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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
angle. Airflow is at its maximum
velocity at the CP in level flight. It
moves aft (and towards the wing
root) when flap is lowered.
The centre of lift is an imaginary point
on the airframe where the total force
of all the lift producing surfaces is
said to act. Normally, it should be
slightly behind the centre of gravity,
which is equally imaginary, and
where the weight forces act through.
The reason for this is to produce a
Principles of Flight 23
slight couple that will tend to make
the machine fly nose down, which is
useful without engine power (thrust
will normally make the couple
ineffective, as will application of the
elevator when thrust is reduced for
low speeds).
You will not be surprised to hear
that there is a formula for calculating
lift, which is:
L = CL½ rV2S
where L=Lift, CL is the coefficient of lift
(the product of aerofoil design and
angle of attack), r(rho)=air density,
V=TAS and S is the wing area. The
coefficient will be at its maximum at,
or just before, the stall, which you
can see from the formula – if it (CL)
increases on one side, the other side
will increase also, until lift can no
longer be produced. Similarly,
increasing speed (V) will increase lift.
Weight
The opposite of lift, acting through
the Centre of Gravity.
Thrust
The force that makes the aircraft
move through the air, and the
opposite of….
Drag
It's not all plain sailing for anything
forced to move through air, as a
certain resistance tries to prevent it,
caused by friction from air molecules
as they are forced out of the way
(skin friction). This tendency to stick
is called drag, which both absorbs
energy and produces heat, so it
needs to be reduced as much as
possible. Again, to fly, the thrust
(from the engines) must always be
greater than the total drag produced
by an aircraft.
　
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