Wooden It Be Lovely? — Part Five: Details, Details

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In this installment of his series documenting construction of a Pietenpol homebuilt, AVweb's Matt Paxton discusses things big and small -- attaching the empennage, control sticks, and instrument panel. Now in the fourth year of construction, Matt has some expected and unexpected hurdles to overcome.

Homebuilts

It's wintertime again; the third winter since I started on this little adventure in airplane building. The fourth year on the Pietenpol project rolled around in late November. As I've said before, trying to predict when a project like this will be finished, or even when a component will be done, is a dangerous thing. What I have come to appreciate ever more is that the building process, a journey of sorts, is very satisfying in its own right. I think I realized this before I even started -- I doubt if I would have begun if I didn't believe this initially. But I've come to the point where building the airplane is such a part of my daily routine that I know I will miss it when I'm finished. But then, I'll have the airplane to fly, so there will be a tradeoff.

Pietenpol Air Camper
Pietenpol Air Camper
When I ended my last installment in this series, the wood fuselage structure was built and the various pieces of the empennage were done. I said I planned to work my way forward on the fuselage, and that is what I did over the past nine months or so. Of course, my prognostications of when I might be through were wildly off. As they say, the devil is in the details.

Empennage Attachment

With the stabilizer and elevators built, it was time to attach them to the fuselage. Before doing this, I applied several coats of epoxy varnish to the wood of the various empennage components. Then I bolted the stabilizer to the aft fuselage, using AN3 aircraft bolts and blind nuts that had been screwed to the undersides of the fuselage longerons. The plans don't give much guidance as to the incidence of the stabilizer, so I shimmed it to have a neutral incidence. If this proves incorrect, I can remove two bolts and take out or add a shim, as needed.

(Click photos for larger versions)
Tail Brackets
The angle brackets that attach the tail surfaces to the fuselage are shown in this view. Note that one bolt in the lower fitting lacks a nut. The empennage components will have to be removed for covering and the nut will be installed on re-assembly.
The vertical fin and rudder required making some right-angle brackets out of steel to fasten the fin to the top of the stabilizer and the trailing edge of the stabilizer to the tailpost. I also had to mortice the lowest rudder hinge into the tailpost, but really couldn't do that until everything was pretty much in place and I could see exactly where the hinge needed to be. Drilling the mounting holes for those right-angle brackets at the trailing edge of the vertical fin and the stabilizer was tricky because as I went around the tailpost, from the starting point of the top left bracket, there were four brackets to install, but when I got around to the lower left bracket, its holes would need to line up with the holes already drilled for the top left bracket. It was kind of like a builder's version of the game of gossip in that you worked to make sure no mis-measurements crept into locating any of the holes. I needed to end up exactly where I started. I wish I could tell you I had this elaborate system that guaranteed success, but truthfully, I measured three times before drilling each hole, started with small pilot holes in case I was off line, and just took great care to line everything up, and it worked.

The forward mounting of the vertical fin also required a couple of steel brackets, and these bolted through the stabilizer into plywood webbing on the fuselage. I incorporated a one-quarter inch of offset to the leading edge to counteract the p-factor.

The brace wires were fun to make, and I did all eight in a night using 3/32 aircraft control cable and nicopress fittings. Each brace wire gets a turnbuckle and those are expensive items. Lots of Piet builders try to find inexpensive turnbuckles but I bit the bullet and ordered mine new from Aircraft Spruce.

Control Stick Torque Tube

With the tail done, I decided to deviate from my plan a bit and tackle the forbidding control stick torque tube. I'd tried twice before to cut and weld this assembly but wasn't happy with the results. This time, I had my own welding rig and I practiced welding for several hours before I attempted any welding on the real tubing. This time, the welds went a lot better, probably because I was getting better, and because of the old saw about the third time being the charm. That certainly holds true in building -- you learn enough screwing up the first two times that by the third try, you can often get it right.

After cutting and deburring all the tubing, and cutting out the flat steel for the control horn that is welded to the torque tube, I started in. Fabricating the control horn went smoothly. This is made from two pieces of thin steel plate, flexed so that there is a quarter-inch gap in the middle of the horn. This gives the horn strength, like an eggshell, so that it can be as strong a solid piece of steel, but much lighter. The two halves of the horn are edge welded, and the holes drilled in the ends for the cable shackles. I had to do some filing to make the horn fit onto the torque tube but once in place, I welded the horn to the tube. One thing I learned from the previous tries at welding this part was to make a jig to hold the torque tube and the horn in place while I welded. Some scrap lumber and welding clamps did the trick. The bushings that carry the control stick bolts though the torque tube came next. So far so good.

Next, two sets of tabs to hold pulleys carrying the elevator cables from the aft control stick had to be cut out and jigged into place. The jig I used for the control horn welds wasn't too badly burned and I modified it to hold the pulley tabs. Blocks of wood of the exact thickness of the pulleys clamped between the tabs held them in proper spacing. Once those welds were made, the torque tube was finished. Some of the welds weren't the prettiest but they have every indication of being good, with good penetration of the base metals.

Control Sticks

Front Cockpit
This view of the front cockpit has the seat bottom removed to see the rudder bar mounting. Note the second steel tube that forms a parallelogram with the sticks and the torque tube and keeps the sticks aligned.
Control Stick
The attachment and routing of the elevator cables is clearly shown in this image. The stick grips are BMX bicycle grips from a local discount store.

The control sticks were next. The rear stick needs tabs on the front and back of the stick where the elevator cables attach. Both sticks require a piece of quarter-inch tubing welded in and protruding out the right side of each stick. These form pivots for another tube with flattened ends and appropriate holes to fit over these pivots. This tube forms a parallelogram between the torque tube and the control sticks, and ensures that both sticks move fore and aft in unison. Steel ears also have to be bent and welded to the bottom of both sticks for the through bolts that run through the torque tube bushings and attach the sticks to the tube. It sounds like a lot, but it went relatively quickly. The hardest part was getting the geometry of those ears that fit over the torque tube right so that both sticks moved fore and aft, and were generally aligned longitudinally. After both were installed, I was off longitudinally by about a half-inch, but I think I can live with this.

The two brackets that hold the torque tube were fabricated next and with that, the entire control stick assembly was done. I'd completed in a weekend what had been on my mind for at least two years.

The rudder bar assembly and the front rudder pedals went pretty smoothly. The pedals are made up using tubing cut at a 45-degree angle at the corners and flattened at the attachment end to accommodate bolts from the mounting brackets. Once all this hardware was built and primed with epoxy primer, and painted by a local body shop, I drilled the holes in the floor and installed everything. One thing that made this easier was that I still had not attached the plywood to the fuselage sides.


Control Cables, Bellcranks

Cable Fairleads
The control cables through the rear fuselage, the elevator bellcrank and the fairleads for the rudder cables can be seen in this image.
After the controls were done, I turned to the elevator bellcrank. This is a simple walking beam in the aft fuselage that ensures that the elevators move together. The beam is made from a piece of streamline tubing flattened on the ends and drilled for cable shackles. A shaft runs through the center point of this beam and the shaft turns in bushings mounted on the fuselage sides. Building this part went smoothly; indeed, my prettiest welds to date fused the beam and shaft. Considering the importance of this assembly, that was very gratifying. The mounting bushings are welded to flat steel for bolting to a fuselage upright, and because of the taper of the fuselage, wedges of spruce need to go under the bushing assemblies to make them line up with the shaft and not bind. The plans don't mention this little detail, and it's noodling out these kinds of details that make this project interesting.

Another place where I have "enhanced" the plans is in the control cable runs. My friend Jack Phillips in North Carolina showed me where he installed additional pulleys in the elevator and rudder control runs to cut down on friction and this looked like a good idea. The plans call for the cables to run through the rear seat support and to rub on the wood as though the holes were fairleads for the cables. This didn't appeal to me so I made three brackets for pulleys -- single pulleys on the outboard sides of the rear of the back seat for the rudder cables and a wider bracket for two pulleys in the center to carry the two elevator cables.

When those were made and installed, I started in with swagging the cables for the various controls. The rudder cable runs were pretty long without any support, and the cables rubbed on the fuselage frame members in places, so I made up a couple of fairlead brackets out of steel and installed nylon fairleads in them. These I bolted behind the elevator bellcrank to move the rudder cables away from the structure and to ensure they stayed away from the bellcrank. When everything was attached, it all moved appropriately and without binding. One additional touch I added was stops on the front control stick to keep the elevator from moving beyond a range of about 25 degrees up or down.


Fuselage Plywood

Fuel Tank Template
A template for the fuel tank was made out of cardboard. The cardboard pieces will function as a pattern to cut the aluminum for the tank.

Now that the controls were installed, the plywood could go on the fuselage sides. The one-eighth birch ply came cut into two-foot-by-eight-foot pieces, and very little had to be trimmed to fit. Both sides went on easily with lots of clamping and use of half-inch brass aircraft nails. My only regret was that I opted to use gorilla glue, a urea foam glue that uses a catalyst on one glue surface and the adhesive on the other. The adhesive then foams when it comes in contact with the catalyst and fills the joint. But if you use a bit too much of the adhesive, the glue foams out of the joint and leaves a lot of squeeze-out. I got this on a couple of frame diagonals. I was able to scrape off most of this but it still shows.

After the plywood went on the sides, a couple of stringers that give the sides some shape had to be trimmed, sanded and glued in place with corner blocks. I also glued one-quarter by one-half-inch by one-eighth-thick ply strips along the edges of the top and bottom fuselage longerons to provide some standoff for the fabric. This gives a much better covering job without a lot of bumps created by the gussets that cover every frame joint.

Next, the steel fittings for the cabane struts had to be made, primed and the hole drilled in the fuselage. Accurate measurement is critical for all these fittings, as the fittings will be holding a substantial part of the weight of the airplane in flight.


Fuselage Front
The engine mount brackets are clearly visible here. The fuselage front panel has been removed to make installation of the landing gear/lift strut brackets easier.
The engine mount fittings came next, and they also required accurate measuring and drilling to ensure proper alignment of the engine. The front plywood was cut and slots cut with a Dremel tool for the engine mount brackets to protrude through the plywood. I didn't glue the front ply in place immediately, leaving it held in place temporarily with four half-inch nails. I figured I might need to take it off to install the landing gear/lift strut brackets, especially the front ones.


Instrument Panels

Access Panel
I cut out a panel in the front cockpit seat back that will be hinged with a latch at the top to allow access to the instruments.
Instrument Panel
The panels feature an inlay of mahogany ply inside the birch plywood. A slip/skid ball will be in both panels and the required placards will complete the panels.
The landing gear brackets and the whole method of hinging the cub-style gear had been on my mind for quite a while. So, while I though and sketched exactly how the brackets would work, I took a detour and fabricated the instrument panels. I'd seen some very creative panels, showing beautiful workmanship. I'll be looking at it a lot once I start to fly the airplane, and it also seems to be one thing that most people want to look at when they look at the plane. I decided to go with a two-tone wood panel, with an ovoid inlay of mahogany ply inside the lighter birch plywood. This wasn't technically hard to do, but took time to get the joints just right. The holes for the instruments also took time to get just right. in both cases, I had to be careful not to take off too much material or the piece would be ruined. So, I was constantly fitting, marking the areas needed a little more filing or sanding and then refitting. In the end, I was pleased with the results. The airspeed, tach, and altimeter are grouped across the bottom of the inlayed area, with the oil temp and oil pressure above. The antique mag switch I bought off eBay for $10 is on the far left and the compass is on the far right. A slip/skid ball will be centered below the instrument. The front cockpit got the inlayed panel treatment but no instruments other than a slip/skid ball.


Landing Gear Brackets

Landing Gear Fitting
This is a close-up of one of the completed landing gear/lift strut brackets. The holes for the gear hinge pin have not been drilled yet.
With the panels done, I turned to the landing gear brackets. Bernie Pietenpol used some cylindrical steel, drilled for a bolt, with two pieces welded to the landing gear "V" and the larger center cylinder welded to the bottom of this fitting. The plans stress how critical it is to get the alignment just right. My problem with this setup was that it looked to me like the welds holding this hinge to the "V" and to the fitting were also very critical. I'd seen several other designs and settled on one that used large steel ears welded to the fitting with a single piece of tubing welded to the top of each leg of the "V" and an additional steel saddle welded over. A quarter-inch bolt runs through one ear on the fitting, through the tubing on the "V" and out through the other ear. This is very similar to what you'd see on a J-3 Cub.

The fittings required a lot of cutting at my friend Bob Conlon's shop using his power hacksaw and then final shaping with a grinder and wire wheel. Two pieces of two-inch angle iron forming a square section tube made a jig to ensure that the ears were parallel to each other and all the same distance apart when welded -- this also had to be made up. Each fitting assembly was made up of four individual parts welded together. I got smart this time and made additional parts in case I messed up a fitting. That way, I wouldn't have to go back to Bob's and cut more metal, wasting a day. That also made me more willing to discard the worst of the fittings when I was done.


View Rearward
This view gives an idea of the empennage, and also my shop layout. The large rounded gray object in the foreground is the cardboard mock up of the fuel tank.
The gear/strut fittings are now fitted to the fuselage, and the inside brackets made, but the bolts have not been tightened. The holes for the landing gear hinge bolts have not been drilled but will be soon. I need to assure that those holes all line up so that the finished gear "V"s don't bind. I think I'll wait until the "V"s are built and the hinges welded to the top of the "V"s and then use those as guides to ensure that everything does work properly. That's another example of the difference between plans building and a kit. With most kits, the assembly manual will provide a lot of guidance on the order that jobs should be completed. With plans building, the builder needs to think ahead, considering the ramifications of what he or she is building, and how that might affect building other parts of the airplane.

The casual visitor to my shop might not notice many of the things I've done to the project since last spring, but they represent many hours of work, and probably more hours of thought and planning than the actual labor. Building an airplane truly exercises the whole body, physically and mentally. And for many of us builders, that's the appeal.