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Aerodynamics - Four Forces
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Understanding
how these forces work
and knowing how to control
them with the use of power
and flight controls are
essential to flight.
In steady flight, the
sum of these opposing
forces is always zero.
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Drag
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A rearward, retarding
force caused by disruption of airflow by the
wing, fuselage, and other protruding objects.
As a general rule, drag opposes thrust and acts
rearward parallel to the relative wind.
There are two basic types: parasite drag and
induced drag.
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Parasite Drag
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Called "parasite"
because it in no way functions to aid flight.
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Parasite drag is
comprised of all the forces that work to slow
an aircraft’s movement. This includes the
displacement of the air by the aircraft, turbulence
generated in the airstream, or a hindrance of
air moving over the surface of the aircraft
and airfoil.
Parasite drag varies with the speed of the airplane
(higher at higher speeds).
Proportionate to square of the airspeed (2x
as fast, 4x lift), aplane at constant altitude
has four times as much parasitic drag at 160kts
than it does at 80 kts.
There are three types of parasite drag:
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Form Drag
results from the turbulent wake caused by
the separation of airflow from the surface of
a structure. Examples include the engine cowlings,
antennas, etc. (Stuff sticking out into the
air). Related to both the size and shape of
the structure that protrudes into the relative
wind
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Interference
Drag comes from the intersection of airstreams
that creates eddy currents, turbulence, or restricts
smooth airflow. For example, the intersection
of the wing and the fuselage at the wing root
has significant interference drag.
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Skin friction
Drag is the aerodynamic resistance due to
the contact of moving air with the surface of
an aircraft. Every surface, no matter how apparently
smooth, has a rough, ragged surface when viewed
under a microscope.
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Induced Drag
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Is a result
of an any airfoil developing lift.
Inversely proportional to the square of the
airspeed (if speed is decreased by half, induced
drag increases fourfold).
When you practice slow flight, you must pull
back on the yoke to bring the nose higher, increasing
your AOA, and also increasing induced drag.
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Induced Drag - Vorticies
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Generated by
the airflow circulation around the wing as it
creates lift
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The high-pressure
air beneath the wing joins the low-pressure
air above the wing at the trailing edge and
wingtips, which causes a spiral vortex, which
trails behind each wingtip when lift is being
produced
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These wingtip
vortices have the effect of deflecting the air
stream downward in the vicinity of the wing,
creating an increase in downwash, and this deflects
the average relative wind downward (not straight
back). Since the lift produced by the wing is
perpendicular to the relative wind, the lift
is inclined aft by the same amount and not straight
upward
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This component
of lift acting in a rearward direction (lost
lift) is induced drag
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As the air pressure
differential increases with an increase in AOA,
stronger vortices form and induced drag is increased
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Low speed = High
AOA = High Induced drag
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High speed =
Low AOA = Low Induced drag
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You will learn
more about these vortices when you are introduce
to Wake Turbulence and Ground Effect
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