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angle. Airflow is at its maximum
velocity at the CP in level flight. It
Principles of Flight 23
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
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).
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.
If you've ever been through a car
wash, and your car is still wet, you
may have noticed droplets of water
remaining quite still on the hood no
matter how fast you drive. This
happens on aeroplane wings as well
– large specks of dust will remain on
them even through a Transatlantic
flight. On the other hand, there is a
point at which the air will flow
smoothly over any surface. The area
between the two is called the
boundary layer, which ideally should
have a laminar flow (i.e. smoothly
layered) but, in practice, this doesn't
happen much farther back than the
thickest part of the aerofoil. In most
small aircraft, the boundary layer
ranges from about half an inch up to
the transition point (where the air
becomes turbulent), to around three
inches afterwards, where it becomes
the turbulent layer.
Anyhow, back to drag, which is a
force that tends to slow an aircraft
down, acting in the opposite
direction to thrust, parallel to the
relative airflow. In order of priority,
it can be split up into various
components:
·  Induced Drag comes from the
air's reaction to the aerofoil, or
is induced from the creation of
lift, so it comes from liftproducing
surfaces and varies
with the angle of attack, so the
slower the aircraft the more you
get (more lift, more drag). It
may come from wingtip
vortices, for example, and is
inversely proportional to the
square of the velocity, that is to
say, halving velocity increases
induced drag by four times. It
also increases as an aircraft pulls
out of ground effect on takeoff,
as the ground will interfere with
vortex formation, and can be
affected by the aspect ratio of
the wing.
24 Canadian Private Pilot Studies
·  Parasite Drag comes from
anything moving through the air
not actually creating lift, like the
fuselage, undercarriage, etc. It
consists of:
·  Interference Drag, or the
result of the interaction
between various
components, say the wings
and the fuselage. In other
words, if you added the
various types of drag
together, you would find
the result to be less than
the actual total.
Interference drag is the
difference.
·  Profile Drag is made up of:
·  Form Drag, from the
shape of any body
moving through the
air. It is minimised by
streamlining
·  Skin friction, mentioned
above. It's the result of
the smoothness or
otherwise of surfaces.
Profile Drag is proportional
to, and increases as, the
square of the speed.
Aileron Drag comes from downgoing
ailerons, causing a yaw in the
opposite direction of the bank.
Maximum Range Speed
This gives you the most lift for the
least drag, for the most economy,
and the most distance for altitude
lost. Flying either side of that speed
will decrease the range when gliding.
The Lift/Drag Ratio comes from
dividing lift by drag. The angle of
attack for the best ratio varies with
the design of the wing, but is around
　
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