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S300 blade track


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#1..have lots of time available where labor cost is not a concern..basically don't rush it.

#2...optimal twist compatability on all blades

#3...good static balance on blades (when were they last at a blade shop?)

#4...are all reflective targets center points the same distance from each blade.?

#5... be picky about each step before proceeding to the next.

#6...when was the last time the dampers were pull tested and how do those numbers compare to each other or to the max limit? (this is usually most balance issues with the 300 besides subtle blade differences)

#7...balance the tail rotor with the main blades off the helicopter b4 you start the main rotor (if able)

#8... remember the factory sometimes has trouble getting things just right with a set and then they try another set!

 

oh yeah forgot equipment... I use the old chadwick 135 and strobex.. too cheap to buy something new.

Edited by apiaguy
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IMO…

 

Use common sense. If you find yourself bending this and turning that for hours on end (treading water), then it’s time to reset to published nominal and go from there. Additionally, insure your equipment is set-up and functioning properly with the correct paperwork.

 

Lastly and ultra-most importantly, never start the job on Friday afternoon…….

Edited by Spike
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Thanks apiaguy, I will try all of those things. Spike, this is my weekend job so I HAVE to start on a Friday afternoon ;P Tracking chinooks with high tech camera equipment can be a nightmare, so I am just SOO excited to try to track a 300C with low tech equipment... Sure would be nice to find someone who's done it all before in my area and can show me the ropes. Maybe I can convince the lessor to help out, I think he's up here from Vegas for an annual on another ship.

 

Another question, is it common to get a moderate low frequency vibration at 2000 RPM? I have been doing my warmup/cool down idles at 2100 to avoid this. Two different ships I have flown recently that have this issue.

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low freq vib at 2000.... guess what the natural bending freq. for the tail rotor drive shaft is...300 rpm (aka 2050 engine rpm)... also because we set them up to be smooth at 3100 or 3200 we don't really care how much they shake at 2000 (as long as it doesn't progress)

 

2nd thing is strut charge... are they well maintained on charge and equal pressure on each side?

 

FYI...The A model warm up was 1850 and I always liked that lower rpm and lack of vibes better than the B and C model 2000 warm up... the early tachs had a resonance range from 2050 to 2400 (rpm engine) to avoid.

 

I'm totally for setting warm-up/cool-down at a smooth rpm...rather than the book posted.

 

would also mention that if you seem to have vibration issues all around.. these are keys to potential other problems! Maybe you have a bad damper.. or a bad oleo.

 

 

Sikorsky Aircraft Corporation

6900 Main Street - P.O. Box 9729 Stratford, Connecticut 06615-9129 (203) 386-4000 Sikorsky Safety Advisory

 

Document Number: SSA-269-12-0001

 

Date: February 22, 2012

 

Revision (date): Original

 

System: Landing gear

 

Affected Aircraft: 269A thru C versions including TH-55

 

Subject: Ground Resonance events

  1. 1. Introduction

 

There have been occasional occurrences of ground resonance in the subject aircraft. The common factor for all ground resonance events is the inability of landing gear dampers to dampen (attenuate) the rocking motion of the airframe. Airframe rocking can often be precipitated by a MR imbalance, which may be caused by out of phase MR blades or can also be caused by an initial "sharp" or pronounced ground contact on one skid, caused by a rough touchdown that sends the initial vibration back through the airframe and into the rotor system. The FAA Rotorcraft Flying Handbook (FAA-H-8083-21) also has an explanation of ground resonance.

 

There are two designs for landing gear dampers; the 269A, TH55 and 269B aircraft utilize the 269A3110 early series dampers. The 269C and 269C-1 utilize the later 269A3150 series dampers.

 

The landing gear dampers act to slow the airframe rocking motion by forcing fluid through internal orifices during compression and extension. Incorrect fluid level or pressures in these dampers can change the damping rate and consequently the capability to attenuate the airframe rocking.

 

WARNING

 

Ground Resonance can occur in the 269 series aircraft when landing gear dampers do not meet design specifications.

 

Always maintain the aircraft in accordance with Current Maintenance Manuals for the particular model

 

Ground Resonance can be a destructive phenomenon and can destroy an aircraft, cause injuries to personnel and/or fire resulting from fuel spills.

 

SSA-269-12-0001 Page 2

  1. 2. Corrective Action

 

PILOTS:

 

Prior to flight:

 

Read and understand all WARNINGS in the Pilots Flight Manuals .

