 Gravity,
Lift, Thrust and Drag. Gravity is a force that is
always directed toward the centre of
the earth. The magnitude of the
force depends on the mass of all the
aircraft parts. The gravity is also
called weight and is distributed
throughout the aircraft. But we can
think of it as collected and acting
through a single point called the
centre of gravity. In flight,
the aircraft rotates about its
centre of gravity, but the direction
of the weight force always remains
toward the centre of the earth.
Lift is the force
generated in order to overcome the
weight, which makes the aircraft
fly. This force is obtained by the
motion of the aircraft through the
air.
Factors
that affect lift:
Thrust is the force
generated by some kind of propulsion
system. The magnitude of the thrust
depends on many factors associated
with the propulsion system used:
-
type of engine
- number of engines
- throttle setting
- speed
The direction of the force
depends on how the engines are
attached to the aircraft.
The glider, however, has no
engine to generate thrust. It uses
the potential energy difference from
a higher altitude to a lower
altitude to produce kinetic energy,
which means velocity. Gliders are
always descending relative to the
air in which they are flying.
Drag is the aerodynamic
force that opposes an aircraft's
motion through the air. Drag is
generated by every part of the
aircraft (even the engines).
 There are several sources of
drag:
One
of them is the skin friction
between the molecules of the air and
the surface of the aircraft. The
skin friction causes the air near
the wing's surface to slow down.
This slowed down layer of air is
called the boundary layer.
The boundary layer builds up thicker
when moving from the front of the
airfoil toward the wing trailing
edge. Another factor is called the
Reynolds effect, which means that
the slower we fly, the thicker the
boundary layer becomes. Form
drag is another source of drag.
This one depends on the shape of the
aircraft. As the air flows around
the surfaces, the local velocity and
pressure changes. The component of
the aerodynamic force on the wings
that is opposed to the motion is the
wing's drag, while the component
perpendicular to the motion is the
wing's lift. Both the lift and drag
force act through the centre of
pressure of the wing.
 Induced
drag is a further sort of drag
caused by the wing's generation of
lift. This drag occurs because the
flow near the wing tips is distorted
as a result of the pressure
difference between the top and the
bottom of the wing, which results in
swirling vortices being formed at
the wing tips. The induced drag is
an indication of the amount of
energy lost to the tip vortices. The
swirling vortices cause downwash
near the wing tips, which reduces
the overall lift coefficient of the
wing. The magnitude of induced drag
depends on the amount of lift being
generated by the wing and on the
wing geometry.
Long
wing with a small chord (high aspect
ratio) has low induced drag, whereas
a short wing with a large chord has
high-induced drag.
The picture below shows the
downwash caused by an aircraft.
 The
Cessna Citation has just flown
through a cloud. The downwash from
the wing has pushed a trough into
the cloud deck. The swirling flow
from the tip vortices is also
evident.
In order to minimize tip vortices
some aircraft designers design a
special shape for the wing tips.
With drooped or raised wing tips,
the vortex is forced further out.
However this method causes an
increase in weight since they need
to be added to the wing tip.
 An
easier and lighter method is by
cutting the wing tip at 45-degrees.
With a small radius at the bottom
and a relatively sharp top corner,
the air from the secondary flow
travels around the rounded bottom
but can't go around the sharp top
corner and is pushed outward.
There's also the Interference
drag, which is generated by the
mixing of streamlines between one or
more components, it accounts for 5
to 10% of the drag on an airplane.
It can be reduced by proper fairing
and filleting which allows the
streamlines to meet gradually rather
than abruptly.
All drag that is not associated
with the production of lift is
defined as Parasite drag.  |
