500D maintenence question

[fling-wing_1]

I have an urgent question regarding a maintenence issue on a 500D I maintain. I have queried MD regarding this and what I get is "gee never heard of that before, good luck and let us know when you find the problem." I'm about fed up with what passes for customer service at MD and intend to chew my way up the chain of command first thing Monday AM.
In the meantime can anybody make sense or offer suggestions on this problem:

Just spent a large amount of money installing an overhauled rotorhead. Previous to this the rotor system was extremely smooth and beautifully in track.

Once the new head was installed we proceeded to once again balance the head (.16ips) and track and balance the rotor (.04 lateral, blades within .15" spread all the way from hover to 120kts, vertical .25 hover to .35 at 120kts)

Now the problem we have is, starting at ground idle the stick develops a pronounced shudder (feels like a 5p) if moved from it's trimmed position. Once retrimmed to the new position it quells the shudder. At 80-90%NR it goes away. In a hover and forward flight it reduces but is still there. There is also a 1p bump in the stick especially at hover and forward flight. These may or may not be related. At this point I'm extremly reluctant to try PC links or tabs to drop the vertical as I suspect it may be related to the shudder since before the head was installed the vertical was almost negligible at this same track.

The droop stop plungers feel a little stiff (in my experience normal for being new) but not excessively. Anyway this should not be an issue in hover and forward flight as the blades should be off the droop stops. BTW MD's guess for what's going on is the droop stops are too stiff. My guess is there is a stiff bearing or something in the head (perhaps a feathering bearing?) but I haven't been able to find it. Based on what we had before this thing should be smooth as glass.

Anybody ever run into this before? Any ideas on what it could be?
:ugh:

[maxtork]

Hi Fling Wing

I'm not a 500 guy so take this with a grain of salt but could it possibly be something with the swashplate? I know it isn't part of the head but it is exposed while the head was being changed. I would guess it wouldn't be something specific to one blade (feathering bearing) as that would be more of a 1P vibe not a 5P like you describe. A swashplate bearing could give more of a shudder that may go away to a point when pitch is applied.

Just a thought from the admittedly uneducated on type. I know what it is like to troubleshoot with little help so I figured it couldn't hurt to offer a couple cents! Good luck!!

Max

[skidbiter2]

Not sure if it's the same thing you are getting, but had a new head installed once also, the cyclic had a bad shake in it after installation, and if it wasn't centered correctly it was real bad. Spoke to a buddy who had done a lot on 500's and he said check the scissor link, he said it should move freely and not have too much friction.
I think the engineer just had to add some shims to loosen it up, after this was done it was back to normal with no shake.
Not sure if this will help, but it's an easy check to do.

[nouseforaname]

only one person to speak to about the MD500 in this country and i've pm'd you his details.

[Hughes500]

Fling wing

There are no feathering bearings in the head, done by the strap pack. I presume you have balanced the head with no blades on ? Needs to be less than 0.1 ips.If so once the blades are on you will have to adjust the pitch links for the hover. tabs are only done in forward flight. Once the track is good then sweep the blades with the dampers to sort balance out. If the track keeps changing then you have a problem with the bushings in the scissior links. How new is the swashplate bearing, this has the same overhaul life as the head ? Droop stop rollers and plungers will make no difference. The only other thing to check is the uniball. If this is stuffed you will get a boom boom when you lower the lever from a cruise.
Bear in mind that you cannot relate the blades being in track and balance from old to new head as the heads will obviously be different.
I could always buy the d model off you and sort it !!!

[RVDT]

There ARE feathering bearings in the head. Sounds to me like the droop stop is set too high and you are touching the droop stops.
The droop is adjusted with spacer washers between the feathering bearing housing and the pitch housing. Procedure is in the MM.
Easy test - place one blade over the tailboom and on a 5 blader lift the 2 forward blades off the stops. See how much space there is between the blade tip and the boom. (Please don't pull down on the blade!!!!) In service from memory it should be roughly 30 cm. Too low and you run the risk of a tailboom strike, too high and you will pound the droop stops.
Personal preference is to NOT sweep the blades to balance. Use weights in the LL bolt like a C model.

[AirWon]

Sorry, can't let this one go.
If there are no feather bearings in the head then I wonder what the hell my mechanic just changed out last week.
That's got to be a little embarrassing. Might just want to change your name.....

