Wooden It Be Lovely? Part Two: Building the Wings
Few of us have the courage, attention span, or time to build a kitplane, much less a plans-build aircraft from scratch. But AVweb's Matt Paxton does, and he continues his saga of building a classic Pietenpol Air Camper. In Part Two, Matt takes us through the joys and agonies of constructing the most critical part of any airplane: the wings. What's it like to start with a stack of aircraft-grade spruce and some blueprints, and wind up with 32 bent-and-glued ribs, spars, struts, leading and trailing edges, all jigged and mated and trammeled into a wing that flies straight and true? Well, for one thing, plan on it taking 15 months ...
What's the most critical part, the indispensable portion of any airplane? The wings, of course. There have been tail-less designs and flying wings without fuselages, and gliders without engines, but you really don't have an airplane without a wing.
Given the central importance of the wing to any airplane, why would I choose to build the wing first?
Well, in my case, it was a combination of procrastination — I could put off making certain decisions on the building of the fuselage — and the perception that building the wings, with their ribs and spars, represented less of a challenge to my building abilities. Don't get me wrong; the wings actually represent a great deal more assembly work than the fuselage or tail on the Pietenpol. First, a rib jig has to be made, then 32 ribs built, then the ribs assembled onto the spars, then the leading and trailing edges attached, followed by assorted compression struts and drag and anti-drag wires. Then, of course, the ailerons have to be built in place and cut out of the wing structure.
Before the first piece of capstrip was cut or the rib jig made, the first order of business, as suggested by a wise person on the Pietenpol list-serve, was to thoroughly study the plans. I had gotten the plans about a month before the wood arrived, so I had time to look at them closely and begin to figure out the order in which things needed to be done. I also began making a list of the tools that I lacked and would need and a second list of hardware and supplies to be ordered.
The Air Camper, unlike many other homebuilding projects, is not available in kit form. There are suppliers of materials kits, such as Western Aircraft Supply and Aircraft Spruce and Specialty, and there are component suppliers, like Replicraft. But the Pietenpol is what's called a 'plans-built' project. You buy the plans from one source, and you obtain the materials to build it from other sources.
A bit of history as I know it about the Pietenpol Air Camper and Bernard Pietenpol: As I understand it, the original Air Camper used spruce and fir that Bernie Pietenpol got from a local lumber supplier. He did what some Piet builders still do today — he went to the lumberyard and searched through the stock to find the right size wood with the required characteristics of grain, freedom from knots and warp, and lack of other imperfections. Then he took the wood and milled it to his specifications. His hardware — bolts, nuts, wire and cable — came from the local hardware stores. Today, to save money, a builder might take the time to search through a lumberyard's inventory of spruce or Douglas fir and mill his own spars, longerons and capstrips, but most builders use aircraft-grade hardware, particularly in the crucial areas of major component attachment and controls. Still, it is a testament to the soundness of the original design that Pietenpols have been built with hardware store bolts and lumber from the yard, and flown safely. Pietenpol experts say that the airplane is somewhat over-built, meaning that material probably could be safely removed from some areas but most builders, lacking the stress analysis and engineering training, opt to build according to the plans. And experienced Piet builders say that is the best way to get a safe, easy-flying Pietenpol.
By the time my spruce arrived for the wings in late November, 1999, I had been studying the plans for over a month. One area concerned me — my lack of metalworking expertise, particularly in welding. I decided to buy the steel fittings for the wing assembly from Replicraft. Replicraft supplies most of the steel components needed for the Pietenpol, as well as assembled ribs and fuselages. I ordered the wing attach straps, the center section wing strap/cabane attach fittings, the inner aileron pulley assemblies, the flying strut attach brackets, the drag/anti-drag wire attach fittings and the aileron control horns. These were all made from laser-cut 4130 steel, TIG welded where required and primed in zinc chromate. The cost of the steel fittings came to $510.10.
With the question of the steel parts answered, at least for the wing, and the plans now dog-eared by all the attention they had been getting, I felt ready to start with building the ribs as soon as the wood arrived.
First, we need a jig...
