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will decrease. The weight concentrated at the CG will pull
the nose back down. If the nose should drop in flight, the
airspeed will increase and the increased downward tail load
will bring the nose back up to level flight.
If the CG is too far forward, the downward tail load will have
to be increased to maintain level flight. This increased tail
load has the same effect as carrying additional weight — the
aircraft will have to fly at a higher angle of attack, and drag
will increase.
A more serious problem caused by the CG being too far
forward is the lack of sufficient elevator authority. At slow
takeoff speeds, the elevator might not produce enough noseup
force to rotate and on landing there may not be enough
elevator force to flare the airplane. [Figure 1-3] Both takeoff
and landing runs will be lengthened if the CG is too far
forward.
Center of lift: The location along the
chord line of an airfoil at which all the
lift forces produced by
the airfoil are considered to be
concentrated.
Longitudinal balance: Balance
around the pitch, or lateral, axis.
Longitudinal axis: An imaginary line
through an aircraft from nose to tail,
passing through its center
of gravity.
1–5
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.
The efficiency of some modern high-performance military
fighter airplanes is increased by giving them neutral
longitudinal stability. This is normally a very dangerous
situation; but these aircraft are flown by autopilots which
react far faster than a human pilot, and they are safe for their
special operations.
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
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.
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 enough out of balance 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.
Lateral balance: Balance around the
roll, or longitudinal, axis.
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 high-speed jet aircraft operation is critical.
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 book describes the way in
which this is done.
Butt (or buttock) line zero: A line
through the symmetrical center of an
aircraft from nose to tail. It serves as
the datum for measuring the arms used
to find the lateral CG. Lateral moments
that cause the aircraft to rotate
clockwise are positive (+), and those
that cause it to rotate counterclockwise
are negative (–).
1–6
2–1
Chapter 2
Weight and Balance Theory
and Documentation
Reference datum (GAMA): An
imaginary vertical plane from which
all horizontal distances are measured
for balance purposes.
Weight and Balance Theory
Two elements are vital in the weight and balance considerations
　
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