Two forward, one back
Every project has a few low points and stumbling blocks. I've run into two recently.
The plywood sheeting on the leading edge of the first aileron didn't cooperate very well when it was installed. The ply didn't want to take the curve of the ribs and I had a lot of trouble getting it clamped and glued down. While I was able to win the clamp battle, the end result wasn't acceptable. The ply ended up scalloped between the ribs, much like fabric will do. The ply formed to the ribs fine, but didn't hold that curve between them. It wants to flatten out and the result is a dip between the ribs. The biggest dip is almost 1/4" below the rib profile.
Time to suck it up and do it better. I'll remove the existing ply with a heat gun and a putty knife. Once the aileron is cleaned up I can give it another try. I emailed Rob at Raven Aircraft to get his input. He mentioned that this has given him problems as well in the past. His suggestion was to build a form out of PVC pipe. The pipe gets cut down the middle lengthwise to form two halves. The ply is soaked with water and then clamped between the two halves to form the desired curve. It doesn't need much curve to work, but the wood will spring back a bit when it is pulled from the form. I'll pick up the pipe in the next day or two and try forming up the next batch of plywood.
Rob felt bad that I had to disassemble an aileron and generously offered to send me two strips of plywood to replace the botched pieces. This is above and beyond the already excellent support he provides. Thanks Rob!
The exhaust tunnel is the other thing that has been bugging me. The parts I made would be fine mechanically, but look rough. Most of this rebuild is fixing and changing things for the better. I know that if I stuck with the current exhaust tunnel parts, it would haunt me later. Acceptable isn't good enough. The parts need to be done well.
The tunnel is complicated by intersecting bends near the rear of the ramp. For someone with more sheet metal experience, it could be done well. With enough practice and after chewing through a bunch of aluminum, I could probably get it figured out as well. It may come to that, but I decided to give something else a try first.
Aluminum doesn't like to cooperate much with complex or intersecting bends, but composites handle the challenge well. I don't want a fiberglass or carbon fiber panel for the whole part. I just need the bends and tricky part solved. So tonight, I started making a plug for the tunnel out of foam. I got it roughed out and the first layer of drywall spackle is drying as I type this. Once I get the plug into the shape I want, I'll make a mold and then a practice part or two out of fiberglass. I intend to only use the composite parts for the bends and sidewalls of the tunnel. The ramp will be stainless steel and the side panel will still be aluminum. Once I get a fiberglass part to come out the way I want, I'll build the final part out of kevlar. It's a little lighter and tougher than the fiberglass and isn't as noisy as fiberglass or carbon fiber. Mostly, it's tough, which will be good on the belly of the Pitts. Not much of the composite part will be exposed and it shouldn't be noticeable after everything is painted. It should almost certainly look better than my mangling of the aluminum parts. We'll see I guess.
Even if I go back to all aluminum, I've been wanting to try my hand at plug, mold and part making in this manner. It feeds into ideas I have for other projects down the road.
Here's a picture of the first pass on the plug. I'll commence with the sanding and filling tomorrow.
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