Stability

The aircraft has now been considered in both the steady flight path condition and during changes of direction (maneuver). It is now necessary to investigate how the designer includes features in order to maintain or encourage either condition.

For example, it will be presumed that a steady flight path is to be maintained. If the aircraft deviates from this flight path, the aircraft should be able to regain it, without control input from the pilot.

In any dynamic system, the ability of the system to regain the desired (set) condition is termed stability. A pendulum is a classic example. It (the weight) normally hangs vertically. If it is displaced and released, it immediately moves back towards the original position. (In fact, of course, it swings past that position - the restoring force of gravity reverses its effect and it swings back again. It will swing to and fro (oscillate) many times before the oscillations (displacements) die away). Such a system is a stable system. But a system can be unstable.

Note that the above is the initial part of considering stability, the immediate reaction or tendency to movement following initial displacement is used to determine the static stability of the system.

Dynamic stability

So, following initial displacements the system may oscillate about the neutral position if the system is statically stable. The manner of the oscillations (meaning the change in amplitude) is used to describe the system dynamic stability. If the amplitude decreases, the aircraft is dynamically stable; if it increases it is dynamically unstable. When the amplitude remains constant, it is neutrally stable in the dynamic sense. Most systems are designed to be statically and dynamically stable.

Aircraft stability

Considering the stability of an aircraft, we might ask two questions. Can it oscillate, and if so, what are the neutral or zero displacement positions?

The first answer is 'yes', where the oscillations are related to angular displacements about any of the three axes. The zero displacements are considered to be those associated with straight and level flight.

Rotation about the lateral axis is termed pitch. Rotation about the longitudinal axis is termed roll. Rotation about the normal axis is termed yaw.

What the Flaps Do

The flap on each wing is called an aileron. The two ailerons work in flying an Airplane opposite directions. When one wing’s aileron is raised up, the other one is lowered. The pilot uses them to tilt the plane to one side or another. This motion is known as “roll.” The tail area flaps move the plane in other ways. The rudder, which stands upright at the back of the tail, can jut out from the tail to the left or to the right. The pilot uses the rudder to turn the plane left or right. “Yaw” is another name for this motion. Flaps called elevators also are in the tail area. The pilot raises or lowers these two flaps. They make the plane climb up or dive down. This motion is known as “pitch.”

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