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it has never been proven to exist on any other helo type

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,....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​​​​

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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

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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.

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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.

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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?

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