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Both takeoff and landing runs will be lengthened if the CG
is too far forward.
Figure 1-3. If the CG is too far forward, there will not be enough
elevator nose-up force to flare the airplane for landing.
The basic aircraft design assumes that lateral symmetry
exists. For each item of weight added to the left of the
centerline of the aircraft (also known as buttock line
zero, or BL-0), there is generally an equal weight at a
corresponding location on the right.
The lateral balance can be upset by uneven fuel loading
or burnoff. The position of the lateral CG is not normally
computed for an airplane, but the pilot must be aware
of the adverse effects that will result from a laterally
unbalanced condition. [Figure 1-4] This is corrected by
using the aileron trim tab until enough fuel has been used
from the tank on the heavy side to balance the airplane.
The deflected trim tab deflects the aileron to produce
additional lift on the heavy side, but it also produces
additional drag, and the airplane flies inefficiently.
Figure 1-4. Lateral imbalance causes wing heaviness, which may
be corrected by deflecting the aileron. The additional lift causes
additional drag and the airplane flies inefficiently.
Helicopters are affected by lateral imbalance more than
airplanes. If a helicopter is loaded with heavy occupants
and fuel on the same side, it could be out of balance
enough to make it unsafe to fly. It is also possible that if
external loads are carried in such a position to require large
lateral displacement of the cyclic control to maintain level
flight, the fore-and-aft cyclic control effectiveness will be
limited.
Sweptwing airplanes are more critical due to fuel
imbalance because as the fuel is used from the outboard
tanks, the CG shifts forward, and as it is used from the
inboard tanks, the CG shifts aft. [Figure 1-5] For this
reason, fuel-use scheduling in sweptwing airplanes
operation is critical.
Figure 1-5. Fuel in the tanks of a sweptwing airplane affects both
lateral and longitudinal balance. As fuel is used from an outboard
tank, the CG shifts forward.
1–
Weight Control for Aircraft other than Fixed
and Rotorwing
Some light aircraft utilize different methods of determining
weight and balance from the traditional fixed and
rotorwing aircraft. These aircraft achieve flight control
differently than the fixed-wing airplane or helicopter. Most
notable of these are weight shift control (WSC) aircraft
(also known as trikes), powered parachutes, and balloons.
These aircraft typically do not specify either an empty
weight center of gravity or a center of gravity range. They
require only a certified or approved maximum weight.
To understand why this is so, a look at how flight control is
achieved is helpful.
As an example, airplanes and WSC aircraft both control
flight under the influence of the same four forces (lift,
gravity, thrust, and drag), and around the same three axes
(pitch, yaw, and roll). However, each aircraft accomplishes
this control in a very different manner. This difference
helps explain why the fixed-wing airplane requires an
established weight and a known center of gravity, whereas
the WSC aircraft only requires the known weight.
The fixed-wing airplane has moveable controls that
alter the lift on various airfoil surfaces to vary pitch,
roll, and yaw. These changes in lift, in turn, change the
characteristics of the flight parameters. Weight normally
decreases in flight due to fuel consumption, and the
airplane center of gravity changes with this weight
reduction. An airplane utilizes its variable flight controls
to compensate and maintain controllability through the
various flight modes and as the center of gravity changes.
An airplane has a center of gravity range or envelope
within which it must remain if the flight controls are to
remain effective and the airplane safely operated.
The WSC aircraft has a relatively set platform wing
without a tail. The pilot, achieves control by shifting
weight. In the design of this aircraft, the weight of the
airframe and its payload is attached to the wing at a single
point in a pendulous arrangement. The pilot through the
flight controls, controls the arm of this pendulum and
thereby controls the aircraft. When a change in flight
parameter is desired, the pilot displaces the aircraft’s
weight in the appropriate distance and direction. This
change momentarily disrupts the equilibrium between
the four forces acting on the aircraft. The wing, due to its
inherent stability, then moves appropriately to re-establish
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Aircraft Weight and Balance Handbook(7)
