Tail Failure

[Stuart]

Not the tube drive (with some people having problems with TT I though it would be wise to take it out and have a look...100% fine solid and locked in)...however, I've had this tail twitch for a few week's and I could see that the tail blades weren't tracking properly.

I found an issue that mentioned that on some early models the tail thrust bearing would catch the outer of the grip and stop them running true.

Tried the fix to see if it worked.

Well the tail flutter was still there and then the tail blade and grip came off today while FF, heli made a thud noise and heard the crown gear getting chewed up. The other tail blade lost its arm, so it hard to determine what happened first. By this time I had flicked TH. Auto'd from about 30ft and landed fine.

Damage = tail blade grips, pitch slider missing an arm, crown gear.

Feather, main mast all else are fine.

The only other piece of excitment for the Synergy was to lose a paddle while in hover. Quite cool to see a black blur flying off, the heli shook and it landed fine and I found the paddle.

[AndersUK]

Damn, missed that - left the field early. Glad you got it down in one piece

[BobJ]

Hi Stuart

What let go, I presume the bolt backed out and did not shreaded off?

Cheers
Bob

[G-DAVE]

Damn, missed that - left the field early. Glad you got it down in one piece

I saw it -- you should never leave early -- He did a good job getting it down in one piece .
Dave

[Stuart]

Hi Bob,

This is what I think happened.

Bolt back out, blade grip with blade shot forward hit the header tank which was knocked out of its mounting then the blade fell to the ground, leaving the header tank hanging on.

With the loss of the blade caused stress on the other tail blade arm which broke off.

The piro tried to start but the crown stripped, so it locked up, with the TH being hit quick I kept a half decent headspeed, allowing a small auto possible.

Looking through Skylines site for parts....

[BobJ]

Hi

So if the bolt backed out it sounds like not enough locktite then.
It is just possible if you had one of the bad thrust bearings that this caused a lot of drag and so worked the bolt out in time.

Cheers
Bob

[Steve]

Must be a day for it - we had two very near misses with tail up the field this afternoon.

One chap had both the bolts come almost all of the way undone out of the ends of the tail yoke (is that the right word - I mean the bit that fixes across the end of the tail shaft and carries the blade holders) on his Trex 600. He found the problem when he was having problems with the tail control.

Another chap had the same part almost sheer off on his Millenium 2. He's had that Millennium (70 sized with 90 blades) for at least 5 years now and never crashed it - logged over 400 flights with it now. He discovered the problem when he was cleaning it down at home - just rang me to see if I have a spare.

It's a testament to the Millennium though - as far as I know, that's the first thing that's ever failed on his one, and fortunately it didn't go in flight.

[Ade_Law]

the grip bolts backing out does seem to be fairly common. i think trev's have backed out 3 times now.

i think this is due to the bearing spacing not being wide enough and the bearings locking under load, this then cracks off the loctite.

millis are good machines, just dont cut the mustard for modern 3d :-(

Ade

[BobJ]

Hi Steve

That part is called the "tail hub" the one on the Millennium/Nova and Futura SE is a very good design, in all the time I flew them I never had one fail.
The Synergy one is a scaled up version of this, JK did a lot of tests on all the major models hubs and found the Robbe one to be the best by far so just beefed it up even more so it's a part which is very unlikely to fail.

Cheers
Bob J

[Steve]

I hear what you're saying - might be just a symptom of another problem. However, I've known this heli since new, and the pilot for nearly 25 years, and the machine is maticulously maintained and cared for, and always flow very smoothly - no 3D, not his style. The heli is 'sweet as'. It was routine maintenance that discovered the failure. I rekon today was his lucky day.

[BobJ]

Hi Steve

I was not trying to point blame at any one just saying it's a strong design so not likely to fail with out a reason, it could just be old age or more likely crash or hanger rash damage.
I have certainly bent the Robbe ones by walking the machine in to a door frame:-)

Cheers
Bob

[Steve]

Doh!

I think most people will have done something similar at some point!

[Steve]

I've just seen the failed Millennium part. It's not the tail hub, it's one of the M2.5 x 8mm bolts that go in the ends that hold the blade grips and bearings on that's kanckered. The bolt has broken off just behind the head, leaving a thread-locked stud inside the tail hub. Hence his call to me asking if I had a spare tail hub and a bolt.

[Ade_Law]

overtightening can cause this.

Ade

[Steve]

Absolutely Ade, but going back a few posts - this particular heli has flown in excess of 400 flights over a 5 year period, and the builder/owner is by far and away the most experienced and maticulous RC modeller that I know.

But, as has been suggested, it might have been nudged being carried through a doorway, or maybe caught the ground while spinning - Millennium tail blades do tend to sit very low to the ground when you haven't got the metal skid attached to the vertical fin, and the NHP tails that he uses hardly rotate at all in their holders, so are quite rigid in their set-up, regardless of how loose or tight the blade bolt is.

Oh well, no harm done. Routine maintenance picked it up, and the heli lives on unscathed.