Sikorsky S-76: Ask Nick Lappos
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4p in S76
First gather some data from either the Chadwick 8500 or RADS and try to quantify the 4p in terms of IPS. Then you will be in a better position to troubleshoot. Sometimes a 1p can appear to make the 4p feel worse than it is. 
I can't answer the particular question, but I note the manufacturer has put the nose mounted VTA back on the C++ as standard (like it was for the A, but not B/C/C+). Regarding a track/balance being within limits, I've seen a C+ dispatched as serviceable with a 10inch spread in the tip path plane and shaking like a 206 with blade tape on only 1 blade. Doesn't mean it's right, or couldn't be improved with a little effort. So an unexplained new vib shouldn't be fobbed off as being normal only because it's within limits (not saying it is in your case, because it's obviously been investigated, I just make that point). And as far as your 4p lateral, my guess it's a soft blade damper giving you a 1p that makes the 4p feel worse than it is. But as per usual I claim not to know anything about anything and it's only my guess. But I'm interested to know what it gets traced down to.
Hi toolguy and gullibell,
IPS as high as 1,5 have been recorded, but intermittently. Engineers here are very well experienced and already have investigated blade dampers, VTA (rather the MTA...manual tune absorber) change, track and balance improvment.... but this vibe always come back.
IPS as high as 1,5 have been recorded, but intermittently. Engineers here are very well experienced and already have investigated blade dampers, VTA (rather the MTA...manual tune absorber) change, track and balance improvment.... but this vibe always come back.
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4p in S76
If you are seeing 4p 1.5 ips in straight and level on an intermittent basis, try measuring swashplate uniball for clamp up, may have wear on the outer races. Check Bifilar weights, pins and races for wear or grooves. If you are not having luck with rotor system, do not forget airframe and check horiz stab. for security, play, etc, have seen them loosen up over time with worn wedges. Seats and seat tracks for security. If no luck, cowls and floor panels for security, landing gear doors for looseness, and even landing gear for worn components by putting the aircraft on jacks and jiggling the gear to look for loose pivot and attachment hardware.
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cpt,
The change in 4P should be easy to measure, get involved with the maintenance guys when they rig the accelerometers. The pilot's seat track outboard vertical tells most of the story. The centerline vertical (center console) tells the rest.Most likely, it is the bifilar, which sometimes gets dirty and caked up.
The thing most pilots miss is that nothing about the blade track will make 4P, and nothing that you do to the track will make it go away. The 4P is a natural product of the rotor, and also comes from the main rotor wake hitting the horizontal tail (try 1/2 ball of left and then right sideslip at 145 knots to see how much the position of the tail relative to the wake affects 4P in the cockpit.)
The change in 4P should be easy to measure, get involved with the maintenance guys when they rig the accelerometers. The pilot's seat track outboard vertical tells most of the story. The centerline vertical (center console) tells the rest.Most likely, it is the bifilar, which sometimes gets dirty and caked up.
The thing most pilots miss is that nothing about the blade track will make 4P, and nothing that you do to the track will make it go away. The 4P is a natural product of the rotor, and also comes from the main rotor wake hitting the horizontal tail (try 1/2 ball of left and then right sideslip at 145 knots to see how much the position of the tail relative to the wake affects 4P in the cockpit.)
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Hi Nick. Not positive this is the same thing, but we've been experiencing a cyclical (in a time sense) vertical for 3 to 4 beats, every 5 seconds or so. Not always there, but enough to be annoying. Our maintenance folks say it isn't felt in the cabin at all. My thoughts are perhaps in the APs but they have been changed more than once for other reasons and the vibration still comes back. It's the inconsistency of it that puts me in a quandary.
As an aside, we just installed the second bifilar after 12 years. No VTA in the nose. Only real difference it makes with us is decelerating through translational lift on approach, but the difference is exceptional.
As an aside, we just installed the second bifilar after 12 years. No VTA in the nose. Only real difference it makes with us is decelerating through translational lift on approach, but the difference is exceptional.
hi Oldornet and Nick,
This 76 is now under a programmed maintenance, therefore we have stopped tracking this vibration for a while.
After having changed and retuned the MTA as well as removed, inspected and re-installed the biffilar, we have noted some improvement but the cycle of vibrations is still here.
In the past, we had solved a vibration problem almost similar to this one in changing the pilots hub (removed the old version to the new one with larger edges)
We were not expecting this improvement, in performing this operation but it worked. It is difficult still to find a relationship.
Of course we have tried to fly without DAFCS at all, just in case, but no change.
As you say Oldornet, the helicopter is serviceable but it is annoying....
This 76 is now under a programmed maintenance, therefore we have stopped tracking this vibration for a while.
After having changed and retuned the MTA as well as removed, inspected and re-installed the biffilar, we have noted some improvement but the cycle of vibrations is still here.
