Work on tunnel plug
Tonight I got the drywall spackle sanded down and coated the plug with a slurry of epoxy and micro. After the next sanding, I'll probably do at least one more coat of micro, but mixed thicker. Once that is sanded and shaped, I'll try to pull an initial mold off the plug.
This picture shows the area that roughly represents the desired parts. This plug will let me make molds for both the right and left side walls of the exhaust tunnel.
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.
Fuel pump and gascolator
The new Buzz Aero wobble pump showed up this last week. It is (close to) a bolt in replacement for the D2 pump that was in my Pitts. The bolt pattern matches and I just had to flip the pump handle 180 degrees to get it to fit the existing mount. I was pretty happy about that until I tried to install the inlet AN fitting. On the D2, the inlet fitting was towards the bottom of the pump when mounted. The new pump has it clocked near the top. Not a big deal, but now it interferes with the front tube on the seat.
I removed the mounting hardware and rotated the pump backwards until the fitting cleared sufficiently. Then I drilled new mounting holes in the existing mount. It was an easy modification and now all the original fuel lines will work correctly. I'll be replacing the hoses when they get re-installed, but it's nice to see everything fit.
I also pulled the new ACS high-pressure gascolator out of its box. It is much shorter than the old part and won't project out of the belly pan. The old gascolator had a good 3-4 inches that stuck out of the bottom of the plane. The new one fits much better and only the end of the Curtis drain valve will be exposed through the belly. I still need to fab up a mount for the new part.
I had one last welding task on the fuselage. There have been harmonic issues on some S1's with the diagonal tube on the bottom of the fuselage in the section behind the seat. It is the longest tube in the fuselage and resonates at frequencies that are reinforced by some power settings. The result can be a crack near cluster welds. The solution is to change the resonance frequency to something higher. I did this by welding in a short 3/8" steel tube between the lower seat back and the diagonal. This raises the harmonic frequency quite a bit. After 30+ years, I couldn't find any cracks, but this will eliminate one possible cause.
I think I've got the fuselage ready for blasting and shooting primer. I need to order a few supplies and then we'll start working through the steel parts until they are all protected again. I'm planning on using the PolyFiber two-part epoxy primer. I'll use white instead of chromate green, since the topcoat for the tubing will be white.
<EDIT> Added pictures of the new pump and the mod for the bottom diagonal behind the seat.
Shaping aileron leading edge
Tonight I started shaping the leading edge on the first aileron. I started with a half-round mill file and then switched to a sanding block for the final bits. This worked fine on the first section that was only 4-5 inches long. On the long section, I pulled out my hand plane and set it up to barely shave off a bit at a time. The plane was much faster and more consistent than the mill file, which only got used a bit before switching to the sanding block.
I need to make a template when I'm happy with the shape and use that to make sure the whole part is consistent. It will also be used to make sure the other lower aileron is the same shape.
Aileron leading edge
Tonight I installed the 3/8" steel tubing into the leading edge of the first aileron. Not much to it. I added a little flox to the epoxy to keep it from running out and to fill the inside corners. Once it's cured, I can figure out how I'm going to shape the leading edge radius. It shouldn't be hard, but may be a little tedious.
Wrapped up fuselage welding
I spent a couple hours today going over the fuselage and making sure I've got it ready to blast and paint. I think I've got a plan for mounting the transponder without any additional tabs needed. I also taped the old panel in place to take some measurements for clearance with the tank and frame. It looks like the current plan for the panel arrangement will work. I made a list of the new hardware needed for the smoke oil tank, battery mount, tail spring and engine mount. Getting close to putting things back together!
Tailwheel spring mount
Friday, I spent a little time working on the tail wheel spring mounting. It was requiring too many washers to drop the front of the spring to the right angle. I welded up a standoff to take the place of most of the washers, but left enough leeway for adjustment. It worked out well and was just about the last thing to weld on the fuselage. Hopefully in the next couple weeks, I can media blast the steel and get the epoxy primer sprayed.
Battery mount complete
I finished up the battery mount today. The new steel tabs were welded on the seat back tubes and the aluminum mount plate was installed. I mounted the battery to the plate with the battery box that came with the battery. The master solenoid was also installed on the aluminum plate. The whole thing is very stiff. I was a little worried about the gauge of steel used for the tabs, but not anymore. I need to replace the hardware, but otherwise the job is complete.
A couple small parts
Today I installed some gussets to support the rear outside corners of the heel trays. These will get nut plates to help secure the trays. Everything is in place for making and installing the trays.
I also started figuring out the mounts for the battery installation. I made one bracket to see how it worked. It looks pretty good, so I'll make three more and weld them up the next time I'm at the hangar.
Smoke tank mounting finished
I finally wrapped up the smoke tank mounting tonight. I got the bushings installed in the new cross tube for the front of the tank. After that I drilled the tank flanges on the front using the bushings as a guide. Then I drilled the rear tabs and tank flanges. I'll order the right lengths of AN4 bolts for both the front and back in the next big Aircraft Spruce purchase.
I'm having some trouble getting new pictures to display in the galleries, but here is a shot of the finished mounts from the bottom.
UPDATE - Worked with Josh from gpEasy and got the gallery problem fixed. Thanks Josh! There are more mount pictures here.
