood even though it didn't exist. Hover taxiing sideways at 10' in and out of dispersal put us in the avoid curve, just as well the Astazou was such a reliable engine.

[Rotorbee]

I give you that from 2001. PPrune Rotorheads. Interesting read.
eden
19th Feb 2001, 02:22
Having flown the Gazelle in a wide variety of flight regimes ..... and during a time where many pilots were coming unstuck with Fenestron related problems .......and during a time when some very telling tests were done with Aersospatiale using some Fleet Air Arm instructors. I have a few views and observations which would suggest that FENESTRON STALL is infact an excuse used to describe MISHANDLING,rather than a Stall of the blades. Mighty Gem – makes a point with regard to this and I’m sorry that it happened to you …. As long as you only dented the pride as opposed to yourself …. Do we know each other???

1. The term FENESTRON stall became obselete within the RN when - as a result of careful examination of circumstances surrounding the departure from controlled flight of a number of aircraft being flown by student pilots and some very expereienced instructors at low speed or in the hover.

The Navy severely damaged several airframes as a result and felt compelled to look deeper into the problem …….. they visited Aerospatiale and carried out some extreme flight tests in the hover …….all captured on video and all VERY ALARMING to watch. The aircraft was put in a spot turn to the left and then allowed to continue to turn …… the rotation was allowed to accelerate and at a point where the rotation was becoming almost disorientating the opposite (RH) pedal was applied using FULL deflection. It caused a massive sustained overtorque but the aircraft rotation stopped (as I recall) pretty much as described by Mighty Gem(within about 270 – 360 deg). These test were carried out at differing rates of rotation and different amounts of opposite pedal were used. The final outcome – illustrated that the aircraft is able to recover from a high rotational left turn but needed FULL APPLICATION of the opposite pedal. It would also cause a massive overtorque and was obviously a flight regime to be avoided.

2. The RN then decided that the term FENESTRON STALL – was no longer valid and the term YAW DIVERGENCE was born – which actually described the problem fairly well. The recovery action required from any apparent loss of control in a LH spot turn was to apply – FULL Right pedal. If FULL RH pedal was not applied – there was a risk that the aircraft might not recover and continue to suffer the YAW DIVERGENCE – which manifested itself as an increasing acceleration to the left. By way of demonstration – I used to show student pilots an incipient level of the problem:

I used to sit in the hover into wind and I would apply……1-2 inches of RH pedal, the aircraft would Yaw right and settle reasonably quickly within 20 – 40 deg of I/W heading (approx). I would then return the aircraft to the I/ w position and repeat the process with 1-2 inches of left pedal ….the aircraft would continue past the 40 deg point and begin to accelerate, if left uncontained, the rotation would continue to accelerate in a dynamic and unstable way. I used to start recovery after 360 deg before any excessive yaw rate had developed. The demonstration was effective and illustrated the caution required in handling low speed left hand yaw applications.

3. YAW DIVERGENCE – incidents were significantly reduced as a result of the increased awareness and if encountered were prevented by using the FULL application of the RH pedal. It is believed that many of the occurrences and incidents involving alleged F/STALL were infact errors or misinterpretation in handling; and whilst many pilots believed they had applied FULL RH pedal it is considered likely that they never achieved FULL DEFLECTION in order to counter the problem. Many of the incidents were dealt with as if they had experienced a TR FAILURE – ie shutting the engine down . The subsequent high rotation – perceived as a TR FAIL’ and the ensuing EOL combined to make a mess of several airframes.
Sorry that I have taken so much time to explain what MIGHTY GEM said in a few lines – but it perhaps puts some background into the thinking behind the MYTH. It would be good to get the views of the Light Blue and Green – if possible?

[Robbiee]

I almost hate to ask, but what's the difference between "undemanded yaw" and LTE?

[[email protected]]

LTE would be when the TR just can't produce the thrust required to maintain or correct the desired heading. Caused by poor design or exceeding design limits.

Undemanded yaw is when the pilot hasn't applied sufficient pedal to maintain or correct the desired heading. Caused by lack of anticipation, awareness or skill.