 

Perform the required preflight inspections and have discrepancies corrected by maintenance staff.

 

During flight / landing:

 

If a rapidly increasing ground rock condition is experienced at or near normal MR operating RPM, pickup the helicopter to a hover and let vibrations subside. Attempt a landing on a different surface (grass/sod) if possible. Pay attention to ground rock during touchdown. If a landing with power cannot be safely achieved, a hovering autorotation may be performed. After touchdown, DO NOT increase the MR RPM to flight range. Have maintenance performed on dampers prior to next flight.

 

TECHNICIANS:

 

Read and understand all WARNINGS in the flight manuals and maintenance manuals (Handbook of Maintenance Instructions).

 

During periodic inspections:

 

Check the stance of the aircraft and measure damper extension in accordance with manuals.

 

During maintenance or overhaul:

 

Remove discrepant dampers and perform servicing in accordance with proper HMI section.

 

Use the correct HMI section and correct paragraph for the type and part number of dampers being serviced.

 

For additional information or questions, contact your Sikorsky Commercial Product Support Manager, or contact the Customer Service Engineering

Edited by apiaguy
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Oleos are well maintained, charged and balanced. I haven't done squat to the rotor system yet so I don't know how the dampers are doing. I do know that last year, when I flew this aircraft for Airwork LLC (WA), it was tracked like a top, one of the best tracks I had seen on a 300. The aircraft went back to NV for 6+ months before we leased it. Now I can actually see a split. Don't know if blades got changed or if a damper is out. I'm looking into it. it also feels rough in flight. A lot of cyclic movement.

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sounds like dampers are out of phase. Maybe just needs a tweak... but if it continues to deteriorate or won't hold a good balance I'm 99% sure you may have one or more dampers that are near limit. The stupid things are good for 6000 hours but very few seem to make it... and Sikorsky won't let lord rebuild them like they will on the 500... very frustrating... I have a box of them that all have thousands of hours remaining but are out of limits.

I can get alot of ships to register 0.2 ips lateral and vertical but the cyclic will still shake and it is annoying... it is always a damper that is pushing 0.048.

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Spike, this is my weekend job so I HAVE to start on a Friday afternoon.

 

The gist of this statement derives from, T&B is usually the last task completed after an inspection. If the inspection has taken more time than originally estimated, then the pressure is on to complete the task ASAF’nP. Therefore, quite often what you’ll get is; a T&B completed in the afternoon of the last workday of the workweek. Instead of working the vibes down to the lowest possible ips, the mechanic will usually just get it within limits and be done with it, cuz beer-thirty and the subsequent weekend is fast approaching. As a mechanic even I have done this all the while knowing, after a few flights and the machine has settled down, that pesky vibe is right back again……

Edited by Spike
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  • 4 weeks later...

How are you trying static balance ?

Are the blades the same weight, we asked 3 repair facility's the weight of master blades, they came up with 3 different figures, so if blades sent out at different times what hope.

 

Have you made tool for checking damper on machine ? only a check but can save hours of head scratching

The pull test is a pain we find MOST dampers will fail, had a new set 18 months ago not one of the 3 would pass, Ser No consecutive.

It seems to be a test for separation rather than flyability.

Phasing dampers does help but will in our opinion radically shorten damper life, the problem is blade weights.

You can get rid of the run up \ down vibes.

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500E brings up a great point about blade static weights... I have a set of blades on one helicopter right now that just had new abrasion strips put on... we put them on the helicopter and tracked it... we could phase it and make it smooth (under .2)...but during run up and down it shook like crazy...

WHY?

because the blade weight on each blade was slightly off causing us to "phase" the blades to extremes where the disc will fly smoothly in flight but when on the ground the odd angles between blades (one span greater than 120 degrees and two blades much closer together) caused it to vibrate and oscilate on the gear to an annoying state.

If I spread the phasing in one direction the ground vibs get better but in flight gets worse... go in the other and ground res increases but in flight is smooth... problem.... blade weight. Start adding tip weights or take the set off and reweigh them.

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What do you mean when you say track?

The track is the flight of the blades in the same path... it is set by the pitch change links at the rotor head... it is quantified by shooting a strobe light at the tip plane path and having a different shaped target on each blade tip to identify each blade and align them within 1/2 inch during flat pitch... hover.. and a number of forward flight maneuvers...

so we discuss track vibration usually as a vertical hop.. a spread in the targets... or a combination.

lateral vibration is usually associated with balance or "phasing" where we adjust the lead / lag "phase" of the m/r blades to minimize the shake in the helicopter or oscilating associated.

