Helicopter down outside Leicester City Football Club
The reference datum is a reference plane that allows accurate, and uniform, measurements to any point on the aircraft. The location of the reference datum is established by the manufacturer and is defined in the aircraft flight manual. The horizontal reference datum is an imaginary vertical plane or point, placed along the longitudinal axis of the aircraft, from which all horizontal distances are measured for weight and balance purposes. There is no fixed rule for its location, and it may be located forward of the nose of the aircraft. For helicopters, it may be located at the rotor mast, the nose of the helicopter, or even at a point in space ahead of the helicopter. While the horizontal reference datum can be anywhere the manufacturer chooses, most small training helicopters have the horizontal reference datum 100 inches forward of the main rotor shaft centerline. This is to keep all the computed values positive. The lateral reference datum is usually located at the center of the helicopter.
You will not have "CG move in front of the nose" even you lose the TR while keeping the H stab. Thats BS unless helicopter had been loaded nose heavy anyways. Most of the weight are lining up with the MGB and the moment of inertial will hard to overcome. FM says it all.
Join Date: Jan 2008
Location: Herts, UK
Posts: 748
Likes: 0
Received 0 Likes
on
0 Posts
From the picture it seems improbable that both locking means were missing/misinstalled.
There is a more subtle issue of the effects of the washer (140) either missing or on the wrong side. This would not jump out on a visual inspection like missing locking means would.
Not discernible from the drawing but appears that the shoulder the washer sits on is probably just a bit larger than the hole in the hinge bracket (90). Lack of washer would allow the system to operate normally but might cause high stress on the hinge bracket leading to a fracture. Even without a fracture it could dig a hole in the hinge bracket resulting in play that could cause other issues such as the nut being loose which in turn could cause other damage.
There is a more subtle issue of the effects of the washer (140) either missing or on the wrong side. This would not jump out on a visual inspection like missing locking means would.
Not discernible from the drawing but appears that the shoulder the washer sits on is probably just a bit larger than the hole in the hinge bracket (90). Lack of washer would allow the system to operate normally but might cause high stress on the hinge bracket leading to a fracture. Even without a fracture it could dig a hole in the hinge bracket resulting in play that could cause other issues such as the nut being loose which in turn could cause other damage.
e.g. all bolts in a locality doing much the same job being of the same length, usually arranged by say stepping a casting thickness .. EVEN IF IT COSTS WEIGHT..
Nailed it!
Join Date: Dec 2006
Location: UK and MALTA
Age: 61
Posts: 1,297
Likes: 0
Received 18 Likes
on
4 Posts
He has not really "Nailed" it. the "Unless" statement is still hogwash.
To have the CG in front of the nose of the helicopter, you would need a MRB sticking over nose, the portion beyond the nose, weighing more than the entire helicopter behind it.
To keep this sensible and simple, The Helicopter CG should be directly under the Main Rotor Mast centroid both Longitudinally and Laterally. That's the optimum ideal position giving 100% MR flight control in all directions.
However, practically speaking, there has to be some loading of the helicopter which would cause the CG to move around the rotor centroid. It cannot move that much. Even in a large helicopter like an EC225 it can only move 0.5m longitudinally.
There are limits to how far the CG can move away from the rotor centroid before effective flight control is compromised.
If the TRGBX and its blades depart the airframe, the CG will make a significant move forward. It is highly likely in ALL helicopters that this will result in a significant compromise of the forward CG limit and effective aft cyclic to counter the forward movement of the CG will reach the aft stop before the pitch moment forward can be cancelled. The results.....well not pretty.
The Datum, as someone has already posted, is simply a point in space to facilitate all calculations to be in the positive range. Usually it is quite a way out in front of the helicopter nose to facilitate the fitment of long PITOT probes during certification. For simplicity, it remains there. Therefore, the limits for longitudinal CG are expressed in positive numbers and to keep the theme at the EC225 4.4m to 4.9m. This actually means, behind the datum. Not in front of the rotor centroid.
I know 99% of Rotor heads know this but to clear up the incessant garbage postings by the odd individual who seems confused....enjoy!
To have the CG in front of the nose of the helicopter, you would need a MRB sticking over nose, the portion beyond the nose, weighing more than the entire helicopter behind it.
To keep this sensible and simple, The Helicopter CG should be directly under the Main Rotor Mast centroid both Longitudinally and Laterally. That's the optimum ideal position giving 100% MR flight control in all directions.
