Tail Rotor Problems
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
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
], 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????????
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
], 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????????
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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
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
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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.
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 
Many thanks.
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??
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??
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
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
JJ: I knew someone would say something I could recognise and understand. I've always been slow on the uptake
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.
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.
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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 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.
