# Aircraft Stability and Control

Updated on July 31, 2013

## Stability and Control

While an aircraft is in flight it will experience disturbance forces from sources like the weather. To have a high performance the aircraft will need to be able to recover from these disturbances quite quickly. When discussing stability and control there are three terms that always come up, these are:

1. Stability: How the aircraft keeps straight and level along the desired flight path.
2. Controllability: How good the response of the aircraft is to the pilot's commands.
3. Manoeuvrability: Following the desired path will combating disturbances.

## Longitudinal Stability

This is in the pitching plane and about the lateral axis. For an Aircraft to be longitudinally stable it must have one of two options. It can either have a natural or an inbuilt tendency to return to the same attitude in after any disturbance (will return to the original angle of attack before the disturbance).

This may be easier to understand with an example, so imagine an aircraft in flight and a gust of wind changes the attitude of the aircraft making the nose pitch upwards. The aircraft will then continue on it's original flight path but coming in contact with the airflow at a higher angle of attack. This causes the tailplane to move upwards thus creating a downwards force on the aircraft nose. The picture above shows that the distance of the moment arm is quite long, meaning the force created by the tailplane doesn't need to be large in order to have a big effect. This happening will force the aircraft back to it's original angle of attack. The tail is an example of a natural tendency of an aircraft to restore itself after a disturbance.

If an aircraft is longitudinally stable it must have a means to correct the forces on the aircraft after a disturbance to return it to the original position. The Centre of Gravity needs to be close to and in front of the Centre of Pressure. The Centre of Pressure moves with the angle of attack, it moves forward as the angle of attack is increased. This movement will then make the aircraft tend to instability.

## Lateral Stability

Lateral Stability is either the natural or inbuilt tendency of an aircraft to recover from disturbances in the lateral plane. Rolling about the longitudinal axis. Disturbances in roll will cause one wing to drop and the other to rise. When the aircraft is banked, the lift vector is inclined and produces a sideslip. As a result of the sideslip the aircraft is subjected to a sideways component of the relative airflow, thus producing forces to restore the aircraft wings to it's original position.

The main factor affecting lateral stability is the wing, Dihedral increases lateral stability. The Dihedral is the amount of incline of the wing from the fuselage. As the aircraft sideslips then the lower wing due to the Dihedral will meet the the relative airflow at a greater angle of attack and will produce increased lift. The upper wing meets the relative airflow at a lower angle of attack and therefore produces less lift. The rolling moment produced turns the aircraft to its original position. Anhedral decreases lateral stability, this is when each wing is inclined downwards from the fuselage. For this case the lower wing due to Anhedral meets the airflow at a reduced angle of attack, reducing lift. The upper wing meets the airflow at an increased angle of attack thus producing more lift. The overall effect increases the roll and therefore reduces lateral stability.

Other factors affecting lateral stability are High Keel Surfaces (high fin / high wings) and a Low Centre of Gravity.

## Directional Stability

Directional stability of an aircraft is the natural or inbuilt tendency to recover from a disturbance in the yawning of a plane. Basically it is how the aircraft moves the nose back into original position after a disturbance. It works along the normal axis. Once an aircraft is disturbed causing the nose or tail to be pushed to one side or yawed as it is called, the aircraft would continue moving in this new direction. The aircraft is now kind of moving sideways through the air, this is known as yaw. However this would divert from the flight path and so must be prevented.

The vertical fin is now experiencing an angle of attack, thus it will generate a sideways lift which tends to take the fin back to its original position. The vertical fin has such a large area so it produces a powerful turning effect, this force is what returns the nose back to position.

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