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Neoprene Rubber Replacement (SJ cowl)

Raymo

Raymo

Well Known Member
The neoprene is not holding up as well as I'd hoped on my SJ cowl. I've had to replace the intake tube after about 70 hours of flying and the tubes into the plenum are showing signs of degradation too.

Wondering what others have used that holds but better over time and is flexible enough for the purpose.

Thanks in advance.
 
My plenum transitions are doing fine but my engine air intake is showing definite signs of distress with 150 hours on it. I believe it's just wetsuit material but where to get it would be the question.
 
On my Sam James ram air intake I used marine exhaust water hose for the ram air to cowling connection such as this. https://www.fisheriessupply.com/trident-marine-series-321-ocean-flex-hose. You should be able to find it cheaper and it works great. I secure the hose using a hose clamp to the intake ring glued to the cowling. This hose overlaps the fiberglass ram air intake by about 1" and no clamp is needed on the ram air intake. This makes removing the cowling a snap.

Bill
 
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airguy said:
My plenum transitions are doing fine but my engine air intake is showing definite signs of distress with 150 hours on it. I believe it's just wetsuit material but where to get it would be the question.
Click to expand...

Thanks. Getting the same material is not a problem. I want something that will hold up better but still be flexible.
 
William said:
On my Sam James ram air intake I used marine exhaust water hose for the ram air to cowling connection such as this. https://www.fisheriessupply.com/trident-marine-series-321-ocean-flex-hose. You should be able to find it cheaper and it works great. I secure the hose using a hose clamp to the intake ring glued to the cowling. This hose overlaps the fiberglass ram air intake by about 1" and no clamp is needed on the ram air intake. This makes removing the cowling a snap.

Bill
Click to expand...

Thanks! I will check this out.
 
Raymo said:
Thanks. Getting the same material is not a problem. I want something that will hold up better but still be flexible.
Click to expand...

Make what you want. Hose material is too stiff.

These are at 675 hours now, still doing fine...a single ply of 9oz plain weave glass soaked in two-part urethane rubber, and wrapped over a form. Andy made some too.

 
DanH said:
Make what you want. Hose material is too stiff.

These are at 675 hours now, still doing fine...a single ply of 9oz plain weave glass soaked in two-part urethane rubber, and wrapped over a form. Andy made some too.

Click to expand...

Dan that's awesome. Can you give any details of the rubber?
 
crabandy said:
Scott,
I believe the rubber DanH used isn't made anymore, I've used a similar formulation from the same company "Smooth On" Urethane rubber "PMC 780 Dry" with excellent results.
https://www.smooth-on.com/products/pmc-780-dry/
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Correct. I was using PMC 870, which turned out to be water-sensitive. The mixed liquid rubber was tinted black before application. The finished black rubber will turn white and faintly sticky when wet, then black again when it dries. Doesn't seem to hurt anything (and it's been parked in the rain more than a few times), but if I build more parts in a similar fashion, I'll use the 780 Dry.

https://www.smooth-on.com/products/pmc-780-dry/

Note that urethane rubber formulations are either polyester or polyether. Polyester formulations handle oils and fuels, but not water. Polyether formulations handle water, but not oils and fuels.

I soaked 870 (a polyester) in oil for a long time and could find no issue, but as noted, it is sensitive to water. Andy soaked 780 in two-stroke gas and reported some cracking.

There are no perfect materials, but 780 is thus a better choice for cooling ducts, which are not soaked in oil or fuel
 
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Thanks, Dan. I was wondering what products you used for your transitions and wondered about the flexibility of the marine hose.

I have the fiberglass; just need to order the rubber.

EDIT: Just ordered two PMC-780 Dry pint kits from a vendor in Atlanta.
 
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crabandy said:
Short term soak of PMC 780 Dry in oil and water was fine, the sample I soaked in 2stroke gas showed a little aging.
Post 434
http://www.vansairforce.com/community/showthread.php?t=94648&page=44

Raymo,
If you read the specs on the PMC 870 it says you can thicken it with their thickening agent, the specs of their thickening agent looked a whole lot like Cabosil. I found thickening the rubber made it easier to paint onto my parts.
Click to expand...

I recently ordered some Cabosil to help smooth all the fiberglass parts.

Thanks!
 
Do what works, but for reference, mixed West 105/206 has a viscosity of 725 cps at 72F, while PMC-780 is 2000 cps. Personally I wouldn't bother with the cabosil; 780 is already thicker than a typical laminating resin.

Judging by appearance, Andy wanted thicker coats, but the rubber is really just a sealing media. The strength is in the cloth.
 
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