Tail Rotor Problems
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TC, as you already know, all helicopters have a short shaft that connects the engine to the MGB. Most helicopters have a TR shaft that connects the MGB to the TRGB. Some helicopters have a different TR shaft that connects the engine to the TRGB instead, it's usually a continuation of the short shaft but goes to the back of the helicopter rather than to the front.
1. TR shaft from MGB to TRGB
When short shaft failure occurs, TR RPM and MR RPM remain coupled with 0% engine power available to both TR and MR.
2. TR shaft from engine to TRGB
When short shaft failure occurs, TR RPM and MR RPM decouple with 100% engine power available to the TR, and 0% available to the MR.
1. TR shaft from MGB to TRGB
When short shaft failure occurs, TR RPM and MR RPM remain coupled with 0% engine power available to both TR and MR.
2. TR shaft from engine to TRGB
When short shaft failure occurs, TR RPM and MR RPM decouple with 100% engine power available to the TR, and 0% available to the MR.
Last edited by cattletruck; 21st Feb 2013 at 23:33.
Maybe it's too early in the morning but, I am not sure what you are describing.
The shaft from ECU to MRGB is called the high speed shaft, over here. On some a/c failure of this causes the engine to either trip offline or sit at max cont. If you are a twin engine cab you can still fly.
Can you tell me which helo(s) have a shaft from the ECU t the TRGB, out of curiosity? I am curious as to where the speed gearing takes place. Surely the small TRGB doesn't do all the speed reducing?
Don't understand your point number 2? Sorry.
The shaft from ECU to MRGB is called the high speed shaft, over here. On some a/c failure of this causes the engine to either trip offline or sit at max cont. If you are a twin engine cab you can still fly.
Can you tell me which helo(s) have a shaft from the ECU t the TRGB, out of curiosity? I am curious as to where the speed gearing takes place. Surely the small TRGB doesn't do all the speed reducing?
Don't understand your point number 2? Sorry.
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Ok, best to use pictures e.g. the Ariel 1B as installed on the AS350.
As you can see from this schematic, the engine reduction drive turns one integrated drive shaft that has a connection spline fore and aft. The foreward spline attaches to a shaft that connects to the MGB which is called the short shaft. The rear spline connects to a shaft that connects to the TRGB.
http://blog.aopa.org/helicopter/wp-c...Picture123.jpg
(Credit: Tim McAdams, AOPA Hover Power)
Here is a nice pic showing the TRGB shaft connected directly to the engine drive shaft.
http://images2.jetphotos.net/img/2/4...1233818594.jpg
(Credit: Hendrik Kruger, Jetphotos.net)
Hope this helps, even if short shaft failures are quite rare.
As you can see from this schematic, the engine reduction drive turns one integrated drive shaft that has a connection spline fore and aft. The foreward spline attaches to a shaft that connects to the MGB which is called the short shaft. The rear spline connects to a shaft that connects to the TRGB.
http://blog.aopa.org/helicopter/wp-c...Picture123.jpg
(Credit: Tim McAdams, AOPA Hover Power)
Here is a nice pic showing the TRGB shaft connected directly to the engine drive shaft.
http://images2.jetphotos.net/img/2/4...1233818594.jpg
(Credit: Hendrik Kruger, Jetphotos.net)
Hope this helps, even if short shaft failures are quite rare.
Last edited by cattletruck; 24th Feb 2013 at 01:53. Reason: images too big, converted to links.
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cattletruck,
I've never worked on the Ariel, so forgive my lack of understanding. You wrote that...."The foreward spline attaches to a shaft that connects to the MGB which is called the short shaft. The rear spline connects to a shaft that connects to the TRGB." (bold italics mine for emphasis)
When looking at the cutaway that you posted, it seems that the transmission shaft is driven through a reduction gearbox by a free turbine, and the same transmission shaft drives both the MGB (forward) and the TRGB (aft).
I do not see a "short shaft" except maybe the small shaft driven by the compressor shaft, which drives the accessory gearbox (AGB). But that shaft does not drive the MGB, just the AGB.
What am I missing?
I've never worked on the Ariel, so forgive my lack of understanding. You wrote that...."The foreward spline attaches to a shaft that connects to the MGB which is called the short shaft. The rear spline connects to a shaft that connects to the TRGB." (bold italics mine for emphasis)
When looking at the cutaway that you posted, it seems that the transmission shaft is driven through a reduction gearbox by a free turbine, and the same transmission shaft drives both the MGB (forward) and the TRGB (aft).
