The wrong farf (Tetropters VI)

I made a few animations especially for the Loncon3 convention, some of them concerning tetrop
ters (see here for the previous tetropter post). The reason was that I wanted show some of the 'flight platforms' that tetropters could conceivable evolve into. So far, there are the 'standard, 'rowing', 'helicopter' and 'farf' modes.

Click to enlarge; copyright Gert van Dijk

These modes all have to do with the relative amount of movement in all the four ways a tetropter wing can move. The image above shows the idea: there is a general tetropter body, characterised by its vertical position, four jointed legs at the bottom and a head with sensors at the top (there is a head with smaller eyes and a mouth at the bottom end of the body, not visible here). The red, blue and green axes run through the attachment point of one wing and concern the movement of that wing. There are similar axes systems for the other wings, but these are not shown (the wings are, though, just). The arrows indicate the direction of rotation of each axes. A to and fro movement around the blue axis will result in a clockwise and anticlockwise movement. If you combine that with an up-and down movement around the red axis you get interesting patterns: the wing could describe a circle, but the most common pattern is a horizontal figure of eight. The wing moves clockwise and down, then at the end moves up quickly, so it can move down again while moving anticlockwise. That just leaves the green axis, which rotates the wing around its own longitudinal axis, allowing it to achieve the proper 'angle of attack'.

I said there are four ways to move a tetropter wing, and the fourth is not a rotation around an axis as are the first three, but warping the plane of the wing. Well, if you followed that an can envisage it, top of the class. Its more or less what you need to describe the movement of the wings of animals with hovering flight, so we are on common ground here.

I will probably come back to the other tetopter flight modes later, but let's talk about the farf mode. A farf is short for farfalla, the name the Furahan citizen-scientists gave to tetropters with a very long wing base. In fact, the image above has just such a wing membrane: you can see that the membrane lies against the vertical blue axis over its entire length. Actually, the wing membrane shown here would not be an actual one. It is just a rectangular placeholder, but is does show the principle of the thing nicely. This arrangement means that movement around the green axis cannot take place, and to get a good angle of attack the wing will have to warp considerably. If you think this scheme reminds you of a butterfly, you are right: butterflies also have wings with a broad long wing base. In fact, 'farfalla' is Italian for butterfly.

So here is an animation of a farf, made for this post, showing the placeholder wings. Not too bad, is it?

And this is the one I showed at Loncon3, with colours etc. Just about the day before I showed it, it dawned on me that I probably made a mistake in warping the wings. When the wings clap together, they have to be more or less flat, and then they should peel apart, first at the top, and then downwards towards the bottom. Well, that bit worked, but for some reasons I had also warped the wings in such a way that the distal end of the wings –that is the bit farthest away from the body- leans into the movement, so it moves before the part near the body. But the wings would encounter resistance from the air, and so the tip of the wings should probably lag behind the proximal part instead of leading it.

I do not think anyone noticed, but I also did not give the audience a long time to think about it. I will have to do another animation with the opposite effect, to see whether that looks better. But there's no time for that yet... Meanwhile, I hope you still enjoy the 'wrong farf', warped as it is.