M20J Cowling rebuild

[Elijah]

9 hours ago, Fly Boomer said:

Pictures?

I posted some pictures over here: 

 

[Bartman]

Since I am grounded due to a broken spinner bulkhead I decided to bring the upper cowl home and start the rebuild process.  I have this 2013 thread from @jetdriven saved and will use that as a guide, but the pictures did not migrate as we changed servers.  I have some supplies gathered, including the MGS epoxy and hardeners, carbon fiber in both woven sheet and unidirectional as well.  Gathering supplies to do bagging, and I'll do it right to the best of my ability, but I'm certain it will be better than what I have now.

Here are some pictures before, and during the removal process.

 

[Bartman]

I bought 1TF in 2007, and this repair was done before I purchased it.  As you can see, it was just brushed on thick with only a couple of layers of mesh, and it delaminated, so I suspect it is not the approved MGS epoxy.  I started with a 40-grit disc on a drill, and that worked pretty well, but honestly, it came off easier with just using a knife and one of those painter's multi-tools.  I also burned thru one small areal so I found it best not to use the sanding disc for the big stuff.  The knife would get under the edges, but it was too thin to get leverage, and the painter's tool worked perfectly for this.  I did hit the entire area with 40 grit sandpaper and then washed with soap and a scrub brush.  I'll have to clean with MEK and do some additional preparation, but I have a good start.

As you can see, the previous repair extended all the way to the honeycomb, and I wanted to remove all the old delaminating resin from the picture.  One question for Byron @jetdriven.  Did you completely remove the honeycomb, or did you cover it with carbon fiber?

Jamie

[jetdriven]

I would recommend putting some trapezoidal shaped blue foam as a stiffener maybe just after of what’s there for factory …..make it an inch wide then half an inch tall. Then when you vacuum bag your carbon over that, it acts more like a beam. Mine has held up well but it has one crack along the left side of the spinner about 4 inches long but this is after 800 hours of flying or more actually.

[jetdriven]

2 minutes ago, Bartman said:

I bought 1TF in 2007, and this repair was done before I purchased it.  As you can see, it was just brushed on thick with only a couple of layers of mesh, and it delaminated, so I suspect it is not the approved MGS epoxy.  I started with a 40-grit disc on a drill, and that worked pretty well, but honestly, it came off easier with just using a knife and one of those painter's multi-tools.  I also burned thru one small areal so I found it best not to use the sanding disc for the big stuff.  The knife would get under the edges, but it was too thin to get leverage, and the painter's tool worked perfectly for this.  I did hit the entire area with 40 grit sandpaper and then washed with soap and a scrub brush.  I'll have to clean with MEK and do some additional preparation, but I have a good start.

As you can see, the previous repair extended all the way to the honeycomb, and I wanted to remove all the old delaminating resin from the picture.  One question for Byron @jetdriven.  Did you completely remove the honeycomb, or did you cover it with carbon fiber?

Jamie

I ended up removing the honeycomb that was in the section of the spinner hump in about an inch or two on either side of that. We thought that we could stack up 1 inch wide layers of carbon, but looking back I think the better way to do it is to put a piece of foam, blue foam in that area, but make it taller than the factory did,  say a half inch or 3/4 of an inch instead of the factory 1/4 inch. Then vacuum bag over that you have a real beam. 

[Flyler]

I remember the guys in the composites shop used to "debulk" with vacuum bags. Precut your fiber patches, lay them in place (no resin or prepreg and leave cold) bag and vac and then let it sit overnight.  The fibers would tend to stay-put a lot easier after that. 

[hammdo]

I’ve done ‘slurry’ over foam with a transition, bid/uni/triax ‘tapes’, peel ply, then ‘stipple’ out the excess when doing repairs. Worked well with the Velocity I did.  May be option if you want another acceptable approach…

-Don

[PT20J]

Mod Works had a mod that worked really well. I’d try to find someone with pictures of it and copy it. 

[Bartman]

Final plan.
 

