Jan 13, 2024     Rigging study - (9.5 hours) Category: Research

Spent some time looking into the how-to's of rigging a biplane. Ron Lynn from the Hatz Biplane Assn. wrote about two of these in the newsletter, so I managed to find and read them.

The most recent publication was a two-part series of articles published in Sport Aviation in Feb & Mar 1963 by Bob Whittier: "The Fine old art of rigging a biplane". Quite helpful, condensed the process to the basics, suitable to address a range of designs.

Next publication was recommended by Chris Bobka A&P, IA. It was the PT-17 (Stearman) Repair Manual (AN01-170AC-3) ca. 1945. The section on rigging was very informative, though specific to the Stearman of course. Even though it's similar to the Hatz, some of the wires and struts are set up a little different, so much of the specifics addressed had to be looked at in the larger sense to develop the overall concept and how to get there.
Notably, there were several tips and techniques mentioned that I hadn't seen elsewhere and should be very helpful going forward. Particularly, the streamline wire tightening tool looks like the best design I've seen so far.

The first manual I read I think was the most informative (maybe because I read it first) was titled "Aircraft Maintenance" by Daniel Brimm & H. Edward Boggess, ca. 1940. The chapter on rigging was very clear and concise. Detailed enough to accomplish the task, yet still present a wide enough view of the process that the reader could employ the information on any biplane or monoplane so rigged.
The book was written for aircraft mechanics in the field at the time, not a school textbook and so approaches the topics in the pragmatic manner required of someone needing t do this, not some theoretical abstract. A very good resource.

  Sep 08, 2023     The Aileron Fabrication Process - (10.0 hours) Category: Research

The corrugated aluminum aileron construction process appears at first pretty straightforward to someone familiar with sheet metal construction. I thought I was but subsequently schooled on some of the nuances unique to this type of build. Others have come to grief building multiple twisted and unusable sets along with all the usual possible missteps involved trying to build a left and right and an upper and lower of anything. This is one way to do it.

I attempted to enumerate each step I followed in producing a flat aileron, but it wound up being two and a half pages of single-spaced text. Even then, as a comprehensive list didn't include many cleanup or turnover steps necessary.
I thought instead I would include here sort of the framework of the fabrication process followed by the assembly framework. Both are separate and distinct processes. (And it will be a lot shorter to read, hopefully making more sense instead of getting caught up in the nuances)

The first of a couple concepts is a corrugated 0.025" sheet of aluminum laying flat on the bench in front of you no longer acts like a flat sheet of aluminum. It's best considered an accordian. The edges will flex around, accordian like unless captured and attached to something that won't. Hence, the need for insert strips along the trailing edges. If the top and bottom skins are attached to the spar but not attached at the trailing edge, you can slide the two trailing edges past one another inducing enormous amounts of twist into the panel. This should be noted when it comes time to drill and attach the trailing edges.
A useful aspect is the top of the aileron is flat. As much drilling and riveting as possible should be accomplished with the top on a flat surface (inverted).

The drilling process follows a slightly different process from riveting due to access limitations to buck/drive the rivets.
I began by fabricating the spar and ribs with the trailing edge pieces set aside for later. Next, I cut the topskin to final size using the template and measurements taken from the respective wing. Since the top is flat, drilling the spar to it while it is inverted on the bench begins the skinning process. After the spar is drilled, drill the trailing edges and inserts, clecoing them to the jig board. After disassembly, backdrill the hinge half to the upper spar flange holes. Mark the hinge for trimming at the outboard end as hinge eyes will not extend the full length of the aileron.

A note about the trailing edge inserts: It's a lot of busy work to produce the elements for the trailing edge insert pieces. There's a curved piece separate from the straight piece which has an angle bent into it and a tapered "hardwood" piece that nests into the bent angle. None of which has thickness dimensions, only the angle dimensions are specified... There is I believe a better way. Vans Aircraft uses a trailing edge wedge in many of their designs control surface trailing edges. It's called "AEX Wedge" and comes in three different lengths that I could find: 20, 51 and 82.5 inches. I discovered this a little too late to backtrack, but if I were to do it again I would seriously consider this stuff to simplify the trailing edge process.

