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Aircraft Static and Dynamic Stability

An aircraft is in static equilibrium when the sum of all forces and moments acting on it are equal to zero, experiencing no acceleration. There are three types of static stability: positive stability causes an aircraft to return to equilibrium after a disturbance, negative stability causes it to move further from equilibrium, and neutral stability causes it to remain at the new position. Dynamic stability deals with the motion over time following a disturbance - positive dynamic stability means the amplitude of oscillations decreases over time while negative dynamic stability means the amplitude increases. Aircraft must demonstrate both positive static and dynamic stability to be safely flyable.

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176 views2 pages

Aircraft Static and Dynamic Stability

An aircraft is in static equilibrium when the sum of all forces and moments acting on it are equal to zero, experiencing no acceleration. There are three types of static stability: positive stability causes an aircraft to return to equilibrium after a disturbance, negative stability causes it to move further from equilibrium, and neutral stability causes it to remain at the new position. Dynamic stability deals with the motion over time following a disturbance - positive dynamic stability means the amplitude of oscillations decreases over time while negative dynamic stability means the amplitude increases. Aircraft must demonstrate both positive static and dynamic stability to be safely flyable.

Uploaded by

Ali Çakirca
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Static Stability

An aircraft is in a state of equilibrium when the sum of all the forces acting on the aircraft and
all the moments is equal to zero. An aircraft in equilibrium experiences no accelerations, and
the aircraft continues in a steady condition of flight. A gust of wind or a deflection of the
controls disturbs the equilibrium, and the aircraft experiences acceleration due to the
unbalance of moment or force.

The three types of static stability are defined by the character of movement following some
disturbance from equilibrium. Positive static stability exists when the disturbed object tends to
return to equilibrium. Negative static stability, or static instability, exists when the disturbed
object tends to continue in the direction of disturbance. Neutral static stability exists when the
disturbed object has neither tendency, but remains in equilibrium in the direction of
disturbance. These three types of stability are illustrated in Figure.

Three types of
stability

Dynamic Stability

While static stability deals with the tendency of a displaced body to return to equilibrium,
dynamic stability deals with the resulting motion with time. If an object is disturbed from
equilibrium, the time history of the resulting motion defines the dynamic stability of the
object. In general, an object demonstrates positive dynamic stability if the amplitude of
motion decreases with time. If the amplitude of motion increases with time, the object is said
to possess dynamic instability.

Any aircraft must demonstrate the required degrees of static and dynamic stability. If an
aircraft were designed with static instability and a rapid rate of dynamic instability, the
aircraft would be very difficult, if not impossible, to fly. Usually, positive dynamic stability is
required in an aircraft design to prevent objectionable continued oscillations of the aircraft.
Longitudinal Stability

When an aircraft has a tendency to keep a constant AOA with reference to the relative wind
(i.e., it does not tend to put its nose down and dive or lift its nose and stall); it is said to have
longitudinal stability. Longitudinal stability refers to motion in pitch. The horizontal stabilizer
is the primary surface which controls longitudinal stability. The action of the stabilizer
depends upon the speed and AOA of the aircraft.

Directional Stability

Stability about the vertical axis is referred to as directional stability. The aircraft should be
designed so that when it is in straight-and-level flight it remains on its course heading even
though the pilot takes his or her hands and feet off the controls. If an aircraft recovers
automatically from a skid, it has been well designed for directional balance. The vertical
stabilizer is the primary surface that controls directional stability. Directional stability can be
designed into an aircraft, where appropriate, by using a large dorsal fin, a long fuselage, and
sweptback wings.

Lateral Stability

Motion about the aircraft’s longitudinal (fore and aft) axis is a lateral, or rolling, motion. The
tendency to return to the original attitude from such motion is called lateral stability.

Dutch Roll

A Dutch Roll is an aircraft motion consisting of an out-of-phase combination of yaw and roll.
Dutch roll stability can be artificially increased by the installation of a yaw damper.

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