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Heath Robinson isn't dead


alan_k
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Conditions were superb last night so I went for a fly around.

Did all the usual pre-flight checks, as always, everything AOK. I'm getting the feel of the new harness now so off on the first attempt.

As the conditions were so calm I did some low level flying near to where some friends of ours are over in France at the moment. After wafting about for a while and enjoying the view I thought I would get a view from higher up so went into a climb at full throttle. Something didn't seem quite right.

There was more vibration than just a two stroke motor revving away, the cage outer hoop was vibrating so I reduced the revs gradually to get a feel for the problem. It wasn't a huge amount of vibration, so I concluded the prop hadn't disintegrated, but it was more than normal. Reducing revs reduced the vibration, mabye no surprise but good all the same, plus there was a slight 'squeeky' noise that wasn't normal also.

I was heading back towards the field at this point so I went into 'emergency landout mode' incase things got rapidly worse. By controlling the revs and allowing myself to loose height very gradually I made it back to the field without anything sounding or getting any worse.

Once I had landed I had a feel of the propellor rotation and it felt a little notchy, so I thought it's probably the hub bearings on the way out. This morning I bought a couple of new bearings and stripped the prop hub down this afternoon. This is where Heath Robinson comes alive as the bearing puller I have wasn't quite the right size and shape for the job. So a bit of ingenuity from the junk box and here it is:

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Well it's definately got a smoother sound and feel to it, but.......

at full throttle and at a point lower in the rev range the exhaust expansion chamber is vibrating, which I hadn't really noticed before. It's all mounted on rubber bushes with flexible joints so I'd expect a resonance somewhere :?

Could it be the different conditions of temperature and air density :?: It has been quite warm over here.

Cheers,

Alan

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Well there's definately vibration there that shouldn't be :!::(

I flew this morning and was thinking there is something not right and trying to figure out what it could be.

I'd been going about 15 minutes and there was this muffled 'phuit' type of sound and a bit more vibration :?:

I immediately looked for somewhere to land but realised that I was still flying without being too shook up so turned towards the T/O field. I kept the revs fairly low and landed 10 minutes later back in the field. When I looked at the business end the drive belt was half on and half off the pulleys...oops :shock: .

Upon further investigation I found that the cranckcase had fractured:

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57336400d6e36_Ohdear!.jpg.f97d6068816c0e

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I've got the motor stripped down and learnt a bit about the way they are put together, thanks pete_b.

There is a fair bit of play in the big end bearing, big expense probably too :?

The small size of the crankshaft means that it is forced together with an interference fit, something that is beyond my skill and tool set to dismantle and reassemble accurately :(

I'm enquiring with FB to see if they can do it, otherwise it's a complete cranckshaft assembly replacement, a lot to replace when it's just the bearing that's at fault.

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Alan any motorcross shop will replace the crank bearings.they will have the v/blocks etc....The crank will need testing for true running as well on a micrometer.i would do that first....small end and big will need to be done as well/ good luck.....

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  • 2 weeks later...

Well now I'm really confused. :?

I took the engine to a motorbike mechanic and he said 'c'est normale'. There is a small side ways movement at the little end of the con rod but no play down the axis of the con rod.

I was quite relieved really as I woudn't have expected the big end to wear out after only 90ish hours, particularly in an engine that only revs to 6350 max.

So I've had the replacement 1/2 crankcase and bolts and gaskets and put it all back together. Hardest mechanical engineering I've ever done with the reassambly of the 'interference fit' parts but it runs.

Guess what, it's fine. Needs a slight adjustment to the timing as I've set it at 1mm instead of 2mm from the stop, but NO VIBRATION.

It's a bit clattery at 3600 revs (it's a 2 stroke) but at full revs it's smooth as a nut. Even Shirley thinks it sounds 'new'.

Where's the vibration gone, or more pertinent what was causing it before? :?:

I've found two very minor things during the stripdown and reassembly. There was a slight leak on the inlet flange, but it looked like a little oil was seeping out rather than any air being sucked in. Cured by tightening the bolts.

The second was that the two halves of the prop didn't weigh the same, only by the weight of a large cable tie layed half way down the blade when on the balancer. I've got round that with 2 extra nuts on the hub on the side of the lighter blade.

Still doesn't answer the conundrum :?

Looking like Sunday might be the earliest to have a test flight,

Cheers, Alan

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Did you replace any bearings?

The prop not being in balance would not be so noticeable when all the mounts were good but with the vibration from the prop not able to go any where this is (in my opinion) what caused the stress fractures in your crank case

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Did you replace any bearings?

The prop not being in balance would not be so noticeable when all the mounts were good but with the vibration from the prop not able to go any where this is (in my opinion) what caused the stress fractures in your crank case

I've replaced the prop hub bearings.

I would have thought exactly the same with the vibration causing the fractures.

I'l see how it goes now, I've reset the timing and all seems well :D

Cheers,

Alan

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  • 2 weeks later...

Hi Duncan,

I've had 5 flights now since the rebuild, 2 hours 40 mins altogether. I suppose I am going to be a bit more paranoid about vibration so I'm still thinking there is more than there used to be.

It's probably not the case though as I have the alternative harness to help cloud the mix.

I have noticed today though that the reduction pulleys were not lined up perfectly, now I have thought about the construction and stripped and rebuilt the reduction drive I can make the following conclusions:

Earlier in the life of the motor, when ground contact was an occasional occurence :oops: , the bending also distorted the central motor frame lower bar.

When I put packing behind the lower engine mount to make the engine more upright in the frame I should have removed or reduced it when I later did a large cage straightening exercise.

I believe the additional stress being thrown onto the upper crankcase (just below the top engine mount) along with the additional vibration was what caused the crankcase fracture.

The reduction drive mounting is an assembly of flat plate and square extrusion and over time has taken on a slight 'set' leading to the misalignment. I have now reversed the assembly and with the removal of the extra packing behind the lower engine mount the pulleys are now aligned correctly.

Last nights flight was a quick one over at the 'chillie and cheese' do at Wanafly, this morning was out of the question due to beer :D

Cheers,

Alan

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  • 10 months later...
Beer........? I thought everybody in France drank wine :lol:

A common mistake, mate. They do have beer but it's not made from hops, barley and all the other yummy things we associate with beer. Oh no. What they do is all gather round a horse's nether regions with their plastic bottles, etc., give the horse a large pail of water and await nature taking its usual course. Funny people, the Frenchies. Nelson had the right idea.

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Beer........? I thought everybody in France drank wine :lol:

A common mistake, mate. They do have beer but it's not made from hops, barley and all the other yummy things we associate with beer. Oh no. What they do is all gather round a horse's nether regions with their plastic bottles, etc., give the horse a large pail of water and await nature taking its usual course. Funny people, the Frenchies. Nelson had the right idea.

Yes very funny people :D

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Beer........? I thought everybody in France drank wine :lol:

A common mistake, mate. They do have beer but it's not made from hops, barley and all the other yummy things we associate with beer. Oh no. What they do is all gather round a horse's nether regions with their plastic bottles, etc., give the horse a large pail of water and await nature taking its usual course. Funny people, the Frenchies. Nelson had the right idea.

Let me enlighten you.

You must try 'farmhouse beer' biere de garde, I think the best is Jenlain, try that and then say french beer is p*ss water :?

Cheers,

Alan

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  • 8 months later...
I flew this morning and was thinking there is something not right and trying to figure out what it could be.

I'd been going about 15 minutes and there was this muffled 'phuit' type of sound and a bit more vibration :?:

I immediately looked for somewhere to land but realised that I was still flying without being too shook up so turned towards the T/O field.

AND?????

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