Ref the 2001 article - all good except for the term Yaw Divergence - if it was used in the military, it was only by the RN, not the RAF or AAC.

From what I understand, the term divergence is usually paired with another axis implying cause and effect - ie yaw/roll divergence (Puma).

[Rotorbee]

From Webster:
Definition of divergence

1a : a drawing apart (as of lines extending from a common center)
b : difference (https://www.merriam-webster.com/dictionary/difference), disagreement (https://www.merriam-webster.com/dictionary/disagreement)
c evolutionary biology : the development of dissimilar traits or features (as of body structure or behaviour) in closely related populations, species, or lineages of common ancestry that typically occupy dissimilar environments or ecological niches : divergent evolution (https://www.merriam-webster.com/dictionary/divergent evolution)
2 : a deviation from a course or standard
3 : the condition of being mathematically divergent (https://www.merriam-webster.com/dictionary/divergent)

I would say 2 applies here. Yaw divergence would be that the tail rotor doesn't do, what it is supposed to do.

[Robbiee]

LTE would be when the TR just can't produce the thrust required to maintain or correct the desired heading. Caused by poor design or exceeding design limits.

Undemanded yaw is when the pilot hasn't applied sufficient pedal to maintain or correct the desired heading. Caused by lack of anticipation, awareness or skill.

Ref the 2001 article - all good except for the term Yaw Divergence - if it was used in the military, it was only by the RN, not the RAF or AAC.

From what I understand, the term divergence is usually paired with another axis implying cause and effect - ie yaw/roll divergence (Puma).

Ah,...another one of those, "we're from different countries" differences. I'll add it to the list.

[AMDEC]

I almost hate to ask, but what's the difference between "undemanded yaw" and LTE?

LTE was used by US Army to designate unanticipated yaw, when they had the problem in the 80's, mainly on OH-58. Bell already noticed in a Service Letter dated July 1984 that the term was misleading.

FAA used the LTE wording in AC 90-95 as a synonym of "unanticipated yaw"

Do not mind what Loss, Tail Rotor and Effectiveness mean, LTE is only a synonym of "unanticipated yaw". I cannot imagine that FAA, as a Certification Authority, would consider certifying a rotor that could lose efficiency and, to my knowledge, nobody has ever demonstrated such a phenomenon.

Before LTE, "Tail Rotor Stall" was used... untill they demonstrated that no stall occurred. But they replaced a myth by another one, because nobody remember the origin of LTE. This is why many think, erroneously that it designates a loss of effectiveness of the tail rotor that does not exist.

[Robbiee]

LTE was used by US Army to designate unanticipated yaw, when they had the problem in the 80's, mainly on OH-58. Bell already noticed in a Service Letter dated July 1984 that the term was misleading.

FAA used the LTE wording in AC 90-95 as a synonym of "unanticipated yaw"

Do not mind what Loss, Tail Rotor and Effectiveness mean, LTE is only a synonym of "unanticipated yaw". I cannot imagine that FAA, as a Certification Authority, would consider certifying a rotor that could lose efficiency and, to my knowledge, nobody has ever demonstrated such a phenomenon.

Before LTE, "Tail Rotor Stall" was used... untill they demonstrated that no stall occurred. But they replaced a myth by another one, because nobody remember the origin of LTE. This is why many think, erroneously that it designates a loss of effectiveness of the tail rotor that does not exist.

If you understand LTE (as it is presented in the Rotorcraft Flying Hanbook) then there really should be no such thing as "unanticipated" yaw.

In other words, if I understand that getting below ETL in a left quartering headwind will cause the nose to want to yaw right, then I will be prepared for it, with left pedal ready to go.

Anyway, that's how I look at it in FAA land.

,...but yeah, the words themselves don't represent the condition very well.

