Police helicopter crashes onto Glasgow pub
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Thomas Coupling # 1068
TC, thanks for encouraging us newbies - although I have observed for a long time but posted very infrequently. I'm not sure I can agree with your "we should be all singing from the same hymn sheet." Alternate views and opinions are healthy. We know that eye-witnesses can be notoriously inaccurate due to the way the brain interprets what we see and hear - we fill in the gaps and interpret what we perceive based on our own experience and mental models. The witness outside seeing the event may well have been interpreting what he saw and heard in the dark. However, the TV footage of those witnesses inside was quite compelling and lucid. They all related an initial "event" and a minor roof issue (comment by the band "we brought the house/roof down") followed after a pause by the collapse and dust cloud. If the aircraft hit very very hard as you say [30g+] would not the impact and roof collapse have been instantaneous, the aircraft destroyed to a greater extent and the roof be damaged more substantially than it was? The TV footage did not seem to support that kind of catastrophic impact and the aircraft appeared erect but nose down in the building. Hence, my implied opinion that it might have been an EOL with some form of control until the cushion but ending up very heavy after which the roof gave way. Like you, I hope the AAIB get some definitive evidence to help solve this tragic event - at the moment we are all really just speculating - which after such a disaster is good as we all want to share our thoughts, ideas and feelings.
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Arkroyal # 1073
Arkroyal. No offence taken sir. Just trying to ask questions based on previous experience and add to the discussions. I have just responded to TC as I cannot reconcile that NR was lost totally in this tragic event. Given that AAIB have yet to find a technical cause, it just seems against all logic that the aircraft could have fallen from a great height with NR drooped to such a level. Lets hope AAIB get an answer.
Klingonbc
Klingonbc
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RVDT, thats exactly what I did as I thought that surely it can't be a good thing to have a rad alt unavailable in such circumstances. On ours the Rad Alt is still on, does yours drop off, as your earlier pits suggests?
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Police helicopter crashes onto Glasgow pub
Klingonbc
That's a good point re the roof. However, the structure effectively comprised 3 parts. The top roof and then an intermediate structure before the ceiling. The top deck was severely impacted the middle section appears to have collapsed and the combined weight if the machine etc has settled/ collapsed into the pub bringing everything with it.
The initial impact created a clear "cut out" and if you look at the photographs the felt covering appears to have been blown back around the edges as air was fired out of the resultant hole.
I think the roof structure did a good job at absorbing the energy, despite the tragic loss of all crew and 6 patrons of the bar. Had the impact been at street level, one could imagine a worse scenario, possibly with fire and flying debris. Looking at the cab the impact didn't look survivable.
That's a good point re the roof. However, the structure effectively comprised 3 parts. The top roof and then an intermediate structure before the ceiling. The top deck was severely impacted the middle section appears to have collapsed and the combined weight if the machine etc has settled/ collapsed into the pub bringing everything with it.
The initial impact created a clear "cut out" and if you look at the photographs the felt covering appears to have been blown back around the edges as air was fired out of the resultant hole.
I think the roof structure did a good job at absorbing the energy, despite the tragic loss of all crew and 6 patrons of the bar. Had the impact been at street level, one could imagine a worse scenario, possibly with fire and flying debris. Looking at the cab the impact didn't look survivable.
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LoneWolf50,
The fenestron has a geared, direct drive from the main gearbox. If the main rotor rpm is gone, then so proportionally is the possibility of thrust from the fenestron. The blade pitch of the fenestron (or any other tail rotor type) is controlled via the yaw pedals.
Feel free to shoot holes in that idea.
There has been a lot (an awful lot) of theorising about fuel starvation and how bizarrely both engines could stop at the same time due to lack of it.
My guess reading the AAIB special is that fuel is not the issue. Just like their reference to mechanical issues or the lack of them, the mention of 95lts of fuel being removed is meant to indicate that lack of fuel was not the problem.
Now it may be that lack of fuel will prove to be the problem but not because it was not in the tanks but because it could no longer get to the engines.
