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  #1  
Old 05-27-2014, 07:08 AM
Arlen's Avatar
Arlen Arlen is offline
 
Join Date: Sep 2013
Location: New Smyrna Beach, FL
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Default Paint blistering on cowling from heat below the engine

When I bought my -6, there was an area below Cylinder #2 where the paint had blistered off. Presuming it was from excessive heat in that area, a heat shield was laid inside the bottom cowl in that location and the glass and paint were repaired.

I believe it's a standard -6 cowling, and there's a Lycoming O-360 inside. The paint used was PPG Concept polyurethane. There do not seem to be any exhaust leaks.

In about 25 hours of flight since the repairs and re-painting, I now see the beginning of some tiny paint blisters again. Most of them are in a line around what corresponds to the outside edge of where the heat shield sits inside the cowl, but a couple are "through" the heat shield.

Is this a known issue with RV cowlings? Is there a fix?
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  #2  
Old 05-27-2014, 09:42 AM
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Mike S Mike S is offline
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Do you have room to add a stand-off heat shield on the pipe???
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  #3  
Old 05-27-2014, 09:50 AM
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Arlen Arlen is offline
 
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Quote:
Originally Posted by Mike S View Post
Do you have room to add a stand-off heat shield on the pipe???
Hmmm.

I don't think so, but I'll check.

Is there any special kind of heat shield that may be better than the standard one?
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  #4  
Old 05-27-2014, 04:16 PM
LarryT LarryT is offline
 
Join Date: Aug 2006
Location: Battle Ground, WA
Posts: 427
Default Make the clearance

I posted about this several years ago but I am too dumb to provide you with a link to the thread.

According to Larry Vetterman, Van's made a running change to the cowl that resulted in a tight clearance to the cowl. He modified his exhaust system for more clearance but I didn't want to spend the $$$$ for an entirely new exhaust system.

Here's what I did:

1. Loosen exhaust system fasteners so you can drop #2 exhaust pipe from the stub pipe that bolts to the cylinder.

2. Cut off about 3/16 - 1/4" of that stub pipe.

3. Reassemble exhaust system.

4. drill a new hole in the "dogbone" for the #2 joint attachment.

You will now have enough clearance to install one of the Van's heat shields. I riveted a piece of high temp (orange) silicone rubber onto the heat shield. Use flush type rivets and countersink so rivets are slightly below the surface. This will protect the lower cowl if there is contact (probably is) during the "wet dog shake" engine start-up and shut-down. I also put the Van's reflective aluminum foil heat shield on the lower cowl.

I have flown the pane for 100+ hrs. in conditions that included 95 degree ambient temperatures and long climb-outs. The repainting on the cowl has not blistered in that time.

Larry Tompkins
544WB -6A
W52 Battle Ground WA
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  #5  
Old 05-27-2014, 04:36 PM
kdedmunds kdedmunds is offline
 
Join Date: Jul 2013
Location: Tallahassee, FL
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Default

You may also look for contamination in the fiberglass. I have the same problem and attributed it to heat also. However in seeking an answer to prepping the surface for repaint I found that this is a common problem on Corvettes. (fiberglass bodies) The surface has to be completely cleaned and the contaminant removed or the pain will blister again. Something you may want to look at.
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Old 05-29-2014, 09:58 AM
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Bump.

Some nice feedback so far, but wondering if anyone has any other ideas....before my friend and I look it over and discuss options this weekend.

Thanks!
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  #7  
Old 05-29-2014, 10:22 AM
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N941WR N941WR is offline
 
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Add a heat barrier to the inside of the bottom cowl:



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  #8  
Old 05-29-2014, 10:37 AM
rightrudder rightrudder is offline
 
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Nice work, Bill. Does that foil come adhesive-backed? If not, what glue do you use?
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  #9  
Old 05-29-2014, 11:46 AM
deek deek is offline
 
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Fiberfrax and heavy-duty foil bonded with RTV makes an excellent cowl insulator. Keep the Fiberfrax about 1" inside the foil border to ensure a good seal.
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  #10  
Old 05-29-2014, 12:24 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by deek View Post
Fiberfrax and heavy-duty foil bonded with RTV makes an excellent cowl insulator. Keep the Fiberfrax about 1" inside the foil border to ensure a good seal.
Right on...a reflector over an insulator.

Fiberfrax felt works well under the Vans self-stick reflective aluminum foil.

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