[Rotorbee]

Right ... err ... about that Rotorcraft Flying Handbook. Don't get me wrong, it meets the purpose, but ... well ... it has a bit too many "lies to children" in it.
It isn't worse than other basic books about helicopters - after all they all copy from each other - but I would not take it too serious. It is alright for the average pilot to learn the basics, but too many subjects are oversimplified. Some things are plain wrong. For example, the last time I looked there was still gyroscopic precession mentioned for the rotor as an explanation, why the swashplate moves the blades before the intended place where the action actually happens. Well that is (almost) plain wrong, that would only happen in a vacuum (Prouty). But it is good enough for the average Joe pilot, because the real reason is quite a bit more complicated. Still the book teaches all the right manoeuvres one should know. It isn't bad in that regard, just the explanations are ... well ... sometimes...
Some years ago Shawn Coyle asked here on Pprune, if somebody would help to find all the incorrections in that book so I started to read it again. We did not get very far in the project, but I found quite a few things that are questionable in the first 50 pages or so. But it is getting better, I think.
Having said that, put yourself in the shoes of the poor guy who has to write that book. There is no doubt, that many FAA experts (and CAA experts and so on) know exactly how reality looks, but you can't put that in a book for the average student pilot with a very basic graps on physics.
For me, Prouty's books were my starting point and quite the eye opener and quite entertaining sometimes. Worth the read. Shawns books are also very helpful to get a better understanding.

[Robbiee]

Right ... err ... about that Rotorcraft Flying Handbook. Don't get me wrong, it meets the purpose, but ... well ... it has a bit too many "lies to children" in it.
It isn't worse than other basic books about helicopters - after all they all copy from each other - but I would not take it too serious. It is alright for the average pilot to learn the basics, but too many subjects are oversimplified. Some things are plain wrong. For example, the last time I looked there was still gyroscopic precession mentioned for the rotor as an explanation, why the swashplate moves the blades before the intended place where the action actually happens. Well that is (almost) plain wrong, that would only happen in a vacuum (Prouty). But it is good enough for the average Joe pilot, because the real reason is quite a bit more complicated. Still the book teaches all the right manoeuvres one should know. It isn't bad in that regard, just the explanations are ... well ... sometimes...
Some years ago Shawn Coyle asked here on Pprune, if somebody would help to find all the incorrections in that book so I started to read it again. We did not get very far in the project, but I found quite a few things that are questionable in the first 50 pages or so. But it is getting better, I think.
Having said that, put yourself in the shoes of the poor guy who has to write that book. There is no doubt, that many FAA experts (and CAA experts and so on) know exactly how reality looks, but you can't put that in a book for the average student pilot with a very basic graps on physics.
For me, Prouty's books were my starting point and quite the eye opener and quite entertaining sometimes. Worth the read. Shawns books are also very helpful to get a better understanding.

I don't mind being the average joe, I'm a pilot not an engineer, and so that book has served me just fine these past twenty years.

You may not agree with what it says about things like LTE, but that knowledge has kept my nose straight in all sorts of wind.

As for gyroscopic precession? Right or wrong, what I learned about it from that book allowed me to pass the test. After that, I have no use for that knowledge.

[[email protected]]

LTE was used by US Army to designate unanticipated yaw, when they had the problem in the 80's, mainly on OH-58. Bell already noticed in a Service Letter dated July 1984 that the term was misleading. read what Ascend Charlie posted above, #20 I think, LTE was made up by Bell to cover the fact they put an inadequate TR on the 206.

Rotorbee - I was talking about using the term divergence in an aviation context not a literal dictionary one.

[Ascend Charlie]

The FAA handbook has a lot of people still believing that "flapping to equality" is happening all the time, and that the advancing blade is flapping up in forward flight. Never mind the reality of looking at the disc tilted forward, that is just a distraction. Sure, flapping does happen as soon as you move forward, but the pilot puts in forward cyclic to stop it, and continue moving forward. Go faster, need more and more forward stick to keep that disc tilted forwards.

Search for "helicopter urban myths" posted on this forum many years ago by Nick Lappos. It clears up a lot of misunderstood things.

[[email protected]]

In other words, if I understand that getting below ETL in a left quartering headwind will cause the nose to want to yaw right, then I will be prepared for it, with left pedal ready to go. yes, you are anticipating the weathercock effect. Pilots who don't anticipate that yaw are more likely to experience undemanded/unanticipated yaw.

Especially with fenestrons.

