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  #1  
Old 07-17-2021, 09:54 AM
bill.hutchison bill.hutchison is offline
 
Join Date: May 2020
Location: Northern VA
Posts: 298
Default Cowl Mod/Improvement Suggestions Sought

And the answer may be, "do nothing", but I'm curious and open to ideas.

The two photos show the bottom left and center of my cowl, which wraps around a carbureted O-360.

Yes, it's messy and yes, I did clean it up after I took the photo. It's blowing out some excess oil because I put 6 quarts in it and it prefers 5.5. This should stabilize soon with less mess, although I'm open to ideas on that one, too.

The main thing I'm interested in is cleaning up/fixing the crack and hole. The hole is for access to the gascolator, but it's not aligned very well and the actual gascolator drain is a bit high, which makes only certain sampler cups viable and they need longer wires to touch the drain.
  1. Do I make the hole a little wider/bigger for better access to the gascolator?
  2. Do I just put filler in there, close it up and sand/paint?
  3. Is there something else better I should consider? What am I not thinking of?

Finally - would it make sense to install an air/oil separator? I'm dimly aware of how they work...the crankcase breather tube reaches down and vents just to the left of the left exhaust pipe. I'd like to do something cleaner/better but not sure what yet.
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  #2  
Old 07-17-2021, 11:28 AM
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DanH DanH is offline
 
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Bill, the only long term repair is new glass, scarfed in. There is no type of filler which will stay.

I've attached a scarf joint illustration below.

The general area here is trashed two ways, structural cracking and probably oil soaking. Best bet is to simply cut out the section and build a new one. It's a small flat section, not as hard as it sounds. Scarf sand the edges of the old glass with an 80 grit block, plus about an inch of paint removal all around the perimeter of the scarf. Temporarily fasten a sheet of some non-stick material to the inside cowl surface, so as to span the missing section. You'll do the new scarf layup on it, remove it after cure.

Given this job has full access to the backside, and you plan to cut a new hole for the gascolator, sand the backside of the new scarfed-in panel and lay three more cap plies. It provides local thickening around the hole, and the screw attachment points.
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  #3  
Old 07-17-2021, 11:46 AM
Ralph Inkster Ralph Inkster is offline
 
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Location: Calgary, Alberta
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Quote:
Originally Posted by DanH View Post
Given this job has full access to the backside, and you plan to cut a new hole for the gascolator, sand the backside of the new scarfed-in panel and lay three more cap plies. It provides local thickening around the hole, and the screw attachment points.
Well described repair process Dan, as ever.

I would suggest more cowl support to avoid the crack reappearing in that area, by installing an additional nutplate & screw along the firewall outboard of the gascolator hole position.
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  #4  
Old 07-17-2021, 11:50 AM
bill.hutchison bill.hutchison is offline
 
Join Date: May 2020
Location: Northern VA
Posts: 298
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Quote:
Originally Posted by DanH View Post
Bill, the only long term repair is new glass, scarfed in. There is no type of filler which will stay.

I've attached a scarf joint illustration below.

The general area here is trashed two ways, structural cracking and probably oil soaking. Best bet is to simply cut out the section and build a new one. It's a small flat section, not as hard as it sounds. Scarf sand the edges of the old glass with an 80 grit block, plus about an inch of paint removal all around the perimeter of the scarf. Temporarily fasten a sheet of some non-stick material to the inside cowl surface, so as to span the missing section. You'll do the new scarf layup on it, remove it after cure.

Given this job has full access to the backside, and you plan to cut a new hole for the gascolator, sand the backside of the new scarfed-in panel and lay three more cap plies. It provides local thickening around the hole, and the screw attachment points.
Dan, if I'm reading your instructions correctly, I'll build up new glass from the outside, right? Laying the new pieces on one after the other, and then I'll remove the temp piece? Then cut the new gascolator hole, prime and paint?

Also - what would you recommend for non-stick material?

And would an oil/air separator be advisable?
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  #5  
Old 07-17-2021, 12:58 PM
RV10Pilot RV10Pilot is offline
 
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Location: Medford, NJ USA
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Quote:
Originally Posted by bill.hutchison View Post
Dan, if I'm reading your instructions correctly, I'll build up new glass from the outside, right? Laying the new pieces on one after the other, and then I'll remove the temp piece?
Then sand the inside with 60 or 80 grit sand paper and lay in some more fiberglass (23 layers). This will bond the new patch in from both sides.
Quote:
Originally Posted by bill.hutchison View Post
Then cut the new gascolator hole, prime and paint?

