Nose Cowl
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rw_flyer
New member
I split mine last summer. It worked well. Clean up the inside of the bowl by sanding smooth the chopper-gun finish from the factory, glass in any reinforcements you want to add, then prepare to use the unsplit bowl as a mold to make the flanges that will connect top and bottom halves of the split bowl. I used wet layup with about 6 layers of cloth. Overlap the intended seam line about 2-3 inches on either side of the line. Make sure you use plenty of mold release so you can peel the flange pieces back out prior to the split.
Prior to removing the flanges, drill some #30 index holes to use when repositioning the flange for final bonding. Also prior to removal of the new flange, drill the top & flange for the cowling screws. Finally, remove the new flanges. This will be difficult and take a lot of prying even with the mold release. If you want more assurance of release, use a shiny celophane-style tape on the inside of the nose bowl prior to laying up the flanges. It won't make as nice of a part due to the folds of the tape, but it will come out a lot easier and the flange is internal, so who cares if it is slightly ugly.
To separate the halves, after removal of the molded flanges, I used Fein Multimaster tool with the 3/4 round blade, worked great with a thin, controllable kerf. That's expensive if you don't already have a Multimaster. The hack saw solution would work too. If you're willing and able to refinish/repaint the nose bowl after separating the halves, then the sawcut kerf doesn't really matter if you fill the cut after separating the two halves. That way you'll end up with a knife edge. Here's how:
Sand the top half so that it has a straight edge. If you remove a little extra material it won't matter, having it straight is the important part. You're going to build up the bottom half of the bowl separation line to but up against the top half edge.
Prior to bonding in the new flanges, using a sander, bevel the edge of the lower bowl to 45 degrees. If it gets sanded back a bit that's ok as long as not excessive.
Bond in the new flanges to the bottom half of the bowl. Use the cleco holes you drilled to reposition. Put in nut plates in the upper half of the flanges and trim.
Use blue tape to cover the top half edge. Use plenty of tape on both inside and outside. This tape will be your mold release. Some blue tape on the flange area that's under the upper nose bowl half is also a good idea (but don't put blue tape beyond the bottom of the upper half).
Assemble the two halves and now you'll have a "trough" in the beveled out edge of the bottom half. Pull apart some cloth to get long 3 to 6" fibers, make a big pile of fibers. Then do a wet layup with the fibers laid in running down the trough. This will fill the gap and make for a very strong reinforcement that will resist damage when you remove/reassemble the cowling. Lay in extra fibers and resin so it will end up mounded. After curing sand down the mound to be flush with the upper and lower halves. Remove the upper half and release tape.
Sand and fill pinholes to prepare to finish. You'll also need to fill the index holes. Rather than just stuffing the holes with resin, which will crack, you should countersink both sides of the hole and fill with a structural filler slurry in your resin, then sand flush.
My cowling was also cracking quite a bit around the leading edge opening, and the rivets around the upper half bracket. I spent some time cleaning up and reinforcing these areas with extra layups. Lastly, to get a good flange "mold" inside of the nose bowl it is necessarry to remove the "ramps" that are glassed in on either side of the nose opening. Cut and sand them out to make your flanges and cut to split the bowl, then after the flanges and fillings are complete just glass the ramps back in. Make a mold out of cardboard and blue tape to build the replacement ramps.
Prior to removing the flanges, drill some #30 index holes to use when repositioning the flange for final bonding. Also prior to removal of the new flange, drill the top & flange for the cowling screws. Finally, remove the new flanges. This will be difficult and take a lot of prying even with the mold release. If you want more assurance of release, use a shiny celophane-style tape on the inside of the nose bowl prior to laying up the flanges. It won't make as nice of a part due to the folds of the tape, but it will come out a lot easier and the flange is internal, so who cares if it is slightly ugly.
To separate the halves, after removal of the molded flanges, I used Fein Multimaster tool with the 3/4 round blade, worked great with a thin, controllable kerf. That's expensive if you don't already have a Multimaster. The hack saw solution would work too. If you're willing and able to refinish/repaint the nose bowl after separating the halves, then the sawcut kerf doesn't really matter if you fill the cut after separating the two halves. That way you'll end up with a knife edge. Here's how:
Sand the top half so that it has a straight edge. If you remove a little extra material it won't matter, having it straight is the important part. You're going to build up the bottom half of the bowl separation line to but up against the top half edge.
Prior to bonding in the new flanges, using a sander, bevel the edge of the lower bowl to 45 degrees. If it gets sanded back a bit that's ok as long as not excessive.
Bond in the new flanges to the bottom half of the bowl. Use the cleco holes you drilled to reposition. Put in nut plates in the upper half of the flanges and trim.
Use blue tape to cover the top half edge. Use plenty of tape on both inside and outside. This tape will be your mold release. Some blue tape on the flange area that's under the upper nose bowl half is also a good idea (but don't put blue tape beyond the bottom of the upper half).
Assemble the two halves and now you'll have a "trough" in the beveled out edge of the bottom half. Pull apart some cloth to get long 3 to 6" fibers, make a big pile of fibers. Then do a wet layup with the fibers laid in running down the trough. This will fill the gap and make for a very strong reinforcement that will resist damage when you remove/reassemble the cowling. Lay in extra fibers and resin so it will end up mounded. After curing sand down the mound to be flush with the upper and lower halves. Remove the upper half and release tape.
Sand and fill pinholes to prepare to finish. You'll also need to fill the index holes. Rather than just stuffing the holes with resin, which will crack, you should countersink both sides of the hole and fill with a structural filler slurry in your resin, then sand flush.
My cowling was also cracking quite a bit around the leading edge opening, and the rivets around the upper half bracket. I spent some time cleaning up and reinforcing these areas with extra layups. Lastly, to get a good flange "mold" inside of the nose bowl it is necessarry to remove the "ramps" that are glassed in on either side of the nose opening. Cut and sand them out to make your flanges and cut to split the bowl, then after the flanges and fillings are complete just glass the ramps back in. Make a mold out of cardboard and blue tape to build the replacement ramps.
rw_flyer
New member
Here is the detailed document with some illustrative photos. I had sent to Robert Szego for publication a few years ago but I guess the article was too big to fit into the printed newsletter, so it has just been sitting on my hard drive.
http://antiqueairfield.com/attachment/showu/e3cf70dc-2daf-11df-9c61-00304889f980
It is a 3.8MB PDF file. Hope you find it useful.
http://antiqueairfield.com/attachment/showu/e3cf70dc-2daf-11df-9c61-00304889f980
It is a 3.8MB PDF file. Hope you find it useful.