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.

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.

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.

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?

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.:ok:

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.

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....

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

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:eek::eek:

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:eek::eek::eek:>

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

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

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.

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?]

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.

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?

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

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

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

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.

TC

I've just read your earlier post where you state "TRE diminishes to below acceptable on 'normal' helos as the TR speed slows even by as little as 5%, never mind even slower"

I don't think that's helpful in understanding the dynamics and capability of a tail rotor. If you're demanding max anti-torque thrust from your T/R, say high AUM, OGE Hover and spot turn, then yes, the TRE may be insufficient.

However, the T/R is perfectly capable of maintaining effectiveness at reduced RPMs if the pilot reduces the thrust demand of the T/R. In the case of an autorotative descent and landing, the T/R will be effective at massively reduced RPMs because there's not much work for it to do.

JJ

Hi Anfi: Its a scary thought that operators of said aircraftdonj't really know what's going on after a hi spd shaft failure, though isn't it. I have my "research team" onto it and if anyone knows whats going on - they will. I will revert.

Meanwhile:

Your last sentence concerns me a little. It may be the printed word that is relaying the wrong message but when the 'propeller' called a tail rotor, slows down by a relatively small amount [In the Sea King, when Nr decays from 102 to 82% - below that TRE is almost zero:eek:], the TR becomes a useless piece of rotating junk (to exagerate)...

Now my previous question was/is:

Yes, I agree that during auto, a TR needs to do not a lot, as a matter of fact it may just as well be stationary. BUT:

During the descent, if the pilot wishes to manouevre in the descent either to avoid something or to turn, say 180 degrees round the corner to get into wind, he/she will need directional assistance.
also
at the bottom, when they flare like a ding bat to wash off descent, the Nr goes up and (normally) the pilot maintains directional control with pedal BEFORE this directional control rapidly dies to zero as the Nr decays (engine off or at idle).

How then - in an AS350 helicopter does the pilot achieve any of these objectives above, if the TR is outside its TRE????????:eek:

Hi TC

No problem with T/R thrust at reduced T/R RPM - there's no major torque to counter - and the flare/level/lever part won't vary the gearbox drag all that much (not significant) - the reduced effectiveness of the T/R will easily be enough... (especially not a problem in manoeuvering turns on the way down - any kind of airspeed (30kts +) will keep that tail near the back - only tiny T/R thrust to stay in balance if you really want to...)

T/R thrust is just about proportional to T/R RPM squared - so 50% RRPM implies 25% of max thrust available - easily enough.

If you think about a running engine off landing you do need much larger T/R pedal inputs to make T/R thrust changes...



The interesting points in this thread is how many people think cutting the throttle is the answer to achieve reduced torque on loss of tail rotor. (as opposed to putting the lever down - Brazilian Squirrel)

AND

The greatest danger when teaching T/R loss is that the student has afterwards an increased chance of mis-diagnosing a tail rotor failure when really they were just caught out by the wind etc

T/R control loss
 

Mainly for TC ... thanks for the note and wish I could exchange my words for a couple of gold bricks!

To answer your note.

Following recognition of the TRF situation, I teach an immediate reduction in power ... (lever fully lowered) My use of the word 'immediate' depends on the power in use and airframe speed at the moment of failure. High power, low speed ... no upper limit. Low power ... highish speed, the lever can be lowered more leisurely. To enhance the simulation, I also 'induce' some airframe fibration with collective lever 'fluttering.'

Either way, the aim is to regain control and prevent further RIGHT yaw (USA machines) by allowing the airfame to return from 'right to left' past the 'dead ahead' position to establish a steady state descent in the '10 o Clock' position which requires right cyclic. With control regained, a combination of speed and power allows stable flight in the '10 o Clock' position to be established AND/OR a variation in distance flown and rate of descent as necessary.