However, practically speaking, there has to be some loading of the helicopter which would cause the CG to move around the rotor centroid. It cannot move that much. Even in a large helicopter like an EC225 it can only move 0.5m longitudinally.
There are limits to how far the CG can move away from the rotor centroid before effective flight control is compromised.
If the TRGBX and its blades depart the airframe, the CG will make a significant move forward. It is highly likely in ALL helicopters that this will result in a significant compromise of the forward CG limit and effective aft cyclic to counter the forward movement of the CG will reach the aft stop before the pitch moment forward can be cancelled. The results.....well not pretty.
The Datum, as someone has already posted, is simply a point in space to facilitate all calculations to be in the positive range. Usually it is quite a way out in front of the helicopter nose to facilitate the fitment of long PITOT probes during certification. For simplicity, it remains there. Therefore, the limits for longitudinal CG are expressed in positive numbers and to keep the theme at the EC225 4.4m to 4.9m. This actually means, behind the datum. Not in front of the rotor centroid.
I know 99% of Rotor heads know this but to clear up the incessant garbage postings by the odd individual who seems confused....enjoy!
The only occasion I know off where the main door departed into and then removed the tail rotor assembly resulted to a massive pitch down which the pilot tried to correct with aft cyclic. The caused the main rotor, in sympathy with the pitch up of the tail boom to slice off the boom just aft of the mounting point.
The ensuing pandemonium resulted in the aircraft descending almost vertical and killing everybody on board.
The ensuing pandemonium resulted in the aircraft descending almost vertical and killing everybody on board.
Bristow lost a 412 in Nigeria and the evidence showed a Cabin Door had come into contact with the Tail Rotor.
The flight was at night in IMC weather.
The aircraft and occupants were not recovered as I recall.....just some odd bits and pieces.
A ROV was lost on the first dive and the search was called off afterwards.
The flight was at night in IMC weather.
The aircraft and occupants were not recovered as I recall.....just some odd bits and pieces.
A ROV was lost on the first dive and the search was called off afterwards.
Bristow lost a 212 in the Sumatran jungle when the T/R departed in the cruise with half of the 90 box. The T/R was not found within a few hundred yards of the crash site. It later found by some Indonesians who were paid a lot of rupiahs, the distance from the locus could not be determined. The rotor was examined by the AAIB and it was determined that it had been hit by an oil cooler duct. The aircraft had caught fire and the port side was burnt out but the starboard panel was still attached. It was assumed that a faulty catch on the panel had failed and the panel had flown over the tail boom and hit the rotor.
For; Blade Slap,totti,M3- perhaps I should have put the phrase about the CG` beyond the nose`` in italics/commas,or a couple of smilies.It was meant to imply that the loss of a tail-rotor and gearbox will have a very significant FWD C o G change,irrespective of where it was originally,and if it was FWD AT THE TIME you will get a significant NOSE DOWN pitch.
In my case ,I had 2 engineers at the front of the cabin,looking at the rear of the engine and reduction g.box checking for oil leaks,as we had just done an engine out,change a component /pipe on the back of the gearbox,, engine back in,and RTB.,so my CoG was well Fwd anyway.
I might suggest if you are pilots or gingerbeers that you go ,find the appropriate tech manual which shows all the weights for tailrotor ,g/box,etc and work out the resultant change of C o G ,if they should depart,......
SAS,DB, thanks....
In my case ,I had 2 engineers at the front of the cabin,looking at the rear of the engine and reduction g.box checking for oil leaks,as we had just done an engine out,change a component /pipe on the back of the gearbox,, engine back in,and RTB.,so my CoG was well Fwd anyway.
I might suggest if you are pilots or gingerbeers that you go ,find the appropriate tech manual which shows all the weights for tailrotor ,g/box,etc and work out the resultant change of C o G ,if they should depart,......
SAS,DB, thanks....
For; Blade Slap,totti,M3- perhaps I should have put the phrase about the CG` beyond the nose`` in italics/commas,or a couple of smilies.It was meant to imply that the loss of a tail-rotor and gearbox will have a very significant FWD C o G change,irrespective of where it was originally,and if it was FWD AT THE TIME you will get a significant NOSE DOWN pitch.
In my case ,I had 2 engineers at the front of the cabin,looking at the rear of the engine and reduction g.box checking for oil leaks,as we had just done an engine out,change a component /pipe on the back of the gearbox,, engine back in,and RTB.,so my CoG was well Fwd anyway.