This is a fact the AAIB could have covered in its bulletin but did not because clearly further investigation is required. By this I mean the engine fuel switch positions.
My guess reading the AAIB special is that fuel is not the issue. Just like their reference to mechanical issues or the lack of them, the mention of 95lts of fuel being removed is meant to indicate that lack of fuel was not the problem.
Now it may be that lack of fuel will prove to be the problem but not because it was not in the tanks but because it could no longer get to the engines.
This is a fact the AAIB could have covered in its bulletin but did not because clearly further investigation is required. By this I mean the engine fuel switch positions.
If the main rotor rpm is gone, then so proportionally is the possibility of thrust from the fenestron.
I am not sure you understood what I posted.
Please go back and consider what I asked about, with the initial point being possibly stalled main rotor blades (decelerating, yes, but still turning for x seconds as they head toward about zero, which is what we are led to believe at this point. )
For that period, however brief and certainly transient, MR stalled but finestron/anti torque not yet stalled. One produces no thrust, the other "some value" of thrust about the vertical axis, as designed.
The blade pitch of the fenestron (or any other tail rotor type) is controlled via the yaw pedals.
The finestron (or a tail rotor on othe models) provides thrust that, due to the lever arm of the tail boom, works to rotate the helicopter about the vertical axis. When we yaw in a hover, our rotation rate about that axis is based on the differential of the torque from the MR trying to rotate us one way, and the finestron acting in rotation in the other direction. (Idealized vectors for simplicity of explanation.) We both know this, it's the basics of what makes helicopters such fun to fly.
MR blades provide us torque (that which rotates the body of the aircraft) as a by product of lift production. Equal and opposite reactions, Newton, etc.
If the main rotor blades are stalled ... NO lift and so I infer no torque reaction to the body (edit: though certainly some precession due to mass and inertia) ... but the finestron is still turning (be it with a lot of pitch or a little pitch, control position dependent) then I'd expect there to a value of thrust, however modest, whenever that set of blades is spinning until they too either
1. stall
or
2. the lack of V gets down to a low enough value that V^2 isnt big enough to matter.
So back to my point, which you did not answer, nor address.
There is a transition point between NR = ~ 100% and, as we are led to believe, NR = ~ 0 at (or even a bit before) time of impact.
It is that transition period that I am concentrating on, specifically in the case that henra mentioned about main rotor blades having a stall point -- so no lift, no torque.
This threshold does not necessarily coincide with a condition that would stall the finestron blades: a differently shaped and oriented set of rotating airfoils.
The period I am asking about may have only lasted a few seconds as NR decayed. I am puzzling over a few seconds of thrust from back there, shafts still turning albeit slowly, where the usual torque/antitorque counter is cast aside by a stalled condition in the main blades. X seconds of anti torque, nominal value, acting agianst no torque. Resultant dynamic action? Rotation about vertical axis ... maybe.
After an initial impulse lasting x seconds or x fractions of seconds, momentum carries that rotation forward. As the major rotating body (MR) slows towards zero, thrust in whatever direction the finestron can act (even to counter the modest impulse noted above) is rendered moot by a continued reduction in its rotation speed too an amount lowered, by a couple orders of magnitude or so, until it too reaches nil when the MR stops.
Now do you see what I am trying to sort out?
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This may be trying to state the bleedin obvious, but the crash site does appear to be a selected site. I am no pilot but I am interested in human factors.
The reasons, as it appears to me, are
1. The surface was flat.
2. The area was large enough to put a helicopter down on (Even though the area was very tight)
3. The helicopter crashed away from the edges of the building (as dropping off or hitting the edge would be most likely unsurvivable)
4. The helicopter did not hit anything else on its descent
5. It was not populated by anything. Not a car park where there might have been cars parked awkwardly for an emergency landing to take place. Lamp standards may well make a car park look like a very difficult place to set down in an emergency without hitting one of them. This might not be so on a flat roof.
