Shifting the CG

Shifting the CG

One common weight and balance problem involves
moving passengers from one seat to another or shifting
baggage or cargo from one compartment to another to
move the CG to a desired location. This also can be
visualized by using a board with three weights and then
working out the problem the way it is actually done on an
airplane.

Solution by Chart

The CG of a board can be moved by shifting the weights
as demonstrated in Figure 2-10. As the board is loaded,
it balances at a point 72 inches from the CG of weight A.

Determining the CG

Determining the CG

One of the easiest ways to understand weight and balance
is to consider a board with weights placed at various
locations. We can determine the CG of the board and
observe the way the CG changes as the weights are moved.
The CG of a board like the one in Figure 2-4 may be
determined by using these four steps:

1. Measure the arm of each weight in inches from the
datum.

2. Multiply each arm by its weight in pounds to determine
the moment in pound-inches of each weight.

3. Determine the total of all weights and of all the
moments. Disregard the weight of the board.

4. Divide the total moment by the total weight to
determine the CG in inches from the datum.

The Law of the Lever

The Law of the Lever

The weight and balance problems are based on the
physical law of the lever. This law states that a lever is
balanced when the weight on one side of the fulcrum
multiplied by its arm is equal to the weight on the opposite
side multiplied by its arm. In other words, the lever is
balanced when the algebraic sum of the moments about the
fulcrum is zero. [Figure 2-2] This is the condition in which
the positive moments (those that try to rotate the lever
clockwise) are equal to the negative moments (those that
try to rotate it counter-clockwise).

Aircraft Arms, Weight, and Moments

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
and others negative. To simplify weight and balance
computations, most modern helicopters, like airplanes,
have the datum located at the nose of the aircraft or a
specified distance ahead of it.

A moment is a force that tries to cause rotation, and is the
product of the arm, in inches, and the weight, in pounds.
Moments are generally expressed in pound-inches (lb-in)
and may be either positive or negative. Figure 2-1 shows
the way the algebraic sign of a moment is derived. Positive
moments cause an airplane to nose up, while negative
moments cause it to nose down.

Weight and Balance Theory

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.

Weight Control for Aircraft other than Fixed and Rotorwing

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
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.

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.

Weight Changes

Weight Changes

The maximum allowable weight for an aircraft is
determined by design considerations. However, the
maximum operational weight may be less than the
maximum allowable weight due to such considerations as
high-density altitude or high-drag field conditions caused
by wet grass or water on the runway. The maximum
operational weight may also be limited by the departure or
arrival airport’s runway length.

One important preflight consideration is the distribution
of the load in the aircraft. Loading the aircraft so the gross
weight is less than the maximum allowable is not enough.
This weight must be distributed to keep the CG within the
limits specified in the POH or AFM.

If the CG is too far forward, a heavy passenger can
be moved to one of the rear seats or baggage can be
shifted from a forward baggage compartment to a rear
compartment. If the CG is too far aft, passenger weight or
baggage can be shifted forward. The fuel load should be
balanced laterally: the pilot should pay special attention
to the POH or AFM regarding the operation of the fuel
system, in order to keep the aircraft balanced in flight.

Weight and balance of a helicopter is far more critical
than for an airplane. With some helicopters, they may be
properly loaded for takeoff, but near the end of a long
flight when the fuel tanks are almost empty, the CG may
have shifted enough for the helicopter to be out of balance
laterally or longitudinally. Before making any long flight,
the CG with the fuel available for landing must be checked
to ensure it will be within the allowable range.

Airplanes with tandem seating normally have a limitation
requiring solo flight to be made from the front seat in
some airplanes or the rear seat in others. Some of the
smaller helicopters also require solo flight be made from a
specific seat, either the right, left, or center. These seating
limitations will be noted by a placard, usually on the
instrument panel, and they should be strictly adhered to.

As an aircraft ages, its weight usually increases due to
trash and dirt collecting in hard-to-reach locations, and
moisture absorbed in the cabin insulation. This growth in
weight is normally small, but it can only be determined by
accurately weighing the aircraft

Changes of fixed equipment may have a major effect upon
the weight of the aircraft. Many aircraft are overloaded by
the installation of extra radios or instruments. Fortunately,
the replacement of older, heavy electronic equipment with
newer, lighter types results in a weight reduction. This
weight change, however helpful, will probably cause the
CG to shift and this must be computed and annotated in
the weight and balance record.

Repairs and alteration are the major sources of weight
changes, and it is the responsibility of the A&P mechanic
or repairman making any repair or alteration to know the
weight and location of these changes, and to compute the
CG and record the new empty weight and EWCG in the
aircraft weight and balance record.

If the newly calculated EWCG should happen to fall
outside the EWCG range, it will be necessary to perform
adverse loading check. This will require a forward and
rearward adverse-loading check, and a maximum weight
check. These weight and balance extreme conditions
represent the maximum forward and rearward CG position
for the aircraft. Adverse loading checks are a deliberate
attempt to load an aircraft in a manner that will create the
most critical balance condition and still remain within
the design CG limits of the aircraft. If any of the checks
fall outside the loaded CG range, the aircraft must be
reconfigured or placarded to prevent the pilot from loading
the aircraft improperly. It is sometimes possible to install
fixed ballast in order for the aircraft to again operate within
the normal CG range.