The bottom skin is smaller, but is purposely cut a bit oversize and laid over the inverted panel. The raised beads in the panels need to align with one another, so sliding them to and fro until a satisfactory alignment is achieved, then driling and clecoing them together on the straight portion of the trailing edge only. But since you can't see thru the oversize bottom skin, that step needs to be done with the panel flopped over on the jig or otherwise clamped flat with the topskin (final size) accessible for back drilling to the bottom.

Full disclosure: I bought a set of corrugated aileron skins from Mr. Tom Brown in Wisconsin, just to cut my losses early instead of trying to stamp out the skins on my own... because I know me. They were worth the money. Very nicely, uniformly stamped and in great condition. All the beaded ends along the trailing edges mostly matched until I got to the last aileron. The two skins on that last one would not line up, they were uniform but just enough different spacing that they were going to look awful along the trailing edges where the beaded ends meet. These skins were uniformly stamped, but got rolled and stuffed in a box to get shipped. The inner skins in the roll got rolled a little tighter than the outer ones and sort of took a set. The bottom skin that looked to need a bit of stretching also really had a curl to it, so I rolled it opposite to the set it already had. That stretched it, and it began to get closer to the upper skin. A little more working it with opposite rolling and the two skins matched perfectly.

Next, put the panel back on the bench topside down and drill the bottom skin to the spar short of the tapered portion. When you flex the skin over the tapered spar, it changes dimensionally enough you need to pay attention to the trailing edge ensuring the raised beads remain aligned. Here's where being oversized helps out. The extra edge margin allows you to flex as necessary keeping the trailing edges aligned without running an edge off the spar flange.
I should mention I used small 9-lb sandbags to apply clamping pressures to these skins while doing this. Easier to reposition everything as necessary and less fiddly than spring clamps or otherwise.

Completing drilling the skins together, I used another 3/4" piece of MDF cut about 3/4" undersize around the perimeter of the panel. With the panel bagged down flat on the undersized board, allows access from below for drilling or clecos. The curved portion of the panel/board can then be flexed/aligned, bagged flat and accessed from below to drill and cleco together. Finally, the skin is drilled to the tapered portion of the spar.
The trailing edge inserts get installed and backdrilled through the bottom skin.

Rivet holes for the internal ribs are located and drilled in the skins, then reassembled and backdrilled through to the rib flanges. Similarly, the slots in the skins necessary for the steel actuator tabs are located once the rib positions are finalized, then cut and cleaned up.

Assembly starts after edge trimming the lower skin, deburring everything and, if desired, notching and crimping the bead ends on both skin panels.
Ribs, tabs and spars are first riveted together. The bottom skin gets riveted along the spar initially. Finish up the bottom skin riveting by riveting the ribs to the skin.

The top skin riveting begins by clecoing the inboard end as far as the first internal rib. Then rolling/peeling the skin back to allow access to buck the rib rivets from the inside and rivet gun access from the outside. once riveted, continue by clecoing to the next rib and repeating the process.
Once the ribs are riveted, invert the panel on the undersized jig board, bag and fully cleco in place including the trailing edge inserts and hinge half along the leading edge.
Keeping everything sandbagged flat, continue riveting both leading and trailing edges together. It should be flat.

The above process took me about 50 hours to complete an individual panel.