I might suggest if you are pilots or gingerbeers that you go ,find the appropriate tech manual which shows all the weights for tailrotor ,g/box,etc and work out the resultant change of C o G ,if they should depart,......
SAS,DB, thanks....
In my case ,I had 2 engineers at the front of the cabin,looking at the rear of the engine and reduction g.box checking for oil leaks,as we had just done an engine out,change a component /pipe on the back of the gearbox,, engine back in,and RTB.,so my CoG was well Fwd anyway.
I might suggest if you are pilots or gingerbeers that you go ,find the appropriate tech manual which shows all the weights for tailrotor ,g/box,etc and work out the resultant change of C o G ,if they should depart,......
SAS,DB, thanks....
I am aware of more than one 407 TRGB and a portion of tailboom departures back in '98/2000 where a successful autorotation was made at least once to the water (PHI GoM 1998 or '99) with a full load of passengers, the pilot reported no excessive pitch down moment.
As far as consulting the appropriate tech manuals, I have access to a couple of types (which I cannot disclose) and we ran calculations confirming that the loss of the TRGB and the TR blades does not move the CG forward to a catastrophic unrecoverable value.
HTH
Totti, OK,I`ll` add a couple of other bits as your calculator is warmed up;
IF the helo now enters a full autorotation,how much aft stick movement is produced at a variety of airspeeds,to keep a level attitude;
now do it all at a fwd C o G...
now consider the fact that the aircraft has yawed(depending on airspeed and fin size)which may also have a `pitch` effect..
now consider where the stick/disc relationship is to tail- boom.....and your stick margins vs control stops
Answers on a postcard,,,...or sheets of A4.....
IF the helo now enters a full autorotation,how much aft stick movement is produced at a variety of airspeeds,to keep a level attitude;
now do it all at a fwd C o G...
now consider the fact that the aircraft has yawed(depending on airspeed and fin size)which may also have a `pitch` effect..
now consider where the stick/disc relationship is to tail- boom.....and your stick margins vs control stops
Answers on a postcard,,,...or sheets of A4.....
Not rocket science to suggest that the longer the tailboom, the further from the C of G is the TRGB and the more marked effect its loss will have on longitudinal C of G.
Totti, OK,I`ll` add a couple of other bits as your calculator is warmed up;
IF the helo now enters a full autorotation,how much aft stick movement is produced at a variety of airspeeds,to keep a level attitude;
now do it all at a fwd C o G...
now consider the fact that the aircraft has yawed(depending on airspeed and fin size)which may also have a `pitch` effect..
now consider where the stick/disc relationship is to tail- boom.....and your stick margins vs control stops
Answers on a postcard,,,...or sheets of A4.....
IF the helo now enters a full autorotation,how much aft stick movement is produced at a variety of airspeeds,to keep a level attitude;
now do it all at a fwd C o G...
now consider the fact that the aircraft has yawed(depending on airspeed and fin size)which may also have a `pitch` effect..
now consider where the stick/disc relationship is to tail- boom.....and your stick margins vs control stops
Answers on a postcard,,,...or sheets of A4.....
Crab, you are absolutely correct.
Join Date: May 2010
Location: Boston
Age: 73
Posts: 443
Likes: 0
Received 0 Likes
on
0 Posts
MWR ... If you are in any way correct, this implies a really poor original design... a prime goal in mechanical components design just as important as stessing and robustness is clarity of assembly whilst minimising potential for mis-assembly...
e.g. all bolts in a locality doing much the same job being of the same length, usually arranged by say stepping a casting thickness .. EVEN IF IT COSTS WEIGHT..
e.g. all bolts in a locality doing much the same job being of the same length, usually arranged by say stepping a casting thickness .. EVEN IF IT COSTS WEIGHT..
GeordiMike also responded that lack of washer would likely affect the centering of the control system enough that it would at least be noticed if not totally unserviceable.
(My thoughts based on his feedback)
Again most likely they suspect something in that mechanism but do not know (or cannot say with certainty) exactly what so the goal is to have it looked at for anything suspicious.
From the picture it seems improbable that both locking means were missing/misinstalled.
There is a more subtle issue of the effects of the washer (140) either missing or on the wrong side. This would not jump out on a visual inspection like missing locking means would.
Not discernible from the drawing but appears that the shoulder the washer sits on is probably just a bit larger than the hole in the hinge bracket (90).
There is a more subtle issue of the effects of the washer (140) either missing or on the wrong side. This would not jump out on a visual inspection like missing locking means would.
