Fenestron stall - Fact or Myth?

[Hoverman]

I've heard conflicting views.
What do other Rotorheads think?

[PurplePitot]

Myth - There is absolutely no such thing as Fenestron stall as proved by Aerospatiale at the time. The only people who refuse to accept reality are the RAF who simply stuck to this ridiculous line in order to get funding for a new basic trainer (got more than they bargained for with DHFS!)

IMHO of course...........

[MightyGem]

It was a favourite topic of the military during the 80s/early 90s. Pilots were losing yaw control in the hover or at very low forward speed. Extensive test flights put it down to coarse pedal inputs and it was possible to recover by simply putting in opposite pedal and waiting for the rotation to stop, sometimes 180-270 degrees.

It was certainly pilot error when it happened to me!

[whatsarunway]

Definatly Myth

[Thomas coupling]

Concur.
http://www.gograph.com/Images-7298/AnimatedGif/redstar.gif

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Thermal runaway.

[eden]

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?

[fishboy]

Exactly the same symptoms as one type of "Loss of Tail Rotor Effectiveness" (LTE). If you let too much yaw rate build - with the tourqe - you will not be able to stop it, even with full application of opposite pedal. The only recovery is to get some airspeed. Prevention is better than cure! turn against the tourque.
I would imagine that's the same with Fenestron systems.
Does the Fenestron suffer from a problem associated with main rotor vortex interference, the same as conventional tailrotors?

[The Nr Fairy]

I was always taught ( in an R22 ) to turn left - against the torque - wherever possible, because if you turned right and raised the lever or had a failure which required more right pedal, you were already using right pedal so the chances were the problem became serious much more rapidly. Besides, if you took off the left pedal input, the aircraft naturally wants to stop rotating rather than requiring a bootful of left to stop.

Because Gazelles swing the other way, is this not the same sort of phenomenon ?

[This message has been edited by The Nr Fairy (edited 19 February 2001).]

[Flying Lawyer]

Thanks for the detailed explanation Eden.

Taking up Nr Fairey's point, and with apologies if I'm being slow here:
Is there any real difference between what used to be called Fenestron Stall, and the risks associated with turning with the torque in a conventional TR helicopter?


[This message has been edited by Flying Lawyer (edited 20 February 2001).]

[212man]

I flew the EC 155 at Marignane last month with a Eurocopter TP, and asked if there was a yaw rate limitation. He said there was not (unlike most conventional types) and we then went on to look at some interesting spot turns!

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Another day in paradise

[Hoverman]

212man
Sounds interesting - please tell us more.

[eden]

I've only had 1 flight in an R22 - so I am speaking from a position of weakness. That said - The extreme dynamically divergent characteristic that is present in the Gazelle I have never experienced elsewhere.

It has caught out many gazelle pilots of variable experience levels. I have only heard of the early Lynx helos with a certain direction(can't remember which way) of TR blade rotation losing TR effectiveness during DL's / when operating at the extremes of wind limits to ships. The direction of rotation was changed and I think it solved the problem.

It is always good practice to spot turn using the power pedal on the first spot turn of the day. It's like a datum setter - it sets the scene for the conditions of the day. Any turn away from this in whatever aircraft should ALWAYS be treated with care and smooth control inputs. The fact of the matter is that the aircraft are there to be flown and you've gotta turn whichever way you like - just take it gentle in the manouevres you now know to be ready for.

I'm going to have to try and trace this video shot at aerospatiale - The rate of rotation was eye watering and the Torquemeter (which had it's own close up camera) was pinned to the max end stop for a for what seemed like an eternity.

I think both NR fairy and Flying Lawyer are probably much better qualified to comment on the R22 a/c characteristics so how do you feel the TR responds in the spot turns left and right or in low speed flight to the left or right?

[MightyGem]

Eden, from your post you appear to be RN. If we know each other then you must have been on a QHI course at Shawbury in '91.

Oh yes, the good old Mk 1 (Army) Lynx. Spot turn (left I think it was) in anything more than a 10kt wind and you'd run out of left pedal. The only way was to dump the lever slightly to get it going again. Those were the days.

[The Nr Fairy]

I wouldn't claim to be an expert on the R22, with some way to go before I can even go on the JAR CPL(H) UK(Restricted) modular course. However, I have listened to everything that anyone cares to tell me about the R22 in an effort to keep myself alive and fly the beast better. I've also no time in other helicopters, except an hour or so in a H300 and a small amount of time in a Chinook sim, so can only relate my experiences of the R22.

My understanding of the tail rotor power available in an R22 ( and I'm told an R44 ) is, that despite the fact it rotates so the forward blade is the downgoing one, the blades are designed to be extremely efficient. Frank Robinson apparently was involved heavily in the redesign of the OH-6 tail rotor after incidents and accidents related to LTE in the mid - late 60s.

