Shuddering Vibration In A 407?

[helidude]

Make sure to fill out an SDR for this issue. These are the kinds of things that should be in the Transport Canada data banks.

[twinstar_ca]

Make sure to fill out an SDR for this issue. These are the kinds of things that should be in the Transport Canada data banks.

 

good idea!! :up:

[fix_heli]

Okay...where to begin...

 

First of all, I am only going to touch on a couple of things and try to dodge the bullets that "could" start flying in this forum, Please remember I am just trying to help another engineer out....

 

It sounds like alot of your problems are in the balancing of the rotor.

I believe Bell only supports using the RADs system for balancing the 407, but the RADs can be both good and bad in making corrections. We balance our fair share of 407's and have a general practice that we use.

 

If the corner mounts are good on the machine, and I believe that they were just changed...if I'm not mistaken.... I would pull the Restraint mounts for the tranny and make sure that they are adjusted as per the manual...meaning one is set longer and one is set shorter....

 

After that....

 

I would remove all hub weight off the head....set the pitch links to nominal settings (see Maint Manual for numbers). Using the RADs and the "Initial" program and balance her, don't balance her separately for Idle and 35% TQ unless you can't get to 100% for the 35%TQ shot after resetting the links and weight.

NOTE: Do what the RADs tells you during this phase but keep the hub weight off...Finish Initial balancing with links and tabs and only put hub weight on at the end of the Initial......BUT...BUT...BUT....If it wants 300 grams, only give her 200.....The RADs seems to compensate for everything with hub weight...according to the co-efficient chart for the RADs the Hub weight has the most dramatic effect on the Rotor over Links or Tabs...

After your done the Initials, goto "Flight" mode and fly her.....Never take a reading above 130 Knots in the cold....leave the 140 shot alone....you can "Save & Exit" without taking that shot. It is proven that the blades become unstable in cold weather and can be seen with the RADSCOM program...If you take the 140 shot and a blade is unstable the RADs will record it and try to correct for it causing problems in the rest of the corrections....

After the flight let the RADs plot the corrections and then "Edit Defaults" and set the RADs for BEST 3 moves. Make adjustments based on the BEST 3 moves not the normal 10 move calculation.

Generally the Hover, 60 Knot, and Let Down vibrations are all tied together and are adjusted with hub weight. If you have to much hub weight on board you get all kinds of goofy things happening.

As far as intermittent vibes in the hover this could be part of the problem, If the machine doesn't have its FRAHM on board, the 407 is known to get buzzy with 4/Rev, generally when low on fuel or internal weight.

Elastomerics are extremely sensitive to cold and heat. We balance at least twice a year, spring and fall just for that problem alone. Hub Shift can also be a problem, A Tech Bulletin and now Main Man shows how to shim the ends of the cones and lockwire them to reduce movement. The RADSCOM program will show you hub shift. After you do the flight with the RADs, transfer the file from the RADs to a desktop PC with the RADSCOM program on it, do a BACKUP from the CADU and unpack it with the RADSCOM program. When you open the flight and look at the track it tells you precisely in mm's the height of the blades, if a blade that is flying high in one flight suddenly reverses on the chart and is low, you'll see it right away. Thats hub shift and you'll have to pull the head and re-set the center cones. Age of Elastomers is also a problem, the older they get the more sensitive they get, check the age of the head and the last time the elastomerics where changed.

 

Some people will agree, some will not, some will scratch there heads...I have included a picture of the RADSCOM program in use to show you what I am talking about. (hope i uploaded it okay)

Product Support Engineering can usually help out alot if your willing to call them, and I have to say they are a great bunch of folks....And if you get a chance to use a Bell Tech Rep named Tom Grover....That guy is like Yoda on the 407....

 

Hope it helps....

[fix_heli]

While driving home, I had a couple of other things to add to my previous post.