[Hughes500]

Let me expand to save face ! The 500 head does not have feathering bearings in the sense of a " bearing " like the 300 has ie the bearing takes the load. the 500 has spigot bearings that allow the blade to feather, the load is taken by the TT straps. I still think your problem will be the uniball although if the spigots or rollers are stuffed then you will never track it

[Cyclic Hotline]

The MD 500 is the worst helicopter ever built.

That said, in response to your question.

Stick shake has nothing to do with your new rotor-head. The old rotor system was simply nicely balanced and this one currently is not.

Follow the entire process in the manual, to ensure you systematically address each of the parts of the system that could induce the vibe. The problem is created by balance across the rotor system (disc) and can only be eliminated by adjustment, or combined adjustment, of the pitch links, dampers, and tabs.

Couple of things:

1. Zero all your trim tabs and start from scratch.
2. Forget the track, go for smoothness.
3. Do not be afraid of starting from scratch.
4. You may have to move blades across the head.
5. You may have to move dampers across the head, in addition to sweeping them.
6. Do not try to reason this in a scientific method, or you will go completely insane - try random solutions.
7. Follow the combined adjustments in the book for tab, link and damper adjustments.

If all else fails - start again from scratch. It will work out.

The 500 is like this every time you have to track and balance. It uses the most bizarre method I have ever seen, which has never been replicated by anyone else - but it works.

Refer to the track and balance information in Chapter 66, as it is real info on how to succeed with this challenge.

What kind of tracking and balance gear are you using?

Did I mention how much I :mad: hate 500's!

I have vowed that I will never, ever operate one again or have anything to do with them again. So far I have been clean for 6 years, and I can guarantee that I will never fall off the wagon.:ooh:

[Scissorlink]

Have you tried spraying dry graphite (been awhile so you better check first if thats the right stuff) on the all 5 droop stoppers and giving the blades a good working up and down? Had a cyclic that would start shuddering at ground idle if in wrong spot, bit of lubrication fixed it right up

SL

[tbtstt]

Some good(ish) advice from Cyclic Hotline there, however I'd argue "The MD 500 is the worst helicopter ever built." and strongly argue "6. Do not try to reason this in a scientific method, or you will go completely insane - try random solutions."

I had my first attempt at balancing a 500 this year (having previously balanced two and three bladed aircraft) and the process was trickier, but by no means an unscientific one. The process of tracking, correction and consequence can be followed, though it can be a lot harder with a multi bladed head (as I found out!). It is absolutely true though that, if it all starts to go wrong, the easiest thing to do is is to start from scratch.

Hopefully nouseforaname has pm'd the name of the 500 man I'm thinking of to fling-wing_1!

[SASless]

Tracking and balancing a five bladed Hughes/MD rotor head can be a real soul searching exercise. Step one (not really highlighted in the maintenance manual) is to take a few minutes and check the flight control system from the cyclic and collective sticks clear to the coolie hat on top of the mast to ensure everything is tight, round (not elliptical holes in bellcranks), lubed, free, shimmed, aligned, and as they should be.

After all that is known to be in order.....waving some old chicken bones, hawk feathers and shaking dry handfuls of dusty dirt about the ramp while incanting ancient Medicine Man chants to the various Gods....is the second step.

Tracking and Balancing are step three....through about one hundred and three....before returning to step one to see what you overlooked....followed by a submitting more offers to the Great God of Rotor Tracking.

Other than that....tracking a Mad Dog (500) is easy as can be!

[Hughes500]

Anyone had problems with htc blades in the sense of problems with the bungee and overcentre bolt ?
Have had to put orange area trim tabs ( the never touch area ) to -1 degrees as recommended by MD, but no more. The lever still pulls pitch at high speed flight and is heavy in hover. Run out of bungee adjustment. Anyone taken blades past - 1 ?

[fling-wing_1]

Thanks to everyone who posted a reply. I'm slowly working through things. If the weather cooperates we should have a go tommorrow.

I was a bit peeved the other day and ranting a bit. It's mainly not so much about the current situation but really just the symptom of a lot of festering frustration with MD going back a couple years. That said there are some good people with the company and John Kerr and Chris deserve kudos for their help with offering solutions. Contrary to Cyclic's opinions I have great affinity for the bird and find it to be a great machine to work on especially in a field environment. Maybe one of these days I'll offer some polite constructive criticism relative to my frustration with the company. Till then I do wish them the best.