The A&P mechanic or repairman conducting an annual or
condition inspection must ensure the weight and balance
data in the aircraft records is current and accurate. It
is the responsibility of the pilot in command to use the
most curre hands off the controls. Longitudinal stability
is maintained by ensuring the CG is slightly ahead of the
center of lift. This produces a fixed nose-down force independent of
the airspeed. This is balanced by a variable nose-up force,
which is produced by a downward aerodynamic force on
the horizontal tail surfaces that varies directly with the
airspeed.

Effects of Weight

Effects of Weight

Most modern aircraft are so designed that if all seats
are occupied, all baggage allowed by the baggage
compartment is carried, and all of the fuel tanks are
full, the aircraft will be grossly overloaded. This type of
design requires the pilot to give great consideration to the
requirements of the trip. If maximum range is required,
occupants or baggage must be left behind, or if the
maximum load must be carried, the range, dictated by the
amount of fuel on board, must be reduced.
Some of the problems caused by overloading an aircraft
are:

• the aircraft will need a higher takeoff speed, which
results in a longer takeoff run.
• both the rate and angle of climb will be reduced.
• the service ceiling will be lowered.
• the cruising speed will be reduced.
• the cruising range will be shortened.
• maneuverability will be decreased.
• a longer landing roll will be required because the
landing speed will be higher.
• excessive loads will be imposed on the structure,
especially the landing gear.

The POH or AFM includes tables or charts that give the
pilot an indication of the performance expected for any
weight. An important part of careful preflight planning
includes a check of these charts to determine the aircraft is
loaded so the proposed flight can be safely made.

Weight and Balance Control

There are many factors that lead to efficient and safe
operation of aircraft. Among these vital factors is proper
weight and balance control. The weight and balance
system commonly employed among aircraft consists of
three equally important elements: the weighing of the
aircraft, the maintaining of the weight and balance records,
and the proper loading of the aircraft. An inaccuracy in any
one of these elements nullifies the purpose of the whole
system. The final loading calculations will be meaningless
if either the aircraft has been improperly weighed or the
records contain an error.

Improper loading cuts down the efficiency of an aircraft
from the standpoint of altitude, maneuverability, rate
of climb, and speed. It may even be the cause of failure
to complete the flight, or for that matter, failure to start
the flight. Because of abnormal stresses placed upon the
structure of an improperly loaded aircraft, or because of
changed flying characteristics of the aircraft, loss of life
and destruction of valuable equipment may result.
The responsibility for proper weight and balance control
begins with the engineers and designers, and extends to the
aircraft mechanics that maintain the aircraft and the pilots
who operate them.

Modern aircraft are engineered utilizing state-of-the-art
technology and materials to achieve maximum reliability
and performance for the intended category. As much
care and expertise must be exercised in operating and
maintaining these efficient aircraft as was taken in their
design and manufacturing.

The designers of an aircraft have set the maximum weight,
based on the amount of lift the wings or rotors can provide
under the operation conditions for which the aircraft
is designed. The structural strength of the aircraft also
limits the maximum weight the aircraft can safely carry.
The ideal location of the center of gravity (CG) was very
carefully determined by the designers, and the maximum
deviation allowed from this specific location has been
calculated.

The manufacturer provides the aircraft operator with the
empty weight of the aircraft and the location of its emptyweight
center of gravity (EWCG) at the time the certified
aircraft leaves the factory. Amateur-built aircraft must have
this information determined and available at the time of
certification.

The airframe and powerplant (A&P) mechanic or
repairman who maintains the aircraft keeps the weight and
balance records current, recording any changes that have
been made because of repairs or alterations.
The pilot in command of the aircraft has the responsibility
on every flight to know the maximum allowable weight
of the aircraft and its CG limits. This allows the pilot to
determine on the preflight inspection that the aircraft is
loaded in such a way that the CG is within the allowable
limits.

Weight Control

Weight is a major factor in airplane construction and
operation, and it demands respect from all pilots and
particular diligence by all A&P mechanics and repairmen.
Excessive weight reduces the efficiency of an aircraft
and the safety margin available if an emergency
condition should arise.
When an aircraft is designed, it is made as light as the
required structural strength will allow, and the wings or
rotors are designed to support the maximum allowable
weight. When the weight of an aircraft is increased,
the wings or rotors must produce additional lift and the
structure must support not only the additional static loads,
but also the dynamic loads imposed by flight maneuvers.
For example, the wings of a 3,000-pound airplane must
support 3,000 pounds in level flight, but when the airplane
is turned smoothly and sharply using a bank angle of 60°,
the dynamic load requires the wings to support twice this,
or 6,000 pounds.
Severe uncoordinated maneuvers or flight into turbulence
can impose dynamic loads on the structure great enough
1–
to cause failure. In accordance with Title 14 of the Code
of Federal Regulations (14 CFR) part 23, the structure of a
normal category airplane must be strong enough to sustain
a load factor of 3.8 times its weight. That is, every pound
of weight added to an aircraft requires that the structure
be strong enough to support an additional 3.8 pounds.
An aircraft operated in the utility category must sustain a
load factor of 4.4, and acrobatic category aircraft must be
strong enough to withstand 6.0 times their weight.

The lift produced by a wing is determined by its airfoil
shape, angle of attack, speed through the air, and the air
density. When an aircraft takes off from an airport with a
high density altitude, it must accelerate to a speed faster
than would be required at sea level to produce enough
lift to allow takeoff; therefore, a longer takeoff run is
necessary. The distance needed may be longer than the
available runway. When operating from a high-density
altitude airport, the Pilot’s Operating Handbook (POH)
or Airplane Flight Manual (AFM) must be consulted to
determine the maximum weight allowed for the aircraft
under the conditions of altitude, temperature, wind, and
runway conditions.