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As with the
flapping mechanism, there are many ways to build an ornithopter
wing. The simple "membrane" type of ornithopter wing is
the most commonly used. This is not only because it is easy to build.
It is also the most consistently successful ornithopter wing design.
The membrane wing consists of a spar, at the leading edge of the
wing, and a membrane, which extends backward from the spar and attaches
to the body of the ornithopter.
parts of the ornithopter wing include the leading edge spar
and the wing membrane.
the rigid structure is concentrated at the front of the wing. The
same is true for birds, bats, pterosaurs, and almost all insects.
The more flexible membrane portion of the wing will passively lag
behind the rigid wing spar. When the wing is going up, it will also
tilt upward. When the wing is going down, it will angle downward.
We refer to this as "twisting" of the wings. It is an
important fundamental motion of flapping wings, equally as important
as the flapping motion.
twisting of the wing allows it to take on a different angle,
for the upstroke and the downstroke. The wing points in about
the same direction that it is travelling through the air.
You can see how that works in the wing cross-sections shown
here. (It's like a side view of the wing.)
stabilizer, or tail surface, holds the ornithopter at a slight upward
angle. This means the wing during the downstroke doesn't point exactly
in the direction it is going, but it has a slight angle called the
"angle of attack". As with any airfoil, the angle of attack
causes the wing to produce a lift force that is roughly perpendicular
to the travel direction of the wing through the air. This is the
blue arrow in the diagram. This also drives the ornithopter forward
through the air.
What about the
upstroke? People often wonder why the upstroke of the bird's wing
doesn't push the bird back down. It's because the angle of attack
is adjusted, so that the wing can pass easily through the air. When
the angle of attack is near zero, there is a minimum of air resistance
on the wing, and no perpendicular force is produced. That is what
we want, for the upstroke. (Actually though, the inner part of the
wing doesn't have much up-and-down motion. It acts a lot like the
fixed wing of an airplane, and still produces lift during the upstroke
as well as the downstroke.)
it is possible to improve the basic membrane wing, by adding
a diagonal brace to the wing structure. This picture shows
one of the "Seagull" engine-powered ornithopters,
developed by Percival Spencer in the 1950s. He built a whole
series of these models, up to eight foot wingspan. The brace
holds some tension on the wing membrane. This prevents the
wing from having too much twist. The diagonal brace itself
must be quite flexible. It must bend so that the wing can
take on a curved or cambered airfoil shape while it is flapping.
If the bracing rods are too stiff, they will cause a discontinuity
in the cross-section of the wing, making the airfoil less
can also be added, to help extend the outline of the wing
membrane. Here the battens have been highlighted in red. Battens
can be used in combination with the diagonal brace described
previously. The battens should radiate outward from the front,
inside corner of the wing. In this way, they do not interfere
with the natural cambered shape of the wing.
may be used for the construction of membrane wings. Traditional
rubber-band-powered ornithopters are made from balsa wood, with
either a lightweight tissue paper, or a plastic film forming the
wing membrane. Radio controlled ornithopters mostly use carbon fiber
rods for the wing spars. It is also possible to make wing spars
from bamboo. The wing membrane can be made from a plastic film.
This is suitable, even for very large RC ornithopters. Look for
a crisp, "cellophane" type of plastic, instead of a limp
saran-wrap type. Sometimes a woven fabric is used. The fabric wings
can look very professional. However, it will be difficult to find
a woven material that is crisp, lightweight, and airtight. Some
of the fabrics specially made for kites may be appropriate. The
Ornithopter Design Manual has more information on assembling fabric
It is possible
to actively drive the twisting of the wing, instead of relying on
the flexible membrane structure. In principle, this would allow
more control over the angle of attack, perhaps resulting in more
efficient flight. However, the active wing twisting requires a more
complex wing design and flapping mechanism.