6. The helicopter landed approximately upright, (not inverted or sideways, spinning or any other position which would obviously indicate that the aircraft was well beyond any control)
So if I am correct, and IF is purely conjecture, then we might consider that the pilot had time to make decisions. This does not appear to be a situation where the pilot flew into the ground due to inattention or anything like it. The workload on the pilot must have been huge, but for each of the factors above, there may have been a conscious decision making process by the pilot. The maximum time for the pilot to make decisions would be largely outlined by the last radio message and the impact. That gives the outline, and the time between the aircraft encountering difficulties and the impact may well be far smaller, but at least there we would have some measurable time.
The reasons, as it appears to me, are
1. The surface was flat.
2. The area was large enough to put a helicopter down on (Even though the area was very tight)
3. The helicopter crashed away from the edges of the building (as dropping off or hitting the edge would be most likely unsurvivable)
4. The helicopter did not hit anything else on its descent
5. It was not populated by anything. Not a car park where there might have been cars parked awkwardly for an emergency landing to take place. Lamp standards may well make a car park look like a very difficult place to set down in an emergency without hitting one of them. This might not be so on a flat roof.
6. The helicopter landed approximately upright, (not inverted or sideways, spinning or any other position which would obviously indicate that the aircraft was well beyond any control)
So if I am correct, and IF is purely conjecture, then we might consider that the pilot had time to make decisions. This does not appear to be a situation where the pilot flew into the ground due to inattention or anything like it. The workload on the pilot must have been huge, but for each of the factors above, there may have been a conscious decision making process by the pilot. The maximum time for the pilot to make decisions would be largely outlined by the last radio message and the impact. That gives the outline, and the time between the aircraft encountering difficulties and the impact may well be far smaller, but at least there we would have some measurable time.
I wonder if trying to shut down an engine in an emergency situation could also have resulted in accidental application of the rotor brake.
Any radio hams out there - listening in?
Will aircraft routinely be returning from missions lower on fuel than previously was the case?
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Police helicopter crashes onto Glasgow pub
Mercury dancer, it is difficult to disagree with your logic - in the absence of a better alternative within range it is easy to reach that conclusion. Best of a bad bunch? It would not necessarily have been obvious a) that it was a building- just a dark area surrounded by other buildings b) that below the dark area was a pub.
However, and we keep returning to this key point- why and when did the rotor stop? To choose a landing site is one thing, to guide 2.5tonnes of metal there with no apparent control is another.
However, and we keep returning to this key point- why and when did the rotor stop? To choose a landing site is one thing, to guide 2.5tonnes of metal there with no apparent control is another.
If the main rotor blades are stalled ... NO lift and so I infer no torque reaction to the body (edit: though certainly some precession due to mass and inertia) ... but the finestron is still turning (be it with a lot of pitch or a little pitch, control position dependent) then I'd expect there to a value of thrust, however modest, whenever that set of blades is spinning until they too either
1. stall
or
2. the lack of V gets down to a low enough value that V^2 isnt big enough to matter.
The question of torque on the airframe does not necessarily depend on Lift. Any kind of drag applied to the rotor will create torque as long as the engines are creating torque.
This drag can either come from producing Lift or from being stalled.
So the only question relevant for the torque on the airframe will be if the engines were operating at anything above idle or even being shut down.
Regarding effectivesness of the Fenestron at RPM whre the main rotor is stalled: That is a good question.
My personal feeling is that it will continue to create lift/yaw significantly beyond the critical RPM for the main rotor. The high number of blades and small spacing between them will cause downwash from one blade interfering with the next blade thereby decreasing local AoA. Additionally the blades of the Fenestron have a rather large camber also improving behaviour at high AoA.
Therefore I tend to believe that it will create meaningful yaw/torque probably down to quite low RPM's.
So if the engines were shut down and stalled it will depend on the position of the pitch change links (pedals) if the helicopter will start spinning significantly pro Fenestron or not. Friction in the main rotor drive will compound to this yaw.