I will remove the two sections of damaged honeycomb as @jetdriven suggested and go all the way to the edge of previous repair that I just removed. Create a ridge of blue foam for both of those sections as Byron suggested. If I extend the foam about 2 inches beyond where it always cracks on one, and then about 3 inches on the other, it will stagger the lateral ends of the blue foam ridge. That may help relieve stress, but may not really matter since it will have carbon fiber strips extending beyond those transitions.
 

I have a roll of unidirectional carbon fiber tape 2 inches wide. I’ll lay that up in layers all the way from end to end covering what remains of the existing honeycomb ribs laterally, and the new blue foam rib in the center. Finally, I have carbon fiber weave fabric and will lay that in a trapezoidal shape over the center section. Wider forward, and narrowing as it transitions aft. I can cut it down the center and overlap so that it lays flat. 
 

Thanks to all for the help. Will document in pictures as the project progresses. 

[jetdriven]

i put a 1.5" wide strip of unidirectional carbon fiber tape on the lower cowl to replace the aluminum 90 degree stiffener that was riveted into the cowl just forward of the cowl flaps. it was springy and flexible like gluing a fishing rod into the cowl there.  I suppose you need the bidirectional cloth and the foam for a square-ish shape to provide real stiffness.  We used 10K and 12K cloth for this reinforcement and you can put 80 lbs of force on the top cowl  in the middle without any visible deflection. 

[jetdriven]

here are some of those photos from august 2013. 

 

As ive learned, some 1/2" tall and 1" wide blue foam with this work over it would be much stronger.  We built a 3/8" high stiffener from carbon fiber strips. 

 

My PA28-1981 Archer has this scheme. The inside of the top cowl has some 1" tall foam shaped into a half moon. With fiberglass over that. Its not cracked. 

 

[Bartman]

Progress report. I did much of the preparation at home and today I brought the cowl and all of my supplies back to the airport and began the finishing work. 

I prepared and cut all of the supplies before mixing the MGS 285L epoxy.  The first thing was to add the sealing tape and cover with painters tape which provided a barrier and keep resin out of that area.  I also chose to leave the existing honeycomb in place, and that turned out to be a good decision. Since part of the honeycomb was exposed, I covered these areas with 2 layers of fiberglass cloth to provide a barrier to the carbon fiber and to prevent galvanic corrosion. Since one area was lateral, I had to extend my strips of carbon fiber to cover that. 

I used the same mix on the epoxy as Byron. 50:50 of slow and fast hardener added to the epoxy and the 5% Antimony trioxide powder turned it a milky color. After covering the aluminum with fiberglass I laid 4 strips of unidirectional carbon fiber to cover each of the existing honeycomb ribs and the fiberglass base. I intended to use the blue foam to make a stiffening ridge, but it was not stable as I added the layers and it kept falling over so I abandoned that part. If I had done individual ribs it probably would have worked, but I needed to compete this in one bagging. Then I added 3 layers of the 19oz weaved carbon fiber cloth to cover the fiberglass and carbon fiber strips, and this layer was much heavier than the unidirectional strips. 

Since I already had the peel ply, the fiber, and the vacuum bag ready, all I had to do was add the layers on top of the epoxy and strips. Instead of using a fitting, I just made a wrinkle in the bagging film and used the sealing tape to seal the hose itself. I bought a cheap 3cfm vacuum pump a Harbor Freight and it worked well. The existing cracks cause excessive leaking so I added some painters tape on the outside to cover the cracks. That worked well and I achieved a good seal. 

[Bartman]

It may not be the prettiest result, but I’m pretty sure it will be much stronger than what I had. Thank you to Byron for chronicling your 2013 rebuild, and for the updated information and photos you added. 

After removing all of the vacuum bagging materials I’ll post a picture of the final result. 

[Fly Boomer]

11 hours ago, Bartman said:

I did much of the preparation at home and today I brought the cowl and all of my supplies back to the airport and began the finishing work. 

Nice description!  This will help someone else with less experience.

[Bartman]

Decided to put two more squares of cloth weave to clean it up and give it a more finished look. 

Final results with the vacuum bagging layers removed. Still had to remove the sealing tape. I had it sitting in the sun for the heat to accelerate the curing process but the sealing tape became very sticky and needs to cool before removal.