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Helicopter down outside Leicester City Football Club

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Helicopter down outside Leicester City Football Club

Old 6th Nov 2018, 13:33
  #621 (permalink)  
 
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Is the Tail Rotor designed to go to neutral pitch or some position such as the BK is which uses counter weights?
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Old 6th Nov 2018, 13:49
  #622 (permalink)  
 
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Originally Posted by GrayHorizonsHeli View Post
My thoughts, and reading the wording of the inspection, makes me think that perhaps the safety wasn't there at all or failed, and the nut backed off completely. That of course would remove your pitch control completely

you would still have your T/R drive all the way thru this event, and that would explain the strike damage in the picture we debated earlier.
The pin plus wire locking would provide a fail safe. Unless both were missing.

As a critical component, this particular job would involve a duplicate inspection so although not impossible for both to be missing, serious questions would have to be asked of the 2 certifying engineers if this were the case.
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Old 6th Nov 2018, 13:51
  #623 (permalink)  
 
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I would think most TR's would find their neutral spot. No idea what this TR assembly would do.
I look at item 90 as a part that might affect that point should it get cockeyed or jammed somehow once everything got loose. Pin 80 must get pinch fitted somehow under torque of the nut to prevent it from coming out under normal circumstances

No idea how it is put together though, so I'm only guessing
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Old 6th Nov 2018, 13:59
  #624 (permalink)  
 
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Originally Posted by SASless View Post
Is the Tail Rotor designed to go to neutral pitch or some position such as the BK is which uses counter weights?
If the yaw was full left/right pedal and then that nut fell off and pilot lost all control to bring it back round, would it spin and plummet the way it did? I guess it did on the West Franklin with the 92 where yaw control was completely lost under similar circumstances (although clearly that was defect related). If the pedals were neutral and the nut fell off, would it have went into a spin with the Tail rotor still being driven and the TR in neutral? Was a yaw input the last controlled input that forced the nut off and then it went into a spin?
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Old 6th Nov 2018, 14:06
  #625 (permalink)  
 
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Mitch....we are talking about a 169....not a 92.

My question was very much Type specific.

How the Tail Rotor would react if this component came adrift would determine how the aircraft reacted to the failure.

Would the Servo(s) go hard over in one direction or another....or would there just be a loss of Pilot input and the Tail Rotor itself be free to find some position it liked?

Perhaps some Engineers that work on the 169 can advise us on that?
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Old 6th Nov 2018, 14:19
  #626 (permalink)  
 
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Yes, it's a good question SASless.

If it is indeed this failure mode, speculating of course, It crashed within 45secs of take off so that nut is likely to have worked its way off on the leg into the stadium. If that is the case then it's likely it was on it's very last threads (or even missing completely) as it was taking off, a commanded yaw input would have then extended or shortened the shaft forcing the nut (or the lever feedback link item 110) off its last thread/shaft and all control to the tail rotor was then lost. The pitch shaft would have been in the position of the last yaw action. As we know it lost control, it's unlikely it found a central position as a fail-safe mechanism and more probable that it was stuck in its last commanded position.

All theory of course, like everyone else, just thinking out loud.

Last edited by Mitchaa; 6th Nov 2018 at 14:30.
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Old 6th Nov 2018, 14:19
  #627 (permalink)  
 
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Originally Posted by SASless View Post
Mitch....we are talking about a 169....not a 92.

My question was very much Type specific.

How the Tail Rotor would react if this component came adrift would determine how the aircraft reacted to the failure.

Would the Servo(s) go hard over in one direction or another....or would there just be a loss of Pilot input and the Tail Rotor itself be free to find some position it liked?

Perhaps some Engineers that work on the 169 can advise us on that?
The nut in question is on the end of the pitch change shaft, after the pilot input and servo assy, so neither of these would be able to transmit any movement to the TR pitch.

Last edited by FlightSpanner; 6th Nov 2018 at 14:20. Reason: Spelling!
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Old 6th Nov 2018, 14:23
  #628 (permalink)  
 
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Originally Posted by FlightSpanner View Post
The nut in question is on the end of the pitch change shaft, after the pilot input and servo assy, so neither of these would be able to transmit any movement to the TR pitch.
Yes, item 110 would have flapped loose of the TR Pitch shaft which means the TR pitch shaft would have been stuck in its last commanded position with no way to counter it?

A link to the video of the crash again...


From take off, It doesn't look like there was any substantial yaw movements until about a second before it lost control. The initial turn was most likely commanded which then forced the feedback link (item 110) off the pitch change shaft (Item 150) and the aircraft continued with the shaft now stuck in that last commanded position. You can see in the video the initial yaw movement before it then speeds up and carries on spinning.


Last edited by Mitchaa; 6th Nov 2018 at 15:48.
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Old 6th Nov 2018, 14:33
  #629 (permalink)  
 
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Thank You!

That is very helpful insight into the mechanism of how this thing might have failed.

What drove my question is the BK-117, for example, in the event of a loss of pilot input (linkage failure for example) the tail rotor has counter balance weights that are supposed to return the Tail Rotor to somewhat a neutral position rather than it being able to hunt for a position it likes which could cause severe handling issues.

