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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. The transition
point moves forward with increases
in speed and angle of attack. Vortex
generators, mentioned above, are
used to try to control the boundary
layer and stop it separating (by reenergising
it), as are thin slots in the
wing that create a vacuum to suck it
down (the suction method). You can
get extra thrust as it is forced
backwards. Heavy showers of rain
162 Canadian Professional Pilot Studies
can affect the boundary layer enough
to decrease lift.
Alternatively, you could use a
specially designed thin wing called a
laminar flow aerofoil with a later
transition point. It has a more
pointed leading edge and is nearly
symmetrical, being thickest about
halfway across (50% chord, if you
want to be technical). One
disadvantage is that the transition
point moves more rapidly forward
when it stalls.
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. Winglets are small
vertical aerofoils on the wingtip
that cut down induced drag by
reducing the area of the wing tip
affected by vortices, by using
side forces, making the wing
span larger at the expense of a
small increase in form drag:
In fact, any method of stopping
vortices from being generated
from air spilling over the wing
tips improves efficiency – these
may also include wing tip tanks,
wing tip plates, or droop tips. This
is because induced drag
originates at the wingtip.
· 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
Principles of Flight 163
· 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
a third to a quarter of the size of the
stalling angle. It never changes, but
without an angle of attack indicator,
you need an indirect method of
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