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Strofio
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Joined: 08 May 2013
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PostPosted: Wed May 08, 2013 5:22 pm
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Hello everyone,

Recently I had a crash with my Belt CP (v1), that resulted to the usual damage: main gear was stripped, feathering shaft and flybar were bent, and the main rotor blades were destroyed. The problem is that after repairing the heli, I still have significant vibrations.

I changed all the broken parts, and I balanced both blades and flybar very accurately. I am always very precise when balancing the blades, as it is the main reason for vibrations on the rotor.

The assembly of the flybar was also done very precisely, using measuring gauge and electronic scales.

All the above said to underline that it is not possible that the blades, nor the flayer, cause the vibrations.

After careful examination, I found out than there was a very small crack on the head block housing, right where the housing of the main shaft meets the main part of the block.

After replacing the fractured head block with a new one, I was very surprised to find out that the heli was still vibrating like hell.

I then tore the hole system down, and I begun reassembling it step by step, part by part, and every time testing it out. I started by installing only the main gear and main shaft, and putting it into spin. Everything was perfect, as expected. No vibrations at all. No need to mention that during the hole process of rebuilding the rotor, the tail rotor belt was removed, to be sure that during the tests there wouldn't be any vibrations coming from the tail.

When I added the new headlock, and put the main shaft into spin, it resulted in a minor vibration. When I eyeballed it, during low speed spin, it was obvious that the top part of the head block, where the feathering shaft joint is housed, was not just spinning, but also moving in the sense of the horizontal plane.

The low amount of vibration produced was normal, due to the low spinning mass combined with the small eccentricity.

As I was adding more parts to the rotor, the vibration, as expected, was getting worse every time.

A careful examination of the new head block, lead to the conclusion that the block's eccentricity was due to the not-so-tight fit of the main shaft into the head block. It seamed like the inner diameter of the hole where the main shaft is inserted, was slightly larger than the diameter of the main shaft, resulting in a very small angular movement of the block, in accordance with the main shaft's axis.

I then decided to repair the old head block, by glueing the crack, using a very strong glue with no elasticity, because the old head block had a significantly tighter fit on the main shaft.

No luck with that either…

My conclusion is that I was very unlucky with the new head block, as it seems that I bought a faulty part. And it may be that when I tried to repair the old one, some very tiny part of the glue that entered the main shaft;w housing on the head block, causes the head block to fit on the main shaft with a slight angle.

I am thinking of buying a main rotor upgrade set, to avoid future problems with faulty - low quality - plastic parts, although I am not yet 100% sure that the head block is faulty, as I haven't yet bought a new part and repaired the helicopter.

Anyone had any similar experiences before?

Is it that common that those parts are faulty?

I have to note that I have already replaced all the parts of the rotor with new ones, except from the swash plate, that doesn't seem to have any problems at all.

Thank you for your time, and I'm looking forward to your responses.
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tombo242
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Joined: 04 Nov 2008
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Location: Santo Estêvão, East Algarve, Portugal. Now 82, but still feels 22.

PostPosted: Wed May 08, 2013 6:15 pm
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Hi Strofio,

Just a thought, you don't mention checking the mainshaft for bends in your detailed description. A slight bend here could affect the head in the same way regardless of the new rotor equipment.

I would have thought that the 'Jesus Bolt' would tend to lock the head to the shaft. I have noticed a slight clearance on my helis but the vibration you quote is not present.

One other thing to check could be the pitch of the tail belt and gears. All belts are not of the same pitch. e.g. Fitting a TRex tail and belt to a B400 will cause vibration unless you use the TRex drive gear to match belt and tail gear.

Tom.
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Strofio
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PostPosted: Wed May 08, 2013 6:32 pm
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Thanx for the reply Tombo,

You're right: I did forget to mention that although the main shaft looked to be ok, I replaced it, just to be sure.

About the "Jesus Bolt": Even if the "Jesus Bolt" does lock the head to the main shaft, if it is locked with a slight angle in respect to the shaft's axis, this will result in a significant vibration of the rotor, due to the added mass of the various components of the rotor head.

As for the tail system, I mentioned that prior to the testing I did, I removed the belt to be sure that the tail rotor would not add to the total vibration.
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Burgess
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Joined: 27 Mar 2012
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Location: Wales

PostPosted: Wed May 08, 2013 7:15 pm
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Hi Strofio,

I think you were just unlucky with that component, I once received a faulty Align plastic rotor housing was malformed, and Cheep clone CNC rotor heads can be worse.

Try shimming the gap with aluminium foil. Trial and error to determine the amount required to rap round the shaft to fill the gap just right.

If you're going to change to a CNC rotor, then have a look at this site their doing Belt CP V2 CNC parts at sale prices.
http://www.rcmods.co.uk

Burgess
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Strofio
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PostPosted: Thu May 09, 2013 5:01 am
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Thanx Burgess,

I was afraid that cheep CNC heads could be worse. Now I have to decide wether to spend money on a costly upgrade or to try my luck with a new plastiv part...
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