Some continued unrelated assaults on my mental health, in addition to hitting a bit of a wall with D-Orny-One-III1 hereafter referred to as D-III , mean I haven’t done much the last few months, but I thought I’d get back into my flight research by building Nathan Chronister’s Flex ornithopter, from his instructions uploaded last August. This design is based on an interesting idea for using cardstock to form a flexible flapping mechanism – the wings are entirely supported by it, and not directly attached to the wooden body at all.
Only a few measurements were left unspecified, most notably the shorter side of the trapezoidal piece that forms the base of the tail. Given that small changes to the angle of the sides will have a larger effect on the tail area, I would think this would be rather important. Working from the image given in the video, I made mine 5/8” and did my best to ensure the angles were the same on each side, though after assembly it is apparent that I didn’t entirely succeed. Since I didn’t have any 3/16” width balsa sticks, I made the width of the body 1/8” with no apparent ill effect.
The lengths of the wire pieces and the single-ended conrods is also not specified, though this is less important. The wires I estimated more loosely from the video (I have limited skill in precise loops anyway) and the conrods are just the half the length of this random single conrod that was sitting on my workspace for some reason.2 After some mild bewilderment I decided it’s the old one from D-III. The exact arrangement of the long balsa sticks is also not specified – my first instinct would be to assign all the longer pieces to the wings, but it becomes clear later on that the two shorter lengths form the wing segments parallel to the body, and the tail is made from 7” pieces.3 I cut the wrong number of each at first, but I think that was just me spacing out the instructions. Luckily I noticed long enough to be confused.
One thing I did not forsee was the wings not lining up well. I tried to make them match of course, but with the conrods connected I could only try in vain to find a way to get both the parallel spars straight along all axes.4 The entirety of my attempts being to see if putting one or the other conrod in front makes a difference – it doesn’t really. I think I also expected the cardstock to bend more easily for some reason, to just naturally fall into an optimal, symmetric equilibrium. This does not happen.
I also managed to snap one of the wing spars before even putting on the wing membranes. That was just me being clumsy. I glued it back together and pressed on. Yes, I probably do have multiple previous records of me stating that this does not work.
I took it out on a nice warm day,5 for March anyway and it flew well for an increasing number of turns. One of the tail spars came unglued after the second flight, but the membrane wasn’t torn, so I used a small piece of tape6 I keep telling myself I’ll start bringing glue and then not on it and that held up for a while longer. I got to 175 turns on the rubber, and then as soon as it left my hand both wing spars snapped. I would have expected one to break at the place I’d already repaired, but oddly enough it didn’t.7 I hypothesize that the first one held because the break was more or less random, in the sense of being unrelated to stresses normally experienced in flight (or to any natural weak point in the wood? – I’m guessing that the force of my hand would render that insignificant but less certain here), while damage accrued in-flight falls more at natural weak/high stress points which remain despite repairs.
I again tried to fix the wing spars without replacing anything, but naturally this was futile and wasted a week and I had to do it anyway.
I had hoped to be able to remove the broken spars without disturbing the rest of the hard8 non-membrane bits, but started ripping the fold in the cardstock in the attempt, so off came both wings. I glued everything back onto new cardstock, but cut out most of the middle of the piece so that there was only a small section on each side to attach to what was still on the body.
As I was finishing the second wing, I was frustrated to realize that the spars along the body were wonky again, but I could have easily fixed it if I had only noticed two minutes earlier, before the glue set. I tore the cardstock again trying to make it go anyway, and then tried to reinforce the breakage with tissue paper. And then realized that that fold gets the most stress out of any of it, and tissue paper had no chance of holding up.9 though now that I say that I want to test it out anyway So I added another piece of cardstock.
they all return to frankenthopter sooner or later ㅠㅠ 
the wings are straighter though… ?
The next time I took it out after all this, it definitely flew worse than before. I noticed halfway through that the U-shaped part was much more lopsided and I’m not sure why. I tried to push the cardstock into a better position but it still tended towards tight circles into the ground. This testing ended when one of the single-ended conrods came unglued from the wing.10 I believe it was after this point that I finally threw some glue into my flight testing bag. Better late than never.
Further work will have to wait a while, as I now unfortunately do not have access to my “workshop” for a few months, but this was a fun build, and on my return I hope to repair it for real. Someday I will learn to play nice with the sensory, and not half-ass my repairs. Someday.