Experimental Cowling Repair Pics
Posted: Wed Feb 26, 2014 12:21 pm
Experimental Cowl Repair
https://www.dropbox.com/sh/jpf36jtj5lb8qeg/6yRK0072yP
Took some pics of a cowling that i'm playing with. After all this work, the $3500 for a cowling doesn't sound too unreasonable lol..
Common Problem being the original dzus style fasters ripping out and wallering out of the fiberglass. Also, when you pull the upper cowl off, the dzus's aren't self ejecting, and they tend to catch in the holes. I decided to do a layup of aluminum on the inside of the cowling which would be held on by Epoxy Resin, and the new style Self Ejecting Dzus Fastners themselves.
The previous repair from the old owner consisted of putting an aluminum doubler along the inside AND outside of the cowl. It had two rows of rivets spaced every 2 inches. This was very ugly and the butt's of the rivets were facing outward which isn't smooth. You can't sand that down and paint over it very well either in the future. It had also failed by the nose and cracked out by the two #10 machine screws.
It had 9 coats of paint on it! I have spent 3 weekends so far with 40 grit sandpaper and a Palm Sander with a bad case of the glass itch ... Also these little 3M Roloc sanding discs came in very handy for grinding down fiberglass and tight spots. Their kinda spendy though. like $3 each
I used West Systems Epoxy Resin with 205 Hardener, and some of the Glass Powder for a filler. This stuff is super expensive (250/gal) but it's worth every penny. It doesn't stink (unlike polyester resin), One pump from each container for a mix ratio, and it's much stronger than the original polyester. To do my layup's I just took a piece of 6 mil plastic clear sheeting cut into a 2'x4' square, folded it in half, placed the fiberglass and resin between the fold, and pushed it around with a scraper untill it had a nice even wet coating. This method keeps you from Gobbing it onto your piece. It also wastes way less epoxy and is stronger. Lots of you tube video's on this method.
I decided against a carbon fiber repair because it's so darn expensive and won't gain much. Just used the regular cross hatch grain mesh stuff. If I were to build a cowl from scratch, maybe I would go carbon. One day..
So after sanding down the area to repair, took measurements and cut the aluminum to length and width for a perfect fit. Laying it up inside the cowl, clamping it down, and tracing out the exact shape it needed to be. Made the strip about 2" wide, although now I wish I went wider. I sanded down the piece of .020, (Would go .025 next time) aluminum with 40 grit so that It would stick good to the Resin. Then mixed up some powder/resin and smeared it down the area where I wanted to stick on the aluminum. (Making sure all oil and grease was removed and area on cowl was sanded, It WILL NOT STICK TO OILY SURFACE) Clamped the aluminum on, filled in all the holes I could with epoxy powder mixture, and let it dry. It helped to use some Mould Release Agent on the clamps, helps keep from breaking them to get them off. Epoxy is tough stuff.
After it dried, I removed the clamps, sanded down the outside of the cowl, and then did another two or three layup of fiberglass on the outside to strengthen it even further and continue filling in all the damaged holes that were torn out. After sanding everything down, I ended up with about 1/8" of fiberglass on top the aluminum.
The self ejecting dzus fastners will be installed with a countersunk rivet, head being inside. I might put a few extra regular rivets on to further secure the aluminum. I worry about the aluminum expanding and contracting at different rates than the fiberglass, therefore separating the bond. These would go just above the Upper/Lower Cowl overlap in a line from front to back, but the round head would be outside so that it was sandable. More rivets at the front because that seem's to be where most of the stress/failure point is.
Decided to go ahead and do the same thing for both the upper and lower cowl where it touches the firewall. I'll just have to put anti Chafe fabric strip around the entire boot cowl before I re-fit the cowl and drill all new holes for everything. I'm a little worried about losing the original alignment of the cowl and re-fitting all the new holes, but with some patience and a hole doubler, it should all work out.
I'll post more pics after getting it primed with EP420.
