R22 Vibration Levels & Cyclic Control

[TiPwEiGhT]

Am in the middle of a course of which I am running the same R22, have noticed over ther past 2-3hours on it that vibration levels have increased significantly.

Have noticed it through the floor and back of seat, as my students are doing the majority of the flying with little input from me on the controls. When I have flown, the controls appear normal with regards to vibration, however the cyclic tends to stay in the same position if you let go of it throughout the speed range, never met this in an R22, any ideas? (I am maybe worrying about nothing).

The vibrations tends to change significantly at approx 70KIAS, I have asked engineering to track and balance on the next maintenace flight in a few hours.

Any suggestions welcome.

TiP

[helimutt]

If you're in OZ, i'd buy a new one. :E



Are they high or low frequency vibes? Main Rotor, Tail, engine?
Cyclic staying in place can be something as simple as a student not undoing the cyclic friction knob.
Let us know what engineering find but I wouldn't neccesarily wait until next inspection. Get it done asap especially if the check is just about due.

[Graviman]

Just a thought, try cleaning bug splatter from the rotors. This may be increasing advancing blade turbulence or reducing retreating blade stall AOA (transition from laminar to turbulent flow). If no joy do the track and balance, and let us know what you find. If no cyclic, i would guess balance more than track - anything starting to wear like an elastomeric bush? Any chance of a bolt allowing some slipage?

[Alloa Akbar]

R,T & B is a pain on these things due to the nature of the threads on the pitch change links fella. Your men need to be delicate and precise, I suggest a locally made clip on pointer and graduated scale sticker wrapped around the PCL to help. My learned colleague below (3Top) describes the whole procedure perfectly

Also check balance the fan on the back of the engine, when the fan is out of balance it shakes like buggery through the floor and seat backs.

As a previous poster said, depends on the vib freq. One more thing, I have had problems with an R44 which turned out to be driveshaft originated due to incorrect set up of the rear drive shaft belt tensioner.

Any pedal buzz??

Hope that helps.

[3top]

MR vibration (low frequency) or high (TR or fan)?
T&B is no big deal if you get the pitchlink adjustment sorted:

Don't try to "measure the changes" with pitchlink rodend against pitchlink rodend.
The only change you make and register is the big self-locking nut on the pitchlink barrel. To do that you loosen one (generally the upper) rodend completely, adjust the barrel nut and re-tighten the rodend - hold the lower rodend so that it touches one end and tighten the upper one in the same position - touching the same side. This way both rodends have their max moving range and are always in the same position.

You mentione that vibration changes above 70 kts.
Does it the same in a hover and just gets stronger with speed or is it not there at all in a hover?

You get a rather strange shaky vibration when the friction in the MR head bushings (main bolt and/or coning hinge bolts) looses or tightens...
Part of this problem comes from mechanics touching the bushings and shims with their hands, introducing small amounts of grease which shows effect only after about 15 to 50 hrs.
Friction values generally stay putt for a long time if done correctly.
But of course everything wears.
Loss of friction does not mean that the bolts have to be loose! They be better be not!!

Do a friction test with 2 brooms, according to the manual - it might show right there!

3top:cool:

[rotorspin]

sounds like t&b

used to have them all the time over 70knots on the 22's and 44's

[TiPwEiGhT]

Bug track and balancing session tomorrow, will let you know the results. Thanks fot the replies so far, all very helpful.

TiP:E

[Peter-RB]

If your not the only pilot to fly, look out for overspeeded Main Rotor, it causes the pitch change bearings to stretch and then you find your self in very unfriendly areas, then again I am paranoid about this one because some jerk did it to me a few years back and never wrote it up in the Tech log, I was lucky to get back onto terra firma ( in one piece).

Peter R-B
Vfr

[Vertolot]

TIPwEIGhT,

If you are worried about the vibrations have an engineer to check up the track and balance of the M/R and the T/R, and also the balance of the fan. And if you are seriuously worried about the vibrations have the engineer to do it today.

If these actions doesn´t help have the engineers to check the "clearance" for the valves in the engine (stucked valve). We have had some times problems with valves and they might give you some indications of low frequency vibrations.

Also an overspeed can be the reason, usually indicated as vibrations thrue the cyclic due to the spindle bearings, also can be reqognised with cracks of the weldings at the fan of the engine.

But anyway, before the next flight: take your time and do a really good pre-flight, check that there is no cracks in the blades etc.:ok:

Cheers,

[TiPwEiGhT]

Other pilots do fly the aircraft and it does cross my mind what happens on flights ie, problems encountered, overspeeds etc.

Engineering is doing a detailed inspecton of the aircraft today before a flight with myself and one of the engineers.

MR and TR blades are OK as I am very particular about checking them before every flight and also cleaning if required. Engineering confirmed this also.

Keep you posted and thanks again for all your assistance.