As some posted previously, yes the FRAHM could cause some weird vibrations. Bell has a fairly new 600 Hr inspection of the FRAHM. We have had our share of issues with the units, some of our birds fly with and without the FRAHM. Take the top lid off and look for broken springs, 2 of ours broke at the very end of the spring where it was difficult to see and then dug into the lower case and cause the weight to push up into the lid. Below the weight is a rubber "bumper" gasket for the weight to bump into and also a rubber gasket around the bearing inside the FRAHM. If the gaskets have come apart you could be getting metal on metal contact at certain times accounting for the vibration.

A flight with the RADs will also show you the 4/Rev's in the machine and they can be dampened by adding or subtracting weight from the FRAHM.

Additionally, if the machine is parked near a "Radiant Tube Heater" like the kind that are usually hung from the ceiling, it will cause problems. 407 blades are very touchy when it comes to heat. At the factory they are twisted and set. Should the blade be heated above approx 180 F, which is very easy to do near an overhead radiant heater, the blade's glue or composite matrix will soften and allow the blade to untwist...once that happens all your balancing work is out the window. That blade or blades with now fly in a completely different plane and the machine will be unbelievable rough.

Hope the second post helps too...

[Jet B]

Thanks very much for the wealth of information.

 

I'm just the weak link between the seat and the cyclic, but I know my engineer has been reading this thread and should have been able to understand all the tracking info.

 

As for the strange vibration, someone else on the forum passed on a similar experience to me and it turned out they had a chattering tailrotor servo in their case. That gives us something specific to look at which is nice.

 

I'm at home right now, but I know our 407 engineer does have a good working relationship with the Bell tech rep and he was planning on talking to him earlier this week.

 

Cheers,

Jet B

[helofixer]

A problem we had on our 407s were the old style universal bearings in the pitch change links at the upper or blade end. They have 4 needle grease fitting holes but when they are installed you can only get to two of them. On our 50 hour inspections we remove them from the aircraft, grease the crud out of them and feel them for hanging in their rotational axis. More often than not you will feel the bearing jam or be rough. Now on our 50 hour lubes we remove them, flip them over,grease all points till fresh grease pours out and reinstall. That way you wear the bearing more evenly and they dont seem to get ratchety feeling. The new part number universal bearing is supposed to be better, but we flip em and grease em like the old part number and seem to get longer life. It may not even be part of your problem, but it couldnt hurt looking at that.

[Jet B]

Hey guys, we solved the problem of the howling vibration.

 

It turned out to be a hydraulic servo. Don't ask me how a servo can start randomly howling like a banshee but it did.

 

If anyone ends up with a similar problem, the way to troubleshoot it is to just shut off your hydraulics when it starts howling. If it's the servo, then the vibration stops instantly with the hydraulics off.

 

I guess Bell has had other people with the same problem in the past. I don't know if it's a quirk with the 407 or just a freak occurence, but I have never heard a noise like that from a hydraulic system... And it wasn't the pump, just the servo itself. Thanks to the individual who sent me that information about your tail rotor servo making the same noise. It ended up being one of our upper deck cyclic servos doing it, but you got us on the right trail to tracking down the cause.

[DGP]

We we're looking for a vibration problem not a noise problem...if you had said ...have you ever heard this weird howling noise coming from somewhere then I am sure everyone that flys a 407 would have said...oh that noise...but glad to hear that it may be the servos as bell seems to think it is coming from the freewheeling unit...they changed out my buddy freewheel with about 300 hrs on the aircraft...don't know how many times that the weird noise problem gets talked about...if you want to hear some really weird sounds...change the hydraulic fluid and then run her up...until all the air is bled from the system you will get some real good noise happening from up top...joy

[Skidbiter]

We we're looking for a vibration problem not a noise problem...if you had said ...have you ever heard this weird howling noise coming from somewhere then I am sure everyone that flys a 407 would have said...oh that noise...but glad to hear that it may be the servos as bell seems to think it is coming from the freewheeling unit...they changed out my buddy freewheel with about 300 hrs on the aircraft...don't know how many times that the weird noise problem gets talked about...if you want to hear some really weird sounds...change the hydraulic fluid and then run her up...until all the air is bled from the system you will get some real good noise happening from up top...joy

 

 

 

Seems to be a common occurrence with the things, guess because the servos run on condition, they can do a fair few hours before getting any sort of over haul.

