Aerodynamics Static Stability and Type

Question:-

What is the static stability and type of static stability?

Answer:-

When the sum of forces is zero and the sum of all moments is zero, an aircraft is considered to be in a state of equilibrium. The aircraft will continue to be in a steady flight. Now when the aircraft is being disturbed by any external deflection the aircraft will experience a change due to an unbalance of moments of forces. Initial Tendency is what defines the type of static stability an aircraft possess.

Static Stability are divided into three main types.

• Positive Static Stability
• Neutral Static Stability
• Negative Static Stability

The term static is given to this form of stability because only the “initial tendency” to return to equilibrium is considered in the static stability and the resulting motion is not considered.

Positive Static Stability

When an aircraft in its steady state of flight is disturbed from its equilibrium it has the tendency to return to the equilibrium, then the aircraft is considered to be having positive static stability. Figure 1 explains the condition of static stability or positive static stability. Let’s consider the ball is disturbed from equilibrium at the bottom (Nelson 1997). The ball will return towards the equilibrium condition, if the disturbing force is removed. The ball will have the tendency to move backwards and forward through the point of equilibrium but the displacement on either side creates a tendency to return.

Neutral Static Stability

An aircraft is said to be inheriting the property of a neutral static stability when it is subjected to a disturbance and it does not has the tendency to return or the tendency to continue in the direction of displacement. Let us consider the same example of the ball, Fig 2 explains the condition of neutral static stability. The ball when its disturbed from its state it acquires a new state of equilibrium at any point it has been displaced and has no tendency to return to its original state of equilibrium.

Negative Static Stability

              In other words this is considered to be static instability. It is said to exist when the aircraft has the tendency to continue in the direction of the disturbance. Fig 3 illustrates the condition of negative static stability or static instability. We can consider the ball on top of a hill and if it has been displaced from tbe state state of equilibrium over the hill it results a tendency of  greater displacement.