Aerodynamics STATIC LONGITUDINAL STABILITY

Question:-

What is the STATIC LONGITUDINAL STABILITY?

Answer:-

STATIC LONGITUDINAL STABILITY

      The Longitudinal stability is a movement about the lateral axis. We shall have a clear understanding once we get to understand the axis of aircraft which shall be discussed later. But for now to let us consider the each axis at which the specific type of stability exists in place.For example the lateral stability is about longitudinal axis (Rolling axis) and Longitudinal stability is about lateral axis (Pitching).  The static longitudinal stability of an aircraft can be explained by the displacement from trimmed angle of attack. If the aerodynamic pitching moments created by this displacement tend to return the aircraft to the equilibrium angle of attack the aircraft is said to be positive static longitudinal stability (Kendall,E,1984)

If the aircraft is neutrally stable it will tend to remain at any displacement to which it was disturbed. The neutral static longitudinal stability is always considered to be the boundary between stability and instability.

The aircraft which is unstable will continue to pitch in the disturbed direction until the displacement is stopped by an opposing force. In this case the aircraft cannot be trimmed and it requires irrelevant control forces and deflection of control column is required to provide equilibrium which cannot be accepted to execute a stable flight.

The relationship between angle of attach and pitching moment is what the static longitudinal stability is depended upon. Therefore it is necessary to understand the contribution of each component of the aircraft for the purspose of pitching movement. We can explain this in a coefficient form.

Where,

M = Pitching moment about the CG

Q = Dynamic pressure

S = Wing area

MAC = Mean aerodynamic chord

Cm = Pitching moment coefficient