Not discernible from the drawing but appears that the shoulder the washer sits on is probably just a bit larger than the hole in the hinge bracket (90).
did you magnify the photo?
If you haven't, please do. I see the arrangement in the photo as indicated in the drawing.
Join Date: May 2010
Location: Boston
Age: 73
Posts: 443
Likes: 0
Received 0 Likes
on
0 Posts
Quote:
If you haven't, please do. I see the arrangement in the photo as indicated in the drawing.
I saw that the photo matches the drawing as expected since the photo is of a correct assembly. My question was about possibility of an incorrect assembly with the washer on the wrong side, under the nut, and if that was even possible without causing an obvious problem. If it is possible then the hardened rod shoulder could stress the 'hinge bracket element (90)' and cause a failure,especially if the shoulder was significantly smaller than the outside diameter of the washer.
Without detailed drawings and system knowledge whether misassembly is even possible is an open question though feed back from GeordiMike suggests it probably would be obvious due to offsets in the servo loop.
Also if washer misplacement (or missing) is possible seems that it would have been an explicit check in the AD which does not mention the washer.
Then again something did go wrong, as others have said will be clear in final report.
Originally Posted by MurphyWasRight 
From the picture it seems improbable that both locking means were missing/misinstalled.
There is a more subtle issue of the effects of the washer (140) either missing or on the wrong side. This would not jump out on a visual inspection like missing locking means would.
Not discernible from the drawing but appears that the shoulder the washer sits on is probably just a bit larger than the hole in the hinge bracket (90).
did you magnify the photo?
From the picture it seems improbable that both locking means were missing/misinstalled.
There is a more subtle issue of the effects of the washer (140) either missing or on the wrong side. This would not jump out on a visual inspection like missing locking means would.
Not discernible from the drawing but appears that the shoulder the washer sits on is probably just a bit larger than the hole in the hinge bracket (90).
did you magnify the photo?
Without detailed drawings and system knowledge whether misassembly is even possible is an open question though feed back from GeordiMike suggests it probably would be obvious due to offsets in the servo loop.
Also if washer misplacement (or missing) is possible seems that it would have been an explicit check in the AD which does not mention the washer.
Then again something did go wrong, as others have said will be clear in final report.
thats why I asked, I figured that as well, if the washer was suspect for any reason it would be part of the AD.
Incidentally for simple random information totally unrelated to this case, I was disassembling some AS350 control rods today, and while cutting the lockwire, the nut started turning. the lockwasher design didn't stop the nut as it was just a place for the lockwire to terminate.
The "double locking" in this case did its job although the proper torque was not on the nut.
However, nothing further would have happened because the lockwire, torgue and locktabs, on the other end would have all had to fail too before bad things happened.
Incidentally for simple random information totally unrelated to this case, I was disassembling some AS350 control rods today, and while cutting the lockwire, the nut started turning. the lockwasher design didn't stop the nut as it was just a place for the lockwire to terminate.
The "double locking" in this case did its job although the proper torque was not on the nut.
However, nothing further would have happened because the lockwire, torgue and locktabs, on the other end would have all had to fail too before bad things happened.
thats why I asked, I figured that as well, if the washer was suspect for any reason it would be part of the AD.
Incidentally for simple random information totally unrelated to this case, I was disassembling some AS350 control rods today, and while cutting the lockwire, the nut started turning. the lockwasher design didn't stop the nut as it was just a place for the lockwire to terminate.
The "double locking" in this case did its job although the proper torque was not on the nut.
However, nothing further would have happened because the lockwire, torgue and locktabs, on the other end would have all had to fail too before bad things happened.
Incidentally for simple random information totally unrelated to this case, I was disassembling some AS350 control rods today, and while cutting the lockwire, the nut started turning. the lockwasher design didn't stop the nut as it was just a place for the lockwire to terminate.
The "double locking" in this case did its job although the proper torque was not on the nut.
However, nothing further would have happened because the lockwire, torgue and locktabs, on the other end would have all had to fail too before bad things happened.
earlier posters describe uncommanded full actuation which would certainly be possible probably likely, with failures on the subject side of servo. A neutral tail rotor would not cause aircraft to keep spinning after main rotor torque was dropped, it would slow down, and if biased it would actually turn the aircraft to the left. Yes, a missing nut could do this, but for all we know, they found a fractured end of servo. One cause of that could be... incorrect or inadequate torque on the nut. An initial inspection is prudent.