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little drag. It isn't just the ailerons –
the wing rolling motion doesn't help
the situation, but the point is that, if
you don't use rudder to counteract
this, the wing causing the yaw slows
down and produces less lift, which
will stop it rising as told to by the
aileron. The other wing moves
faster, and gets more lift. The end
result is that you roll the wrong way,
or at least the Wright Brothers did.
Modern design methods have
reduced the risks in the normal flight
envelope, but when in extreme
situations, such as in a steep turn,
near the stall (and don't forget that a
stall can happen at any speed with
the wrong angle of attack) it may
well catch you by surprise if you use
the ailerons too abruptly, especially
when the lifting wing stalls and puts
you in a spin, which is just what you
don't want at 200 feet (remember the
aileron's purpose is to temporarily
increase the angle of attack). Tip:
use ailerons last out of a steep turn.
Put the control column forward and
use opposite rudder first,
remembering that the controls are
much less effective at slower speeds.
This can also be a problem when
taking off from a short strip, with
both wings at a high angle of attack.
Sharp movement one way or the
other will increase the angle on the
lifting wing and stall it the wrong
way. Again, modern design, such as a
twist in the wing that makes the tip
ride flatter, has improved matters,
but try it in a Cessna 152 (a long way
off the ground!) to see what I mean.
Stability
The stability characteristics of an
aeroplane describe its ability to
return to its flight path after a
disturbance without input from the
controls. A stable aircraft is easier to
fly and more pleasant, but one too
Principles of Flight 21
much that way will not be so
manoeuvreable. The static stability is
the initial tendency, while the dynamic
stability concerns the overall tendency,
after a series of ever-decreasing
oscillations – having one does not
necessarily lead to the other. Its
significance lies not just with you
nudging controls by accident, but if
you encounter turbulence, which has
the most to do with knocking your
aircraft off its flight path.
If positive stability is a tendency to
return to the flight path, negative
stability tends to move it further
away in increasing movements:
You could then say that the aircraft
is unstable. This could be a problem
when the increasing oscillations lead
you to stall or dive. Be aware that a
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):
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
22 JAR Private Pilot Studies
yaws, the surface is struck more
from the side to force the nose back.
　
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