I do not see a "short shaft" except maybe the small shaft driven by the compressor shaft, which drives the accessory gearbox (AGB). But that shaft does not drive the MGB, just the AGB.
What am I missing?
It is my understanding that the 'short shaft' is not depicted in the cutaway that cattletruck has provided. It must be located where the power drive is pointing towards.
Great food for thought TC.
Great food for thought TC.
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Hard to find a good picture of what I was trying to illustrate, their is a freewheeling unit connected ahead of the forward spline that then goes into the MRGB, so in the case of the Arriel engined AS350 I guess you could argue the transmission shaft is the short shaft (I'm not an engineer).
The point I was trying to make was about the potential for decoupling of MR and TR RPMs with these types of configurations (engine drives MRGB and TRGB directly).
I think I have confused myself now, with the engine rather than the MRGB driving the TRGB, what happens when the engine stops and the freewheeling unit separates the engine from the MRGB? There must be some other magic to this.
The point I was trying to make was about the potential for decoupling of MR and TR RPMs with these types of configurations (engine drives MRGB and TRGB directly).
I think I have confused myself now, with the engine rather than the MRGB driving the TRGB, what happens when the engine stops and the freewheeling unit separates the engine from the MRGB? There must be some other magic to this.
Cattletruck: Some helos have clutches others have freeewheels. All helos allow the MRGB and or the TRGB to disconnect from the engine in the event the engine stops. Simples.
Engine stops: both MR and TR continue to spin (provided you are in auto!
TRGB stops, engine and MGB continue to work.
MRGB stops - you die!
"short shaft" snaps/shears, engine runs up out of control and either switches itself off or hangs at max cont. MGB/TRGB continue to work provided you go into auto.
Beautiful photos though....
Engine stops: both MR and TR continue to spin (provided you are in auto!
TRGB stops, engine and MGB continue to work.
MRGB stops - you die!
"short shaft" snaps/shears, engine runs up out of control and either switches itself off or hangs at max cont. MGB/TRGB continue to work provided you go into auto.
Beautiful photos though....
TC
Wrt the Ariel engine.
"short shaft" snaps/shears, engine runs up out of control and either switches itself off or hangs at max cont. MGB/TRGB continue to work provided you go into auto.
Not quite......
The governor senses Nf at the accessory gearbox on the engine, not the MGB. Therefore, there would be a momentary rapid increase in Nf at the moment of short shaft breakage but this would be quickly countered by the governor bringing the engine down to idle. Now the MRGB needs autorotative flow to keep turning but the TRGB is still driven by the idling engine so yaw control is still possible. When conducting the cushion on at the end of the EOL, the collective anticipator will throw a bucket load of fuel into the engine and the T/R will increase RPM rapidly and possibly uncontrolably with ensuing 'unexpected' yaw.
Best advice, I believe, is to keep the engine running on the way down but shut it off prior to commencing the landing phase.
JJ
Wrt the Ariel engine.
"short shaft" snaps/shears, engine runs up out of control and either switches itself off or hangs at max cont. MGB/TRGB continue to work provided you go into auto.
Not quite......
The governor senses Nf at the accessory gearbox on the engine, not the MGB. Therefore, there would be a momentary rapid increase in Nf at the moment of short shaft breakage but this would be quickly countered by the governor bringing the engine down to idle. Now the MRGB needs autorotative flow to keep turning but the TRGB is still driven by the idling engine so yaw control is still possible. When conducting the cushion on at the end of the EOL, the collective anticipator will throw a bucket load of fuel into the engine and the T/R will increase RPM rapidly and possibly uncontrolably with ensuing 'unexpected' yaw.
Best advice, I believe, is to keep the engine running on the way down but shut it off prior to commencing the landing phase.
JJ
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Question JJ
is it true to say that the Ariel set to idle under those circumstances would still turn the tail rotor (at less effective TRPM) and negate the anticipator problem on collective raise?
JJ: Thanks for this. Are you saying that the TR runs down to the equivalent of ECU flight idle speed during a high speed shaft failure?
How much a reduction does the TR experience as a percentage of its normal operating speed?
TRE diminishes to below acceptable on 'normal' helos as the TR speed slows even by as little as 5%, never mind even slower
On face value this sounds a very archaic way of driving a gearbox (directly from engine speed.
