Stability
If you have ever been on a commercial flight when the aircraft flew into turbulence, you know what the opposite of stability is: "instability". This section will discuss stability, what it is, and how to make it happen.
Stability is the tendency of an airplane in flight to remain in straight level, upright flight, and to go back to flying like this after something happens, without the pilot having to take control.
For example, you may fly through a patch of turbulence (bumpy air) and your aircraft may pitch up. Once you are clear of the turbulence, without you, the pilot, doing anything, the aircraft returns to flying in the same way it was before the turbulence.
Static Stability is the initial or original tendency of an airplane, when disturbed, to return to flying normally.
Dynamic Stability is the overall tendency of an airplane to return to its original position, following a series of "oscillations" (waving motions).
Stability can be positive, nuetral or negative. Negative stability is the same thing as instability. With negative stability, if the aircraft is disturbed from its flight path, it will just keep moving farther and farther away from its original path, unless the pilot takes control and fixes the problem. Nuetral stability means that the airplane won't go farther from its original path, like with negative stability, but it won't go back to its original path either. It will just stay where it is. With positive stability, the airplane will go back to its original flight path, on its own, without needing the pilot to take control.
In most airplanes, it is best to have positive stability. You are probably expecting to here that we would want all airplanes to have positive stability. Actually, there are some times when nuetral or positive stability is good. Can you think of any times? Click the link below to find out when.
When is it better to have an airplane wth nuetral or negatve stability?
An airplane isn't neccesarily stable in all axis, depending on how it is constructed. Stability may be:
Lateral Stability
Lateral stability is stability around the longitudinal axis, and is also known as roll stability. It is known as roll stability because the airplane rolls around its longitudinal axis.
Many people become confused, and don't understand why lateral stability is stability around the longitudinal axis. Think of it in this way: Rolling is the movement around the longitudinal (nose to tail) aixs. The lateral axis runs from wing tip to wing tip. If the airplane does not roll easily, it is stable from wing tip to wing tip. The wing tip to wing tip axis is the lateral axis, and the airplane rolls around the longitudinal axis, so therefore, lateral stability is stability around the longitudinal axis.
Aircraft that are laterally stable resist being caused to roll by outside forces.
Four factors affect lateral stability.
1. Keel Effect
Many boats, especially sailboats have a part called a keel. The keel is a large weight (usually streamlined to avoid friction) that is attached to the bottom of the boat and is completely underwater. The function of the keel is provide extra balance for the boat and to keep it from tipping over.
In high winged airplanes, there is a similar effect from the weight of the entire fuselage. With high winged airplanes, the majority of the weight is below the level of the wings. If the airplane is caused to roll, the weight of the airplane hanging below the wings will cause the aircraft to return to flying straight and level, in a similar way to a boat.
2. Sweepback
An airplane with swept back wings has wings which clope backwards instead of being straight out from the fuselage of the airplane.
When the airplane is caused to roll by outside forces (not the pilot), the lower wing has more lift (because of the sweepback) than the top wing, and this causes the wing to rise to its normal flight.
3. Dihedral
The dihedral is a special angle that the wings make with the fuselage. Dihedral wings are tilted upwards and are not parallel with the ground. If one wing drops lower than the other (the airplane rolls) the air will flow at a greater angle of attack against the lower wing. This will cause the lower wing to have more lift and the airplane to roll back into its normal flying position.
dihedral
When one wing is lower (the left wing), it recieves greater lift, causing the airplane to roll back to normal flight.
4. Proper Distribution of Weight
If too much weight is placed on one side of an aircraft, or if one wing is full of fuel and the other is empty, this can have an effect on the lateral stability of the aircraft. Think of a see-saw with a heavy person on one side and a light person on the other. It will naturally tip so that the heavy end rests on the ground and the light end is in the air. The wings of the aircraft work in the same way. If there is extra weight on one side of the aircraft, it will naturally tip so that the heavier wing or side is lower than the lighter side or wing. Using common sense when loading the aircraft will avoid potential problems.
Longitudinal Stability
Longitudinal stability is stability around the lateral axis, and is also called pitch stability. It is called pitch stability because an aircraft pitches around the lateral axis.
To keep lateral and longitudinal stability straight in your head, remember that they are opposites:
Lateral stability = around Longitudinal axis
Longitudinal stability = around Lateral axis
Airplanes that have longitudinal or pitch stability are constructed so that the nose does not easily pitch up or down. Two main factors cause an airplane to be longitudinally stable:
1. Size and Postion of the Horizontal Stabilizer
You will remember from some of the earliest classes that the horizontal stablizer is found on the tail of the airplane. The angle that the horizontal stabilizer is set and the size of the stabilizer has an effect on the longitudinal stability of the aircraft, becuase of the way that the downwash from the wings strikes the surface of the stabilizer.
2. Position of the Centre of Gravity
In much the same way that proper distribution of weight affects lateral stability, the position of the centre of gravity affects longitudinal stability, if there is too much weight aft (near the tail) the nose will tend to pitch up.
Directional Stability
Directional stability is stability around the vertical (also known as the normal) axis. Many things affect directional stability, including keel effect and sweepback. The most important is the fin.
The Fin
You will remember from your early classes that the fin is the vertical surface at the tail of the airplane, consisting of the rudder and the unmoveable vertical piece. Most of us are familiar with a weather vane. A weather vane is a device that is made of a pointing device, such as an arrow, and parts that indicate direction, North, South, East and West. The wind blows the arrow so that it shows the direction that the wind is blowing.
An aircraft's tail section works in a similar way to the arrow of a weather vane. If the aircraft yaws away (turns) from its normal flight path, the wind will hit the side of the aircraft and puch it back into straight flight.