In the past, we had solved a vibration problem almost similar to this one in changing the pilots hub (removed the old version to the new one with larger edges)
We were not expecting this improvement, in performing this operation but it worked. It is difficult still to find a relationship.
Of course we have tried to fly without DAFCS at all, just in case, but no change.
As you say Oldornet, the helicopter is serviceable but it is annoying....
Vibs can be mysterious
You need to identify exactly what the frequency of the vibration you are feeling is. You may be able to do this by just holding an accelerometer where you feel the vibration the most and take multiple readings. From your description it sounds as if you have an intermittent harmonic or beat frequency. I have seen this in other models of Sikorsky’s. Some of the resultant causes have been, improperly seated rotor hub, improperly torqued main rotor hub, main gear box mounting bolt torque broken, cracked MGB mount. The oddest one was a bad thrust bearing in the main gear box of an H-53E. I lost a keg of beer to the engineer that ferreted that one out.
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oldhornet,
You described, "a cyclical (in a time sense) vertical for 3 to 4 beats, every 5 seconds or so."
That could be many things, and also by being intermittent, will drive the maintenance guys to try and smother you in your sleep, so I share your frustration.
If you turn all sas/AP's off, does it still happen? I ask that because there is a phenomena where sometimes the rate gyros can create a slight but perceptible low frequency vibration close to 4P that wiggles the cyclic and the rotor, and that the pilots can usually feel. Was fixed a while back when the vertical gyros were redesigned (a 4 bearing vice 3 bearing design, my deep memory wants to say) to eliminate a gyro output mode.
If it occurs when the sas/AP's are all off, forget that theory, and try this (which is close to Jack's explanation of a beat frequency): Sometimes one blade can have a bit more 3P than you'd like, and it can mix with the 4P to create an intermittent odd harmonic that does not show up on the vibe gear, except as a somewhat higher 3P level on the meter. This can many times be cured by tabbing that blade (get a Sikorsky rep to help). This is rare, but possible. It is the kind of vibe that is not awful, but just makes you want someone to identify it and fix it. If you made the 4P come down with bifilar maintenance, this type of beat frequency will also come down, but not go away.
PM me and I will get you in touch with Sikorsky's best vibe engineer, who can usually solve everything, even Santa's sled with a bent runner.
You described, "a cyclical (in a time sense) vertical for 3 to 4 beats, every 5 seconds or so."
That could be many things, and also by being intermittent, will drive the maintenance guys to try and smother you in your sleep, so I share your frustration.
If you turn all sas/AP's off, does it still happen? I ask that because there is a phenomena where sometimes the rate gyros can create a slight but perceptible low frequency vibration close to 4P that wiggles the cyclic and the rotor, and that the pilots can usually feel. Was fixed a while back when the vertical gyros were redesigned (a 4 bearing vice 3 bearing design, my deep memory wants to say) to eliminate a gyro output mode.
If it occurs when the sas/AP's are all off, forget that theory, and try this (which is close to Jack's explanation of a beat frequency): Sometimes one blade can have a bit more 3P than you'd like, and it can mix with the 4P to create an intermittent odd harmonic that does not show up on the vibe gear, except as a somewhat higher 3P level on the meter. This can many times be cured by tabbing that blade (get a Sikorsky rep to help). This is rare, but possible. It is the kind of vibe that is not awful, but just makes you want someone to identify it and fix it. If you made the 4P come down with bifilar maintenance, this type of beat frequency will also come down, but not go away.
PM me and I will get you in touch with Sikorsky's best vibe engineer, who can usually solve everything, even Santa's sled with a bent runner.
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Nick,
76C. Both TQ's fluctuate constantly and rapidly 5% at 107% Nr with no corresponding fuel flow, N1 or T5 movement. At minimum beep TQ fluctuates slowly through 25% once again with no movement in other parameters. Any ideas? Factory guys are in the shop investigating at the moment. Over the years this is the forth occasion and been solved in the past by changing main gearbox.
Thanks in advance - Brian
76C. Both TQ's fluctuate constantly and rapidly 5% at 107% Nr with no corresponding fuel flow, N1 or T5 movement. At minimum beep TQ fluctuates slowly through 25% once again with no movement in other parameters. Any ideas? Factory guys are in the shop investigating at the moment. Over the years this is the forth occasion and been solved in the past by changing main gearbox.
Thanks in advance - Brian
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Tq fluctuation
Four times is very strange especially if you have changed the box several times. If the only parameter that is changing, is Tq, with no yaw kicks (SAS/AFCS off) and no engine parameter change, look closely at the Torquemeter indicating system. Had a very similar problem with a S61, changed both engines, gearbox and still the same. Turned out to be wiring of the Tq through an aftermarket HUMS system. If you have IIDS, it will be tougher.