[Ascend Charlie]

a left quartering headwind will cause the nose to want to yaw right,

Why are fenestrons different? In a real helicopter, a wind from the left makes the nose yaw left, into the wind. Ignoring the horsefeathers about the LTE diagrams.

[AMDEC]

read what Ascend Charlie posted above, #20 I think, LTE was made up by Bell to cover the fact they put an inadequate TR on the 206.

Rotorbee - I was talking about using the term divergence in an aviation context not a literal dictionary one.

LTE is not the explanation of unanticipated yaw, it is unanticipated yaw. Unanticipated yaw exists on any single rotor helicopter, even NOTAR helicopters (see the MD902 Grossglockner accident) and the LTE wording in that case is quite strange. Installing a more capable tail rotor will allow to expand the flight envelope but not cancel the risk of unanticipated yaw. Unanticipated yaw is not a problem of tail rotor sizing.

Bell seems to have been very reluctant to use the LTE wording.

[[email protected]]

LTE is not the explanation of unanticipated yaw, it is unanticipated yaw. No, you can get unanticipated yaw in a helicopter without loss of effectiveness, it just means you don't have enough pedal applied.

If a gust of wind hits you and the heading changes, the TR hasn't lost effectiveness.

If it was LTE you wouldn't be able to correct it.

[AMDEC]

No, you can get unanticipated yaw in a helicopter without loss of effectiveness, it just means you don't have enough pedal applied.

If a gust of wind hits you and the heading changes, the TR hasn't lost effectiveness.

If it was LTE you wouldn't be able to correct it.

But loss of effectiveness does not exist. There is no unanticipated yaw you are unable to correct as long as you keep your RPM. LTE was only used as a synonym of unanticipated yaw (#45)

https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/597x193/sans_titre_e654260e9d94c3c534ebc36f37863685b4cd3a65.png
extract from AC 90-95


I am not an english speaker but I understand from that extract of AC 90-95 that "unaticipated yaw" and "LTE" are two ways to designate the same phenomenon.

[[email protected]]

AMDEC - that is the same Bell myth used to disguise a poor design and unfortunately reproduced by the FAA.

If you put a TR on a helicopter that can't cope with certain wind conditions then you either impose handling/wind limits on it or put a bigger TR on it.

Bell sales would have been affected by a wind limit that competitors did not have so they invented LTE as an explanation.

If your TR can't produce enough thrust to maintain or correct a heading then you have a loss of TR effectiveness. That is not the same as not putting enough pedal in.

Plenty of people have been spun round by weathercocking in helicopters but it is often lack of anticipation rather than poor TR thrust.

Losing ETL on the MR also affects the TR and you can suddenly experience an increase in Power required from both rotors - you raise the lever to compensate and also require more power pedal - both to compensate for the loss of ETL on the TR and to counter the increased torque reaction caused by you pulling more collective.

[[email protected]]

LTE was also associated with helicopters that had the TR rotation going the wrong way ie not upwards into the MR downwash - the Lynx is a typical example, it had issues with running out of yaw control until they changed the TR direction and it suddenly had awesome TR capability

[AMDEC]

In the certification process you have to claim for a flight envelope (in our case wind envelope) and you have to demonstrate the maneuverability (pedal margin) in that wind envelope. If your tail rotor is limited, it will make a limitation on your hover performance and it will be clearly visible.

If your TR is limited and you are unable to maintain the critical heading, you will stabilize at the heading corresponding to your pedal stop but in no case you will start spinning.

Yawing right with full left pedal is nonsense. It could happen in pure hover without wind if you exceeded the MR torque that you are able to counterbalance with the full TR thrust. Certification does not allow that. The TR thrust is designed to cope with at least 17 kt in the critical azimuth and hover performance is limited to that point. You have little pedal margin at maximum hover performance in the critical azimuth but you recover a lot when the wind comes from a different direction.

[Agile]

https://www.aerotime.aero/upload/files/1250x420/Sikorsky_VS300_crop.jpg

Sikorsky had it right, we just messed it up after that :)

On a good windy day regularly do some 360 slow rotations on the taxiway for a good 15 minutes to get your feet moving and you will be fine.
Control authority is there, plenty of it, if you push early and strongly enough.