TiP

[VfrpilotPB/2]

Tipweight,

The problem with the overspeed causing the spindle bearings to stretch or distort is that you cannot really see what has happened, it relies on the honesty of the Pilot or FI who was incharge at the time of the overspeed, the example I cited was caused when a young low hours pupil was allowed under supervison to enter an Auto rotation to power recovery, the young FI didnt keep a tight grip on what was happening and the pupil attained 116% to 120% overspeed on the main rotor, alas nothing was reported.

I came along to SFH with the same machine, I am renowned for being picky on my walkround pre-flight but dispite this even though I looked and touched the hub and spindle boxes nothing seemed untoward, the problem only became apparent at Transitional flight speed by this time it was nigh on impossible to direct the R22 anywhere where I wanted it to go, I could bore you with many more details but to say this occurance changed my way of trusting people is a slight understatement,

ASK THE ENGINEER TO LOOK!!

Peter R-B
Vfrpilotpb

[3top]

Well if the overspeed goes off the scale, there is some major work ahead (Worst case: overhaul)!!
A slight over speed may brinell the spindel bearings (rpm still ON the scale - rpm needle free to move. Once it is at the top stop you don't know where the rpm went!).
Maintenance would turn the spindels to move the brinelled area.
But as was mentioned it takes that the pilots who had the incindent to have the guts and report....

Otherwise, a experienced Robby mechanic may be able to find a rough spindel by disconnecting the pitchlink, raising the MR blade off the droop stop and move it pitchwise by hand. To go more sensitive he might have to empty the spindel bearing area and remove the boot.....

Good luck!

Curious what it was! (.... will have been?)

3top:cool:

[Graviman]

What was the verdict on the track and balance TiPwEiGhT?

[chcoffshore]

Go and turn your selection of lawn mowers into xmas trees and find some spare's!:p

[VeeAny]

The only time i've seen a cyclic not move when you let it go in an R22 was the one which had its blade almost split in half about 10 flying hours later.

The main rotor teeter friction was very obviously too high, you couldn't tilt the head when pushing it from below.

It had a very odd vibration in flight and when you lowered the lever in flight the cyclic would come back at you.

The Crack was a scary sight, I have some photos somewhere I'll see if I can find them.

GS

[TiPwEiGhT]

Track and balances on rotor and engine fan were well within limits which was a surprise!

As for the "steady" cyclic, well that has baffled most people, a full control linkage check has been done with no problems found.

As for balde cracks... I'll be having a good look tomorrow morning again!

Another pilot came forward the other day and reported similar occurrences, so there is much curiousity from both engineers and pilots.

Will keep you posted, am curious to see these picture of the rotor crack.

TiP

[K48]

Check for peeling main rotor weartape..? it lifts and settles back depending on angle of attack/load of blades... symptoms: vibration and slightly more MP required.. plus cyclic control anomalies. It may not be visible.... surely your engineers would have thought of this one but thought it worth mentioning for the idea-pot. It happened to me recently and kicked off a panic of instrument checks and possible eol fields. Is any tape missing?
Another thing is the recent directive on blade delamination near the tips/underside.. tap tap with finger nail..?

[TiPwEiGhT]

K48,
Blade tape is not allowed on the Robbo. Certainly none of our machines have them. I presume you mean blade tape (wear tape?).

[K48]

TipWght
I have not seen it on the R22 first hand. It was removed by the time I next flew the specific craft. To fill you in: For me it happened at a climb out from 500ft to 800ft and disappeared as I eased collective down and levelled out. I was about to make a precautionary landing but as there were no warnings and it had been a low rpm vibration I elected to continue to the field, albeit very watchful of all indicators. It didn't appear again but I relayed the incident and could find nothing myself. Later the lifting tape was found as the answer, found by the cfi who had an inkling from previous experience with the same and plastic flotsom etc.. I have flewn it since and it runs smoothly.
Not sure what BladeTape is (if that is different), perhaps I should not have used that term if it is applied to something else specific? It must be weartape? I have seen it on the 206... for leading edge wear protection etc.

[Graviman]

TiPwEiGhT,

If your engineers are equiped to do it, then you could fly with an accelerometer stuck to the fuselage. This could be inside the cockpit, near where you felt the vibration, to avoid any complications. The idea is to convert the signal into a frequency plot, to understand the cause. It works because engine, main rotor and tail rotor are geared to rotate at different frequencies.

You haven't mentioned tail rotor by the way - any constraints on flapping? Is drive shaft running out of balance?

For general vibration work, with variable RPM, a Campbell diagram is a great way to pinpoint the source of a vibration
http://www.diracdelta.co.uk/science/source/c/a/campbell%20diagram/image001.jpg

[TiPwEiGhT]

Grav,

You have just sprung a thought into my mind sir!

Could excessive wear in the TR elastomeric bearing (but within limits according to engineering) cause this strange vibrations?

I have asked them on several occassions about the condition of the bearing, which is starting to debond on the outer edge, but as I mentioned it is apparently within limits.

I'll try get a picture tomorrow of it.

