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Security of your exhaust springs


Guest v23nb
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After an e-mail discussion with ParajetSimon I thought I'd put a thread on this forum as it seems that several Parajets have the same feature as my PAP.

Last week I was 30 minutes into a flight just south of Farnborough when I heard a ping noise. Everything felt fine so I carried on. 30 minutes later I came back to the field near Guildford to land and noticed that at reduced revs there was a small amount of mild vibration. After landing I found a small amount of damage to the leading edge of my prop. My exhaust system contains a ball joint set up that is held together by three exhaust springs and 2 of these springs were now missing. The M25Y Black Devil engine appears to be the basis of the Parajet design and many Parajet motors seem to also have this ball joint and spring set up. I came accross a recommendation a while ago that suggested wire locking these springs so that in the event of failure they would not detach and go through the prop. I did the mod which involved a single long continuous piece of locking wire threaded through the center of each spring and the loops the spring attaches too in turn. It appears that in my case a single spring broke followed by the wire breaking. This then pulled off a second spring but fortunately the third one held. I am now replacing all the exhaust springs with new ones and I shall be wire locking them individually.

A further security feature is to completely fill the inside of the springs (after fitting) and the hooks with clear silicone. This needs to be left for 24 hours to set. This acts as anti-vibration and also catches anyfailed springs and holds them in situ. Sleeves over the springs will not catch a failure at the hook ends (these are under tension load rather than the shear load on the main part of the spring) whereas the silicone will. By leaving the outside of the spring uncovered you can easily see if the spring has failed at any point.

I include a picture in my album in which you can see the springs covered in silicone (this is a picture of a friends machine who did cover the outside of his springs).[/img]

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OK, just to make this clear,

this is not a Parajet / PAP issue, not even a Paramotor 'specific' issue to be fair!

Silicone in exhaust springs has been used this way in Aviation, microlights and the like for many moons! as has it also been used in Motorsport for even more moons.

SW :D

Edited by Guest
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Simon, this effects all of us with springs in our exhaust systems so it is about as relevant as you can get. I hope that by posting this I can save someone their prop and a possible forced landing! I am not havinf a bash at Parajet, PAP or anybody, just trying to add a 'safety' topic to the 'safety' forum. In other sports/applications an exhaust spring can fail with minimal consequence. The effect in paramotoring can be far more destructive as I found out.

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...It just could have been read as a Parajet / PAP fault...

Perhaps if you are a Parajet dealer :wink: but it was written by the ordinary man for the ordinary man without bias.

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Ok, I have read it again and all I can see is a genuine concern for anyone with springs in their exhaust, whether they be on a Parajet, PAP, Black Hawk, Fly Products or anything.

Perhaps we can get back to the topic of safety???

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I received an e-mail from a guy in the US yesterday giving a superbly simple solution to this problem. His idea is that after you have fitted your springs, use a set of grips to close up the ends of the hooks so that in the event of a spring failure the remains cannot slip off the loops they hook on to.

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  • 9 years later...
On 11/8/2007 at 19:42, admin (Simon W) said:

OK, to end this.....

For all of those of you who have a Paramotor with exhaust springs, it is a good idea to fill them with silicone to prevent vibration damage.

They should also be looked at with each pre flight check.

SW :D

not any type of silicone ?  as many use an acid to cure while drying that causes the springs to rust , bostik build silicone is neutral curing acid free. bit late to answer this 2007 topic but hopefully the information will feed through.

Edited by bazz
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