I have been shown, and flown myself in the same sortie, spot turns at 2' skid height with the governor off and Nr down to 80% ( normal is 104% ) and there was no problem in left turns. I don't recall if we did right hand turns. Apparently B206 pilots turn an odd shade of green when shown this as apparently the JetRanger suffers from a lack of T/R power in some conditions.

[Robbo Jock]

Yes, the Jetranger and Robbo TR power do differ - I've flown most of my hours in the R22 with a few latterly in the 206 and I am always being ranted at for not using enough boot. Pull in power on a Robbo, a little bit of left pedal and you're happy, pull in power on a Jetranger, a little bit of left pedal and you've got the Instructor sitting in your lap shouting 'look at the bl**dy ball!'

[Neil R D Kay]

Eden's submission is spot on. Early concentration on Fenestron Stall and then the realisation that Yaw Divergence was the issue.

QHI in 91 - not the naughty boys who wrecked the Corbett Room, I hope?

[MightyGem]

Niel, not that I can recall, but it was a while ago!

[L J R]

Arrived on this thread [from Military Pilots] after answering Hovermans invitation. Sounds too techo for a Mach 2+ Fixed wing chap.

Back to the Military..

Cheerio.

[Arkroyal]

eden's said it all really.

Do your first spot turn of the day using power pedal and treat any othe direction with extreme caution and you will be OK.

Talking of the Lynx (which I didn't fly), can anyone confirm that when Westlands trialled the reverse direction T/R (Clockwise from the left instead of the earlier anti-clock), they forgot to alter the spiral groove oil feed to the outer bearing and seized the thing?

[R O Tiree]

I remember Tim C telling us about this in his series of helo PofF lessons, aka "101 ways to kill yourself in a helicopter". He showed us some interesting pics of the tip vortices off a Gazelle gathering together and inpinging quite neatly right where the Fenestron was, once you had yawed about 30 - 40 degrees. The rotation direction of this vortex was the same as that of the Fenestron, thus giving rise to the theory that the Fenestron might lose effectiveness. There was also the eye-watering footage of the French tests, but I seem to remember him saying that the French might have changed the rotation direction of the Fenestron on their Gazelles. Any truth in this?

[Lu Zuckerman]

To: Arkroyal

You asked,” Talking of the Lynx (which I didn't fly), can anyone confirm that when Westlands trialled the reverse direction T/R (Clockwise from the left instead of the earlier anti-clock), they forgot to alter the spiral groove oil feed to the outer bearing and seized the thing”?

This was the subject on another thread. My response at that time was that if Sikorsky could do it then I was sure that Westland could be guilty of the same error. During the design of the H 37, Sikorsky found that the tail rotor drive shaft was contacting a part of the primary structure. In order to correct this condition they had to lower the drive shaft. This involved putting an additional gear set on the main transmission causing the tail rotor to turn in the wrong direction. They modified the intermediate gearbox and the tail rotor gearbox by reversing the pitch on the gears. In the process, they forgot about the Archamedes (sp) pump on the outboard end of the tail rotor drive spindle and instead of delivering oil to the outboard bearing is actually took oil away from the bearing. In light of that, I would say that the Lynx story is true.



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The Cat

[Arkroyal]

Thanks Cat.

[PFL]

Having been in a ******* in which the HP lost tail rotor control I think I have an idea on why this happened. The statements put forward by eden were borne out in this instance.

The HP was a young pilot (tw@t) who was not as good as he believed and who had a history of over-controlling. 2am over a well lit city to the W of GB ;-) We were hovering downwind (in limits, honest). Young chap decided to lose a bit of his limited concentration and sent us backwards. Wind crossed from one side of the tail to the other, then under the horizontal stab. The result was a spectacualr >360 which included a healthy dose of 80 deg nose down.

The next day he was heard spouting on about Fenestron Stall until it was pointed out that his poor handling had intitiated the whole sorry affair.

Regards PFL

[HeliAviator]

In reply to Arkroyal, yes you are correct old chap, when the ex-garage door company first changed the Lynx tail rotor over the spiral oil groove forgotten about, causing an angle gearbox to fail, that was when the error was discovered.

As to Fenistron Stall, speaking from experience as a past and current Gazelle pilot, there is no such thing as Fenistron stall. Large and rapid imputs of right boot and high power settings can (if you try hard enough) cause a lack of directional control, its called gross miss handling! I managed it only once, that was during an "AAC Blue Ego's" pactice and yes, adrenaline is brown in colour.
:rolleyes:

[Thud_and_Blunder]

PurplePitot,

Speaking as a Crab who was on loan to the AAC around the time all this started, and speaking also as someone who was able to get the information as supplied by Eden from the other Crabs at CFS(H) - what's your point? Can't say I remember any fiendish Brylcreem-smeared plot to rid the forces of the chicken leg before its due date. Do enlighten us.

Oh yes, and I do remember having to take a Lynx Mk1 bombed-up with 8 (empty) tubes down to the <1 foot hover to get a nose-left turn going. Hat well and truly doffed to any HAS shipmates trying to get that lump onto a heaving deck in anything more than Mr Beaufort's size 5.

