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with the square of the velocity. Doubling the velocity means
“q” increases by four times. Increasing the velocity by a
factor of three means that the dynamic pressure (q) increases
by a factor of nine. This is a very important concept in
understanding the aerodynamics of WSC.
Formula for dynamic pressure: q = V2 x ρ/2
V = velocity
ρ = density factor
Lift
Lift opposes the downward force of weight and is produced
by the dynamic effects of the surrounding airstream acting
on the wing. Lift acts perpendicular to the fl ightpath through
the wing’s center of lift. There is a mathematical relationship
for lift which varies with dynamic pressure (q), AOA, and the
size of the wing. In the lift equation, these factors correspond
to the terms q, coeffi cient of lift (CL), and wing surface area.
The relationship is expressed in Figure 2-13.
Figure 2-13 shows that for lift to increase, one or more of
the factors on the other side of the equation must increase.
Generally, the lift needed is about the same for most fl ight
situations. A slower speed requires a higher AOA to produce
the same amount of lift. A faster speed requires a lower AOA
to produce the same amount of lift.
2-8
Eliptical
lift distribution
Weight-shift control
lift distribution
Figure 2-14. Elliptical lift distribution compared to lift distribution
of a WSC wing.
Figure 2-15. Front view with projected area shown that produces
drag.
Because lift is a function of dynamic pressure (q), it is
proportional to the square of the airspeed; therefore, small
changes in airspeed create larger changes in lift. Likewise, if
other factors remain the same while the CL increases, lift also
increases. The CL goes up as the AOA is increased. As air
density increases, lift increases. However, a pilot is usually
more concerned with how lift is diminished by reductions in
air density on a hot day, or if operating at higher altitudes.
All wings produce lift in two ways:
1. Airfoil shape creates a higher velocity over the top of
the wing and a lower velocity over the bottom of the
wing with Bernoulli’s venturi effect.
2. Downward deflection of airflow because of the
curvature of the wing with the principle of Newton’s
Third Law of Motion: for every action, there is an
equal and opposite reaction.
Both principles determine the lifting force. Review the
Pilot’s Handbook of Aeronautical Knowledge to understand
Newton’s laws of motion and Bernoulli’s venturi effect.
Figure 2-14 (top) shows the amount of lift produced along
the wing for an airplane wing with an elliptical planform.
Notice how the amount of lift generated is smallest at the
tips and increases slightly towards the root of the wing. This
is known as the “elliptical lift distribution.”
The WSC wing lift distribution is different because the wing
twist at the root is at a higher AOA than the tips. Most of
the lift is produced at the center of the wing with less lift
produced at the tips. The WSC lift distribution is compared
to the lift distribution for an optimum design elliptical wing
in Figure 2-14.
Drag
Drag is the resistance to forward motion through the air and
is parallel to the relative wind. Aerodynamic drag comes in
two forms:
1. Induced drag—a result of the wing producing lift.
2. Parasite drag—resistance to the airfl ow from the
carriage, its occupants, wires, the wing, interference
drag from objects in the airstream, and skin friction
drag of the wing.
Induced drag is the result of lift, and its amount varies as
discussed above for lift. Induced drag creates organized
circular vortices off the wingtips that generally track down
and out from each wingtip. Refer to the Pilot’s Handbook of
Aeronautical Knowledge for additional discussion on wingtip
vortices formation.
These wingtip vortex formations are typical for all aircraft
that use wings including WSC, PPC, helicopters, sailplanes,
and all fi xed-wing airplanes. The bigger and heavier the
aircraft, the greater and more powerful the wingtip vortices
are. This organized swirling turbulence is an important
factor to understand and avoid for fl ight safety. Refer to
the Aeronautical Information Manual (AIM) or the Pilot’s
Handbook of Aeronautical Knowledge (FAA-H-8083-25)
for additional discussion.
Parasite drag is caused by the friction of air moving over
all the components of the aircraft. Just as with lift, parasite
drag increases as the surface area of the aircraft increases,
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Weight-Shift Control Aircraft Flying Handbook(19)
