Carbon Spinner Bulkhead
The 172 went through its annual a couple weeks ago and while most of it went pretty uneventful, we found some cracking in the forward spinner bulkhead plate. After some deep digging for a new one, we couldn’t find any used/serviceable ones available and new ones from Textron were about $1400. There is a rule in 21.9(a)(5) that states replacement parts can be “produced by an owner or operator for maintaining or altering that owner or operator’s product.” I decided to make my own out of carbon fiber to save some money. Obviously there are some differences between the original metal part and a carbon fiber part. First, I didn’t really care about making the new part the same thickness as the old one, but rather the same stiffness. I found the new thickness required using relative stiffnesses and decided the part needed to be made of 4 plys of carbon. I decided to just use 3k plain weave carbon and West 105/205 cause that’s what I had.
tcarbon = tmetal * (Emetal/Ecarbon)
I started off making a mold by laying up fiberglass over the original part. I released the part real well with wax and then applied about 6 layers of glass and then envelope bagged it to get a nice tight contour. After popping it off the piece, I sanded down some of the rough areas, applied some micro all over, and sanded some more until I was happy with the surface quality and shape of the mold. Thinking about how the carbon was going to wrap around the edge of the part when I vacuum bagged it, I decided to make a cardboard flange to go around the outside of the mold so that the carbon didn’t have to make a 180 degree turn around the edge. I sprayed the mold with some gloss clear coat to seal it and then released it with wax.
I then wetted out each ply of carbon in the mold individually with resin and made sure they were fully contacting the mold. The layup schedule was 0,45,45,0 to make the layup symmetrical to avoid warping and get the part to be as quasi-iso as possible. I trimmed the edges so the carbon didn’t extend too much beyond the flange, put peel ply on the back side so I could bond nut plates later on and then bagged it up!
After taking it out of the bag, I cleaned up the edges nicely with a Dremel and some sandpaper. The next part was drilling the holes accurately. There are two sets of holes, the holes for the propeller bolts and the holes for the spinner attach bolts. Each set needed to be located with respect to each other and also centered so the plate was balanced.
In order to accomplish this, I made a secondary mold off the original part to find the location of the holes with respect to one another. I just used one ply of fiberglass so I could press the glass into the holes of the original part to make a little mark. Once it dried, I popped it off the original part, put it on the carbon part, and had the locations of all the holes.
I designed a drill jig that allowed the prop bolt holes to be drilled according the hole pattern and also allowing the holes to be upsized in steps to avoid damaging the carbon while drilling. The drill jig was water jet cut out of HDPE and then I pressed some drill bushings into it and held it in place with dowl pins/drill bits. The whole setup cost like $20 which was pretty worth it to be able to drill the holes accurately.
After all the holes were drilled, I did a final sanding/bond prep on the part and bonded click bond nut plates to the back so the spinner could bolt on. Bonded nut plates seem to scare a lot of people, but we use them all over the place at work. As long as you properly prep them, they can be a pretty awesome tool.
Last thing was to put a couple light layers of clear coat on to protect from galvanic corrosion with the metal parts it will be touching. I’m not super worried about that because the prop is painted and we should have a good layer between bare carbon now.
I was slightly worried about the carbon getting crushed when we torqued the prop bolts down so I waterjet a “washer” out of .040” aluminum to go between the carbon and prop bolts to spread out the load a little bit. I primed it with zinc phosphate to mitigate corrosion issues and with that it was pretty much done!
When we torqued the bolts down, there was no carbon crushing sounds or any signs of damage. I have put about an hour on the airplane with this and inspected for damage after the hour with no signs of anything bad. I thought this was a pretty fun and useful project and learned a lot doing it.