I was curious if the 169 was similarly designed in some way.
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Old 6th Nov 2018, 14:35
  #630 (permalink)  
 
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Originally Posted by Mitchaa View Post
Yes, item 110 would have flapped loose of the TR Pitch shaft which means the TR pitch shaft would have been stuck in its last commanded position with no way to counter it?
Could stick, go to a designed position or zero pitch, sorry don't know the answer
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Old 6th Nov 2018, 14:40
  #631 (permalink)  
 
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Originally Posted by FlightSpanner View Post
Could stick, go to a designed position or zero pitch, sorry don't know the answer
True, neither do I. However, we have the video footage to go off so it's very likely that it was stuck in its last commanded position rather than returning to a central datum.
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Old 6th Nov 2018, 14:59
  #632 (permalink)  
 
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General comment on servos:-
If the feedback fails then the servo will drive to the end stop.
Feedback is negative, it tells the servo when to stop.
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Old 6th Nov 2018, 15:00
  #633 (permalink)  
 
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Originally Posted by Mitchaa View Post
True, neither do I. However, we have the video footage to go off so it's very likely that it was stuck in its last commanded position rather than returning to a central datum.
The result was like a TR failure, so no more thrust. That indicates, that the TR went in a kind of neutral position, not stucked in high power position.

skadi
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Old 6th Nov 2018, 15:06
  #634 (permalink)  
 
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Originally Posted by skadi View Post
The result was like a TR failure, so no more thrust. That indicates, that the TR went in a kind of neutral position, not stucked in high power position.

skadi
Have to agree, the acceleration in rotation to me indicates this possibility.
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Old 6th Nov 2018, 15:13
  #635 (permalink)  
 
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If the TR went into a neutral position, why did the rate of rotation speed up rather than slow down? There is no sign of a correction in the video, only a deterioration. The speed of rotation gains momentum which suggests (to me anyway) the yaw was fully extended with no way of it resetting/correcting. As it gained momentum it made the spin even more violent.

Are you suggesting drive to the tail rotor failed so the TR was no longer being driven or along the same lines of it being a control issue?
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Old 6th Nov 2018, 15:25
  #636 (permalink)  
 
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Originally Posted by Mitchaa View Post
If the TR went into a neutral position, why did the rate of rotation speed up rather than slow down? There is no sign of a correction in the video, only a deterioration. The speed of rotation gains momentum which suggests (to me anyway) the yaw was fully extended with no way of it resetting/correcting. As it gained momentum it made the spin even more violent.

Are you suggesting drive to the tail rotor failed so the TR was no longer being driven or along the same lines of it being a control issue?
The TR was in a fairly high load, drive remained but the pitch decreased due to its neutral position. Hence the spin
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Old 6th Nov 2018, 15:28
  #637 (permalink)  
 
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I think there are two different "neutral" concepts in play here now. Neutral can mean no TR blade pitch, which should be roughly the same as a TR drive failure, but neutral can also mean "provide enough torque to counter the torque from the main rotor".

I would think that if left to "adjust itself", the first kind of neutral would be most likely, while the second kind of neutral would be what one would want as a safety fallback for control failure.
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Old 6th Nov 2018, 15:34
  #638 (permalink)  

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If a tail rotor control servo runs away to full negative pitch (some most definitely can do this and Iíve written about this before in previous threads) it will give a situation worse than a failed tail rotor driveshaft. The aircraft will continue to yaw away from the main rotor direction despite all main rotor torque being removed by fully lowering the collective and shutting down the engines. The pilot would be unable to correct this. A horrible situation.
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Old 6th Nov 2018, 15:43
  #639 (permalink)  
 
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Originally Posted by FlightSpanner View Post
The TR was in a fairly high load, drive remained but the pitch decreased due to its neutral position. Hence the spin
Yes, okay I see your point now. Nadar's response above picks up on what I was thinking. (TR still applying a counter action at neutral)
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Old 6th Nov 2018, 16:00
  #640 (permalink)  
 
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Mitchaa wrote "The speed of rotation gains momentum which suggests (to me anyway) the yaw was fully extended with no way of it resetting/correcting. As it gained momentum it made the spin even more violent."

My speculation from experience of other designs; if the T/R control became disconnected, the blades would revert to a pre determined position by design. This position is a balance between aerodynamic and centrifugal turning moments which are usually designed to apply some positive pitch to the blades to allow a running landing. In a vertical climb OGE, this pitch setting would not be sufficient to prevent yaw developing (quite rapidly).

Now to address Mitchaa's observation of the rate of turn accelerating; as the yaw rate develops, the effective NR reduces, and more collective is required to maintain height, the torque reaction increases and yaw rate increases further, requiring increased collective pitch due to an even greater reduction in effective NR. It's a viscous circle. This would apply in the first few seconds before the descent was initiated. After descent is initiated, this theory falls down as the yaw rate would reduce noticeably and the video didn't really show this.

If the yaw servo did indeed travel to full deflection due to lack of feedback loop, then the rate of deflection, and consequent yaw, would be a function of how much the servo control valve was open at the time of failure. This could explain the relatively progressive increase in yaw rate initially.

If it was indeed a servo 'runaway', then deselection of the appropriate hydraulic channel should allow the blades to revert to the position mentioned above and permit some reduction in yaw rate and recovery of control.

I now refer you to a post I made https://www.pprune.org/rotorheads/528810-category-takeoff-background-post10298782.html
There I tried to explain that tail rotors don't tend tofail 'quietly'. However, servo runaways are 'quiet' and should not be mis-diagnosed as a t/r drive failure, despite similar yaw rates.

JJ

Last edited by jellycopter; 6th Nov 2018 at 16:13.
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