Birds & Ornithopters Fly
You may be surprised
to learn that a bird or ornithopter flies in much the same way as
an airplane. The wings produce lift in the same way as an airplane,
simply by their forward motion through the air. To understand how
this works, let's start with the simple case of gliding flight.
When a bird
is just gliding, it moves forward through the air, with its wings
held in a fixed position. The wings are at a slight angle, so they
deflect the air gently downward. Pushing the air downward causes
a reaction force in the opposite direction. You will notice a reaction
force, any time you push against anything! The reaction force is
called lift. Lift is a force that acts roughly perpendicular to
the wing surface and keeps the bird from falling.
gliding flight, a bird's wings deflect air downward,
causing a lift force that holds the bird up in the air.
There is also
air resistance or drag on the body and wings of the bird. This force
would eventually cause the bird to slow down, and then it wouldn't
have enough speed to fly. To make up for this, the bird can lean
forward a little and go into a shallow dive. That way, the lift
force produced by the wings is angled forward slightly and helps
the bird speed up. Really what the bird is doing here is giving
up some height in exchange for increased speed.The bird must always
lose altitude, relative to the surrounding air, if it is to maintain
the forward speed that it needs to keep flying.
tilting forward and going into a slight dive,
the bird can maintain forward speed.
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.
wing usually consists of a thin fabric membrane, which takes on
a curved or cambered shape, when it pushes against the air. Birds
have more of a rounded leading edge to help reduce air resistance.
The inner part of the wing, near the bird's body, is more curved
than the outer part. As you read on, see if you can figure out why!
work on the same principle as an airplane propeller, except they
are moving back and forth. The wings flap with an up-and-down motion,
usually. But as 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. Airplane
propellers also have a twisted shape, but their continual rotation
means the shape doesn't have to change.
As the wing
moves downward and twists, 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. Often people have the wrong-headed notion that
the upstroke will somehow cancel out the downstroke. But remember,
the force produced depends on the angle of attack. It can be controlled.
And here is what birds do to make the upstroke more efficient:
- 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. Ornithopters do this
- The bird
partially folds its wings. This reduces the wingspan and eliminates
the draggy outer part of the wing. This is not strictly necessary
though. Most insects and most ornithopters lack this capability.
The inner part
of the wing is different from the outer part. There is little up-and-down
movement there, so that part of the wing continues to provide lift
just as a result of its forward motion. Only the inner part of the
wing produces lift in the upstroke, so 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
even during the upstroke.
outer part of the wing is angled to pass through the air with
So, like an
airplane, lift and thrust functions are separated. The outer part
of the wing provides thrust. The inner part of the wing produces
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 tiny insects fly this way, but it doesn't work well
at larger scales.
a bird separate its feathers during the upstroke?
A: Yes, but only in special maneuvers like takeoff or landing. The
wings beat horizontally and the upstroke produces lift. The feather
separation allows them to maintain the correct angle of attack and
is not to acting as a one-way valve or to reduce air resistance
on the upstroke.
Q: Do birds
move their wings in a figure-8 motion?
A: Hummingbirds do this when they hover.