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MTBF ............ fatigue and time to change ?


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solentlife
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Joined: 30 Dec 2010
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Location: Latvia / UK

PostPosted: Sun Jun 26, 2011 6:48 pm
PostPost subject: MTBF ............ fatigue and time to change ?
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Well ... seems that numerous mishaps, hard landings, crashes etc. are starting to catch up with my 450.

I am getting stupid failures of ball links, threads etc. ... ie flybar control arm ball link breaking of .... main rotor holder ball link thread stripped ..... odd plastic links unclipping of balls ......

Most of course happen when it's in the air and not much you can do to save the heli !

I've now changed whole head for new and will replace near all on old to renovate.

Anyone else had similar ?? I put it down to fatigue / stress and in time it fails.

Considering the loads and stress that must prevail on such machines ... is it any wonder ?
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tombo242
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Location: Santo Estêvão, East Algarve, Portugal. Now 82, but still feels 22.

PostPosted: Sun Jun 26, 2011 6:51 pm
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Agreed there Solentlife,

The way some helis are thrown about, it's amazing that they last the flight.

Tom.
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admiral
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Location: Brisbane, Australia

PostPosted: Mon Jun 27, 2011 3:52 am
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Hi Solentlife,

As some of Helicopters my get older I have found that weekly checks and preflights are becoming more important, it's amazing how often you find a loose screws or bolts, missing nuts, cracked ball links.

I think it's just the nature of the beast, a complex piece of equipment with rapid moving parts needs regular inspection like their full size counterparts.
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platinum
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PostPosted: Mon Jun 27, 2011 5:05 am
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admiral wrote:

I think it's just the nature of the beast, a complex piece of equipment with rapid moving parts needs regular inspection like their full size counterparts.


I agree with ...admiral.
My fellow 450 the blades and grips flown off inflight. Lucky no body got hurt.

platinum
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solentlife
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PostPosted: Mon Jun 27, 2011 8:34 am
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Pre-flights and general tightening etc. in fact - my 1.5mm hex driver is now worn out ... I've had to resort to grinding back the worn bit to good square section. These are NOT chinese cheap jobs either !
I have a complete new set of drivers in post coming to me.

My 450 is about 6months old ... and I was not expecting such failures yet ...
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solentlife
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PostPosted: Wed Jul 27, 2011 1:35 pm
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No g'tee of quality !

With my 450 suffering various silly mishaps despite checks before flights ... I decided to invest in a second set of hex drivers ...
As we all know various screws on the frame, feathering shaft ... requires having 2 drivers at same time. Well I bought what was apparently high quality not cheap drivers.

First job with the most common sized driver we need ... 1.5mm - the 1.5mm bit snapped of ! I couldn't believe it.

But the major problem in this fatigue matter is the screws themselves I reckon. Unfortunately not all the silver screws can be replaced with superior 12.9 machine jobs and I'm finding it's generally those that are failing. One common one is the self-tappers into back of frame to secure the tail support rods. Being self-tappers into CF frame - you cannot locktite them, If you superglue them = then you ruin the frame.

One modification that seems to be working ....... a replacement rotor head assembly had cross point screwdriver instead of 1.5mm hex driver heads ... the balls etc. have stayed tight and no more rounding out with driver.
Another guy who built a CopterX version - he had cross-point screwdriver heads as well and is very pleased with them.
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platinum
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PostPosted: Thu Jul 28, 2011 5:27 am
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solentlife wrote:
No g'tee of quality !
...........
One modification that seems to be working ....... a replacement rotor head assembly had cross point screwdriver instead of 1.5mm hex driver heads ... the balls etc. have stayed tight and no more rounding out with driver.
Another guy who built a CopterX version - he had cross-point screwdriver heads as well and is very pleased with them.


It sounds interesting. Thanks for sharing. Smile

platinum
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tombo242
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PostPosted: Thu Jul 28, 2011 4:59 pm
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solentlife wrote:
One common one is the self-tappers into back of frame to secure the tail support rods. Being self-tappers into CF frame - you cannot locktite them, If you superglue them = then you ruin the frame.


I use a smear of 'Blue Tack' on the threads of these. Easy to take out with tools but not so easy for vibration to loosen them.

(For those who do not know 'Blue Tack' is a sort of 'sticky' playdough used to put posters etc on inside walls.)

Tom.
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Dumb Thumbs
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PostPosted: Sat Jul 30, 2011 9:00 am
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When I built my clone 450 I tossed the links in the trash right off. Compaired to the quality of the link parts of the Align 450 I have, I just didn't trust the Hobby King junk and used Align links. Never had a Align link fail yet, but I do keep an eye on them fore ware & tare (lots of tare more then ware) Rolling Eyes
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solentlife
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PostPosted: Tue Aug 16, 2011 3:40 pm
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Dumb Thumbs wrote:
When I built my clone 450 I tossed the links in the trash right off. Compaired to the quality of the link parts of the Align 450 I have, I just didn't trust the Hobby King junk and used Align links. Never had a Align link fail yet, but I do keep an eye on them fore ware & tare (lots of tare more then ware) Rolling Eyes


Basically links don't fail - its the rounding out of the driver slot.

I have had a couple of plastic links come of balls ... but I think I maybe clicked those on wring way round when adjusting and destroyed the ridge moulded in them.
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