Twitchy Tail in the S61
 

Just looking for a couple of ideas here gents. N model S61 on descent around 35-40% Torque, tail wags back and forth a bit. Yaw channel off, still there, AFCS off still there, Aux off still there, but more pronounced. Does not do it in a hover or climbing flight, only descent. Came to light about 6 weeks after a T/R gearbox change. Aux servo has been replaced since, problem still there.
Its aerodynamic so im leaning towards NFG problem? Open to suggestions.....

We call that tail shake or tail wag, it is caused by the air from the rotor head and transmission area that strikes the horizontal tail especially in moderate descents. The "horse collar" around the upper transmission fairing was shaped to minimize the influence.

Could it be that this was always there, but not noticed until the mtf flight after the tail maintenance? I do think so.

No, i don't think its the "sikorsky shuffle" Couple of 10,000 hour pilots flying the ship, and it started out intermittently, and now happens all the time. With aux off, the tail shake is getting quite violent.................

Sounds like TR cable tension? Slop in NFG spring system? The key is "quite violent," a very credible description.

Cable tension checked ok. The NFG spring has been looked at and spring tension was checked on an arbour scale ( by the other crew) Spring has not been replace yet, but thats on the list of things to try.
Thanks for your input, and i will double check that the cable tensions are good...

What is the yaw control like with aux out in other stages of flight? I presume the TR track and balance is OK. Maybe the new TR gearbox isn't quite so new?
It would be odd if everything else was completely normal in terms of TR handling and the fault only ever manifested itself in the conditions you describe, what is it like in auto?

Brinelling on the TR shaft might cause such a problem but it would be evident in other stages of flight as well.

I had a almost violent vibration (fairly high freq and med amplitude) on our S61 during my last OPC, and it turned out to be one tail blade being out of balance. Weights were added if I remember correctly and LC flew smoothly again after.

Si

P.S. that was on approach to land as I recall, but it was only when AFCS out that the vibration was noticed.

Check mixer bearings, linkages, and look for properly routed tail rotor control cable is not over pins. Then Aux servo tail moog valve. If all else is okay, look at TGB Pitch change shaft bearings and TR spindle stack bearings.

toolguy has it there.

I would put my money on the Tail Rotor Stack Bearing(s) being bad as a next troubleshooting step. Bigglesbutler observation is also correct regarding not typically being able to feel the problem with the aux on, and it will certainly become more apparent with the aux off.

Change out the tail rotor head and let us know.

Tail Rotor Stack Bearings and Flapping hinge bearings are easy and quick to check. Disconnect the pitch links, rotate the bearing stack and check for roughness and brinneling, do this before greasing, if nothing found then purge lots of grease through with the blades at 12 o'clock, this will remove any water trapped. Any signs of cocoa at all... replace. :)

Mixing unit is smooth as silk. On the ground T/R controls operate normally and smoothly. Moving T/R controls by hand from forward quadrant to P/C shaft ( ie T/R blades not connected) everything is smooth, no binding. As mentioned before the aux servo has been replaced since this problem has shown up, and it made no difference. I will have them double check the stack bearings, and see if they want to change out the head...........the opinion seems to be to put the old T/R gearbox back on and see what happens.

thanks for the suggestions!

Just a thought, many moons ago we also had an 'undiscoverable' vibration which manifested itself initially as 1r then as 5t.

After changing out most of the tail rotor gear train, the battery balance tray and the tail rotor tip weights it turned out to be excessive play in the rotating scissors on the MRH. Confused? Yep we all were.

Good luck in the investigation.

Turn and purge will not reveal the binding of the stack bearings, remember these are thrust bearings retaining the tail rotor cuff and blade, allowing feathering - the problem occurs when the tail rotor system is rotating and the bearings are loaded. I have seen this occur in a single specific phase of flight, and no effect on any other phase.

This is not a speculative troubleshooting procedure, but one I have experienced quite a number of times.

You can change the tail rotor head, or alternately if you have a donor aircraft, pull the flapping hinge bolts and swap each entire spindle and blade assy onto your old hub, maintaining the same positions to keep the balance (the hub has very little effect on balance). You will still have to balance the tail rotor before flying, but you should get a flight out of it before lunch if you do it this way and either eliminate or confirm this as a cause.

Was this ship operational before the TGB change, or parked?

Flapping hinge bearings could also feel smooth on the ground, but when spinning and loaded the could be rough, beter to pull them and look. After re reading your post and seeing how recently you changed the TGB, I would double check pitch cange shaft and pitch change shaft bearing. A little bit of play while loaded means a bunch while in flight.

I need some help here from the long time 61 guys, please...

Considering that the inner race the outer races of the stack bearing are fixed onto the spindle and into the housing respectively and that the pitch link is a fixed length won't rotating the stack bearing simply return everything back to the same point it started from??
I am all for grease purging as I have seen the results first hand, but as far as the rotating goes, I cannot see it changing anything.

I still do it though, regardless.

If the pitch beam cannot move one spindle, it will not be able to move the other four. If you have a bad flapping hinge bearing it could upset the track on all five. Since the problem shows up in descent, pitch should be reduced, but if you are stuck at higher pitch/flap, you are not getting the reduction.

More likely is the pitch change shaft since you changed the TGB. Stuck or too much slop when reducing pitch. Could not hurt to double check rigging as well, including the NFG.

Unstable load.
Here is my version of the reason why. Im sure others will have other opinions, so be easy on me!
Grease, like electricity and apprentices, tends to find the path of least resistance. If the pitch links are hooked up and you are greasing away, the grease tends to "tunnel" and just finds its way past the seal. Being able to turn the spindle through out 360 degrees, sort of makes the grease go everywhere and all the bearings get a good lube job. By rotating the blade vertically while greasing, if there is any entrapped water, it should get purged out with the old grease as well.

my 2 cents worth......

S61 twitchy tail
 

This is typical of brunelled bearings in the tail rotor. Disconnecting the pitch-change rods and rotating sleeves while greasing is a temporary cure but bearings should be changed.

I hope this isn't overly obvious, but have you checked torque on the TGB mounts?

Consensus was to put old T/R gearbox back on and see what happens, as was noted allready, things were fine until shortly after gearbox change.
Weather finally co-operated this morning and after 45 minute test flights can NOT induce the problem again. Looks like P/C shaft bearing or shaft problem!