Quote from Lonewolf 50:
If the main rotor blades are stalled ... NO lift and so I infer no torque reaction to the body (edit: though certainly some precession due to mass and inertia) ...
Forgive a possibly naive question from a fixed-wing pilot, but are you sure that a "stalled" (but still turning under power) blade would produce "NO lift" and (consequently) "no torque reaction"?
henra?
If the main rotor blades are stalled ... NO lift and so I infer no torque reaction to the body (edit: though certainly some precession due to mass and inertia) ...
Forgive a possibly naive question from a fixed-wing pilot, but are you sure that a "stalled" (but still turning under power) blade would produce "NO lift" and (consequently) "no torque reaction"?
henra?
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Stalled blades produce torque?
I believe that they do. In fact, more than un-stalled blades subject to the same power.
Driven, stalled blades produce more drag than driven, un-stalled blades.
It's the drag rather than the lift from the blades that imposes torque on the rotor mast, which in turn produces a reaction on the bearings, given that the angular momentum of the helicopter, and the air that flows over it is fixed.
Driven, stalled blades produce more drag than driven, un-stalled blades.
It's the drag rather than the lift from the blades that imposes torque on the rotor mast, which in turn produces a reaction on the bearings, given that the angular momentum of the helicopter, and the air that flows over it is fixed.
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Additionally the blades of the Fenestron have a rather large camber also improving behaviour at high AoA. Therefore I tend to believe that it will create meaningful yaw/torque probably down to quite low RPM's.
So how would you explain that in the event of a tail rotor control failure, where the tail rotor blades are stuck at fixed pitch, resulting in the aircraft yawing off heading on the ground during a running landing, bleeding off main rotor rpm has been shown to reduce tail rotor effectiveness and bring the aircraft back on heading? Unless there is a tail rotor control failure, the pilot can still "fly" the tail rotor with his yaw pedals.
Also, a known result of reducing rotor rpm too far in "normal flight" is loss of tail rotor effectiveness.
Henra: A partial answer to your Q:
The fenstron blades on a 135 are irregularly placed around the hub. They are not equally spaced. For the very reason you mentioned - to avoid dirty vortices off the blade in front. I believe if my mind serves me well they are also configured along two tip path planes (similar to the Apache et al).
In answer to a previous enquiry about Gazelle fenestron stall: The EC135 [it being a mix of the German BO105 and the French Aerospatialle]. The phenomenon experienced by some Gazelle drivers was machined out when it came to the 135 by champfering the edge of the fenestron ring thus widening the "acceptance angle" of the fenestron and reducing (not fully eliminating) the risk of 'fenestron stall'. Technically the 135 can still suffer from the effects of FS due largely to the huge shroud masking the incoming airflow and hindering its throughput. Very few cases (if any) have been reported though.
Finally - TRE. The Main rotor has only to lose a small amount of its energy for the TR to lose a major portion of its effectiveness. We have the use of Westland data to prove this when we simulate decaying Nr scenarios or tail rotor malfunctions. The computer displays the decaying effectiveness as it happens, so, say for EG the nr drops 5%, the TRE drops 20%. There is a disproportionate and large loss of effectiveness from the tail when the MRB's slow even a little. Hence the beauty of controlling Nr with throttles to keep the a/c straight during a TR malfunction.
The fenstron blades on a 135 are irregularly placed around the hub. They are not equally spaced. For the very reason you mentioned - to avoid dirty vortices off the blade in front. I believe if my mind serves me well they are also configured along two tip path planes (similar to the Apache et al).
In answer to a previous enquiry about Gazelle fenestron stall: The EC135 [it being a mix of the German BO105 and the French Aerospatialle]. The phenomenon experienced by some Gazelle drivers was machined out when it came to the 135 by champfering the edge of the fenestron ring thus widening the "acceptance angle" of the fenestron and reducing (not fully eliminating) the risk of 'fenestron stall'. Technically the 135 can still suffer from the effects of FS due largely to the huge shroud masking the incoming airflow and hindering its throughput. Very few cases (if any) have been reported though.
