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Stability and Control
Getting an ornithopter
to fly is only half the battle. For the sake of esthetics, we often
depart from the tried-and-true control systems found in airplanes.
If we insist on using a flat, triangular tail like a bird, this
will increase the challenges of stability and control.
ornithopter will have a horizontal stabilizer in the back. The tail
usually provides a downforce to keep the nose up, and therefore
the tail incidence or angle relative to the wing is much more than
you would find in an airplane. This angle is typically about 15
degress, but it may be less or more depending on where the center
of gravity is located and other aspects of the ornithopter design.
In certain rubber-powered
ornithopters, the motor stick has been elongated in an effort to
increase duration. With the center of gravity farther to the rear,
the stabilizer provides lift instead of downforce, and the incidence
is decreased. Obviously, the lifting stabilizer is more efficient,
and this is what birds use. However, the more rearward center of
gravity location can decrease the directional stability of the ornithopter.
Your ornithopter might need a vertical fin.
between the two wings can cause the ornithopter to turn left or
right. If the wing spars are made of wood, their weights will naturally
differ. You can also have asymmetry in the mechanical properties
of carbon rods, or in the tension of the wing membrane. If the
ornithopter wants to turn to one side, this can be corrected by
adding weight to the wingtip on the outside of the turn. Through
inertia, the added weight acts to supress the flapping motion of
the weighted wing, while increasing the flapping amplitude of the
unweighted wing. The difference in travel results in a difference
in thrust that will compensate for any unexplained pull to one side.
usually done by the tail. The wings can be used for steering, but
this is less consistently successful and more difficult to implement.
A simple elevator and rudder system is very effective for ornithopter
steering. For a more birdlike appearance, though, a flat, triangular
tail is more often used. The tail may swing out to the left and
right sides, so that the downforce of the tail causes a rolling
moment on the ornithopter. Alternatively, the tail may rotate about
its long axis. In this case, the downforce is redirected in a way
that provides yaw control. In some RC ornithopters, the tail rotates
about an oblique axis, combining the two motions described here.