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Flight in Birds
to understand gliding flight before we begin to talk about flapping
wings. Go back and read the gliding flight
page if you haven't already done so. Recall
that the wings are angled slightly, which allows them to deflect
the air downward and produce lift. The slight angle of the wings
is called the angle of attack. If the angle of attack is too great,
the wing will produce a lot of drag. If the angle is too small,
the wing won't produce enough lift. The best angle depends on the
shape of the wing, but it's usually just a few degrees! Notice that
what matters is the angle relative to the direction of travel, not
relative to the horizon.
angle of attack determines the amount
of lift and drag made by the wing.
bird wing has a cambered, or curved, cross-section.
has a rounded front edge to help reduce air resistance. Some wings
also have a curved or asymmetric shape that helps deflect the air
downward. While not strictly necessary, this is typical for birds.
The inner part, near the bird's body, is more curved than the outer
part. As you read on, see if you can figure out why!
The wings flap
with an up-and-down motion. This may change in special situations,
but we aren't going to talk about those until later. When the wings
move up and down, they are also moving forward through the air along
with the rest of the bird. Close to the body, there is very little
up and down movement. Farther out toward the wingtips, there is
much more vertical motion.
As the bird
is flapping along, it needs to make sure it has the correct angle
of attack all along its wingspan. Since the outer part of the wing
moves up and down more steeply than the inner part, the wing has
to twist, so that each part of the wing can maintain just the right
angle of attack. The wings are flexible, so they twist automatically.
shows how the wing must twist in the downstroke, to keep each part
of the wing aligned with the local direction of travel.
As the wing
twists, and as the outer part of the wing moves downward, the lift
force in the outer part of the wing is angled forward. This is what
would happen if the whole bird went into a steep dive. However,
only the wing is moving downward, not the whole bird. Therefore
the bird can generate a large amount of forward propulsive force
or thrust, without any loss of altitude.
The wing twists as shown to
maintain the correct angle of attack for the downstroke.
bird's wing produces lift and
thrust during the downstroke.
The air is not
only deflected downward, but also to the rear. The air is forced
backward just as it would be by the propeller of an airplane. You
can feel this blast of air when a bird takes off from your hand.
If thrust is
produced in the downstroke, you might be wondering what happens
in the upstroke. Since the wing is travelling upward, shouldn't
there be a lot of drag, tending to slow the bird down? To avoid
this, the bird does two things:
- The outer
part of the wing points straight along its line of travel so it
can pass through the air with the least possible resistance. In
other words, the angle of attack is reduced to zero.
- The bird
partially folds its wings, which reduces the wingspan and eliminates
the draggy outer part of the wing. This is not strictly necessary
though, and most insects lack the capability.
The inner part
of the wing is different. There is little up-and-down movement there,
so that part of the wing continues to provide lift and function
more or less as it would when gliding. Because only the inner part
of the wing produces lift in the upstroke, the upstroke as a whole
offers less lift than the downstroke. As a result, the bird's body
will bob up and down slightly as the bird flies.
The inner part of the wing produces
even during the upstroke.
outer part of the wing is angled to pass through the air with
read so far is a basic description of how birds fly, when they are
already up to speed and just cruising along. Birds also have other
flying techniques, which they use when taking off or landing, or
for other special maneuvers. Books on bird flight will tell you
more about these techniques as well as the special adaptations birds
have for flight.
a bird push its wings backward against the air?
A: No. Some insects fly this way, but it doesn't work well at larger
a bird separate its feathers during the upstroke?
A: Yes, but only in special maneuvers like takeoff or landing.
Q: Do birds
move their wings in a figure-8 motion?
A: Hummingbirds do this when they hover.