Finally - TRE. The Main rotor has only to lose a small amount of its energy for the TR to lose a major portion of its effectiveness. We have the use of Westland data to prove this when we simulate decaying Nr scenarios or tail rotor malfunctions. The computer displays the decaying effectiveness as it happens, so, say for EG the nr drops 5%, the TRE drops 20%. There is a disproportionate and large loss of effectiveness from the tail when the MRB's slow even a little. Hence the beauty of controlling Nr with throttles to keep the a/c straight during a TR malfunction.
@ ShyTorque:
The scenario you point to, and which I understand and have practiced, is at flight NR and in powered flight. MRH torque is orders of magnitude larger than tail torque, so I don't think your objection fits the scenario. Controlling the direction with slight variations in pitch and airspeed/streamline is in part made possible due to that signal difference in magnitude of torque, and due to aerodynamic forces on the fuselage aiding in direction control ... but thanks for making me think about that anyway. And I may still be missing a point, so have at me if need be!
@henra: (and awblain)
Thanks for the point on stalled blade still having a reaction/torque effect. Appreciate it. Probably changes the balance of forces a bit.
@Chris:
No, Chris, NOT under power. That's the point of inquiry I am pursuing.
The energy bleed comes from the combination of a lack of motive power to the drive train, low inertia rotor head, and a collective position that (for x seconds or parts of seconds) keeps pitch on the blades ... which of course makes for drag ... drag reduces rotor speed ... that changes AoA due to reduced airflow ... this compounding pair of factors rapidly makes for critical AoA and a stall ... (Vicious circle, pun intended, however tragic.)
How do I paint this in fixed wing?
A never ending floating flare with a glider until wing stall for an analogue, with the only real difference being four blades spinning. But it happens in three axes, not one. Would you enter a spin in a glider a few hundred feet over the ground? No. Nobody would.
To make this extra tasty, chances are one rotor blade stalls first so that you get a rolling (and pitching) moment as soon as that one loses it ... and it gets more bizarre as the rest of the egg beater gives up the fight ... in that way it might have a resemblance to the old retreating blade stall at high speed that we are all taught to avoid.
But, ya see, he's not NOT flying fast, so RBS isn't going to leap to his "what's wrong" screen in the brain, is it?
Where am I going with this?
UPSET! UPSET training.
And, you may smite me for this ... AF 447.
The parallel is bizarre, and it's a reach. A pilot maybe ending up in a condition that he (1) doesn't recognize and (2) is not trained for.
(I don't refer to the UAS, I refer to the actual stall of the A330 as the analogous issue)
In this case, there are immensely good reasons not to train to "lets stall the rotor head and see how she flies!" and I'll leave that to another time.
With the above in mind, let's sit in the captains seat in this EC 135.
For a reason (one I am not sure of) drive train input to rotor ends, rotor RPM decays, and for another reason (one I am not sure of) the response of down collective right now, and a bit of back stick to regain inertia, is either late or applied out of synch or he already had the cyclic back and was slowing down ... or ... something. Don't know.
Anyway, the bird gets to the point henra mentioned previously, wherein the main rotor blades are slow enough to stall, and as I suspect the finestron is for at least a few seconds still providing thrust/yaw, the pilot finds himself in a weird place.
Rotary Wing OCF. Out Of Control Flight. UPSET!
At this point, with the rolling and yawing moments mentioned above due to the force combinations in three axes profoundly changed from the usual proportions, he may well have the collective all the way down expecting it to get him back into the autorotation regime, however hasty, that he knows he can get sorted.
He may not realize that the RH is stalled, and on top of that he's just had serious rolling and pitching moments, at night, low altitude, that he didn't expect. He's trying to get the bird stable to make the best auto he can at the bottom.