Also - what would you recommend for non-stick material?
Anything flat wrapped in plastic packing tape. The resin will not stick to the plastic tape. I would use a piece of plywood cut to fit, but you could use a piece of thicker aluminum sheet wrapped with tape. Fit the aluminum first, before removing the broken section. Drill a dew holes through the aluminum and into the good portion of the cowl (the part you are saving) and Cleo in place. This will retain the shape of the cowl. Then remove aluminum sheet and cut out the bad section, reattach the flat sheet and glass in per above.
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  #6  
Old 07-17-2021, 01:12 PM
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Toobuilder Toobuilder is offline
 
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Couple of thoughts on the fiberglass repair. Yes, its going to take a bit of grinding to create bevel around the hole to scarf in new glass. This is NOT hard. I'd clean up the inside of the cowl to simply sand out the oil contamination and weave, bevel the outside at a reasonable slope (maybe 5 to 8:1), and then superglue or tape a disk of manila folder on the inside of the cowl and lay one layer of glass over the top to seal it in. From the outside apply your lamination until you are a few plies proud of the OML and let it cure. Sand to contour, drill a new hole and paint to match.

Concerning the oil separator - I've used an automotive valve cover breather for quite a while now with good success. Get the breather or the outlet loop as high in the cowl as possible before turning down and out. They are not nearly as complex as a typical oil separator, but are only a few bucks from Summit or Jegs and are quite effective.

Attached is a picture of my Summit breather and the oil cooler duct built almost completely in place with manila folder stock and masking tape under glass and carbon.
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Last edited by Toobuilder : 07-19-2021 at 08:58 AM.
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  #7  
Old 07-17-2021, 01:15 PM
bill.hutchison bill.hutchison is offline
 
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Originally Posted by RV10Pilot View Post
Then sand the inside with 60 or 80 grit sand paper and lay in some more fiberglass (23 layers). This will bond the new patch in from both sides.

Anything flat wrapped in plastic packing tape. The resin will not stick to the plastic tape. I would use a piece of plywood cut to fit, but you could use a piece of thicker aluminum sheet wrapped with tape. Fit the aluminum first, before removing the broken section. Drill a dew holes through the aluminum and into the good portion of the cowl (the part you are saving) and Cleo in place. This will retain the shape of the cowl. Then remove aluminum sheet and cut out the bad section, reattach the flat sheet and glass in per above.
Helpful stuff.

Since posting, I've been researching. Seems that conventional wisdom dictates that I use the same kind of glass and mat that was originally used on the cowl.

What kind of fiberglass was used on these cowls? I do know that I need to use an epoxy resin to keep it from shrinking (have another spot on the airplane where that is happening, unfortunately) but also not sure what stuff is the right stuff to do this job.
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  #8  
Old 07-17-2021, 02:19 PM
RV10Pilot RV10Pilot is offline
 
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Quote:
Originally Posted by bill.hutchison View Post
Helpful stuff.

Since posting, I've been researching. Seems that conventional wisdom dictates that I use the same kind of glass and mat that was originally used on the cowl.

What kind of fiberglass was used on these cowls? I do know that I need to use an epoxy resin to keep it from shrinking (have another spot on the airplane where that is happening, unfortunately) but also not sure what stuff is the right stuff to do this job.
I do not know what specific cloth or resin is used on the cowl and it has probably changed over time. I would use West 105 resin with the appropriate hardener, depending on temperature.

A standard plain weave or boat weave in the 7-9 ounce range would work well. Something like 7500 or 7520-50 from ACS

Or my preferred fiberglass cloth vendor Thayercraft https://thayercraft.com has 9.6 oz 7500
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  #9  
Old 07-17-2021, 02:36 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by bill.hutchison View Post
Since posting, I've been researching. Seems that conventional wisdom dictates that I use the same kind of glass and mat that was originally used on the cowl. What kind of fiberglass was used on these cowls?
Depends on vintage. Early RV cowls were polyester resin and fiberglass cloth, and apparently none had a core material. Those cowls are relatively weak, structurally speaking. Later cowls appear to be epoxy resin and fiberglass cloth, with a honeycomb and foam core.

Either cowl can be repaired with an epoxy resin/hardener and fiberglass cloth. No mat please; that's a heavy boat material. For this job I'd use ordinary 7500 9oz plain weave, four or five plies for scarfed layup, and two tie plies later on the inside, overlapping out onto the old glass about an inch.

A West epoxy quart kit (01-08200, 105 resin and 206 slow hardener) is way more than enough. Alternate plan...buy the composites practice kit, with Rutan's book, as it has everything you'll need plus THE classic instruction manual for composites homebuilders:

https://www.aircraftspruce.com/catal...ickkey=8112553

https://www.aircraftspruce.com/catal...ickkey=8112553

Every surface where you expect a bond must be clean, clean, clean, and free of all paint and filler. Just bare glasswork. Scrub everything twice with a strong detergent, then sand until clear.

Quote:
I do know that I need to use an epoxy resin to keep it from shrinking (have another spot on the airplane where that is happening, unfortunately)
That would be polyester. Some guys love it because it's fast and cheap. To me it's like a blind date with a transvestite...looks good for a while.
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  #10  
Old 07-17-2021, 05:10 PM
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rocketbob rocketbob is offline
 
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I've repaired a number of RV cowls and wheel pants. I cannot remember the last time I used fiberglass for a layup and consider it an obsolete material. In this case I would use 3K carbon and a layer of kevlar to prevent screw holes from opening up.
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