[[email protected]]

AMDEC - I agree with what you say, the ability to cope with at least 17 kts at various test points must be demonstrated.

But what happens in dynamic manoeuvres or when the wind is gusting in excess of 17 kts? That is where a poorly designed TR causes problems..

You said yourself you have little pedal margin when it comes to critical azimuth. Which operator is going to limit themselves to 17 Kts cross or downwind?

[Robbiee]

Just out of curiosity, since you guys don't agree that in certain wind angles the tail rotor becomes "less effective" thus requiring more left pedal, do you also disagree when we (FAA land) say that when the tail rotor passes through ETL on takeoff it becomes "more efficient" thus requiring less left pedal?

[Rotorbee]

As much as I like to discuss this (again), I'd rather like to know, why you can not stop the spinning of a Cabri, if that wasn't a mechanical failure. I don't know, but that looks to me like a design flaw and should not pass a certification.

[Robbiee]

As much as I like to discuss this (again), I'd rather like to know, why you can not stop the spinning of a Cabri, if that wasn't a mechanical failure. I don't know, but that looks to me like a design flaw and should not pass a certification.

Well, like the "design flaw" of the R22, just slap an SFAR onto it, and go about your day. :ok:

[AMDEC]

As much as I like to discuss this (again), I'd rather like to know, why you can not stop the spinning of a Cabri, if that wasn't a mechanical failure. I don't know, but that looks to me like a design flaw and should not pass a certification.
I fully agree Rotorbee and a tail rotor that would lose effectiveness at any time should neither pass.
The spinning of the Cabri is not stopped either because there is a failure or because the pilot has a much too centered pedal position. Full right pedal makes the quickest stop but is not mandatory. The Gazelle tests have shown that the headwind hover pedal position was sufficient to stop the turn in less than 360°.

[Rotorbee]

The thing that puzzles me in these videos is the rate of spin. If you have that kind of a problem in a R22 or even a 206, the whole thing is relatively gentle. You have no problem to shove the cyclic forward and point the nose in a direction and fly out of it, even with an general direction of your choice. But in these videos the Cabri just looked out of control. In the one from Switzerland, he even got the spinning stopped and then it started again. That is really odd.

[[email protected]]

We don't actually know what the problem in the Swiss one was, might have been stuck pedals or something jamming them.

We also don't know what he did with the throttle or collective that might have affected the rate of spin.

[Robbiee]

The thing that puzzles me in these videos is the rate of spin. If you have that kind of a problem in a R22 or even a 206, the whole thing is relatively gentle. You have no problem to shove the cyclic forward and point the nose in a direction and fly out of it, even with an general direction of your choice. But in these videos the Cabri just looked out of control. In the one from Switzerland, he even got the spinning stopped and then it started again. That is really odd.

Its also odd that after he's on the ground, he picks it up again for one more spin? Would you do that if it were a mechanical failure?

[[email protected]]

Just out of curiosity, since you guys don't agree that in certain wind angles the tail rotor becomes "less effective" thus requiring more left pedal, do you also disagree when we (FAA land) say that when the tail rotor passes through ETL on takeoff it becomes "more efficient" thus requiring less left pedal? Those certain wind angles don't make the TR less effective but demand more pedal to counter the effects of the wind on the fuselage - ie weathercocking tendency.

Yes the TR gains from ETL just like the MR does.

[Rotorbee]

Its also odd that after he's on the ground, he picks it up again for one more spin? Would you do that if it were a mechanical failure?
You know, that pilot was purely running on instinct. I don't think there was a lot of thinking going on in these moments of helicopter hell. If he picked it up again, that might have been just a reflex, being so low. I wonder if he remembers everything that happened.

[RVDT]

You have no problem to shove the cyclic forward and point the nose in a direction and fly out of it, even with an general direction of your choice.

I would like to see that - personally I think you may be deluded slightly.

