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Olas

AS350B3 strange vibration

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Hi! One of our Astars has a weird vibration we cant figure out.

Problem: When going from flght idle to ground idle at about 290NR it starts shaking pretty bad up front. The AMM doesnt care about checking vibes at this RPM but our pilots are worried and we cant find anything wrong.

were using the microvibe II. We've tested the track and balance and vibe absorbers under the cabin, checked the tail shafts for runout. Everything checks good but at 290NR we get a .626 ips at the lateral accelerometer at the mast. at 100%NR its about .2 ips wich is passing.

Anyone else had an issue like this?

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Sounds like the anti vibe in the head.  Most likely the bearing is shot, i've also seen damaged springs.  You'll have to remove and disassemble it to really see the problem. 

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Ya, we still need to take that top antivibe off. the TR seems to be all good. were about to throw a scope into the trans to look for messed up teeth

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Is this only on deceleration to ground idle? Is it consistent, and did this just start happening.

I've noticed this before with a tail wind, but imagine you would have checked that already.

 

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Let me start by saying I do not have much wrenching time on Astars but I have flown them. I once went for a ride in a 350B model with our chief pilot as he said he thought it was getting rough. So he fires it up and it felt okay at flight idle and at 100 % nr for take off.We get it into flight and I told him to take it slow with the airspeed  increases so I could see when it got rough.Well when we hit around 80 knots it got crazy bad real fast.I told him to keep it under 80 or slower and get it back to the hangar which took only minutes. They took the blades off and were checking the big spherical bearings on the ends of the star...bear with me as I am not very familiar with the head on the Astar. Anyways...you could almost pull the inner race of the bearing right through the outer race on all 3 bearings on the ends of the star...this probably is not your fix but have a look!

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Was the tail rotor recently installed?  If so try loosing and retightening the attachment bolt.  It can load the hinge bearings up. 

If tail rotor was not recently installed check the laminated bearings.

 

I have had both cause a big shutter through the airframe around 270-320rpm. It would be smooth at idle and at 100%. 

I think if you had messed up teeth in the MGB, I would hope you would have had a chip light

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I don't think a tailrotor imbalance would give you such a high lateral vibration.  But that cross bolt does like to do weird things.  Start with easy things first.  Check trim tabs for proper setting and track too.  Get that lateral vibe down lower too.  

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The only times I've experienced a strange transient vibe at the RPMs you're talking about (would be so bad we couldn't read the gauges momentarily), it was the T/R, specifically the teeter bolt. Flipped it around , re-balanced the T/R, and was good after that. Someone explained it as being a harmonizing of various vibrations in the airframe at that RPM. Don't know if it was a BS explanation of something real.

If that isn't the problem, my next thought would be to re-set all M/R balance, rigging, and trim tabs to nominal and re-balance, or as mentioned, the Anti-Vibe

Would be interesting to hear about the end results in this case...

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