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In-Depth Information
the wing, lowering the air pressure on the upper surface, and additional lift
occurs. However, there are limits. As the angle increases, the air moves less
smoothly across the airfoil's surface and there is an increase in turbulence on
the trailing edge of the wing's upper surface. Along with the increase comes
an increase in drag. Once the angle of attack reaches a critical state, the drag
factor equals the thrust and a stall is the result. Stalling may generally be seen
as a negative factor in general flight and something to be avoided, but it can
be beneficial during landing as a bird attempts to shed air speed.
Symmetrical airfoils are more unusual than asymmetrical ones but are
still able to produce lift, though this is achieved by the angle of attack.
Asymmetrical airfoils are capable of producing lift with a zero angle
of attack and can produce more lift than symmetrical ones. Lift during
inverted flying is achieved by increasing the angle of attack, thereby
making what would generate negative lift into positive lift. In this
way aircraft flying upside down do not fall out of the sky; instead they
demonstrate an elevation of the front end to create the increase in the
angle of attack.
We might consider flight to be the most noticeable feature of bird locomotion,
but not all birds fly, and though all birds have wings or vestiges of wings,
wings are not restricted to birds. Many insects, a few animals, and even a
number of plants have developed flight as a solution to their particular needs.
To do this they have each developed wings of one sort or another. Some
of them allow for gliding, as in the winged seeds, called
samaras
, of certain
plants like the sycamore
Acer pseudoplatanus
, the ash
Fraxinus excelsior
, or
the field maple
Acer campestre
. Other flyers have more sophisticated design
solutions that allow for powered flight.
FIG 4.50
Seed wings.
Nature developed the wing and in so doing created different shapes and sizes
relative to the body that enable different kinds of flight.