  Dec 04, 2022     Degausser use and construction - (12 hours)       Category: Research

Construction has been slowed lately due to family commitments but I have had time for some reading and research into upcoming potential problems.
One of those issues is magnetic fields that may or may not be induced in a welded steel tube fuselage. Reading the Biplane Forum it seems to be a common problem that the magnetic compass often functions only to satisfy a FAA requirement. EFIS magnetometers which provide source info for the glass-cockpitted set also suffer the same ill effects. Some question exists as to the cause of these fields. Some claim it's TIG welding, others say any welding and some claim uncut lengths of tube from the mill have been discovered magnetized... All plausible as the production requires drawing the material through multiple dies and rollers and welding may only interfere or move these existing fields about.
So much for theory and problem definition; now what is to be done about it?
Many have claimed degaussing the welded fuselage can eliminate this. Sometimes it takes a second degaussing until the fields are completely eliminated.
Degaussers have been around for decades for use in erasing magnetic tapes and other magnetic-stored media. Some are still available on ebay and elsewhere for erasing VHS tapes or reel-to-reel audio tapes. An old one of these may work, but again, some have questioned if they are strong enough for this use. I dunno. I did find an article describing how to build one on a webpage devoted to people who are sensitive to strong magnetic fields (www.eiwellspring.org) Enter the geeky retail therapy on Digi-Key and Parts Express.com
It's a pretty simple setup: Line voltage goes to a 120/12 VAC transformer, the 12 VAC is connected to a large copper coil : 18 awg 12 mH air-core coil and driven at about 3-4 amps. This produces an alternating magnetic field, swapping polarity at 60 hertz. When moved over the offending magnetic field, the coil and it's alternating fields "scramble" the existing fields. Apparently, the field coil needs to be kept moving in a fluid motion during this process to achieve the desired result. Also, the coil will heat up while in use and should be allowed to cool after awhile.
Photo below shows the coil on the left and transformer to the right and some supporting tech info.

  Sep 06, 2020     Lofting Scaled Drawings - (5.5 hours)       Category: Research

Traced multiple curved pieces from full-scale templates included in the drawings to make expendable templates glued to cardboard or foam-core throughout this project. But, only a relatively few in the plans have full-scale templates. Most require/involve a grid (sometimes 1" sometimes 2" sometimes 1/2" I think) on which the curved contours are drawn in typically one-quarter scale. One example is the wingtip bows (they required two, one for around the end of the wing and another for the half-camber of the airfoil). Another is the firewall (only half -- centerline forms one edge). After enough of these, I got tired and bored of the tedium (and my lack of precision) attempting to reliably enlarge and duplicate them, it dawned on me that Kinkos can make big copies... kinda cheaply. Took the scale drawings to them I needed enlarged, put 'em in the BIG printer, selected X400%, pushed the big button and YAY! Full scale templates, and gnat's ass perfect!
Having never experienced an original thought in my life, this is about as close as I come.

  Feb 29, 2020     Fabric Class - (16.0 hours)       Category: Research

Attended a fabric covering class in Boyceville, Wisconsin in Feb before the whole COVID thing kicked off in earnest. Time well spent as far as I was concerned since I had zero experience in that department. Class focused on using the Stits Polyfiber process. We went through the Poly Fiber covering manual from front to back in the course of the two-day class. Briefings prior to the hands-on practice for each segment/phase of the process helped with understanding the concepts. On the last day, we used the spray booth, picking up tips and techniques in the spraying process as well.
Joel Timblin and his wife presented the class, provided all the materials and the PolyFiber manuals. They also provided breakfast and lunch! Yay!
They really did a nice job and importantly took the mystery out of this process for me. I'm still a beginner at this point, but now I can at least recognize the difference between a drain grommet and reinforcing ring or a rib stitch. They also explained and demonstrated, like so many other things, that while the process is very straightforward, it's the details where the difference is made between an adequate job and a show worthy finish. Attention to the details of straight edges or lines, even stitch spacing, keeping the pinked edge tips stuck down and proper sanding techniques are just a few areas where taking the time to do it carefully will really show in the final result.
Photos show fellow EAA Chapter25 member and all-around aviation enthusiast John Schmidt in the blue shirt and myself in the gooey chemistry-stained red and blue fleece snipping and sticking our way through parts of the process. Class photo in the hangar and a nice chilly morning before class looking down the runway at Boyceville.