What he won't know is that he has just become a test pilot, with precious little time to figure it all out before the bottom of the auto that he thinks he's shooting ... and when it gets to the pitch/flare and pull ... if that MRH is still stalled and NR decaying ... there isn't lift, there's no bite on the blades, neither to pitch nor to make that one last pull at the end that makes most autorotations turn from a rapid descent into a landing one walks away from.
TC's analysis some pages back got my brain whirling.
This combination of events and unexpected aerodynamic forces more or less fits what TC was getting at: it hit hard.
I'll add the estimation that the pilot was both intially disoriented by unexpected pitching and rolling moments, and was playing catch up as fast as he could all the way to the ground .... as he flew what he thought was the last bit of an autorotation ... but, as I understand henra's point, if the rotor blades are stalled, you can't fly an auto! 
In another difference form AF 447, he didn't have two, three, or four minutes to sort it all out. He had seconds.
I confess to you all that I am not on firm ground with all of this. Please, critique and point to where my reasoning is flawed/broken.
I doubt very many RW test pilots have ventured out into this region of aerodynamic performance of main rotor blades, that is, deliberately causing them to stall in flight.
For good reason, I might add.
The scenario you point to, and which I understand and have practiced, is at flight NR and in powered flight. MRH torque is orders of magnitude larger than tail torque, so I don't think your objection fits the scenario. Controlling the direction with slight variations in pitch and airspeed/streamline is in part made possible due to that signal difference in magnitude of torque, and due to aerodynamic forces on the fuselage aiding in direction control ... but thanks for making me think about that anyway. And I may still be missing a point, so have at me if need be!
@henra: (and awblain)
Thanks for the point on stalled blade still having a reaction/torque effect. Appreciate it. Probably changes the balance of forces a bit.
@Chris:
No, Chris, NOT under power. That's the point of inquiry I am pursuing.
The energy bleed comes from the combination of a lack of motive power to the drive train, low inertia rotor head, and a collective position that (for x seconds or parts of seconds) keeps pitch on the blades ... which of course makes for drag ... drag reduces rotor speed ... that changes AoA due to reduced airflow ... this compounding pair of factors rapidly makes for critical AoA and a stall ... (Vicious circle, pun intended, however tragic.)
How do I paint this in fixed wing?
A never ending floating flare with a glider until wing stall for an analogue, with the only real difference being four blades spinning. But it happens in three axes, not one. Would you enter a spin in a glider a few hundred feet over the ground? No. Nobody would.
To make this extra tasty, chances are one rotor blade stalls first so that you get a rolling (and pitching) moment as soon as that one loses it ... and it gets more bizarre as the rest of the egg beater gives up the fight ...
in that way it might have a resemblance to the old retreating blade stall at high speed that we are all taught to avoid. But, ya see, he's not NOT flying fast, so RBS isn't going to leap to his "what's wrong" screen in the brain, is it?
Where am I going with this?
UPSET! UPSET training.
And, you may smite me for this ... AF 447.
The parallel is bizarre, and it's a reach. A pilot maybe ending up in a condition that he (1) doesn't recognize and (2) is not trained for.
(I don't refer to the UAS, I refer to the actual stall of the A330 as the analogous issue)
In this case, there are immensely good reasons not to train to "lets stall the rotor head and see how she flies!" and I'll leave that to another time.
With the above in mind, let's sit in the captains seat in this EC 135.
For a reason (one I am not sure of) drive train input to rotor ends, rotor RPM decays, and for another reason (one I am not sure of) the response of down collective right now, and a bit of back stick to regain inertia, is either late or applied out of synch or he already had the cyclic back and was slowing down ... or ... something. Don't know.
Anyway, the bird gets to the point henra mentioned previously, wherein the main rotor blades are slow enough to stall, and as I suspect the finestron is for at least a few seconds still providing thrust/yaw, the pilot finds himself in a weird place.
Rotary Wing OCF. Out Of Control Flight. UPSET!
At this point, with the rolling and yawing moments mentioned above due to the force combinations in three axes profoundly changed from the usual proportions, he may well have the collective all the way down expecting it to get him back into the autorotation regime, however hasty, that he knows he can get sorted.