Hughes500: Yep had lots of problems with the o/c and bungee with the HTC blades. Someone on the line at MD told me last year that -2 deg (across the full tab) and nuetral o/c and bungee will give nuetral collective. I've tried it and it does work but the tabs don't seem to like the -2 and try to reflex a bit. An great mech in Panama (Policia {sp?}) got me using -1 (full tab length) and that seems to be the best compromise. It gets close on collective adjustment and gives enough down twist to the blades that you don't have to screw the p/c links all the way in to get auto rpm up. I've used that for a few years now and have been very happy. Fortunately I have 4 MD blades and only one HTC on this machine and everything is very predictable. I've had lots of problems with the HTC blades from chordwise CG issues to erratic track. An essential fix if using the HTC blades is to at least fair the inboard edge of the abrasion strip to the skin with filler or epoxy. That edge seems to generate some kind of aerodynamic disturbance (vortex?) that makes tracking a bear. If you've ever made a tab bend only to see another blade move in a weird way you know what I mean. I've seen guys fair the whole strip but I don't know how effective it is. One thing I do know is that taco style strip certainly robs a good deal of payload. If you ever get a chance to see an old MD blade notice how the abrasion strip is smoothly faired into the skin. I hear MD has their own blades in production flight test right now so happy days may be near again.

MaxTork: Swashplate is good, just had it out, inspected and lubed 100 hours ago and checked again before installing the head.

Skidbiter: Checked the scissors again and it's a little bit stiff. I've had them that stiff before and it didn't cause a problem but I'm going to try to loosen it up. Problem is, at the head there are no shims just a bushing. The only way to loosen it up is to back off the bolt which allows the bolt to turn a bit in the assembly. Can't say I'm wild about that. If anyone has any other suggestions please let me know.

Hughes500: Checked the head balance. It was good but I'm not too concerned about that as the whole disk (of which the head is a part) is balanced at the end. At this point the whole disk system is .04ips. I started with nominal dampers, -1 tabs and left the links as is since the old auto rpm was good. We've swappped blades, dampers and p/c links between ships before and the track and balance did not change except for a slight lateral difference so I'd have to disagree with your statement about how track and balance cannot be related from the old head to the new.

RVDT: I think you may be right about the droop stops being set too high. I did notice even in a hover that a couple blades would occasionally go out of track when the pilot moved the cyclic around to maintain hover. I never thought it could be so high as to have an impact in flight. I also always assumed the factory had a jig to set that at overhaul so I never bothered to check it at install. I'll have help in the morning to check where they are.

Cyclic: Good info. Actually I don't find T&B on the 500 that mysterious. It does make sense (well most of the :mad: time) Lateral is due to a mass imbalance across the disk and is corrected by sweep or weight. It should be done once all else is finished. Vertical is due to aerodynamic (lift) dissymetry across the disk. Track is not necessarily an indication of balanced lift. That's why it's sometimes necessary to move a blade out of track in order to reduce vertical. I did replace the mylar tape with stainless so I do suspect it may be causing a slight lift imbalance from what I had before so I may have to link one or two blades to reduce vertical but I guarantee it's not the source of the shudder. I'm using the MicroVibe. If I had the database option I'd post what I have for feedback :(

Scissorlink: Yep, doused it down with Tri-flow.

SASless: Checked all the linkages as it's just going through annual. Replaced the five lower and one upper p/c link bearings.

Anyway, thanks to everyone for your input. Results to follow... :}

[spinwing]

Have you checked the cyclic "Uni-loc" for correct operation etc??

[fling-wing_1]

Well seems I've finally licked the problem (at least to a manageable level!)

I'll be a monkey's uncle and wouldn't have believed it but after talking with a friend who's had the same problem in the last couple months with heads come back from MD after repair and overhaul I tried swapping blades around on the head. Bloodly hell if it didn't work. Swapping dampners didn't do a thing but it seems the blades just didn't like where they were on the head. The pilot reports that the stick has a different sweet spot as it's a bit more to the rear and left but it's no different than most of the 500's he's flown. I'd like to imagine I could eliminate it completely but at this point I give up. It's relatively smooth and vibe levels are below .1ips so I'll content myself with that. I would have thought that the pitch cases would all be machined to be identical but sure enough swapping blades and then P/C links showed they aren't even close! Seems I don't know nearly as much about the angry egg as I'd like to think!

Thanks to each and every one who provided suggestions. It's much appreciated!

Cheers to all and a safe and prosperious New Year!!!