During flight, it is very important for an aircraft to remain straight, level, and in a constant velocity, as this means all forces which are acting on the aircraft are keeping it in a state of equilibrium. If this equilibrium is deterred by things like turbulence, the pilot might lose complete control, resulting in the plane yawing left or right, pitching up or down, or going into a roll. However, if an airplane has stability, it will return to its state of equilibrium once turbulence or any other form of disturbance goes away.
Static stability in an airplane can be described as the initial response or reflex an aircraft has after its state of equilibrium is disrupted. Once the disruptive force is removed from the situation, the aircraft returns to its original state if there is positive static stability. However, with negative static stability, the plane might give in to the turbulence and will continue to move away from its initial position even when the disruptive force is removed.
If the static stability of an aircraft is negative and its nose is pitched upward due to wake turbulence, the plane will generally continue to pitch up after the turbulence fades away. After such forces are removed, if the aircraft continues to stay in a displaced position and does not tend to move forward with greater displacement, the static stability of the aircraft will be considered neutral.
The tendency of the aircraft nose to pitch down or up while rotating around its lateral axis is known as longitudinal stability, and it is measured from one wingtip to another. A longitudinally stable aircraft returns to its accurately trimmed angle of attack once the disruptive force is cleared from the path. As such, having longitudinal stability is highly beneficial.
A key factor in determining longitudinal stability is the weight and balance of the airplane. This is based on the design specifications and characteristics of the aircraft as well as the way in which it is loaded. The center of pressure or center of lift, which is located on the wing of airplanes, is where all the forces of lifting concentrate. So when in flight, the aircraft acts as if it is being supported or lifted from these points. The center of lift also runs from one wingtip to another.
The amount of time taken by an aircraft to react to its static stability after getting displaced from its state of equilibrium is known as dynamic stability. It revolves around the oscillations that occur as the plane maneuvers back into its original attitude or position. Even if an aircraft has positive static stability, it can also have negative, positive, or neutral dynamic stability.
The movement of an aircraft around its vertical axis and its capability to avoid being adversely affected by a force creating yaw motion is known as directional stability. When one wing of an airplane is positioned lower as compared to the other, the vehicle’s lateral stability will bring the lower wing back to an equal level in accordance with flight attitude. Lateral stability occurs around the longitudinal axis from the nose to the tail of the aircraft. A dihedral is a design characteristic that provides an airplane with lateral stability. Generally, a dihedral is only a few degrees and it can be described as the upward wing angle in terms of the horizontal axis.
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