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Plastic fatigue in clevises


wookman
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wookman

I have in my collection of models Stan's prototype Phoenix Model Products Fun Start (with acknowledgement to PMP for use of the photo), alongside another that I built myself when my daughter showed some, long since evaporated, enthusiasm for model flying.

FunStart.jpg

It is an EPP interpretation of the classic rudder elevator slope trainer and a damn fine aeroplane.

As it was aimed at beginners to modeling simplicity was the watchword. To this end the clevises were the sort that clamp to the solid wire in tube push rods.

IMG_2185.JPG

With these models now being 15 years or so old now the clevis material seems to have dried out and gone very brittle. The slightest of knocks snapped off the pins in the lower two and releasing the other from the horn broke the leg off altogether!

IMG_2187.JPG

IMG_2188.JPG

The solution arose from a quick rummage through the "Control accessories" tin in the form of a slack hand full of solder adapters and snap clevises.

A couple of hours fettling and we have two airworthy Fun Starts again. Pictures of my "daughters" Fun Start because it photographs better than the black one.

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Phil.Taylor

Make that 20+ years old !

I had one of them in 1998 to try & get my kids interested. Wings folded up to 45+ degrees when my son tried a loop! Beefed up the spars & it lasted years. Saw one flying at Whitesheet last year.

Phil.

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wookman

My red one has a completed date of May 07 written on it so the black one may well be over twenty years old.  The red one has a two piece wing as it has been to France a few times and we were using an Escort Cabriolet at the time. The Wanabee was built the same way for the same reason.

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wookman

You had an early one Phil. Released in 98 and just under 200 sold that year. Wonder what happened to all of them? Must still be a few about given how durable they are. So the black one is 23 years old.

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Relating to the snapped Clevis: I wonder if the model was stored in a loft in this hot weather? Nylon in particular requires about 6% moisture in it for maximum strength. When dry is can be quite brittle.

This is from experience of injection moulding nylon where we chucked the freshly moulded parts into a bucket of water over night to allow the parts to adsorb the water which had to be removed from the pellets prior to moulding. The flexing strength difference is very significant. (Ask any tow line guys about it!)

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wookman

Good point. Both models had quite a long summer in my parents loft when we moved house a few years ago. Probably didn't help.

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thermaldoctor

Hi Graham it's pretty common on old plastics. Over time the plasticisers used in initial production to make it tough and flexible start to break down and have lesser and lesser effect until it becomes brittle and fragile. Heat and UV exposure play a big part.

We have this problem when racing the vintage RC cars. Old 80's and 90's plastic wishbones etc break quite easily, yet when replaced with a brand new re-release item they are tough as old boots again

Regards

Neil

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Peter G

Interesting to note 👍

I've got some fairly valuable plastic with these (MP Jet type) clevises on the elevators. Possibly worth changing as they get older for the sake of a few pence.

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When I flew an old Pike that I’d not flown for 10 years the splines on the Volz servos arms took it in turns to strip. After the second one went I twisted the others on the ground and they all failed. It was a bit scary.   

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wookman

That's a scary one! I have an old (your old) Pike will a full suite of volts servos that are going to be twenty odd years old now. Not flown it for quite a while, might be worth considering some new servos before I do. Don't suppose you can get new volts servo arms anymore.

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