ornithopters

Model-making is a time-honoured, effective way of investigating problems. Often things are not quite as you might imagine, and models soon indicate where your understanding is based on shaky foundations! By making model flappers, for example, I discovered quite early on that there is no need to power the upstroke - lifting forces are quite capable of bringing the wings back up ready for the next downstroke. Now this can create a problem if you are using familiar power sources such as twisted rubber, which has an even power output: how can you put more power into one part of the flapping cycle than another?

I found the simplest way to put more energy into the downstroke was to tension the wings in the down position. By experimentation, the result is a slow upstroke with a build up of resistance in the tensioners, followed by a more vigorous downstroke. This simple adaptation leads immediately to a more realistic flapping motion.

A second important point is that the wing on the downstroke must either rotate bodily into a more leading-edge-down position (ie: negative angle of attack), or the wing membrane itself must be flexible enough to twist into this new position. The best way is to pivot the wing root so that it is able to rotate appropriately, and then tension it back into the positive angle of attack position. During the powered downstroke, the torsional effect of having the centre of effort of the wing behind the pivot axis causes the wing to tilt into a nose down position until the upstroke begins.

These two factors can make a lot of difference to the success or otherwise of flapping wing models!

A rubber-powered model emulating insect flight
(1982)

Rubber powered model with articulated fuselage (1999). Click on the thumbnails below for enlarged images.

general view, 64 K	structure of the wing, 25 K	beginning of upstroke, 62 K
detail of wing root, 36 K	incidence changes of wings, 27K	detail of crank, 20 K

To see a proposal for a human-powered ornithopter using the principles derived from model ornithopters, click here