[Thomas coupling]

Crudge and A Greenhall just 2 that I can think off who departed the fore and aft rather rapidly in the horizontal, with the Gaz piece. The former I believe, volunteered for it with the froggie TP's...
The TR was not altered in any way other than 'optimising' it. It seems pilots should have been aware of it if they were fully au fait with their flight envelope...as the rest of us are....no?


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Thermal runaway.

[eden]

TC: Andy G's was during a wingover from several hundred feet up - he showed me the pictures (of the wreckage he walked clear of) and spun the dit. He was very lucky and he knows it. Don't know about Crudge being the guy who did the Aerospat' tests - but thinking back - I recall that it may have been Paul Shawcross who went to AeroSpat' to do the exceptionally scary tests.

I am trying to trace the video .....???

[blind pue]

I would like to agree with PFL,
I also have been involved in 2 cases of the famed Fenestron stall, once in exactly the same manner as described by PFL,
the other on a low approach downwind my pilot decided to pedal turn around instead of flying round, I was distracted with the maps and the first I knew we were in the C***, was as I heard 'you have control' during the 2nd 360, rapid application of the opposite pedal and a nose down attitude to regain airspeed, allowed us to walk away with only a dent in our pride as everyone on the the ground had seen our air display.
On both occaisions my pilots were straight out of MW and graded above average who thought they could do anything.
These were good lessons for both, about the effects of mishandling, and I am glad to say the experience years ago has possibly helped them earn that above average grading they now hold .

[Flying Lawyer]

It's good to see interesting posts from current military pilots adding to the excellent contributions we already get from the ex-mil pilots.
Thanks to you all.

http://www.gkn-whl.co.uk/images/index_ani.gif

[Geoffersincornwall]

Having survived a 'moment' with a 365N during which momentary loss of yaw control was experienced and the situation rectified by full application of opposite (right) pedal I reported the problem to the engineers just in case it was some sort of yaw AP problem (we had been having some problems with the yaw stab). The engineer came to find me the following day carrying a photocopy of a EC Service Letter dated about 10 years previously (so about 1990) drawing the pilots attention to a situation in which there could be a loss of yaw control when taking off with a wind from the starboard quarter. They implored the pilot NOT to take the aircraft flying in an attempt to regain control and instead suggested that he apply full opposite pedal and land as soon as possible.

I had landed and parked into wind but when we came to leave the wind was indeed from the starboard quarter. Never new about that problem until I read the Service Letter. Anyone else heard of it?

G.

[cpt]

I must admitt I don't understand such a big fuss about Fenestrons tail rotors....Having flown several thousands hours on helicopters types ranging from the SA341 to the AS365N3 on military, air transport or aerial work I only have experienced once what can be described as a "mild" LTE: it was on lift-off with an SA365C from an off-shore rig with a 15 kts front right wind....I just landed back and I never experienced this again later despite the great variety of wind/speed/power combinations.

It is true that we need a greater amount of anti-torque pedal to get the same affect as with a conventional tail rotor, but it should never be a great hasle if correctly informed or trained.

[hihover]

I have to admit, I had over two thousand hours in the Gazelle before I realised there was such a term as Fenestron Stall, I had never heard of it, and didn't know I didn't know.

I was sitting in the back of a Gazelle during a short flight on my QHI course when the HP very clumsily tried a lookout turn to the left. The strong wind caught the big tailfin and whipped us even faster to the left, his reaction was full-right pedal, and off we jollywell went. After about 3 revolutions, I thought the shaft had failed, I could see he had full right pedal applied and nothing was happening. He instinctively raised the collective to get away from the ground but this made things worse so he decided to land very heavily whilst in the spot turn.

On further investigation of the twisted Gazelle, there was nothing wrong with the anti-torque system. If you apply too much pitch to an aerofoil, eventually it will stall, he had applied the maximum possible pitch, very quickly, and I would say the fenestron had stalled.

The French TP's answer was to apply full left pedal to unstall then re-apply right pedal gently. I have no doubt that would work, but try doing it when you are spinning at 360 deg in 3 seconds and out of control with your ar$e biting the buttons off the seat cushion.

Call it what you like, I would say the fenestron had stalled.

Tam

[cpt]

Yes Hihover, but this case was initialy caused by a sharp yawing motion (caused by a wind gust on the fin) requiring a quick and wide pedal action to counteract. Don't you think a conventional tail rotor could have develloped an LTE in this situation as well ? Is it really typicall of a fenestron design ?

[hihover]

Well, I would not like to comment on how a conventional tail rotor would have coped in the same situation but with the fenestron, the airflow is all going through the duct, and in this case, the blades in that duct had stalled. There is no way to regain control of the fenestron unless you unstall the blades and re-apply right pedal.

By definition, fenestron stall is LTE, but I do not believe it is the same thing when compared to a conventional tail rotor.

Gladly there were not too many cases like this one.

Tam

[cpt]

I may have been lucky then :hmm: