Wooden It Be Lovely? Part Five: Details, Details
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.
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|
Empennage AttachmentWith 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.
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 TubeWith 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.
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
|The control cables through the rear fuselage, the elevator bellcrank and the fairleads for the rudder cables can be seen in this image.|
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.
|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.
|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.|
Landing Gear Brackets
|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.|
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.
|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 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.