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Gary Manuel17/02/2017 16:33:18
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Posted by john stones 1 on 17/02/2017 16:20:56:

Looks neat, but i would prefer bigger holes myself, it'll vibrate a bit as they do, and it might set stress fractures off ?

I agree - I might make them bigger. Play it by ear till I've got engine running.

Gary Manuel17/02/2017 16:35:11
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1015 forum posts
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Posted by Rich2 on 17/02/2017 16:19:57:

I'm using these **LINK**

M3, and captive T nuts and small rubber washers.

 

They're the sort of thing I had in mind.

Where did you source your T nuts and washers?

Edit - I found and ordered some from ebay.

Edited By Gary Manuel on 17/02/2017 17:02:12

Gary Manuel17/02/2017 16:36:58
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1015 forum posts
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Posted by Rich2 on 17/02/2017 16:24:27:

I'll be using quite a few of them. It will also be reinforced where the bolts are. And I use bigger ones to fix the cowl to the fuselage. You might be right..

I was thinking 10 to 12 down each side.

Cowl to Fus is already sorted.

Gary Manuel17/02/2017 16:53:57
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1015 forum posts
660 photos
Posted by iqon on 17/02/2017 16:17:43:

Why no air out hole........

Because it's got a proper tunnel for the silencers. That's where I want as much air as possible to go.

I've also put a lip on the leading edge of the tunnel outlet to help "suck" air through.

Gary Manuel19/02/2017 15:22:24
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1015 forum posts
660 photos

I've had a read-up about grommet material.

Most of them (including mine) are made from PVC, which starts to decompose at 140C and melt at 160C. Some are natural rubber which is only good for up to about 100C. Not much use if it comes into contact with a hot exhaust pipe then.

The rubber strip used at the bottom of Page 1 of Adrian Smith's thread HERE, got me thinking about using using something similar, but I think that will have the same temperature issues.

I've found THIS stuff, which is made from Platinum cured Silicone. It should be good for up to 230C - 250C. It will also allow me to cut the holes to any shape / size I want. The 25mm grommets I've used at the moment only have about 3mm clearance all round. I'd like double or treble that to allow for vibration and expansion.

 

Edit - here's a LINK to Silex Website

Edited By Gary Manuel on 19/02/2017 15:35:06

john stones 119/02/2017 19:19:50
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7811 forum posts
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You're a very thorough n meticulous man Gary winkme i would have laid some more glass cloth where my cut outs were gonna be.

John

Gary Manuel19/02/2017 21:12:25
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1015 forum posts
660 photos

Haven't ordered any Silicone trim yet. It's just a thought at the moment. All I've done so far is rule out the PVC grommets.

What I'm concerned about is that the silencers are only held onto the headers via the PTFE tubes and spring clamps. I think they might start to move around when the engine vibration starts. I don't want the vibrating silencers to come into direct contact with the cowl - even if it's reinforced at the point of contact. I'm still thinking of a solution but I have one or two ideas.

Suggestions welcome.

john stones 119/02/2017 21:44:23
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7811 forum posts
1273 photos

Rich ? used some heat proof silicone on the joint i think.

Gary Manuel19/02/2017 21:49:43
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1015 forum posts
660 photos
Posted by john stones 1 on 19/02/2017 21:44:23:

Rich ? used some heat proof silicone on the joint i think.

Silicone won't stop it moving it's too soft.

I think you might be thinking about the gap he filled between the fuselage mounted clamp and the silencer - because the fit was too loose.

Rich220/02/2017 06:51:30
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1957 forum posts
550 photos

that's correct, the clamp was slightly loose so I used the silicon to fill the gap.

are you trying to prevent the muffler and exhaust outlet from turning in the header? a small screw through the header/muffler joint?

Edited By Rich2 on 20/02/2017 06:52:29

Gary Manuel20/02/2017 09:43:01
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1015 forum posts
660 photos

That's a good idea Rich.

john stones 120/02/2017 12:51:14
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7811 forum posts
1273 photos

I would look through the Loctite range of product myself, if you remember the demo we had there's a vast range of stuff.

John

Rich220/02/2017 13:48:35
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1957 forum posts
550 photos

i would try a small hole, thread it to M3, put a small M3 screw in there with some Loctite on the threads.

Gary Manuel20/02/2017 14:35:43
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1015 forum posts
660 photos

The problem with a screw is that it needs something to tighten up against, to prevent it coming loose. The PTFE is probably too soft to do this - especially when it gets hot.

What I'm thinking at the moment is to drill a pilot hole through the PTFE tube and into the stainless steel silencer or header pipe below. Then drill the PTFE hole a bit larger and push a servo mount spacer into the PTFE tube. Then screw a self tapping screw coated in strong threadlock into the stainless steel, till it tightens against the servo mount spacer. This should then be nice and solid to hold the screw in place, if all goes well. The servo mount spacer should also spread the load of the vibrations a bit to help prevent the PTFE tube from rubbing against the screw and tearing.

The final step would be a turn or two of silicone self fusing tape around the PTFE tube / screws to hold the screws in place in case it does try to vibrate loose.

I don't need to decide just yet as I will start the engine with the cowl off initially, to see whether I think it needs anything doing or whether I'm concerned over nothing. Waiting for tail wheel / prop order from ProBuild before I finish, then wait for field to dry out a bit before flying.

Rich220/02/2017 15:05:53
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1957 forum posts
550 photos

I see, sorry I misunderstood and thought it was for metal to metal contact. I don't think you will have a problem, I haven't done anything on my Yak. Your holes might be a bit small though, see what happens when you fire it up.....wink

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