[Rotorbee]

I would like to see that - personally I think you may be deluded slightly.
I bet you would. As I explained way up there, I was in that situation. In my case it was main rotor down wash interaction, I suppose and the tail rotor trust was just diminished, not completely lost. Can't prove it, but it was the logical conclusion at that time. Friend of mine had the same problem under the same circumstances. We both had no pedal travel left to push, but it was slow and by far not as violent than what the Cabri had. After less than 180 degrees of "unintentional yaw" I was out of it. I But since we found out how to prevent it, that will not happen again. You are out of luck. As for the delusional part, when you experience something once, one's brain tend to overload with input and things might get a bit fuzzy in your memory, but if you do things several times and you start to recognise the early signs, the old brain is much better in remembering how an event happens. Still, I can't prove that I don't live in the matrix and all this is just a very strange dream.

[[email protected]]

Rotorbee - perhaps things are getting confused here - I think your reference to just push the cyclic forward is about a situation in a hover taxi with some MR/TR vortex interaction giving some undemanded yaw and NOT as a cure to the wildly spinning Cabri - am I right?

We used to demonstrate MR vortex entering the TR on the Wessex, usually at high AUM in a hover taxi with the wind from the 10' o'clock position (anti-clockwise rotor) - it caused vibration and some minor heading variations but nothing more. Changing the speed or the wind angle removed it entirely.

[Rotorbee]

Crab, you are right. I do not suggest in any way, how to stop the spinning in a Cabri. What I wonder is, why the Cabri starts spinning like crazy that fast and can not get stopped by pedal input or naturally by pointing the nose into the wind. My experience was just to explain, that in other helicopters, it does not happen that fast, if the tailwind and the resulting interaction with the down wash was the cause of the problem. If you are in that situation in a Cabri, I very much doubt that you are even able to react properly, unless you expect it to happen. In the Swiss video the pilot literally dove down and the spinning stopped and started again when he levelled out. That is why I do not exclude a mechanical problem. The whole thing is a complete mystery to me, including how to get out of it.
When I had my little incidents, I did not stay there just to find out, if I could go all the way round. Therefore I can not say, if it would have stopped by itself, once the nose got into the wind. Probably would have. A friend of mine flying a 206 at high altitude, between 10'000 and 12'000 and low airspeed (looking at the Eiger north face) did go around a couple of times and entered autorotation. No biggie since he had a lot of free air below. For that Cabri thing, I don't know how the get out of it and why it is so fast and violent. I would have thought that the tail fin would put up quite a resistance. For me, in my little brain - and I do not suggest that this actually happened - it looked like something reversed the pedals by magic and the fenestron pushed the Cabri around. That is only my imagination because I find the rate of spinning is beyond what physics should allow (just my thought on it, not what I think what could be the cause, because I do not have an idea, so don't fixate on it).

[WillyPete]

That is only my imagination because I find the rate of spinning is beyond what physics should allow (just my thought on it, not what I think what could be the cause, because I do not have an idea, so don't fixate on it).

The yaw rate in that video is about 160 degrees per second.
Completely feasible in a left stuck pedal, or operator error.

[fdr]

That is a strong cell around the drivers.

But I learned two methods to stop the spinning when I felt it coming. Just wiggle the cyclic for a second and it disappears. The second is a quick down and up movement of the collective. Don't like that one, but it works, too.

Interesting observation on the cyclic wiggle...It is conceivable that the cyclic motion is affecting the local flow in the downwash that affects the tail rotor, as otherwise, the torque demand would rise for steady rotor height. Am curious if there is a known reason why cyclic motion would improve TR effectiveness. It may be that the cyclic gives also a slight overspeed condition through the hysteresis of the governor, which would give increased TR authority.

[Rotorbee]

Interesting observation on the cyclic wiggle...It is conceivable that the cyclic motion is affecting the local flow in the downwash that affects the tail rotor, as otherwise, the torque demand would rise for steady rotor height. Am curious if there is a known reason why cyclic motion would improve TR effectiveness. It may be that the cyclic gives also a slight overspeed condition through the hysteresis of the governor, which would give increased TR authority.
I think it was just the change in downwash. With that little wiggle and it disturbs whatever is building up around the tail rotor.
For the overspeed condition, I don't think so. The governor of the R22 wasn't very fast. When you are flying with a steady power setting like in that slow flight, I never felt a lot of movement in the twist grip.