Tiafun Cowl repair cont.

 

I got back from Brazil Thursday morning. After a day of nonstop sleep I got back to work on the cowl repair. I finished cleaning up the internal blister layup and blend sanded the new laminate into the existing laminate. 

 

inside ply's trimmed and blended

most structural glass repairs only need one inch per ply overlap to fully develop the tensile strength of the ply. In the case of the blister, this was a 6 ply repair with 3 structural plys on the inside and outside, each layer overlapping the existing laminate by 1 inch. This is truly overkill, but meets the requirement of restoring original capability, plus one ply. I will spray the inside of the cowl with a grey trunk paint and this damage will be completely hidden. The outside micro balloon and epoxy was completely cured when I got back from my trip. I rough sanded the micro with 100 grit sandpaper with my orbital DA sander. This reduced the high points and leveled most of the surface. There will always be slight pin holes left that will need to be filled with Pinhole filler or spot putty.

micro balloon blend sanded

This picture shows the micro balloon after it was rough blended with the DA. The surface is wave free but there are still small pin holes and very shallow applicator marks. I followed up with the DA by using a flexible rubber sanding block with 180 grit paper. This served to further level the surface. This cowl is very easy to blend out because there is severe contour. The hardest surface to make smooth, level and wave free is a flat surface. For that type of contour I will usually stop at this point and spay the surface with a sand able primer. A long sanding stick quickly shows high and low spots. I usually try to level the surface with a combination of sanding and spot putty fill. Spot putty is designed to adhere to primer paint.

catalyzed spot putty

This is my preferred spot putty. This is far superior to the Auto-Zone lacquer based products.  This material is catalyzed like auto body filler. Its ready for sanding in 10 minutes. The cycle time of your products makes all the difference in speeding up the repair. I could have used light weight auto body fill in place of the epoxy micro in the earlier step, but since I knew I would be in Brazil for 5 days, I knew the time didn't matter. Epoxy Micro is a very strong filler and is superior to Automotive bondo.

blended spot putty

 This is what the spot putty looked like after an initial rough sanding with 100 grit. I finished the feathering with 180 grit and there was hardly any left.

There was one other issue with this cowl and that was a corner Dzus fastener had pulled thru the cowl laminate. Normally I would look for an oversize repair fastener and enlarge the existing hole with a step drill or tapered reamer. I would then install a repair fastener. But that would require a crimp tool and a special set of repair washers. This fastener fit just fine but had no retention on the back side of the laminate. So I cleaned the metal with  rotary wire wheel and sand blasted the metal to a dull finish. I then mixed some T-88 structural epoxy adhesive with a small amount of chopped cotton fiber. I then bonded the Dzus fastener back in place and formed a fillet around the inside surface. This will stop the fastener from backing out.The main difference between structural epoxy adhesive and epoxy laminating resin is the addition of molecules of an elastomeric. This dramatically increases the peel strength of the adhesive and stops all tendency for  delamination

This lower cowl had been repaired previously after the original nose gear collapse. I have noticed that the door underwent extensive reconstruction and is over 1/2" thick in some places. The leading edge of the door sits below the existing contour and quite frankly looks terrible. I am thinking about blending the the existing contour to the door contour after I rework the door hinges to slightly lower it. The idea is to get a smooth surface when the climb door is closed for soaring. That will be the subject of the next entry in the blog. My friends at the airport have repositioned the Tiafun fuselage into a larger hanger where I will have full access to the engine. Since the plane skidded on its fwd engine mount, I am unsure of the actual condition of the metal engine mount. There is flaking paint on the mount and I suspect its just a bad paint job, but I will pull the engine and remove the mount so I can sand blast it and inspect the for cracks. Dan has already bought a new Saur fwd engine mount that fits like the original. I will prime the mount with zinc chromate and finish with a polyurethane top coat. The last thing you want to do is powder coat any aircraft parts.Powder coating is a plastic film that is fused to the metal in an oven. It doesn't not have any ability to act as a rust conversion coat and in a short period of time it will develop cracks and water will migrate.