My next victim has been the D-Orny-One-III ornithopter, designed by one Dan Garfinkel, as described in the Flapper Facts plans here. 1I also would be quite curious as to how this name came about, but unfortunately I haven’t the slightest idea. Since up to this point I’ve only built things following basically the same design, I learned quite a bit just through the building process this time, mostly relating to thread hinges and working with aluminum sheeting. None of which turned out to be that hard in the end, but it certainly took longer than it needed to.
It took a while for me to get started because I couldn’t decide where in the process it would be least difficult to figure out how to hinge the wings, but ended up constructing all of the wings and the fuselage first. Partly because I figured I was going to end up having to replace wings anyway. (Spoiler: I did.) The only thing I screwed up was making all of the wing chords an 1/8” wider than intended, because I can’t read, so I lengthened the top pylon to match.
But before that, I had to figure out how to cut aluminum. The plans only specify “soft” and “thin”, so I bought two thicknesses, which both turned out to be too hard and thick. So I tried again and found some .016” sheets, and Google-sourced evidence that OLFA PC-L knives, designed for scoring plastic and laminates, can also be used on thin enough metal. 2As it turns out, they’re also easier for cutting very thin strips of plywood than the saw I had been using. Armed with these and the smallest metal hole punch I could find, I finally managed some usable ‘thopter bits.
I later stumbled upon the idea of reusing the metal from beer/soda cans, which made me feel slightly stupid for buying a bunch of sheet metal without at least testing this first, but oh well – something to try another time. I blame the fact that I don’t normally buy things that come in aluminum cans.
Anyway, with this handled, I had no choice but to turn to the problem of the hinges. Spent a very long time on this, because I couldn’t visualize how to attach one wing, much less four of them, and several times almost gave up and contemplated tissue hinges like what I’ve done before. But I went slowly through this video and got out some chopsticks as visual (tactile?) aids, and realized they’re not nearly as complicated as I thought. I then built a little mockup of the front and top of the pylon, to experiment with how best to sew on multiple wings, and fine-tune the measurements for the metal plate since that was not specified either. For the actual one I ended up punching the holes slightly more than 3/8” apart horizontally, and 3/16” vertically, and glued the top a bit higher than the top of the pylon, because that seemed like the easiest way to sew the wings on straight and all at the same tension. The plans say to “pin and glue” the plate to the pylon, but I still don’t know precisely what this means, or how any of my guesses as to what it means would really help, so I just superglued it and it seems to be working just fine. Sewed up the hinges with some nylon thread I had laying around, and I think they turned out quite well.
So. On to the flapping mechanism.
The plans only specify the radius of the crank, leaving me to figure out the length of the conrods and where to put in the wire pins, and whether or not there was an angle between the two sections of the crank. I had initially put in the wing wire pins at 3 inches out from the center, because that was a dimension that was marked and it seemed reasonable, but by this step I realized that that was not connected at all and probably too far out. So I somewhat arbitrarily made new wire pins at 75% of that distance, 2.25 inches out, and went from there. After some hours of attempting to have a functioning working memory, I decided to try conrods of 3 inches for the lower wings and 4 inches for the upper, and to keep the angle between them at 180°. Mostly because I had trouble even thinking about what other angle might be better or how to make sure none of the moving parts would hit each other if I did.
The first test flights were plagued by problems on several fronts – both of the lower wings snap within a few trials, the front conrods keep hitting the crank wire at the bottom of the stroke, the tail warps as soon as I take it out of the house. The tail I fixed first, around trying to get away with gluing snapped wing spars back together rather than redoing them. Shortly before the test I had stumbled upon an article about how to use a piece of window screen to emboss a grid pattern into tissue, to give it room to deal with temperature changes without warping thin balsa, so I was at least aware of it as a potential problem. 1/16” seems to be some sort of critical width for warping to happen, because the tapered ends of the wings have also become less than straight, but not as dramatically as the tail. In any case I got my grid-patterned tissue (using a piece of screen that was still attached to a door required some creativity, but we got there in the end), and I replaced the tail membrane and there’s been no problems since. (For once!)
Once I gave in and replaced both the lower wings, one after the other, they also started being cooperative and not breaking, though we’ll see how long that lasts. Between these trials, I first replaced the pins on the upper wings with ones 1/8” longer, which helped for a while but as I wound it up more the conrods started hitting it again. Later I pulled the outer section of the crank out a bit to angle it, and that also seems to have helped.
On one of the days when I was messing with the tail angle, the base of the tail snapped off just past the aluminum strip, which is completely random and I have no idea why that part would be weaker than say, the entire rest of the tail stick that is thinner. I just glued it back on with a few layers of reinforcement. It’ll live.
The biggest remaining problem is me trying to give it more power. It just doesn’t fly that great, even when I do manage not to break anything for a few flights, and I don’t really know how to diagnose it beyond guessing that it needs more thrust to get more lift. But every time I try to test this hypothesis, I am stymied by the crank bearing breaking off – I haven’t been able to try above about 300 turns, and I know rubber this length should be able to handle much more than that. I’ve even finally put effort into learning how to make fancy 4-strand motors. But subsequent repairs seem to make the bond worse (pretty sure this is a property of superglue I’ve read somewhere), so it seems I have no choice but to keep redoing both the crank and the small wood piece it attaches to with different reinforcements until I find something that stays…
to be continued…