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tail. As forces such as turbulence and gusts act to displace
the aircraft, the pilot reacts by providing opposing control
forces to counteract this displacement.
Some aircraft are subject to changes in the location of the CG
with variations of load. Trimming devices, such as elevator
trim tabs and adjustable horizontal stabilizers, are used to
counteract the moments set up by fuel burnoff and loading
or off-loading of passengers or cargo.
Aircraft Design Characteristics
Each aircraft handles somewhat differently because each
resists or responds to control pressures in its own way. For
example, a training aircraft is quick to respond to control
applications, while a transport aircraft feels heavy on the
controls and responds to control pressures more slowly.
These features can be designed into an aircraft to facilitate
the particular purpose of the aircraft by considering certain
stability and maneuvering requirements. The following
discussion summarizes the more important aspects of an
aircraft’s stability, maneuverability, and controllability
qualities; how they are analyzed; and their relationship to
various flight conditions.
Stability
Stability is the inherent quality of an aircraft to correct for
conditions that may disturb its equilibrium and to return to
or to continue on the original flight path. It is primarily an
aircraft design characteristic. The flight paths and attitudes an
aircraft flies are limited by the aerodynamic characteristics of
the aircraft, its propulsion system, and its structural strength.
These limitations indicate the maximum performance and
maneuverability of the aircraft. If the aircraft is to provide
maximum utility, it must be safely controllable to the full
extent of these limits without exceeding the pilot’s strength
or requiring exceptional flying ability. If an aircraft is to fly
straight and steady along any arbitrary flight path, the forces
acting on it must be in static equilibrium. The reaction of
any body when its equilibrium is disturbed is referred to as
stability. The two types of stability are static and dynamic.
Static Stability
Static stability refers to the initial tendency, or direction of
movement, back to equilibrium. In aviation, it refers to the
aircraft’s initial response when disturbed from a given pitch,
yaw, or bank.
• Positive static stability—the initial tendency of the
aircraft to return to the original state of equilibrium
after being disturbed. [Figure 5-21]
• Neutral static stability—the initial tendency of
the aircraft to remain in a new condition after its
equilibrium has been disturbed. [Figure 5-21]
• Negative static stability—the initial tendency of the
aircraft to continue away from the original state of
equilibrium after being disturbed. [Figure 5-21]
Dynamic Stability
Static stability has been defined as the initial tendency to
return to equilibrium that the aircraft displays after being
disturbed from its trimmed condition. Occasionally, the
initial tendency is different or opposite from the overall
tendency, so a distinction must be made between the two.
Dynamic stability refers to the aircraft response over time
Positive Static Stability Neutral Static Stability Negative Static Stability
Applied
force
Applied
force
Applied
force
CG
CG
CG
CG
Figure 5-21. Types of static stability.
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