Page 94 - Understanding Flight

CH03_Anderson 7/25/01 8:56 AM Page 81

Wings 81

from zero at the wing surface outward. It is the friction of air molecules
with the surface that causes this change in speed.
The lower speed near the surface translates into lower kinetic energy.
Boundary-layer energy is important because higher kinetic energy will
allow the boundary layer to continue to follow a surface even when the
surface curves away. This is the essence of the Coanda effect mentioned
earlier. As stated in Chapter 2, most of the lift on a wing is in the first
fourth of the chord length. This is where the pressure is lowest. As the air
moves back along the top of the wing, the pressure increases until it
reaches the ambient pressure at the trailing edge of the wing. This is
known as the trailing-edge condition. Thus, the air is moving into an
increasing pressure, which tends to slow it down. If the boundary
layer has enough energy to overcome the increasing pressure, it In March 1945 a C-47 (military
will follow the wing’s surface. When the energy of the boundary version of the DC-3) had its left
layer is not sufficient, the boundary layer will stop flowing and wing severed off just outboard of
separate from the surface. Past the separation point the wing the left engine from a midair
experiences air flowing in the reverse direction. The wing is collision. The pilot managed to
entering a stall. Separation usually occurs near the trailing edge make a controlled crash landing
at the critical angle of attack. As the angle of attack increases, the with only one wing.
point of separation moves forward and lift decreases.
An understanding of the energy in the boundary-layer air is necessary
for wing designers to design wings that hinder separation. If a wing can
reach a higher angle of attack before stalling, it will be able to take off and
land at lower speeds or carry heavier loads. Lower stall speeds translate
to shorter runways, and heavier loads translate to greater revenue.

ICE ON A WING

A wing designed to stall from the trailing edge first may lose
this characteristic when it flies into icing conditions. Ice
forms on airplanes that fly into moisture in a certain
temperature range. Supercooled water drops freeze on
impact and form rime ice, which is rough and opaque.Water
that is warmer will impact the wing and form glaze ice,
which is smooth and clear. Mixed ice is a mixture of glaze
and rime ice.