Thanks.

[ericferret]

Brinneled pitch change bearings are difficult to detect with the blades installed. If nothing else is found and the problem persists I would whip the blades off and check the spindles by hand.

[Graviman]

TiPwEiGhT, like EricFerret says.

It makes sense that this could be the problem, since most others have been eliminated. You describe increased vibration in cruise, which suggests that when the tail rotor is asked to flap there is some non-linear behaviour in the tail rotor bearing. In flight it would be under load while moving, so could judder. Vibration would be almost at the low frequency end of audible - haven't got Prouty to hand so don't know TR RPM...

From ground vehicle design experience i know that elastomeric bearings fail by splitting the rubber. It takes a long time to start splitting, but once started does not take long to break up (from my experience that is not immediately catastrophic, but the bearing does start to float).

[bugdevheli]

Would it be reasonable to assume that if the machine die not have full and free cyclic movement whilst on the ground one would refuse to fly it!

[TiPwEiGhT]

The machine does have full and free control movement on both the ground and in flight. The cyclic simply remains very steady in flight and maintains a pretty good attitude... have I just discovered a stab system for the robbo?

[bugdevheli]

Graviman . Honest, not me guv, never mentioned delta 3 . Just got the impression from the first post that the cyclic stuck a bit and it sounded a bit dangerous. Heres an interesting question though. As the Robinson relies on a stack of bearings to take the centrifugal forces on the blades, does to load get distributed throughout the five (from memory) bearings or is it taken on one bearing. Bug

[Graviman]

Sorry Bug, dumb comment on my part (post now deleted). As well as friction off full and free check, i assume that track and balance would have picked up on any unexpected friction in the MR feathering bearings. Under load friction could be higher though.

TiPwEiGhT are you sure pitch bearings OK?

Not familiar enough with design details to answer question. Pics of main rotor:
http://www.b-domke.de/AviationImages/Rotorhead.html#Robinson
Couldn't find tail rotor images either.

[ericferret]

Track and balance is often ok.
Pitch bearing brinelling tends to come through as a "rough" feel in the controls.

The point graviman makes about flight loads is spot on.

What does not feel like much by hand is very much worse with a few tons of centifugal force thrown into the equation.

[vrtcld]

Hey guys, I just flew an acceptance flight in an R22 and am getting a very similar vibe and centerd cyclic. The cyclic seems to center itself in forward flight and in a hover almost like a trim. I've never experienced this before. Good range of motion before start up and nothing binds up or feels even remotely rough.

Also, at about 70-75knts I'm getting a good bit of stick shake. More than I'm used too. Almost perfectly smooth in a hover. Nice and smooth up to about 65knts and starts to shake, not bad but noticeable. At 70 it starts to get rough.

Little background. New -4 MRB, new tail blades, new engine mounts, new lower sheave, new belts, new longitudinal bungee.

Another thing was that in a hover the ship twitched to the left a couple times. The mech said that the lower plugs may be fowler since the engine was pickled for a while. Has new mags and carburetor.

Any help would be great, thanks!

D

[peely]

Get the engineer to check the T &B a few times in between a few flights up to 75 KIAS and ensure the same result is achieved without any adjustments.
I had an increasing vibration over a period of a week and as I was the only instructor flying the helicopter I could account for it not been oversped etc. Then on one flight half way through the student making a steep turn there was an unusual increase in vibration etc and the cyclic felt different. I made a precautionary landing and when I could not find an issue and having discussed with the engineer I flew back to base at 60 KIAS without incident. A full check was carried out and we concluded the T & B was within limits but decided to make it as smooth as possible so made an adjustment. After a few quick flights and small adjustments we found our selves making changes but with no improvement and the plot moving very randomly around the graph. After 4 moves the engineer said he thought he knew what the problem was - we checked the flapping hinge tension which whilst within limits did not feel right so we pulled the pins and sure enough the bushes were very stuffed. We replaced these, tracked it up and it has flown 600 trouble free hours since. The worn bush has clealry moved under load but the vibration was not felt until above 65 to 70 KIAS.

[Evil Twin]

Beat me to it Peely, nearly rattled my teeth out during the ferry flight up from the South Island that aircraft :hmm:

Was like a different aircraft after the fix.

[blackhand]

Unexplained vibration levels on r22 is cause to ground the machine until you know what the problem is. A senior pilot and instructor were killed in Australia from this very problem. Engineers had tried numerous times to balance the main rotor without disassembly, crack in retention bolt hole propagated and blade departed the machine.

[owen meaney]

Unexplained increase in vibration level on the R22 is cause to ground the machine.
Senior pilot and instructer were killed in Australia from this issue.
Engineers had tried unsuccessfully numerous times to balance the Main Rotor. Crack in blade retention bolt hole propagated until a blade departed the helicopter in flight
http://www.atsb.gov.au/publications/investigation_reports/2003/AAIR/pdf/aair200302820_001.pdf