Diagnosing bad shake/wobble in 6HVU

[simplechamp]

So here is a little history:

One day I noticed a pretty minor skid wobble while hovering my 6HVU. Took it home, pulled the head apart, lubed dampers (lime KBDD) and everything else, put it back together. Took it to a fun fly, did 4 flights and everything was fine, no skid wobble. Then next time out took off, no skid wobble, did a couple moves, came back into a hover and it was wobbling/shaking quite violently to the point I thought it might come down. Brought it in as fast as I could and landed. Looked everything over, didn't notice anything obvious. Spooled it up again, spoolup was smooth, started hovering without any shake, but after 5-10seconds it started the violent shake again. Repeated this several times with same result.

Pulled the head apart again along with pulling apart the gears, mainshaft, one-way assembly, etc. Looking everything over it all seems to be in good shape. Pulled motor apart and all looks OK there. Didn't find any loose parts or screws while tearing it down. Haven't started in on the tail yet. I could be totally off base but it just seems to me like a head problem and not a tail problem.

Nothing has changed with the FBL settings and I haven't crashed or tipped over to cause any damage. Just looking for any troubleshooting tips or theories of what's causing it before I start throwing money at replacing parts. Sorry but I didn't get any video of it before tearing it down.

[quinn]

Did you check if the head block clamping bolts are tight? To check the feathering shaft, remove blades, pop the pitch links loose and turn both straight up so you can line up the 2 link ends, turn the spindle with an Allen wrench in one of the bolts. You will see the link ends go in and out of alignment as you turn the spindle if its bent at all. I've had one mysteriously get a slight bend causing a similar shake. Also make sure tracking is dead on.

What headspeeds are you running? Did you change them at all lately. Sometimes you will get a resonance at specific speeds that will go away from raising or lowering 20 rpm or so. I've had this problem before especially with new dampers if my tracking wasn't absolutely dead on. It was more forgiving after they wore in a little.

Could be none of these things, just a few ideas.

[simplechamp]

Thanks for the ideas quinn.

I have not changed the headspeeds since maiden, running at 2100 all the time. It could be resonance induced because it does spool up smooth then the shake kicks in after a short time.

As soon as I hit TH the shake goes away, does that help in diagnosing it?

[quinn]

Still hard to say. I would check the spindle like I mentioned.

[Scott Bowersox]

I would suggest to replace the gyro tape with fresh tape. Might not solve it but will limit that issue while you raise or lower the head speed.

[ARM_Coder]

I once had an intermittent wobble problem in my old 6HV that was caused by a small vertical play in the mast. Hitting TH made the wobble disappear at once.

Redoing the mast collar adjustment cured the problem.

[Scott Bowersox]

Any updates on this issue?

[simplechamp]

Sorry guys, no update yet. Haven't had much hobby time the past 2 weeks. 6HVU is still torn down, what little time I've had been flying Chronos and Warp.

Should have time to work on 6HVU this week. Going to put a new feathering shaft in, replace the gyro tape, and make sure the mast collar is set right.

Thanks to all for the suggestions.

[Ah Clem]

If it shudders towards the end of a flight, would take the gain settings on the cyclic down a bit.

Just something else to try.

[simplechamp]

Hey all,

Boy the time flies, didn't realize my 6HVU had been down for this long. Decided to put everything back together and fly one more time before replacing parts. Went out to Chesaning Fun Fly today and it still had a minor wobble at 2100RPM headspeed (what I usually fly at) but not violent like it was before. And then I changed it to 2200RPM and the wobble was totally gone. So it seems like it's only happening at the 2100RPM. Next time out I'm going to try 2000RPM as well and see what happens.

Does this help point to a cause? Vibe that's bad at that certain headspeed? Thanks guys

P.S. the shudder/wobble starts almost right after taking off

[Ah Clem]

Almost certainly too high of a gain on the cyclic.

Vibration will result in drift, not shaking.

[simplechamp]

Thanks Ah Clem

I will try reducing the cyclic gains and see what happens. The weird thing is that I haven't touched the cyclic gains since I started flying this heli and it only now started this behavior. Maybe things are starting to wear and get more play which is bringing it out.