 

So will adding tip weight affect blade track? Not usually for the minimal amounts of weight we're talking ... but always check track after an adjustment.

Edited by apiaguy
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Blade tip weights will adjust phasing

Pitch links will adjust blade track

Trim tabs will fine tune tracking in forward flight

 

If you think about those blades flying around... a heavy blade will require the other two to be drawn closer to each other to offset the extra weight of the single blade so it doesn't cause a vibration..

Edited by apiaguy
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  • 3 months later...

Phasing is the quickest way to screw dampers, the blade weight is the important thing, get blades within 2 grams tip 3\4 root & they will fly out of box at 0.2 with Good dampers, & everything set to datum.

Make your self a tool to check dampers on machine Just a bar with 2 pins that fit the damper bolt heads measurement in HMI let the rotors stop, (none of that hand friction on tail shaft) if the tool & bolts don't align in 5 minutes dampers may not be shot but bet the blades have been phased.

We have found new dampers balanced blades no vibes on run up, or rod end problems = smooth flights staring Chadwick.

Balancing just the tip don't work

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apiguy

 

Me to but make a jig check tip weights 2 gram difference max, check root weight max4 grams max on the set.

This will get rid of run up vibes & do away with phasing, if dampers are good, & we are finding appreciable savings in flight control hardware, + damper life

Email & will send you pic of jig

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track has a big influence on balance, balance has a small effect on track. Never try to make a track adjustment unless your IPS is about 0.5 or less. I always preferred tip weights rather than sweeping blades, sometimes to get rid of stick shake tho, sweeping helps. 300 is a tempermental ship, but can be made to fly really smooth, and it will usually stay smooth for many hundreds of hours once you finally get it right. I actually prefer the old friction dampers--you have no option to sweep there.

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500E...are you just talking about weighing the tip weights and root weights of the individual blades or weighing the tips and roots of the entire blade with weights installed?....we made our own weighing fixture to weigh the entire blade...but sometimes still requires a slight weight adjustment when balancing on the heli to get just right.

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Sent you emails

We check all blades for tip & root weights the set should balance within mentioned limits or closer preferred.

The overall weight is a red herring, you can have blade weights the same, but position of weight along length can be wildly different.

It is the spread of weight along blade that is important.

How do you find de-lamination of erosion strips ? it appears to be a problem in UK, we have been doing a voltage check between strip and alloy of blade the worst reading was 0.45volts the average appears to be 30mV not good this would suggest that there is corrosion working even when tap test does not show problem

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we made our own weighing fixture to weigh the entire blade...

 

i would like to see or hear of this fixture,,, does it find the CG of the blade? scales on each end? You have pictures? How exactly does it work?

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oh your gonna love this...basically upsized it, used precision scales, and compared some "other guys" balanced blades to find our "master"

http://www.indyhelis.com/documents/koll.pdf

 

On the delam issue... just tapping... I'm not convinced the voltage test would be worthwhile as varying amounts of epoxy holding the strip may give different signals.

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Above 100mV potential differences or so, you will start to see some ionic migration, its is always there from any metal that at the surface is slightly soluble in an electrolyte at pico-amp levels but when you apply an electric field potential or drift field from the different surface voltages in an electrolyte that supports diffusion you start to get exponential transfer of species.

There is a better more robust way to test but this is simple\quick.

My thought is it gives you a start as to what is happening & it cant be good, the epoxy should be an infinite resistance (in theory) so you basically have a battery using the dissimilar metals & salt water as an electrolyte.

Like the thinking regarding balancer, we have been using ours for 2 years, have another version being built at present.

Some interesting reading.

 

http://www.rwas.com.au/rtb.html

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oh your gonna love this..

 

this is even easier, all ya need is a sawhorse and a 1x1x1/8 alum angle about 12 inches long. Mill or file a knife edge on the angle, screw it down on the horse and balance the blade on it @ 45 degree angles, and draw a line with a pencil at both 45 degrees,, the crossing of the lines will not only give you the blade spanwise balance, but also its cordwise and its exact cg. Unfortunately, it will not weigh the blade---just match them an make sure that the cg is on the cp

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