I hope I never have to look at another cowl again.... Have 48 hours invested so far....
https://www.dropbox.com/sh/jpf36jtj5lb8qeg/6yRK0072yP
Took some pics of a cowling that i'm playing with. After all this work, the $3500 for a cowling doesn't sound too unreasonable lol..
Common Problem being the original dzus style fasters ripping out and wallering out of the fiberglass. Also, when you pull the upper cowl off, the dzus's aren't self ejecting, and they tend to catch in the holes. I decided to do a layup of aluminum on the inside of the cowling which would be held on by Epoxy Resin, and the new style Self Ejecting Dzus Fastners themselves.
The previous repair from the old owner consisted of putting an aluminum doubler along the inside AND outside of the cowl. It had two rows of rivets spaced every 2 inches. This was very ugly and the butt's of the rivets were facing outward which isn't smooth. You can't sand that down and paint over it very well either in the future. It had also failed by the nose and cracked out by the two #10 machine screws.
It had 9 coats of paint on it! I have spent 3 weekends so far with 40 grit sandpaper and a Palm Sander with a bad case of the glass itch ... Also these little 3M Roloc sanding discs came in very handy for grinding down fiberglass and tight spots. Their kinda spendy though. like $3 each
I used West Systems Epoxy Resin with 205 Hardener, and some of the Glass Powder for a filler. This stuff is super expensive (250/gal) but it's worth every penny. It doesn't stink (unlike polyester resin), One pump from each container for a mix ratio, and it's much stronger than the original polyester. To do my layup's I just took a piece of 6 mil plastic clear sheeting cut into a 2'x4' square, folded it in half, placed the fiberglass and resin between the fold, and pushed it around with a scraper untill it had a nice even wet coating. This method keeps you from Gobbing it onto your piece. It also wastes way less epoxy and is stronger. Lots of you tube video's on this method.
I decided against a carbon fiber repair because it's so darn expensive and won't gain much. Just used the regular cross hatch grain mesh stuff. If I were to build a cowl from scratch, maybe I would go carbon. One day..
So after sanding down the area to repair, took measurements and cut the aluminum to length and width for a perfect fit. Laying it up inside the cowl, clamping it down, and tracing out the exact shape it needed to be. Made the strip about 2" wide, although now I wish I went wider. I sanded down the piece of .020, (Would go .025 next time) aluminum with 40 grit so that It would stick good to the Resin. Then mixed up some powder/resin and smeared it down the area where I wanted to stick on the aluminum. (Making sure all oil and grease was removed and area on cowl was sanded, It WILL NOT STICK TO OILY SURFACE) Clamped the aluminum on, filled in all the holes I could with epoxy powder mixture, and let it dry. It helped to use some Mould Release Agent on the clamps, helps keep from breaking them to get them off. Epoxy is tough stuff.
After it dried, I removed the clamps, sanded down the outside of the cowl, and then did another two or three layup of fiberglass on the outside to strengthen it even further and continue filling in all the damaged holes that were torn out. After sanding everything down, I ended up with about 1/8" of fiberglass on top the aluminum.
The self ejecting dzus fastners will be installed with a countersunk rivet, head being inside. I might put a few extra regular rivets on to further secure the aluminum. I worry about the aluminum expanding and contracting at different rates than the fiberglass, therefore separating the bond. These would go just above the Upper/Lower Cowl overlap in a line from front to back, but the round head would be outside so that it was sandable. More rivets at the front because that seem's to be where most of the stress/failure point is.
Decided to go ahead and do the same thing for both the upper and lower cowl where it touches the firewall. I'll just have to put anti Chafe fabric strip around the entire boot cowl before I re-fit the cowl and drill all new holes for everything. I'm a little worried about losing the original alignment of the cowl and re-fitting all the new holes, but with some patience and a hole doubler, it should all work out.
I'll post more pics after getting it primed with EP420.
I hope I never have to look at another cowl again.... Have 48 hours invested so far....