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toolguy, for info. The aircraft are IIDS and a direct reading TQ gauge shows nothing happening, so appears to be electrical as you say. Aircraft has been put into the experimental category while the factory people fit test gear and play/probe.
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Oh please do not change any gearbox for a torque indication - the only thing it can add to a torque problem is the Nr sensor, or a total GB massive malfunction - in which case, you would be writing this!
This surely sounds like an electrical problem, since the beginning, and I would bet it is a bad pin on a connector or bad shielding that is letting some stray signal into the torque indicating system.
To confirm: when you see the torque rising and falling, is there any change in the engine/gearbox noise? Any slight yaw motion? No consistent change in fuel flow and Ng/N1?
I would invite someone to take a roll of aluminum foil along, and wrap all the connectors with it, then duct tape them to keep the foil in place. If the wiggling goes away, then unwrap them in sequence, to see when it comes back. The factory guys know this stuff, they will get it.
This surely sounds like an electrical problem, since the beginning, and I would bet it is a bad pin on a connector or bad shielding that is letting some stray signal into the torque indicating system.
To confirm: when you see the torque rising and falling, is there any change in the engine/gearbox noise? Any slight yaw motion? No consistent change in fuel flow and Ng/N1?
I would invite someone to take a roll of aluminum foil along, and wrap all the connectors with it, then duct tape them to keep the foil in place. If the wiggling goes away, then unwrap them in sequence, to see when it comes back. The factory guys know this stuff, they will get it.
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Nick and Cpt
I have tried turning off the #1 and it does seem to go away. The vibe then becomes essentially steady but of much lower magnitude (just a slight, constant vertical). But changing the AP's doesn't have any effect,nor does swapping sides. (Nick you're right about the maintenance guys wanting to smother me lol). I will ask them, gently, about looking into the things you've mentioned.
As an aside, the #1 AP always always always produces a very obvious stick vibration. Very noticeable when doing the AP check after start. The cyclic just vibrates fore and aft about a 1/4 inch, kind of a buzz.
Nick, would have PMed but not as PC savvy as I thought I was.
As for the second bifilar, we were talked into just one of them 12 years or so ago when we specked out the machine. After installing it I'm sorry we didn't have it all along.
Thanks for your help.
I have tried turning off the #1 and it does seem to go away. The vibe then becomes essentially steady but of much lower magnitude (just a slight, constant vertical). But changing the AP's doesn't have any effect,nor does swapping sides. (Nick you're right about the maintenance guys wanting to smother me lol). I will ask them, gently, about looking into the things you've mentioned.
As an aside, the #1 AP always always always produces a very obvious stick vibration. Very noticeable when doing the AP check after start. The cyclic just vibrates fore and aft about a 1/4 inch, kind of a buzz.
Nick, would have PMed but not as PC savvy as I thought I was.
As for the second bifilar, we were talked into just one of them 12 years or so ago when we specked out the machine. After installing it I'm sorry we didn't have it all along.
Thanks for your help.
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oldhornet,
That is the key! I think so it is a bad gyro, usually in roll. Look at the channel indicator to see if the needle jumps, and look for that cyclic throb (oh, throb! throb!) which is what we call sas nibble, when the sas servo moves the stick instead of the main servo.
The AP check grabs the gyro while it is spinning up, and that makes the nibble far more noticeable.
Good luck.
BTW, just go to the left where the username is on any post by that guy, left click and pull down "send a private message to....." to get to the PM page. Look to the top right of any page where it says Privat Message to see if you have one waiting for you.
That is the key! I think so it is a bad gyro, usually in roll. Look at the channel indicator to see if the needle jumps, and look for that cyclic throb (oh, throb! throb!) which is what we call sas nibble, when the sas servo moves the stick instead of the main servo.
The AP check grabs the gyro while it is spinning up, and that makes the nibble far more noticeable.
Good luck.
BTW, just go to the left where the username is on any post by that guy, left click and pull down "send a private message to....." to get to the PM page. Look to the top right of any page where it says Privat Message to see if you have one waiting for you.
Hi Nick and all,
Is there one table/chart available that compares all the 76 variants up to D model like for like ?
I am not an operator but I need to compare the technical benefits of upgrading to C+ or C++ vs existing A++
Appreciate your help, have searched this thread without luck,
pp
Is there one table/chart available that compares all the 76 variants up to D model like for like ?
I am not an operator but I need to compare the technical benefits of upgrading to C+ or C++ vs existing A++
Appreciate your help, have searched this thread without luck,
pp
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peterperfect,
I don't know of one, I will send your request to the Sikorsky Chief Pilot, he can assign a pilot to make such a matrix - it is a great idea, and perhaps about time!
Nick
I don't know of one, I will send your request to the Sikorsky Chief Pilot, he can assign a pilot to make such a matrix - it is a great idea, and perhaps about time!
Nick