I didn't have the proper bulkhead for my flow guide yet, so I made one out of a bit of 1/4 foam. I used this to position the flow guide on the prop hub. Next I duct taped all around the edges and made some plywood supports both to support everything, and rough out the shape. I covered all this with a sheet of plastic and got out the pour foam. A cup of pour foam didnt go far. My rough guess is that I'll need a couple of gallons of the stuff to do the job. Hmmmm. I dashed over to Home Depot and got three cans of "Great Stuff" expanding foam. The first can did more than the pour foam, but didn't really make a dent in the shape I needed. I decided to try bits of blue foam to fill the voids. I placed chunks of blue foam all over until it seemed to cover the basic shape, then squirted "Great Stuff" in between the blue foam to stick it all together. It looks ugly as hell, but I think I'll be able to carve [read hack] it into shape. I need more "Great Stuff". Another 4 or 5 cans should do it.
The home depot guy recommended the Latex expanding foam, so I got a couple of cans as well as the "great Stuff". I squirted the latex foam into a bunch of voids and went inside to wait for it to set. Two hours later, I found that it was still gooey. I read the instructions which say it takes 24 hrs to cure. The instructions also mention that it's water soluble, so I power washed it out of the way and carried on with the Great Stuff (which sets in about an hour). After another six cans of Great Stuff and a lot of carving I had an approximate shape. Occasionally I'd uncover a void while carving and have to fill it with foam, wait for cure, then carry on. I think this would have been easier with blocks of urathane foam, but I didnt have any and didnt want to pay for all the foam I'd need. Actually, urathane would probably have cost less than 8 cans of Great Stuff at 4 something a can.
You can't sand Great Stuff, but you can carve it fairly accurately. My plan was to make a smooth plug, cover it with something for release and glass it. If I'd used Urathane the shape would have been easier to get, but then I'd still have to figure a way to release the glass. I tried covering the foam with sheetrock, but decided the cure time wasnt good enough and you couldnt fill large holes with it. While at Home Depot on one of my Great Stuff runs I came across plaster of paris. $7 for a 25 lb bag. I bought two bags. You get 30 minutes to apply it and then it sets up like a rock. I covered all the foam with a skim coat, waited for it to set, then sanded it. It doesnt sand very well. It just clogs up the paper. I mentioned the problem to Char. She had worked with plaster of paris before and told me that you can wet sand it. The water soaks into the top layer and it starts to "unset". Cool. I tried this and, sure enough, I was able to wet sand the plug. I'll need some more plaster to fill some dips, but I think this can be done. It's starting to look like a cowling.
I slapped on a skim of this stuff and, sure enough it's hard in 20 minutes. Still a little fast, but its much easier to spread. After a few hours it was still too wet to sand, but I found that I could "scrape" it easily. After a few experiments with various tools I settled on a sharp, clean 3 inch putty knife. I found I could scrape the surface smooth very easily this way, pulling off anything from just a sliver to a 1/2 inch lump as needed. In a couple of spots I got down to the P of P and had to hack it out and replace it with joint compound. I needed to shape the cowl around the bump of the NACA scoop inlet, so I just added an inch or so of sheetrock filler and, after cure, carved it into a "pleasing shape". Don't try this with micro! So far I've worked on the cowl for about 24 hrs total, and I have a fairly smooth plug.
After about five or six sessions with the sheetrock compound, with wet sanding in between, I had a 320 grit plug the shape I wanted. Next day I waited for it to dry out, then dry sanded. Once satisfied with the plug I sprayed it with the latex paint. This dried in a few minutes but seemed a bit soft and was difficult to shine. It'll probably peel off with the epoxy. I had a couple of cans of enamel spray paint I'd been using for guide coats when sanding. They were dark blue, but what the heck. Still not enough paint. Hmmm. I found a large can of epoxy appliance enamel. This did the trick. I was happy with the shine, so I polished the plug with turtle wax and taped the edges with packing tape for a good release. I replaced the duct tape around the edges and epoxied some peel ply onto the bare plug ready for the reinforcements later. This was my first layup for months. I was a bit out of practice. After 4 plys of BID and 5 hours work I was back in practice. I had a bit of a battle with some of the curves. You know, when you squeege the layup into one curve and it pops out of another. I battled the last ply for about half an hour then decided "screw it" and made a cut through the ply about 5 inches long in one of the curves. Now, finally, it would lay down flat. I cut a piece of bit to repair the cut and overlap an inch all around, then covered the whole lot with plastic and squeeged it again. No one will ever know.
Sitting in the hot tub that night we admired the new dimension to the plane. I'm very pleased with the shape. Somehow the cowl just looks "right". Next day I lifted the cowling gingerly. It came off, complete with all the paint and quite a lot of the sheetrock compound. Hmmm. I thought that latex hadn't stuck well. I pulled on my release tapes and peel ply around the edges and the paint came away from the epoxy cleanly. To get the paint and sheet rock out of the middle I resorted to the power washer. It took a while, but eventually it came clean. The inside of my cowl is very smooth. It will make an excellent mold if I decide to make another cowl. So far in my experiments I've proven that neither Turtle wax nor Pledge work for mold release. I think I'll get a can of the proper stuff for next time.
Once I had the rough shape of the cowl outlined in blue foam I used 3 cans of triple expansion Great Stuff to stick it all together. Char was doing some night gardening so I got her to pitch in. Next day I began carving. An hour or two of carving got me to a reasonably even shape.