The first order of business, after taking inventory of the spruce shipment, was to construct a wing rib jig. Basically, this is a form, laid out on a heavy base of plywood or other solid, unwarped material. The outside dimensions, top and bottom, of the rib are accurately drawn on the base and then blocks are attached to the base to hold the top and bottom capstrips and internal braces in place for gluing. Short pieces of spar material are also attached to the base in the correct location to insure that the finished ribs will slide onto the spars and fit properly.
I used three-quarter-inch plywood for the base of my rib jig. The jig base measured about six feet by 18 inches, more than enough to accommodate the 60-inch chord of the ribs. The plans come with a photocopied full-size template of a rib, but my measurements of the template compared to the dimensions called for on the plans didn't agree. The template was about an inch longer than the plans called for, and the distances from a reference line along the chord of the rib to defined points along the top and bottom of the rib didn't match up either. I learned later that the paper the template is photocopied on often stretches some during the copying process. I started from scratch, and measured and re-measured the points that define the upper and lower surface of the rib, as defined in the plans. Then I carefully drew lines connected the points on the jig base. Those lines would define the shape of airfoil of the wing, and I checked and rechecked those measurements to be sure I had gotten them right.
Once I was sure my outline for the rib was okay, I cut several dozen one inch by one-inch blocks from my remaining plywood and glued and nailed them so that the inside edges of the blocks lay just on the outline of the rib outline. Then, I glued and nailed blocks to the inside of the line, offsetting the blocks a quarter inch plus a fraction to allow just enough room to slip the spruce capstrip between the inner and outer blocks. This jig locks the upper and lower spruce strips in place, and if the bracing is installed correctly, will insure that all ribs are uniform and that the airfoil curves are per the plans.
In addition to the blocks that position the top and bottom strips, I marked the locations for the internal braces that make up the truss structure of the rib. I didn't install blocks to locate those pieces, figuring that I could locate them in the proper place by following the outlines I marked on the jig base. In retrospect, I wish I had put in some blocks, because my truss bracing didn't turn out exactly uniform when comparing one rib to another. The bracing is sound and properly located for strength, though, and that's the most important consideration. From a purely aesthetic point of view, perfectly identical ribs would have been nice, but, they'll be covered up with fabric anyhow. One lesson I have learned, though, as I progress with the Pietenpol is that parts that look good and are uniform fit together better and probably result in a better job, so now I pay even more attention to detail than when I started.
...to fab the ribs
Now that the jig was built, it was time to start building ribs. I had read somewhere that rib-building went a lot faster if you measured the various pieces that made up the rib and precut enough pieces to make up many or even all the ribs. Put the individual pieces in bins, and then just take one of each to assemble a rib. That way, once the cutting of individual pieces is done, you don't have to keep going to the saw every time you start building a rib. I took some scrap plywood and made a series of pigeon holes for the rib pieces and cut enough for about a dozen ribs.
Next, I needed to come up with a way to steam or soak my top capstrips to keep them from cracking and splitting when they were bent to form the airfoil surface in the jig. This was particularly critical from the leading edge back to about mid-chord, because of the greater curvature of the top leading edge of the wing. I got my local metal shop to weld a 12-inch diameter steel plate to the end of a three-inch by 18-inch steel pipe, closing the end and forming a base. The pipe was filled about two thirds up with water and the whole thing placed on a propane camp stove. I put several cap strips down into the pipe, and the stove heated the water and softened the wood. After the strips had cooked for a while, at least a half hour, I put them into a simple jig made from a scrap of two-by-ten board that was cut with a curve slightly greater than the wing leading edge. After being clamped in the bending jig overnight, the strips dried with a bend set in them, and there was very little tension on them when installed into the rib jig. The idea for this came right out of Tony Bingelis's terrific book "Sportplane Construction Techniques." Bingelis's quartet of how-to manuals for building aircraft are a must for any builder.