He may not realize that the RH is stalled, and on top of that he's just had serious rolling and pitching moments, at night, low altitude, that he didn't expect. He's trying to get the bird stable to make the best auto he can at the bottom.
What he won't know is that he has just become a test pilot, with precious little time to figure it all out before the bottom of the auto that he thinks he's shooting ... and when it gets to the pitch/flare and pull ... if that MRH is still stalled and NR decaying ... there isn't lift, there's no bite on the blades, neither to pitch nor to make that one last pull at the end that makes most autorotations turn from a rapid descent into a landing one walks away from.
TC's analysis some pages back got my brain whirling.
This combination of events and unexpected aerodynamic forces more or less fits what TC was getting at: it hit hard.
I'll add the estimation that the pilot was both intially disoriented by unexpected pitching and rolling moments, and was playing catch up as fast as he could all the way to the ground .... as he flew what he thought was the last bit of an autorotation ... but, as I understand henra's point, if the rotor blades are stalled, you can't fly an auto!
In another difference form AF 447, he didn't have two, three, or four minutes to sort it all out. He had seconds.
I confess to you all that I am not on firm ground with all of this. Please, critique and point to where my reasoning is flawed/broken.
I doubt very many RW test pilots have ventured out into this region of aerodynamic performance of main rotor blades, that is, deliberately causing them to stall in flight.
For good reason, I might add.
Last edited by Lonewolf_50; 10th Dec 2013 at 23:10.
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@ ShyTorque:
The scenario you point to, and which I understand and have practiced, is at flight NR and in powered flight. MRH torque is orders of magnitude larger than tail torque, so I don't think your objection fits the scenario. Controlling the direction with slight variations in pitch and airspeed/streamline is in part made possible due to that signal difference in magnitude of torque, and due to aerodynamic forces on the fuselage aiding in direction control ... but thanks for making me think about that anyway. And I may still be missing a point, so have at me if need be!
The scenario you point to, and which I understand and have practiced, is at flight NR and in powered flight. MRH torque is orders of magnitude larger than tail torque, so I don't think your objection fits the scenario. Controlling the direction with slight variations in pitch and airspeed/streamline is in part made possible due to that signal difference in magnitude of torque, and due to aerodynamic forces on the fuselage aiding in direction control ... but thanks for making me think about that anyway. And I may still be missing a point, so have at me if need be!
But if the main rotor rpm falls, the thrust from the tail rotor decreases quite rapidly and LTE may occur in extremis.
If the tail rotor runs away to full travel at normal rpm, either to maximum positive pitch, or maximum negative pitch, that is a different scenario. Either may not be recoverable, but that would be type specific and in which circumstances it occurred.
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Irregular fenestron blade spacing
The reason that the fenestron has irregular blade spacing is to do with the blade-wake interaction - but to make it irregular, rather than to reduce it.
To minimize the strength of the interaction, the blades should be as separated as possible.
The irregular spacing cuts down on the high-pitched whine that's very pronounced from a Gazelle - the equivalent of the typical buzz-saw noise from an old large turbofan at high power. The new fenestron produces no constructive interference in harmonics at any multiples of the fenestron fan rpm frequency, and so makes the helicopter sound quieter, by giving the fenestron a quality of tone that is richer and less penetrating and annoying.
I believe this is the same reason that the Apache has a 4-bladed tail rotor in a squashed rather than right-angled cross configuration.
To minimize the strength of the interaction, the blades should be as separated as possible.
The irregular spacing cuts down on the high-pitched whine that's very pronounced from a Gazelle - the equivalent of the typical buzz-saw noise from an old large turbofan at high power. The new fenestron produces no constructive interference in harmonics at any multiples of the fenestron fan rpm frequency, and so makes the helicopter sound quieter, by giving the fenestron a quality of tone that is richer and less penetrating and annoying.
I believe this is the same reason that the Apache has a 4-bladed tail rotor in a squashed rather than right-angled cross configuration.