Fenestron stall - Fact or Myth?
 

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

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...........

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!

Definatly Myth

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

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Thermal runaway.

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?

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?

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).]

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).]

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

212man
Sounds interesting - please tell us more.

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?

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.

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.

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!'

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?

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

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.

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?

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?