 

Yip, the change of hyd fluid noise is REAL comforting when you are new to the thing, and its your first start after a service!

[sod]

Okay...where to begin...

 

First of all, I am only going to touch on a couple of things and try to dodge the bullets that "could" start flying in this forum, Please remember I am just trying to help another engineer out....

 

It sounds like alot of your problems are in the balancing of the rotor.

I believe Bell only supports using the RADs system for balancing the 407, but the RADs can be both good and bad in making corrections. We balance our fair share of 407's and have a general practice that we use.

 

If the corner mounts are good on the machine, and I believe that they were just changed...if I'm not mistaken.... I would pull the Restraint mounts for the tranny and make sure that they are adjusted as per the manual...meaning one is set longer and one is set shorter....

 

After that....

 

I would remove all hub weight off the head....set the pitch links to nominal settings (see Maint Manual for numbers). Using the RADs and the "Initial" program and balance her, don't balance her separately for Idle and 35% TQ unless you can't get to 100% for the 35%TQ shot after resetting the links and weight.

NOTE: Do what the RADs tells you during this phase but keep the hub weight off...Finish Initial balancing with links and tabs and only put hub weight on at the end of the Initial......BUT...BUT...BUT....If it wants 300 grams, only give her 200.....The RADs seems to compensate for everything with hub weight...according to the co-efficient chart for the RADs the Hub weight has the most dramatic effect on the Rotor over Links or Tabs...

After your done the Initials, goto "Flight" mode and fly her.....Never take a reading above 130 Knots in the cold....leave the 140 shot alone....you can "Save & Exit" without taking that shot. It is proven that the blades become unstable in cold weather and can be seen with the RADSCOM program...If you take the 140 shot and a blade is unstable the RADs will record it and try to correct for it causing problems in the rest of the corrections....

After the flight let the RADs plot the corrections and then "Edit Defaults" and set the RADs for BEST 3 moves. Make adjustments based on the BEST 3 moves not the normal 10 move calculation.

Generally the Hover, 60 Knot, and Let Down vibrations are all tied together and are adjusted with hub weight. If you have to much hub weight on board you get all kinds of goofy things happening.

As far as intermittent vibes in the hover this could be part of the problem, If the machine doesn't have its FRAHM on board, the 407 is known to get buzzy with 4/Rev, generally when low on fuel or internal weight.

Elastomerics are extremely sensitive to cold and heat. We balance at least twice a year, spring and fall just for that problem alone. Hub Shift can also be a problem, A Tech Bulletin and now Main Man shows how to shim the ends of the cones and lockwire them to reduce movement. The RADSCOM program will show you hub shift. After you do the flight with the RADs, transfer the file from the RADs to a desktop PC with the RADSCOM program on it, do a BACKUP from the CADU and unpack it with the RADSCOM program. When you open the flight and look at the track it tells you precisely in mm's the height of the blades, if a blade that is flying high in one flight suddenly reverses on the chart and is low, you'll see it right away. Thats hub shift and you'll have to pull the head and re-set the center cones. Age of Elastomers is also a problem, the older they get the more sensitive they get, check the age of the head and the last time the elastomerics where changed.

 

Some people will agree, some will not, some will scratch there heads...I have included a picture of the RADSCOM program in use to show you what I am talking about. (hope i uploaded it okay)

Product Support Engineering can usually help out alot if your willing to call them, and I have to say they are a great bunch of folks....And if you get a chance to use a Bell Tech Rep named Tom Grover....That guy is like Yoda on the 407....

 

Hope it helps....

Just wanted to add a comment in regards to the transmission restraint info that was offered. I was told by bell that they are there only to maintain transmission - driveshaft alignment and that is why they are different lengths?