With a suitable landing site available, a descent is initiated to around 20 - 30 feet AGL at the lowest achievable speed. As the airframe sinks to the surface, collective lever is raised to arrest ROD with a cyclic flare used to reduce forward speed. With the increase in power, airframe commences the original yaw to the RIGHT which is allowed as far as the 'One o Clock' position when throttle is closed for a low speed EOL. Cyclic is pushed forward to co-incide with the airframe approaching the dead ahead 'twelve o Clock' position while allowing the aircraft to partially touch down on the rear skids to assist a dead ahead 'run-on' landing. Even so the airframe is still likely to continue its left yaw, which as noted in a post above, can be minimised using a right cross wind giving greater drag on the right skid.

I hasten to add that my notes must neccessarily apply to the type on which I've experienced my two total T/R failures, being the cable operated Enstrom T/R control system where the failed cable wrapped itself around the T/R to seize the transmission. The other total failure being a straightforward break in the T/R drive shaft.

My 1999 Biggin Hill Air Fare failure being the less difficult left hand CABLE breakage which allowed use of left anti-torque pedal, but leaving no right pedal control for left yaw. I hope these somewhat neccessarily convoluted notes help an understanding of these failures. and I'd happily chat further with interested parties by PMs. Safe flying to all. Dennis K.

Thanks Dennis - copy and paste your contribution into my other thread: TRF for beginners. I want newbies to read it rather than here because this thread is far too convoluted and techie! Newbies are going to switch off :zzz:

Many thanks.

Anfi - what do you fly?

Have you experienced flight with reduced TR speed?

I'm struggling with this scenario (hi spd shaft failure):

If I am to believe what is being described in general with the AS350, if you are flying along in cruise flt and the hi spd shaft fails, you fully lower the lever because the ECU falls to flt idle. At the same time, because the TR slows, the a/c yaws uncontrollably in the direction of MR rotation to an offset position, left or right of the nose dependent on the type of helo.
So now we have a scenario where the a/c is in an auto with no TRE, yawed off. At the flare stage you increase Nr. Because you have no control over yaw, the a/c then yaws further in the direction of MR rotation - no???

Does one then chop the throttle prior to shut down to bleed the Nr down cancelling any further yaw offset?

Why didnt OEM's like Aerospatialle drive the TRGB from the main GB? Much much less complicated and no worries with a hi spd shaft failure. I don't understand why they have gone down this route??:hmm:

TC

Are you trolling? :)

What's not to understand? Just because the Gas Generator has spooled down to idle doesn't mean it's producing no thrust at all. It's residual thrust at idle is more than enough to keep the Free turbine and hence T/R spinning plenty fast enough, possibly even over-speeding when at zero pitch.

"At the same time, because the TR slows, the a/c yaws uncontrollably in the direction of MR rotation to an offset position"

I think the under-lined is the nub of your misunderstanding. The T/R doesn't necessarily slow. Indeed, at time of MGB input driveshaft failure it will temporarily over-speed due to the Free turbine spooling up rapidly as it is instantaneously off-loaded. Remember, the Free turbine is connected, via a reduction gearbox, to the T/R.

JJ

Now come on TC ..... Look into my eyes .... Concentrate :rolleyes:
( I'm keeping v quiet ..)

JJ: I knew someone would say something I could recognise and understand. I've always been slow on the uptake:suspect:

NOW - I understand. It wasn't clear (to me) before that the TR doesn't actually 'slow down' perceptibly does it? The flt idle speed (or whatever the term given) is sufficient to maintain the Nf such that the reduction gearbox output to TR retains its TRE. Cool - now I understand thanks JJ Got there in the end.

So in an AS350 during a hi spd shaft failure - TRE is still maintained during the auto to land.:D

JHC all these emicons annoy. like people with soft toys attached to their dashes

Thank you all and well done. :ok:

So unlike other helicopters where if you incur a high speed shaft failure and enter an auto, and if you have time shut off the fuel valve before touchdown, in the A350/Arriel design you must keep the ENG on to maintain TRE. Is that correct?

Keeping it in context and as was said before, high speed shaft failures (or what I call the short shaft) are very rare, but I have thoroughly enjoyed the discussion nonetheless. Thanks.