I tried both dry and wet sanding. Both techniques have their pro's and cons. An unexpected advantage of wet sanding, if you're small like me, is that the wet sheetrock builds up on the soles of you're sneakers. If you start with the sides you gradually gain height. By the time you're ready to do the top, you can reach it! Dry sanding 80 grit or so is better for fine work, but you have to wait a day or so for the sheetrock to dry. As I got closer to the final shape I removed the PVC exhaust and plugged the hole.
I spent a couple of days filling and sanding sheetrock until I'd got a symetrical and pleasing shape. The cheeks are there because the turbo needs some room and I felt that I needed some kind of structure, otherwise the cowl would be weak and would look like a "lump on the back". The outside edges of the cheeks are parallel with the fairings on the fuselage. So airflow and drag wise, they don't stick out at all. One issue I'm not sure about yet is the fact that the upper and lower cowlings dont match each other. When taken as a whole, they don't merge. This is purely an aesthetic issue. Maybe I'll extend the lower cheeks to match the upper ones later. Maybe not.
I was determined to leave the cowl to cure overnight, but later that evening I went out to inspect the layup. I decided that it would be a good idea to do some finishing work while the cowl was still supported on the plug. I mixed up some dry micro and started in. I put a skim (may 1/16th) over the relatively flat parts, but when I started on the rounded cheeks I came across the old problem of how to apply dry micro to a curved surface. I came up with a new (to me) idea.
How did the waxed sheetrock plug work out?, you ask. Well, it came away with a very thin layer of a new filler I've invented. Instead of micro, mix epoxy with sheetrock putty. When cured the result is hard, heavy and quite difficult to get off. It's saving grace is that its water soluble. I dumped the cowl in the pool overnight. Next day the stuff scraped off without too much effort.
Things I learned about making plugs:
1. The (wood) base you use for you're foam plug needs to be strong and well anchored so that the foam can't move when you press on the plug later. 2. You need to build the initial foam plug at least 1/2 inch smaller than the final shape to be sure the foam doesn't poke through anywhere, so the accuracy of the foam carving isn't really important. Given this fact, you can make the initial plug with just about anything. Lumps of blue foam stuck together with great stuff work fine. Carve the foam fairly close to the shape and dig it out if it comes close to the surface anywhere. 3. Sheetrock compound isnt very strong when thin. I think my plaster of paris coat might have helped because it gave the compound a good solid base to rest on. The only problem with this was that the plaster of paris base was too close to the final shape and it showed through here and there making it difficult to sand the mixed surfaces. On the upper cowl I didnt bother with P of P and the job was much easier. 4. Use a good mold release wax, or paint the plug with something that will peel off the epoxy easily (like latex). Or, don't paint the plug at all. Sheetrock is easy to get off the cured epoxy. 5. I should have done the cowling before painting the fuselage bottom. My duct tape masking pulled off some paint when I removed it. 6. NEVER try to remove paint from epoxy with paint stripper. Thankfully I did a test first. The paint stripper removed the epoxy too. 7. Making plugs is fun!
During the cowling modifications, I took a day off to help Bulent install his spar.
All the above meant that the rear foot or so of my upper and lower cowling needed to be reshaped. With the engine, redrive and flow guide in place, and the cowls screwed on tight, I started in with the dremel. First I cut the curve from the aft wing root to the "cheeks" going through both cowls at the same time. This left me a gradually widening gap between the cowls from nothing at the wing roots, to about an inch where the curve met the cheek. I considered making a vertical join here, or even using this as an air outlet. After staring at it for a while and pacing around it, I decided this would look better if the two cowls met at the back and the trailing edge of the wing simply curved into the cheek. To make this happen I had to cut the join at the bottom of the cheek almost all the way forward to the firewall. Once this cut was made the "flat" part of the cowl could be lowered a little. The net result is that the curvature of the wing is simply extended all the way to the cheek. This was one of those times when, all of a sudden, everything just seemed to look right. I clamped the trailing edges together and bondoed sticks to hold the two parts in place when I remove the upper cowl later and bridge the gap on the inside with glass. Next I followed the curve of the cheek all the way to the spinner and cut the horizontal lips off. Finally I added more sheetrock on top of the existing cowl to give me the 1/2 gap around the spinner and the redesigned boat tail.
I glassed over the new sheetrock with 4 ply BID and then cut away the old cowl from underneath. I added 3 ply BID lips along the base of the top cowl all the way to the spinner so I could attach it to the lower cowl and added a couple more nutplates along the lip.
"Are you planning to put cowl doors where you have those tapes?", he asks. "Uh.. yes", says I. "Why?" was his one word reply. "Well, I've flown cessnas with their tiny little oil check door, and owned a Piper with full size cowl doors. I liked the Piper much better because I could have a good look around during pre-flight", I explained. "How fast did the Piper go?" asked Paul. When I said 110 with the wind behind it, he asked "Do you know how much pressure is on the upper cowl at 200mph?". It wasn't hard to see where he was going with this. Basically, with a series of leading questions, he advised me not to make large doors in my cowl. I took his advise, but I did expand the plans door to about 10 inches * 7 inches. I figured out how to install the Hartzell catch only after looking carefully at Al's catch on his LongEz. I had a spare strip of extruded hinge which was just about the right size. Rather than riveting it in place I installed three nutplates on each side. When the door was done I decided that there was a chance the airflow could get under the leading edge, so I added two nutplates for torx screws just to be on the safe side.
I was just finishing off the door installation when Tom (Glasair builder from across the way) dropped by. "You know, most people do this stuff before final paint", he says smuggly. "Yea, I know.", I replied. Tom's been a good buddy since I left the hangar, but I swear I'll drop him one of these days....
The rest of the cowling work comes under engine and Finishing so this chapter is now, finally, done. But - I'll add here for anyone thinking of making their own cowl - finishing the cowling was a LOT of work.