| Home | Privacy | Contact |

Pilot's Handbook of Aeronautical Knowledge
Weight and Balance
Balance, Stability, and Center of Gravity

| First | Previous | Next | Last |

Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Flying an aircraft that is out of balance can produce increased
pilot fatigue with obvious effects on the safety and efficiency
of flight. The pilot's natural correction for longitudinal
unbalance is a change of trim to remove the excessive control
pressure. Excessive trim, however, has the effect of reducing
not only aerodynamic efficiency but also primary control
travel distance in the direction the trim is applied.

Effects of Adverse Balance
Adverse balance conditions affect flight characteristics in
much the same manner as those mentioned for an excess
weight condition. It is vital to comply with weight and
balance limits established for all aircraft, especially rotorcraft.
Operating above the maximum weight limitation compromises
the structural integrity of the rotorcraft and adversely affects
performance. Balance is also critical because on some fully
loaded rotorcraft, CG deviations as small as three inches can
dramatically change handling characteristics. Stability and
control are also affected by improper balance.

Loading in a nose-heavy condition causes problems in
controlling and raising the nose, especially during takeoff
and landing. Loading in a tail heavy condition has a serious
effect upon longitudinal stability, and reduces the capability
to recover from stalls and spins. Tail heavy loading also
produces very light control forces, another undesirable
characteristic. This makes it easy for the pilot to inadvertently
overstress an aircraft.

It is important to reevaluate the balance in a rotorcraft
whenever loading changes. In most aircraft, off-loading a
passenger is unlikely to adversely affect the CG, but offloading
a passenger from a rotorcraft can create an unsafe
flight condition. An out-of-balance loading condition also
decreases maneuverability since cyclic control is less
effective in the direction opposite to the CG location.
Limits for the location of the CG are established by the
manufacturer. These are the fore and aft limits beyond
which the CG should not be located for flight. These limits
are published for each aircraft in the Type Certificate Data
Sheet (TCDS), or aircraft specification and the AFM or
pilot's operating handbook (POH). If the CG is not within the
allowable limits after loading, it will be necessary to relocate
some items before flight is attempted.

The forward CG limit is often established at a location that
is determined by the landing characteristics of an aircraft.
During landing, one of the most critical phases of flight,
exceeding the forward CG limit may result in excessive loads
on the nosewheel, a tendency to nose over on tailwheel type
airplanes, decreased performance, higher stalling speeds, and
higher control forces.

In extreme cases, a CG location that is beyond the forward
limit may result in nose heaviness, making it difficult or
impossible to flare for landing. Manufacturers purposely
place the forward CG limit as far rearward as possible to
aid pilots in avoiding damage when landing. In addition to
decreased static and dynamic longitudinal stability, other
undesirable effects caused by a CG location aft of the
allowable range may include extreme control difficulty,
violent stall characteristics, and very light control forces
which make it easy to overstress an aircraft inadvertently.
A restricted forward CG limit is also specified to assure
that sufficient elevator/control deflection is available at
minimum airspeed. When structural limitations do not limit
the forward CG position, it is located at the position where
full-up elevator/control deflection is required to obtain a high
AOA for landing.

The aft CG limit is the most rearward position at which the
CG can be located for the most critical maneuver or operation.
As the CG moves aft, a less stable condition occurs, which
decreases the ability of the aircraft to right itself after
maneuvering or turbulence.

For some aircraft, both fore and aft CG limits may be
specified to vary as gross weight changes. They may also
be changed for certain operations, such as acrobatic flight,
retraction of the landing gear, or the installation of special
loads and devices that change the flight characteristics.

The actual location of the CG can be altered by many variable
factors and is usually controlled by the pilot. Placement of
baggage and cargo items determines the CG location. The
assignment of seats to passengers can also be used as a means
of obtaining a favorable balance. If an aircraft is tail heavy, it
is only logical to place heavy passengers in forward seats.

Fuel burn can also affect the CG based on the location of the
fuel tanks. For example, most small aircraft carry fuel in the
wings very near the CG and burning off fuel has little effect
on the loaded CG. On rotorcraft, the fuel tanks are often
located behind the CG and fuel consumption from a tank
aft of the rotor mast causes the loaded CG to move forward.
A rotorcraft in this condition has a nose-low attitude when
coming to a hover following a vertical takeoff. Excessive
rearward displacement of the cyclic control is needed to
maintain a hover in a no-wind condition. Flight should not
be continued since rearward cyclic control fades as fuel is
consumed. Deceleration to a stop may also be impossible. In
the event of engine failure and autorotation, there may not be
enough cyclic control to .are properly for a landing.