carlmeek wrote:Someone invented the myth of “Fenestron stall” and then disproved it.... but there have been some incidents.
I know (well, knew - what follows is memory, and likely in error in detail) a bit about this, as many years ago I was involved in a technical analysis and review of the issue.
There was a slew (close to double figures, I think) of accidents in the UK (and maybe elsewhere: Aerospatiale were cagey about that) late 80s/early 90s where the aircraft were hovering in light winds when, it was thought by the crews reporting what had happened, that the fenestron had suffered an aerodynamic stall. A very rapid rotation occurred, and the handling pilot reaction in many of the accidents had been to dump the collective - leading to contact with the ground whilst still yawing, which typically resulted in a rollover. Fortunately there were (I believe) no serious injuries.
UK MOD, thinking it was actually an aerodynamic stall, then published a Warning in the Aircrew Manual (ACM) to the effect that if you did experience the apparently uncommanded rapid yaw, don't quickly apply anti-yaw pedal because (it was felt) this would drive the fenestron aerofoil further into stall.
The accidents typically occurred in low wind conditions; there was an associated (UK MOD) theory that the main rotor downwash/tip vortices were entrained into the fenestron duct, and it was that which initiated the uncommanded yaw, since - so the theory went - the anti-torque thrust was lost due to the aerodynamic interaction.
Despite ACM advice to avoid certain wind conditions, thought conducive to initiating the "stall", and the Warning mentioned above, the accidents unfortunately kept occurring.
So MOD eventually contracted Aerospatiale to provide advice, and in due course funded a trial with an RN Gazelle, instrumented by them including flow tufts on the fenestron fin and rear fuselage and a camera on a horizontal stab endplate looking at the fenestron. The trials were flown by an Aerospatiale test crew and an A&AEE Boscombe Down 'D' Squadron test pilot (Fleet Air Air Arm, IIRC) went down to Marignane to observe the trials and fly in the aircraft.
The trials were conclusive, in that there was no aerodynamic stall of the fenestron. The trial condition replicated the majority of the UK accidents in that a spot turn to the left was initiated and the resulting yaw left to accelerate - which with a large initiating pedal input it rapidly could, especially with some relative wind strengths and directions (low-ish speed from the rear quarter, IIRC). The yaw became highly disorientating and it was evident that pilots could believe that control had been lost.
The trial showed that the yaw acceleration, and eventually the yaw itself, would be controlled by application of full right pedal deflection; although a significant and sustained over-torque typically resulted. Stopping the yaw took some time; it could rotate greater than 360 degrees following application of recovery pedal IIRC. The videos were very impressive and I think were released to the operational and training Gazelle units on VHS casettes. The edited versions showed the external video of the tufted fenestron with an inserted simultaneous view of the torquemeter. Obviously, there was also a supporting technical report (and A&AEE commentary), and before long the advice in the ACM was amended to reflect what was actually happening, and to provide handling advice and (I think) an amended low speed flight envelope to avoid getting into the problematic situation in the first place. The ACM deleted all reference to "fenestron stall"; I think the term "yaw divergence" was used instead, possibly later still changed to "undemanded yaw".
The above preceded later common UK use of the term "Loss of Tail Rotor Effectiveness" (LTE).
I think there was a little later a similar civil Gazelle accident in the UK. The resulting AAIB report referenced the A&AEE and Aerospatiale reports on the Marignane trials, and described the UK MOD history a little; this I suppose would have been around the early/mid 90s.
Following this change in ACM content and in operator training, the accident rate from this cause decreased (almost?) to zero.
Final anecdote: I remember the first meeting at Aerospatiale Marignane to discuss the issue. There was a large conference room with an enormous oval table. At the side of the conference room there was a long glass windowed enclosure in which the simultaneous technical translators sat, and we all wore translation headphones working via IR transmitters in the roof above the table centre.
The meeting was long and complex, going into some detail about the hazards that (at that stage) the UK felt could exist in the Gazelle design, with around 30-40 people in the room. It was conducted in French and English, and as it came to an end everyone was inevitably tired but still focussed on the serious subject.
The French chairman drew the meeting to a close and said, in French, something like: "Thank you to our English friends for making the trip over to La France; I wish you all a safe flight home."
The French could not understand why all the English speakers in the room suddenly fell about in hysterical laughter.
That was because the female translator had rendered the sentence as: "Thank you to our English friends for making the trip over to La France; I wish you all a safe flight home. But not in a Gazelle helicopter, huh?
It's a fabulous type, and what a view. I flew quite a bit on trials after the events above, and loved every minute of it - but especially the ones where I managed to get a bit of stick time and ad hoc
instruction. I particularly remember one trip with an AAC tp over Salisbury Plain, at high speed following the contours at below tank turret height. Just magic.
(Apologies for hijacking your thread, @carlmeek - it's just that it sent me down an enjoyable part of Memory Lane!)