Would it not be wiser for the TRGB drive shaft to be driven by the accessories gearbox (as it is now) BUT in the event of a high speed shaft failure, the engine could run to idle or even shut down and the MGB could drive the accessories in auto thus allowing TRGB speed to be maintained inside normal operating limits.
I can't believe the TRGB slows to ECU Flight idle speed and still allows any elemnet of authority>
How much a reduction does the TR experience as a percentage of its normal operating speed?
TRE diminishes to below acceptable on 'normal' helos as the TR speed slows even by as little as 5%, never mind even slower
On face value this sounds a very archaic way of driving a gearbox (directly from engine speed.
Would it not be wiser for the TRGB drive shaft to be driven by the accessories gearbox (as it is now) BUT in the event of a high speed shaft failure, the engine could run to idle or even shut down and the MGB could drive the accessories in auto thus allowing TRGB speed to be maintained inside normal operating limits.
I can't believe the TRGB slows to ECU Flight idle speed and still allows any elemnet of authority
>
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TC - I don't think he meant idling in the sense of Nf rather in the sense of No Significant Load - I was suggesting going to 'Idle' ( instead of shut down ) and I should think that 40% TR authority would be a luxury (lad) and easily enough to overcome MRGBx drag and keep ... etc
and asking (confirming?) that the 'anticipation' is not effective at all in Idle
and asking (confirming?) that the 'anticipation' is not effective at all in Idle
AnFI
Very good point re anticipator. And without digging the manuals out I can't confirm one way or the other; I'll check at work tomorrow.
As regards the drive to the main rotor and tail rotor, they share the same shaft that runs under the engine (see Cattletruck's link). Forwards to the MGB, rearwards to the TGB. However, when the short shaft (more correctly called Main Gearbox Input Drive Shaft) shears, the engine instantly off-loads to the MGB, however, due to the configuaration, the shaft is still driving the T/R. In the cruise, since the T/R is doing very little due to the fin producing the anti-torque force, the Nf and therefore T/R RPM increase rapidly but are quickly reduced again because the governor reduces Ng down to it's idling minimum fuel flow. I'm not sure what the Nf or T/R RPM are in this condition as I've (thankfully) never seen one. My guess, is that even with minimum fuel flow, the Nf and T/R may well be overspeeding in the cruise as I would imagine that the residual gas generator thrust, even at idle, would be more than sufficient to turn an off-loaded T/R.
I do recall an incident in the States, sadly fatal, where the short shaft failed and the pilot lost control during the landing due to the anticipator rapidly spooling the engine and therefore T/R up.
JJ
Very good point re anticipator. And without digging the manuals out I can't confirm one way or the other; I'll check at work tomorrow.
As regards the drive to the main rotor and tail rotor, they share the same shaft that runs under the engine (see Cattletruck's link). Forwards to the MGB, rearwards to the TGB. However, when the short shaft (more correctly called Main Gearbox Input Drive Shaft) shears, the engine instantly off-loads to the MGB, however, due to the configuaration, the shaft is still driving the T/R. In the cruise, since the T/R is doing very little due to the fin producing the anti-torque force, the Nf and therefore T/R RPM increase rapidly but are quickly reduced again because the governor reduces Ng down to it's idling minimum fuel flow. I'm not sure what the Nf or T/R RPM are in this condition as I've (thankfully) never seen one. My guess, is that even with minimum fuel flow, the Nf and T/R may well be overspeeding in the cruise as I would imagine that the residual gas generator thrust, even at idle, would be more than sufficient to turn an off-loaded T/R.
I do recall an incident in the States, sadly fatal, where the short shaft failed and the pilot lost control during the landing due to the anticipator rapidly spooling the engine and therefore T/R up.
JJ
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The Arriel have a reduction gearbox(MO5) which reduces the free turbine rpm to app 6000 rpm varying between the Arriel 1 and 2 and variants. The MO 5 is at the back of the engine. On the 350 the MGB is driven by the torque shaft in the eng MO1(the silver circular bit below the gasturbine and free turbine) which drives the MGB driveshaft between the eng and MGB. The tail rotor is driven directly off the back off the MO5 via the short shaft then long 5 bearing tail driveshaft to the TGB which reduces from 6000 something to 4000 something. The purpose of the short shaft between the MO5 and tail driveshaft is to absorb misalignment and movement as on the 350 the eng is not fixed mounted but on elastomers so it absorbs vibrations. On a single there is no overspeed protection and the Arriel 2 with its FADEC by default will freeze where it thinks a failure occurs and not go to grn idle or shutdown by itself. The EBCAU another airframe comp will take over now but lowestN1 will be N1 at failure so you must continue to load the disc if lowering collective to prevent overspeed. So, if the MGB driveshaft fail between eng MO1 and MGB the engine will be offloaded due MGB not driven, yet the TR will continue at free turbine speed due direct drive. This could cause a TR and eng overspeed whilst MR in not driven mode-disaster! Dont know of a case of MGB input shaft failure yet but ASB's on checking couplings with no noise during rotation allowed. Too much typing on a Sunday night.
