Stability and Balance Control

Stability and Balance Control

Balance control refers to the location of the CG of an
aircraft. This is of primary importance to aircraft stability,
which determines safety in flight.
The CG is the point at which the total weight of the aircraft
is assumed to be concentrated, and the CG must be located
within specific limits for safe flight. Both lateral and
longitudinal balance are important, but the prime concern
is longitudinal balance; that is, the location of the CG
along the longitudinal or lengthwise axis.
An airplane is designed to have stability that allows it to
be trimmed so it will maintain straight and level flight with
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
nose-up 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.

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.

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.