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reduced. This system also pulls fuel
into a carburettor, and it's also the
reason why a door closes by itself if
left slightly ajar – there is less
pressure in the gap between it and
the door frame as the air moves
through it.
If you take the top half of the tube
away, the phenomenon still works
on the remaining half, which looks
like the top surface of an aerofoil.
You can see this yourself by taking a
large piece of paper and folding it
back over the top of your hand,
keeping hold of it with your fingers.
If you blow across the top, you will
see the paper rise. Used sideways,
this is how yachts make use of the
wind to get along.
The aerofoil will therefore have a
natural tendency to go up or, looked
at another way, to pull air down, to
the low pressure area on the top and
help the brute force effect. Around
two thirds of the total lift comes
from the reduced pressure effect
across the top, not forgetting the
higher pressure underneath.
The wing loading is the average weight
lifted by each square foot of the
wing. It works in a similar way to
Principles of Flight 159
loadspreading, in that a larger area
has a lesser loading.
At zero angle of attack, a cambered
wing will produce some lift and
some drag, and a symmetrical wing will
produce no lift but some drag.
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
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:
160 Canadian Professional Pilot Studies
In a high wing aircraft, the keel effect
of the fuselage acts like a pendulum
to pull it back to normal. Sweepback
helps here, and it is discussed under
High Speed Flight, below.
Directional Stability
This comes from fins, and makes
you yaw when hit by a gust from the
side. A sweptback wing also affects
this, when one wing presents a
longer leading edge to the airflow if
the aircraft is yawed, producing
more drag and slowing it down,
yawing it back.
The fin acts like a weathercock to
keep the aircraft straight – if it yaws,
the surface is struck more from the
　
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