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badly placed C of G can make a
previously stable aircraft unstable.
Neutral stability occurs where the
oscillations are constant around the
original flight path, or a new one is
taken up completely.
Stability also works in the pitching,
rolling and yawing planes, as in
longitudinal, lateral and directional.
Longitudinal Stability
The Centre of Gravity (which is an
imaginary point around which the
aircraft is balanced) is designed to be
ahead of the centre of pressure, to
make the plane nose heavy so that,
without engine power, the machine
adopts the correct gliding attitude.
In the cruise, the tailplane's negative
lift balances this tendency. With
longitudinal stability, you will pitch
when a vertical gust hits you.
Lateral Stability
This makes you roll when hit by a
gust from the side. You get it if the
wings are not level across their span.
The dihedral is the angle between the
wings and the horizontal, looked at
from the front – a positive dihedral
has both wingtips higher than the
roots, and enhances stability in the
roll plane – if the flight path is
disturbed, and the aircraft sideslips,
the lower wing produces more lift to
restore level flight because of the
increased angle of attack. If you had
your hands on the controls all the
time, of course, like the Wright
Brothers, you wouldn't need it.
Anhedral is the opposite, where the
tips of the wings are lower in the
horizontal than the roots (e.g. the
Harrier, or the BAe 146):
22 Canadian Private Pilot Studies
In a high wing aircraft, the keel effect
of the fuselage acts like a pendulum
to pull it back to normal.
Directional Stability
This comes from fins, and makes
you yaw when hit by a gust from the
side. The fin acts like a weathercock
to keep the aircraft straight – if it
yaws, the surface is struck more
from the side to force the nose back.
The fin acts like a weathercock to
keep the aircraft straight – if it yaws,
the surface is struck more from the
side to force the nose back.
One reason for the roll that results
from using the rudder by itself is the
dihedral effect, otherwise known as the
secondary effect of rudder, if you
remember your instructor's lessons,
or rolling moment due to sideslip (the
wing on the outside of the turn goes
faster, produces more lift and goes
up, to start the roll). Others are
sweepback (above), high wings with
a low C of G or a high fin and
rudder, though the latter will
produce a roll in the opposite
direction (i.e. left with right rudder).
Dutch Roll
A combined effect of disturbing the
yaw and roll axes, with more roll
than yaw (if it were the other way
round, it would be called snaking).
Essentially, the machine rolls as it
yaws, arising out of sideslipping
when the machine yaws. When this
happens, the effective span of the
wings is changed and the forward
one creates more lift for a short
time, because it presents more of a
span to the airflow than the other
one. This makes it rise, hence the
roll. However, the increased lift also
creates more drag to pull the wing
back, starting an oscillation.
Spiral Stability
If you release the controls in a turn,
the machine will either wind into the
turn or come out of it by itself,
indicating negative or positive spiral
stability, respectively.
Forces In Flight
The four forces acting on an aerofoil
are Lift & Weight, Thrust & Drag. The
parts of each pair oppose each other,
and must be balanced for straight
and level flight. Where their points
of action do not correspond, a
couple is created that will affect the
aircraft attitude.
Lift
Lift acts through the Centre of
Pressure at 90° to drag and the
relative wind. You can increase it in
4 ways, in this order:
· Increase speed for more
reaction over the wings.
· Increase the angle of attack (up
to the stalling point).
· Increase the wing area with
extra devices, such as flaps.
· Fly in denser air (that is, lower
or colder).
The centre of pressure on an aerofoil is
the point where the lift is supposed
to act, which varies with the angle of
attack. Its usual position is around a
third of the way from the leading
edge, but it moves forward as the
angle of attack is increased. Its most
forward point is just before the stalling
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