Flight Logs: Freebird

At long last, I am back at building things. The format of this online version of my log will probably change, but my goal is to focus on what I learn from my already numerous mistakes. Mostly involving forcing myself to glue things more times than necessary…

I’m restarting my journey from the basics, with the Freebird ornithopter plans from Nathan Chronister. The intent was to construct it exactly according to plan, and combat my tendency to try and “optimize” instructions whether or not I have any idea what I’m doing, but somehow I have yet to do so.

I headed out to the first test flight very optimistically with my nice pretty model.

It wasn’t until the 3rd day that I managed to come back with an intact Freebird.

First the tail broke – during my first attempt at filming a flight, it ran directly into me because I was looking at my phone screen instead of where it was heading, and in the impact the tail came unglued from the wire. An easy enough fix. I added more tissue paper reinforcements too. And I learned later that it is possible to get something on screen without watching the screen.

Next to go was the motor stick. I had noticed it coming unglued from the body but when I went to fix the tail it seemed fine so I ignored it. I had used superglue on this joint, and it actually broke the wood right above the layer of glue. I remember reading something once about how some glues are more rigid than others; white glue for example having more elasticity than CA glue even after it was dry. Before beginning this project I would never have thought that that would be a desirable trait, but I think it’s more important than I gave it credit for. I redid the joint with white glue, and added a bunch more layers of tissue paper because it was only because of those that it held on for as long as it did. It’s stayed nice and snug ever since.

Lessons learned:

• Using fancier glue doesn’t solve everything.
• That insignificant looking tissue paper is actually kinda important.

And then once I’d ironed out enough structural problems for it to stay in one piece, I got to puzzle over everything else I did wrong. Long story short, I gradually realized that it was locking up at the top of every stroke, so I had jacked up the flapping mechanism somehow. First realized that the angle on the crank was supposed to be more like 120 degrees, not 90, which I really should have known from building the kits. In fact, I remember looking at the directions and thinking, “Huh, weird that this one is 90, I thought having a different angle here was kinda important,” while completely ignoring the picture right next to that showing an angle of clearly not 90.

Lessons learned:

• I’m really not good at following directions. We already knew that.

Another easy fix, but it didn’t help that much. I had also made the wing hinge wires too short. The length is not written out in the directions, but only implicitly included in the picture, and I had focused too much on getting them to be the same length rather than on matching that length to the diagram. So they came out each about 1/16” too short. (A small length change here seems to be more significant than an equivalent length on the crank, at least when looking at flapping amplitude.) Redoing the wing spars sounded like a lot of work, so I messed around with FlapDesign to find a way to redo the crank to reduce the lockup. I shortened the first length to about 0.3 in, down from 3/8 in, and it did help the flapping rate. Also somehow turned it into a wannabe aerobatics bird that tries to do big vertical loops. Still not sure what was up with that. I kind of fixed it by messing with the tail and adding weight to one side, and I’m guessing the wind that day also had some role.^​*​

Sometime after this I snapped one of the wing spars doing nothing more exciting than moving it off my desk, so there went my excuses for not fixing the wire lengths. I redid both wings with the proper measurements this time, and it seems to have helped. I didn’t un-fix the crank, so it’s still not according to plan, and I can see that it’s probably just not converting power efficiently (the change in the crank radius alone makes it lose some 15 degrees of flapping amplitude). Sooner or later I will re-redo it and we’ll see how much of a difference it makes.

Update: Fixing the crank helped, but not as much as I hoped. The next thing to go was the rubber. It snapped when I was trying to wind it up a bit higher just to see what happened, and I didn’t want to slowly wind 120 turns in the opposite direction, so I just launched it anyway and miraculously didn’t break anything else by doing this. I suppose it’s just old – this is the rubber that came with the kits that I bought maybe 4 years ago, that aren’t even sold anymore. I cut a new length and noticed shortly after that it already had a nick in it. How much of a difference does the age make? How do I tell how affected a piece is? If this isn’t the problem I’m pretty much out of ideas.