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only a flat wing like a kite. This, of course, is not the
case at all. The balance of the lift needed to support the
glider comes from the flow of air above the wing.
Herein lies the key to flight. The fact that the most lift is
the result of the airflow downwash from above the wing,
forcing the wing upward, must be thoroughly understood
in order to continue further in the study of flight.
It is neither accurate nor does it serve a useful purpose,
however, to assign specific values to the percentage of
lift generated by the upper surface of the airfoil versus
that generated by the lower surface. These are not constant
values, and will vary, not only with flight conditions,
but also with different wing designs.
DRAG
The force that resists the movement of the glider
through the air is called drag. Two different types of
drag combine to form total drag—parasite and induced.
PARASITE DRAG
Parasite drag is caused by any aircraft surface, which
deflects or interferes with the smooth airflow around
the glider. Parasite drag is divided into three types—
form drag, interference drag, and skin friction drag.
[Figure 3-8]
FORM DRAG
Form drag results from the turbulent wake caused by
the separation of airflow from the surface of a structure.
The amount of drag is related to both the size and
shape of the structure protruding into the relative wind.
[Figure 3-9]
INTERFERENCE DRAG
Interference drag occurs when varied currents of air
over a glider meet and interact. Placing two objects
adjacent to one another may produce turbulence 50 percent
to 200 percent greater than the parts tested separately.
An example of interference drag is the mixing of
air over structures, such as the wing, tail surfaces, and
wing struts.
SKIN FRICTION DRAG
Skin friction drag is caused by the roughness of the
glider’s surfaces. Even though the surfaces may appear
smooth, they may be quite rough when viewed under a
microscope. This roughness allows a thin layer of air
to cling to the surface and create small eddies, which
contribute to drag.
INDUCED DRAG
The airflow circulation around the wing generates
induced drag as it creates lift. The high-pressure
air beneath the wing joins the low-pressure air
above the wing at the trailing edge of the wingtips.
This causes a spiral or vortex that trails behind each
wingtip whenever lift is being produced. These
Figure 3-8. Parasite drag increases fourfold when airspeed
is doubled.
wingtip vortices have the effect of deflecting the
airstream downward in the vicinity of the wing, creating
an increase in downwash. Therefore, the wing operates
in an average relative wind, which is deflected
downward and rearward near the wing. Because the lift
produced by the wing is perpendicular to the relative
wind, the lift is inclined aft by the same amount. The
component of lift acting in a rearward direction is
induced drag. [Figure 3-10]
As the air pressure differential increases with an
increase in the angle of attack, stronger vortices form
and induced drag is increased. The wings of a glider
are at a high angle of attack at low speed and at a low
angle of attack at high speed.
TOTAL DRAG
Total drag on a glider is the sum of parasite and
induced drag. The total drag curve represents these
combined forces and is plotted against airspeed.
[Figure 3-11]
3-5
Figure 3-11. The low point on the total drag curve
shows the airspeed at which drag is minimized.
High pressure air joins low pressure air
at the trailing edge of the wing and wingtips.
Wingtip vortices
develop.
The downwash increases behind
the wing.
The average relative wind is inclined downward and
rearward and lift is inclined aft. The rearward component
of lift is induced drag.
3-6
L/Dmax is the point, where lift-to-drag ratio is greatest.
At this speed, the total lift capacity of the glider, when
compared to the total drag of the glider, is most favorable.
In calm air, this is the airspeed you can use to
obtain maximum glide distance.
DRAG EQUATION
To help explain the force of drag, the mathematical
equation D=CDqS is used. In this equation drag (D) is
the product of drag coefficient (CD), dynamic pressure
(q), and surface area (S). The drag coefficient is the
ratio of drag pressure to dynamic pressure. The drag
coefficient is represented graphically by Figure 3-12.
This graph shows that at higher angles of attack, the
drag coefficient is greater than at low angles of attack.
At high angles of attack, drag increases significantly
　
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