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system (GPS), spare clothes, suitcase, etc.).
During gliding flight, weight is broken down into two
components. The component that opposes the lift, acting
perpendicular to fl ight/glide path, and the component that
opposes the drag and acts in the direction of the fl ight/glide
path. During gliding fl ight, this component of weight is the
weight component providing the forward force which some
call thrust for gliding fl ight.
During gliding, straight, and descending in unaccelerated
fl ight:
Lift (L) and Drag (D) components = Resultant force (RF)
= Weight (W)
Total Drag (DT) = Weight component (WD) in the direction
of fl ight
Lift (L) = Weight component (WL) that opposes lift
Similar to airplanes, gliders, and PPC during gliding fl ight,
less lift is required because the resultant force composed of
lift and drag provides the force to lift the weight. In other
words, in gliding fl ight, drag helps support the weight.
[Figure 2-19]
Thrust
At a constant air speed, the amount of thrust determines
whether an aircraft climbs, fl ies level, or descends. With the
engine idle or shut off, a pilot is descending or gliding down.
Maintaining a constant airspeed, when enough thrust is added
2-11
Total thrust
to climb
Root Wing Chord
Relative Wind
Flightpath
Component of weight
to oppose lift
Lift Resultant force
required to lift weight
Thrust required
to overcome
component
of weight
Excess Thrust
(Total thrust
minus Drag)
used to climb Angle of Attack
CG
Weight
Figure 2-21. Typical forces in climbing flight.
Earth’s Horizontal Plane
CG
Root Wing Chord
Relative Wind
Flightpath
Lift
Drag
Weight
Thrust
Resultant force
Resultant force
Thrust
Angle of Attack
Figure 2-20. Typical forces in level fl ight.
Thrust Required for Increases in Speed
Above the lowest total drag airspeed [Figure 2-18], faster
speeds (lower angles of attack) for level and climbing fl ight
requires greater thrust because of the increased drag created
from the faster speeds.
AOA is the primary control of increasing and decreasing
speeds, and increasing thrust generally does not produce
higher speeds, but additional thrust is required to maintain
level fl ight at higher speeds.
Ground Effect
Ground effect is when the wing is fl ying close to the ground
and there is interference of the ground with the airfl ow
patterns created by the wing. At the same angle of attack,
lift increases slightly and the drag decreases signifi cantly.
The most apparent indication from ground effect is the
unexpected lift given to an aircraft as it fl ies close to the
ground—normally during takeoffs and landings. More details
for ground effect aerodynamics are found in the Pilot’s
Handbook of Aeronautical Knowledge. Flight characteristics
for ground effect are covered in the takeoff and landing
chapters.
Center of Gravity (CG)
The CG is the theoretical point of concentrated weight of
the aircraft. It is the point within the WSC aircraft about
which all the moments trying to rotate it during fl ight are
balanced. The most obvious difference in the CG for a WSC
aircraft is the vertical position compared to an airplane, as
it is always lower than the wing. The Pilot’s Handbook of
Aeronautical Knowledge accurately states the CG is generally
in the vertical center of the fuselage. The same is true for the
WSC aircraft. However, the WSC wing is higher above the
fuselage/carriage and, since most of the weight is centered
in the carriage, the CG is well below the wing.
In a two-seat WSC aircraft, the second seat is typically behind
the pilot’s seat and the CG is usually located close to the rear
passenger seat. Therefore, the CG location does not change
signifi cantly with a passenger. Fuel tanks are typically located
near the vertical CG so any difference in fuel quantity does
not signifi cantly change the CG fore and aft with different
fuel quantities.
For level fl ight, the CG is directly below the wing/carriage
attachment point known as the hang point, and the propeller
thrust line is typically designed to be near the vertical position
of the CG. [Figure 2-22]
2-12
CG
Aircraft center of gravity
• Center of gravity is under wing hang point for
level flight.
• Center of lift is directly above center of gravity,
above the wing hang point for level flight.
Thrust line is typically
designed to be at vertical
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Weight-Shift Control Aircraft Flying Handbook(21)
