My next model to attempt is the Luna, also by Nathan Chronister. It’s similar to the Freebird, but this one is a biplane ornithopter. There is a smaller spar above the main fuselage that the wings actually attach to, and the crank is simpler since getting the wings to flap in sync is less of an issue with four wings. (There’s still only two connecting rods because the four wings are really two wings, hinged in the middle like scissors.) I was a bit confused at first why this one was considered more complicated, until I got to assembling everything and figuring out how to attach the membrane. It took a while to figure that out… also how to keep it intact once I did. Here we go again!
It took some four attempts before I managed to more than a flight or two without breaking something – the wings are very prone to either coming unglued or snapping around the center hole. Partly I think this is construction error; however hard I tried I could never get the wings or the wing lever wires (the wires that attach to the connecting rods) to go on entirely straight, and that would put stresses on it in directions it’s not meant to withstand. (I swear it looks like they go more out of whack as I wind the motor up, even though that makes no sense and I’m probably imagining things.) It also wouldn’t glide very well, and the tendency to almost dive into the ground when it runs out of power probably doesn’t help either.
After a couple of instances of having to redo the exact same part (also the most annoying one, since you have to redo all the wing membranes to replace either wing), I thought maybe it was also the membranes I was screwing up – the instructions say to offset the centerlines slightly to put more tension on the rear wing, but it seemed to me that that was just adding more stress to it. So I resolved to try and lessen that tension – and then promptly forgot the next time I went to repair it, seeing instead how the wings were not going to stay in place without the tension. But I went and tried it again anyway, and it just… fixed itself.
The glide behavior was suddenly better – or at least it drifted toward the ground instead of falling into it, and I managed to go one day without breaking any of the wings. But I still have no idea what was different to cause this change. The only semi-reasonable explanation I can come up with is that it had something to do with having more tension on the rubber – the motor had sort of twisted itself around the rear hook in such a way that it stopped just a few turns short of unwinding all the way. I could see how this tension could keep the wings in a position more favorable for gliding – but not how it could do that so consistently. (I don’t think anyone has figured out a way to get rubber models to consistently end with a good configuration for gliding. Seems to me it should be pretty much random whether the tension happens to stop it wings-up or wings-down.) Or maybe I’m completely wrong and this is completely unrelated. I got no better ideas. I stopped that day because the crank bearing had come loose and was slowly eating away at the tissue paper that was the only thing preventing it from snapping off and probably breaking something.
But aside from the aforementioned glide issues, the Luna’s performance is pretty impressive compared to Freebird. It climbed higher, and usually maintained its altitude until the motor ran out. It really doesn’t need much downforce from the tail at all – setting it at what looks to me like parallel with the body, it flies nice and level, straight ahead. (I tried this with the Freebird, just to see, and it does not work.)
Luna is currently sitting in a closet with yet another broken wing. I don’t really have much for ideas on how to improve it, or how to make it more durable, so we’ll see if or when I find time and motivation to redo it again, despite how fun it is to fly.