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of the flapping mechanism is to convert the rotary motion of your
motor into the reciprocating motion of flapping wings. There are
many ways to do this, and I will describe only some of the more
common ones here. The mechanism must be lightweight and fairly simple.
Yet it must also provide a fairly symmetrical wing motion so the
ornithopter flies straight.
The basis for
most mechanisms is called a "four-bar linkage". There
is a rotating crank shaft, driven by the motor. As the crank goes
around, the connecting rods push the wings up and down.
when a second wing is added, this mechanism will produce asymmetric
flapping. The two connecting rods leave the crank at different angles.
This causes them to act at different times. The
asymmetric flapping lowers the efficiency and makes the ornithopter
want to turn to one side. There are several ways to improve the
Crank: The simplest solution is to use a staggered crank.
Here, the connecting rods go off at different angles, but
their timing is corrected by having them placed on separate
crank throws. The staggered crank is most easily constructed
from bent wire. This limits its use to fairly small ornithopters.
Wing Hinge: If your fabrication methods will not allow
a staggered crank, it might be preferrable to separate the
two wing hinge lines so that the connecting rods pass together
between the wing hinges. Since the angle between the connecting
rods is small, the flapping is fairly symmetric. You will
need a more complicated body structure to support the outboard
Cranks: Another solution is to use two separate cranks.
This requires an additional drive shaft and gear. This mechanism
will probably weigh a little more than the outboard wing hinge
mechanism shown above, but the flapping will be more symmetrical.
Shaft: A variation on the dual crank idea is to use a
single, transverse drive shaft, with cranks at either end.
Since the cranks are not operating in the same plane as the
flapping arc, the connecting rods must have ball joints at
their ends. This results in more friction compared with ball
bearings operating in a single plane.
These are some
of the most commonly used mechanisms for two-winged (monoplane),
ornithopters. A wide variety of different mechanisms have been developed
for special purposes, such as flapping multiple wings, moving the
tail along with the flapping motion, or actively twisting the wing
structure as it goes through the flapping cycle.
Check out the
free FlapDesign Software on this web
site. It's an indispensible tool that allows you to figure out the
correct dimensions for your flapping mechanism before you start