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Pilot's Handbook of Aeronautical Knowledge
Aerodynamics of Flight
Weight and Balance

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Pilot's Handbook of Aeronautical Knowledge

Preface

Acknowledgements

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

Appendix

Glossary

Index

 

Effect of Weight on Stability and Controllability
Overloading also effects stability. An aircraft that is stable
and controllable when loaded normally may have very
different flight characteristics when overloaded. Although
the distribution of weight has the most direct effect on this,
an increase in the aircraft's gross weight may be expected
to have an adverse effect on stability, regardless of location
of the CG. The stability of many certificated aircraft is
completely unsatisfactory if the gross weight is exceeded.

Effect of Load Distribution
The effect of the position of the CG on the load imposed
on an aircraft's wing in flight is significant to climb and
cruising performance. An aircraft with forward loading is
"heavier" and consequently, slower than the same aircraft
with the CG further aft.

Figure 4-55 illustrates why this is true. With forward loading,
"nose-up" trim is required in most aircraft to maintain level
cruising flight Nose-up trim involves setting the tail surfaces
to produce a greater down load on the aft portion of the
fuselage, which adds to the wing loading and the total lift
required from the wing if altitude is to be maintained. This
requires a higher AOA of the wing, which results in more
drag and, in turn, produces a higher stalling speed.

Effect of load distribution on balance.
Figure 4-55. Effect of load distribution on balance.

With aft loading and "nose-down" trim, the tail surfaces exert
less down load, relieving the wing of that much wing loading
and lift required to maintain altitude. The required AOA
of the wing is less, so the drag is less, allowing for a faster
cruise speed. Theoretically, a neutral load on the tail surfaces
in cruising flight would produce the most efficient overall
performance and fastest cruising speed, but it would also
result in instability. Modern aircraft are designed to require
a down load on the tail for stability and controllability.

A zero indication on the trim tab control is not necessarily
the same as "neutral trim" because of the force exerted
by downwash from the wings and the fuselage on the tail
surfaces.

The effects of the distribution of the aircraft's useful load
have a significant influence on its flight characteristics, even
when the load is within the CG limits and the maximum
permissible gross weight. Important among these effects
are changes in controllability, stability, and the actual load
imposed on the wing.

Generally, an aircraft becomes less controllable, especially
at slow flight speeds, as the CG is moved further aft. An
aircraft which cleanly recovers from a prolonged spin with
the CG at one position may fail completely to respond to
normal recovery attempts when the CG is moved aft by one
or two inches.

It is common practice for aircraft designers to establish an aft
CG limit that is within one inch of the maximum which allows
normal recovery from a one-turn spin. When certificating an
aircraft in the utility category to permit intentional spins, the
aft CG limit is usually established at a point several inches
forward of that permissible for certification in the normal
category.

Another factor affecting controllability, which has become
more important in current designs of large aircraft, is the effect
of long moment arms to the positions of heavy equipment
and cargo. The same aircraft may be loaded to maximum
gross weight within its CG limits by concentrating fuel,
passengers, and cargo near the design CG, or by dispersing
fuel and cargo loads in wingtip tanks and cargo bins forward
and aft of the cabin.

With the same total weight and CG, maneuvering the
aircraft or maintaining level flight in turbulent air requires
the application of greater control forces when the load is
dispersed. The longer moment arms to the positions of the
heavy fuel and cargo loads must be overcome by the action
of the control surfaces. An aircraft with full outboard wing
tanks or tip tanks tends to be sluggish in roll when control
situations are marginal, while one with full nose and aft cargo
bins tends to be less responsive to the elevator controls.

 

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