Once the top and bottom capstrips were placed in the rib jig, I went to my pigeon holes and got the truss braces for each location. These required beveling to properly meet the top and bottom capstrips, and I used a tabletop disk sander to shape each joint. Each joint requires a gusset or stiffener of sixteenth-inch aircraft plywood. I cut one-inch and inch-and-a-half strips out of a plywood sheet on my table saw and then used a pair of utility cutters to cut the rectangular gussets out of the strips. Some joints required the smaller one-inch size, others needed the inch-and-a-half, and some needed triangular gussets cut from the inch-and-a-half stock. Once the braces were shaped and the gussets cut out, it was time to glue.
I used T-88 epoxy glue, which is mixed one to one. T-88 is fairly tolerant of temperature, and it will set up even if the shop is cold, which happens after I turn off the propane space heater. In cold weather, the epoxy takes longer to reach its full strength, but it evenutally cures. You just have to leave joints clamped longer in cold weather. T-88 also doesn't require strong clamping pressures and exact fits, like some other glues. In fact, you can overclamp expoxies like T-88 and "starve" the joint by squeezing out too much of the glue.
First I applied glue to the ends of each truss brace and then after they were installed, I applied glue to the area around the joint, going out about an inch, and laid the gussets in place. Then I nailed half-inch aircraft nails through the gusset into the spruce, generally one nail per leg of the joint. This acts as a clamp, holding the gusset and the joint firmly in place while the glue sets up. Some purists come back afterward and pull the nails out in the interest of weight reduction. I left the nails in although I do appreciate that to keep the weight down, you must be concerned with every gram that goes into the airframe.
After the gussets are nailed, I removed the rib from the jig, and glued and nailed gussets to the other side of the rib. When the rib was done, I hung it from a rod fastened to the shop rafters. Eventually, I had quite a collection. By the time I had built a few, my technique was better, and the ribs were more uniform The outside dimensions and the area where the spars passed through were identical for each rib, though, from the first to the last.
I got to the point where I could complete a rib in about two hours, and I made it a point to do some work in the shop every night. The result was I had all the ribs built in a little over a month.
Then assemble the ribs on the spars
After finishing the ribs, I needed a suitable table to assemble the wing on. I got two sheets of three-quarter-inch plywood, some two-by-fours and four-by-fours, and built a table four feet wide by 13 feet long. I shimmed the table top with shim shingles to get it as flat and plumb as I could with a level.
Next, I took the front and rear spars and beveled the tips and the inboard ends per the plans. Then I took my tape rule and measured off the center points for each of the ribs on the spars. Once marked the outboard ribs were slid into place, plywood plates were glued in place on either side of the spars where the flying strut brackets go. After the glue set up, I marked the locations and drilled holes for the bolts that go through the flying strut brackets, plywood and the spars. Installing the aircraft grade bolts finished the job. Then the inboard ribs could be slid in place from the butt end of the wing section.
Finally, small wedges were cut to shim the top of the ribs, which curve, to fill the gap between the top rib capstrip and the top of the spars. A dab of T-88 on the top and bottom of the rib glued the rib to the spars and two one-inch aircraft nails driven through the rib capstrip into the spar held everything in place until the glue set.
After the ribs were in place, I installed the steel wing attach straps at the butt end of the wing panel. Next, compression struts, which keep the spars from pulling together, were cut and glued in place, one pair at the flying strut and one pair at the butt end.
The wing tip, which starts as a one-inch by one-inch by 60-inch piece of spruce, had to be bent to approximate the curve of the midline of the airfoil. This took several baths in the steamer, which got a piece of PVC drain pipe slid over it with a cap to keep the heat and steam circulating around that five-foot stick of spruce. A special bending jig for the wing tip worked to get the curve needed, and the wood bent without splitting. Later, I talked to a builder who laminated his wing tips with thin strips of spruce glued up in a jig to the one-inch thickness. I don't know which method works best, but his sounded very elegant.
As the wing tip was steaming, I installed the brackets for the drag and anti-drag wires. These criss-crossed wires keep the wing from deflecting fore and aft due to drag in flight. The plans called for 14-gauge hard wire. I substituted 11 gauge steel fencing wire I got from the local farm supply store. Nicopress sleeves swaged those in place, with turnbuckles to adjust the tensions. My EAA Technical Counselor pronounced the fence wire to be entirely appropriate for a Pietenpol, even if it did raise some eyebrows with some other Piet builder I talked with. I figured Bernie would approve.