The freewheel is in the front of the MO1 between the torque shaft and the MGB driveshaft so no connection to the tail driveshaft. Also the Arriel 1 free turbine flyweight in the FCU is driven by the free turbine and Arriel 2 is 3 sensors-2 to eng comp lane 1 and 2 and 3rd to VEMD. Anticapator on Arriel 1 is teleflex rod from collective slowing down N2 before it actually does and on Arriel 2 digital signal from mixing unit collective input resetting N1 instantaneously.
The freewheel is in the front of the MO1 between the torque shaft and the MGB driveshaft so no connection to the tail driveshaft. Also the Arriel 1 free turbine flyweight in the FCU is driven by the free turbine and Arriel 2 is 3 sensors-2 to eng comp lane 1 and 2 and 3rd to VEMD. Anticapator on Arriel 1 is teleflex rod from collective slowing down N2 before it actually does and on Arriel 2 digital signal from mixing unit collective input resetting N1 instantaneously.
Last edited by victor papa; 24th Feb 2013 at 18:50.
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Thanks Jelly
and good info VP ( you are clearly a knowing man )- SO VP you say "free turbine flyweight in the FCU is driven by the free turbine" - So it will attempt to govern still even though the load is only the tail rotor?
This leaves us with an interesting question which Jelly raises: is it possible for the FADEC to command a sufficiently low fuel flow to prevent overspeed of Nf (ie T/R)?
[and since you are a knowing person - can I check with you what EBCAU does - surely it can't just freeze N1 (Ng) - it would mean major overspeeds on lever reduction - I think I tried to use the EBCAU and think I remember it doing some Nr (Nf) governing ? Do I?]
and good info VP ( you are clearly a knowing man )- SO VP you say "free turbine flyweight in the FCU is driven by the free turbine" - So it will attempt to govern still even though the load is only the tail rotor?
This leaves us with an interesting question which Jelly raises: is it possible for the FADEC to command a sufficiently low fuel flow to prevent overspeed of Nf (ie T/R)?
[and since you are a knowing person - can I check with you what EBCAU does - surely it can't just freeze N1 (Ng) - it would mean major overspeeds on lever reduction - I think I tried to use the EBCAU and think I remember it doing some Nr (Nf) governing ? Do I?]
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EBCAU is a independant Eurocopter supplied computer that governs the free turbine between 98 and 102 perc or somethingnlike that utilising a seperate independant valve from the stepper motor metering valve and electronic logic in the ASU 3 situated to the right of the coplts feet. The EBCAU is automatically activated by the FADEC incase of total stepper motor or metering valve failure which will be sensed bynthe resolver or both comp fails. No action req by the pilot. This is for the 2B1 and 2 D, the 2B will unlock the sliding lock on the throttle to allow manual control. In both cases the engine comp stops the metering valve where the failure occured thus the N1 and the EBCAU or throttle can control free turbine speed around that position.
Overspeed control is called the pilot on the singles. Not sure how the DECU's will deal with it and not going to try and see what happens if the MGB input shaft fails.
Overspeed control is called the pilot on the singles. Not sure how the DECU's will deal with it and not going to try and see what happens if the MGB input shaft fails.
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VP - that's great information - so EBCAU will govern Nf then so even if DECU's 'flip out' the basic EBCAU should stop overspeed ... ? and so T/R might not overspeed if that all works ...
as an aside (whilst we have an expert on the line) people run helis up without blades sometimes - does DECU have a problem with this ? Is there an issue with feedback/resonance/oscillation ? Is it an 'ok' procedure to run with out blades?
as an aside (whilst we have an expert on the line) people run helis up without blades sometimes - does DECU have a problem with this ? Is there an issue with feedback/resonance/oscillation ? Is it an 'ok' procedure to run with out blades?
ANFI / VP: Let's keep it simple for simple folk like me eh?
