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the desired relationship between these forces. This happens
by the wing flexing and altering its shape. As the shape
is changed, lift is varied at different points on the wing to
achieve the desired flight parameters.
The flight controls primarily affect the pitch-and-roll
axis. Since there is no vertical tail plane, minimal or no
ability exists to directly control yaw. However, unlike the
airplane, the center of gravity experienced by the wing
remains constant. Since the weight of the airframe acts
through the single point (wing attach point), the range
over which the weight may act is fixed at the pendulum
arm or length. Even though the weight decreases as fuel is
consumed, the weight remains focused at the wing attach
point. Most importantly, because the range is fixed, the
need to establish a calculated range is not required.
The powered parachute also belongs to the pendulumstyle
aircraft. Its airframe center of gravity is fixed at the
pendulum attach point. It is more limited in controllability
than the WSC aircraft because it lacks an aerodynamic
pitch control. Pitch (and lift) control is primarily a function
of the power control. Increased power results in increased
lift; cruise power amounts to level flight; decreased power
causes a descent. Due to this characteristic, the aircraft is
basically a one-air speed aircraft. Once again, because the
center of gravity is fixed at the attach point to the wing,
there can be no center of gravity range.
Roll control on a powered parachute is achieved by
changing the shape of the wing. The change is achieved
by varying the length of steering lines attached to the
outboard trailing edges of the wing. The trailing edge of
the parachute is pulled down slightly on one side or the
other to create increased drag along that side. This change
in drag creates roll and yaw, permitting the aircraft to be
steered.
The balloon is controlled by the pilot only in the vertical
dimension; this is in contrast to all other aircraft. He or she
achieves this control through the use of lift and weight.
Wind provides all other movement. The center of gravity
of the gondola remains constant beneath the balloon
envelope. As in WSC and powered-parachute aircraft,
there is no center of gravity limitation.
Aircraft can perform safely and achieve their designed
efficiency only when they are operated and maintained in
the way their designers intended. This safety and efficiency
is determined to a large degree by holding the aircraft’s
weight and balance parameters within the limits specified
for its design. The remainder of this handbook describes
the way in which this is done.
1–
2–
2 Chapter Weight and Balance
Theory and Documentation
Weight and Balance Theory
Two elements are vital in the weight and balance
considerations of an aircraft.
• The total weight of the aircraft must be no greater
than the maximum weight allowed by the FAA for the
particular make and model of the aircraft.
• The center of gravity, or the point at which all of the
weight of the aircraft is considered to be concentrated,
must be maintained within the allowable range for the
operational weight of the aircraft.
Aircraft Arms, Weight, and Moments
The term arm, usually measured in inches, refers to the
distance between the center of gravity of an item or object
and the datum. Arms ahead of, or to the left of the datum
are negative(-), and those behind, or to the right of the
datum are positive(+). When the datum is ahead of the
aircraft, all of the arms are positive and computational
errors are minimized. Weight is normally measured in
pounds. When weight is removed from an aircraft, it is
negative(-), and when added, it is positive (+).
The manufacturer establishes the maximum weight and
range allowed for the CG, as measured in inches from the
reference plane called the datum. Some manufacturers
specify this range as measured in percentage of the mean
aerodynamic chord (MAC), the leading edge of which is
located a specified distance from the datum.
The datum may be located anywhere the manufacturer
chooses; it is often the leading edge of the wing or some
specific distance from an easily identified location. One
popular location for the datum is a specified distance
forward of the aircraft, measured in inches from some
point, such as the nose of the aircraft, or the leading edge
of the wing, or the engine firewall.
The datum of some helicopters is the center of the rotor
mast, but this location causes some arms to be positive
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Aircraft Weight and Balance Handbook(8)