Later, after the outer panels were finished, I got nervous about the mild steel fence wire, cut it out and replaced it with 1/8 inch stainless 7X19 cable. I think the fence wire would have worked, but I figured the downside was too great. In replacing the drag/anti-drag wires, I indulged in a practice all homebuilders are well-acquainted with — redoing portions that you screw up or become uncertain of.
Add leading and trailing edges...
The leading edge comes as a piece of one by two spruce, 13 feet long. It has to be shaped into a parabolic leading edge, and I chose to use my table saw to rough cut the angles at which the leading edge met the top and bottom of the ribs. This cut down on an awful lot of planing. With the leading edge clamped in place on the noses of the ribs, I drilled holes for the small bolts that hold the leading edge to the ribs. Then I glued and bolted the leading edge to the rib noses. After the glue set, I used a Stanley Sur-Form tool to put the proper rounded shape on the leading edge. I made up a simple form from a piece of scrap plywood and used this to ensure that the shape of the leading leading edge was consistent the length of the wing.
The trailing edges came from Western Aircraft Supply already cut to the proper shape. Those were attached with sixteenth inch plywood gussets and glue to the rib tails.
Once the wing panel was essentially built, I trammelled the wing, which means I checked that it was square by measuring diagonally from the root to the tip. By adjusting the tension on the drag/ant-drag wires with the turnbuckles, I adjusted out the approximately half-inch difference in the diagonal measurements. Then the turnbuckles were safety wired to prevent them from turning.
...and ailerons 'n stuff
The ailerons are built in place to ensure that the aileron fits properly. The false spar that frames the aileron opening in the wing, and the leading edge of the aileron both require some precise angle cutting on the table saw. This was definitely a case of "measure twice and cut once," but both pieces came out okay. Another piece of spruce forms the aileron spar. These pieces are slid into the ribs and glued in place. With the addition of some internal bracing, the aileron was complete, though it was still a part of the wing.
Next, it was time to fabricate the three aileron hinges. These start as off-the-shelf hardware-store three-inch strap hinges, which are bent to the proper shape. The non-removable hinge pin gets driven out and removable aircraft grade pins form the hinge pins. I had the screw holes welded up and the loops of the hinges welded to form a closed circle, strengthening the hinges. Then new mounting holes were drilled on the drill press.
Once the hinges were bolted into place, I pulled the hinge pins, and sawed the aileron out of the wing by cutting the ribs where the aileron gap is. Amazingly, when the pins went back in, the aileron worked without binding. I was worried about using "hardware store" hinges, but after looking at the finished product and comparing them to the aileron hinges on production airplanes, I had no worries. The Piet hinges look much stronger and beefier.
There were a few more braces to install, and the outer aileron pulley bracket to make, but that essentially finished the wing panel. The last order of business was to varnish all the wood with two coats of epoxy varnish, making sure to work the varnish into all the nooks and crannies, such as behind the rib gussets and inside the bracing joints. After that dried, I hung the completed wing panel from the rafters, and started on the other panel.
Then do it all again for the other wing
Including making all the ribs, it took me eight months to finish the left wing panel. The right wing took about four months. The wing center section required another three months, mainly because I made some changes off the plans, including moving the fuel tank from the center section to the fuselage, and cutting out a semi-circular area in the trailing edge of the center section to make it easier to get into and out of the rear cockpit.
People who stop by to check on my progress have remarked that the Pietenpol wing really goes together like a big balsa wood model, and they are right. Much of the design, from the built-up ribs to the double spruce spars, looks like a scaled-up model. I built a few flying, or sort-of flying, models in my youth. Based on the performance of some of those models, I'm not sure whether that thought makes me feel comfortable or nervous. Knowing that my rear end will be flying in this big model makes me a lot more thorough than I might have been with the balsa and tissue models from 30 years ago.
Next — the fuselage...