Taking only the AS350 as an example (because other SE helos don't follow this concept):
In the event an AS350 eperiences an engine failure, the a/c has to enter auto immediately for obvious reasons! The MRGB then 'reverse' drives the reduction gearbox off the engine, which in turn drives the TRGB at normal speed - correct?
Now:
In the event the short shaft between the MRGB and engine - fails (high speed shaft failure), the engine reduces to Flight Idle (or similar). The a/c has to enter auto/eol and the MRGB maintains its speed. The TRGB is dramarically reduced in speed - correct?
At the bottom of the auto, prior to touchdown the MRGB increases in speed initially due to flare effect but then reduces as the collective is raised - but the tail rotor speed remains at tick over throughout.
How, then do you stop the undemanded yaw of the a/c due to the TR not being able to offer any anti torque - during the flare effect (provided the engine is at tickover speed)???
Thanks
Taking only the AS350 as an example (because other SE helos don't follow this concept):
In the event an AS350 eperiences an engine failure, the a/c has to enter auto immediately for obvious reasons! The MRGB then 'reverse' drives the reduction gearbox off the engine, which in turn drives the TRGB at normal speed - correct?
Now:
In the event the short shaft between the MRGB and engine - fails (high speed shaft failure), the engine reduces to Flight Idle (or similar). The a/c has to enter auto/eol and the MRGB maintains its speed. The TRGB is dramarically reduced in speed - correct?
At the bottom of the auto, prior to touchdown the MRGB increases in speed initially due to flare effect but then reduces as the collective is raised - but the tail rotor speed remains at tick over throughout.
How, then do you stop the undemanded yaw of the a/c due to the TR not being able to offer any anti torque - during the flare effect (provided the engine is at tickover speed)???
Thanks
Last edited by Thomas coupling; 25th Feb 2013 at 13:07.
AnFI
Why I needed to go to work to check the books, i don't know. Of course throttling back to Idle will inhibit the anticipator. Every time we practice EOLs, the throttle is at idle and therefore there's no problem with the anticipator spooling the engine up.
JJ
Why I needed to go to work to check the books, i don't know. Of course throttling back to Idle will inhibit the anticipator. Every time we practice EOLs, the throttle is at idle and therefore there's no problem with the anticipator spooling the engine up.
JJ
TC
The Ariel has a Free Power Turbine.
In the case of an MGB Input Drive Failure, the governor will sense Nf (N2 on Allisons) being High so will command the Gas Generator (Ng / N1 / CRPM) to minimum RPM. However, as the T/R is driven from the aft portion of the shaft, and the M/R is driven from the forward portion of the shaft, there is a potential disconnect between T/R and M/R rpms. The M/R will require you to enter auto, as per any helicopter that loses drive to the MGB.
The difference, is that the Ariel will still power the T/R in this configuration. The big questions are, since the Gas Generator is at minimum fuel flow, and therefore minimum Ng (CRPM / N1) what is its residual thrust? Will it accelerate the Nf ( and therefore T/R RPM) beyond normal limits as it won't be loaded by the tail rotor at close to min pitch? Will it start to govern effectively again if the tail rotor is loaded for any reason (I suspect it will)?
Any clearer?
JJ
The Ariel has a Free Power Turbine.
In the case of an MGB Input Drive Failure, the governor will sense Nf (N2 on Allisons) being High so will command the Gas Generator (Ng / N1 / CRPM) to minimum RPM. However, as the T/R is driven from the aft portion of the shaft, and the M/R is driven from the forward portion of the shaft, there is a potential disconnect between T/R and M/R rpms. The M/R will require you to enter auto, as per any helicopter that loses drive to the MGB.
The difference, is that the Ariel will still power the T/R in this configuration. The big questions are, since the Gas Generator is at minimum fuel flow, and therefore minimum Ng (CRPM / N1) what is its residual thrust? Will it accelerate the Nf ( and therefore T/R RPM) beyond normal limits as it won't be loaded by the tail rotor at close to min pitch? Will it start to govern effectively again if the tail rotor is loaded for any reason (I suspect it will)?
Any clearer?
JJ
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As said, I am no expert and am definately not going to try a MGB input shaft failure! Dont wanna be close just for incase. Before the 350 doom prophets climb onboard another possible flaw, I think the chances are very slim seeing the fleet has got millions of hrs and we are speculating only.
The Arriel is extremely fast in excelerating especially the Arriel 2 so I do not know whether I would run without blades.
The Arriel is extremely fast in excelerating especially the Arriel 2 so I do not know whether I would run without blades.