4.3.1. Static stability
Definition: The static stability of an object depends on its tendency to return to its initial position after being slightly moved.

Unstable Neutral
Stable
a) Longitudinal stability during steady flight
. Aerodynamic center
The increase (or decrease) in lift due to an increase (or decrease) in incidence (α) always applies on a fixed point of the wing. This point is called the aerodynamic center.
. Static stability
When the aircraft is submitted to a gust, it is equivalent to an increase in incidence or to the
creation of a pitch up moment.
A lift increase is created at the aerodynamic center.

If the CG is located forward of the aerodynamic center, the increase in lift creates a pitch down
moment which reduces the incidence and brings the aircraft back to its initial conditions.

. The aircraft is stable
WEIGHT AND
BALANCE
WEIGHT AND
BALANCE
A. GENERALITIES

If the CG is located aft to the aerodynamic center, the increase in lift creates a pitch up moment which adds to the initial pitch up moment due to the gust. This brings the aircraft further away from its initial conditions.
.
 The aircraft is unstable. The CG must not be behind the aerodynamic center.
Note :
If the center of gravity is located on the aerodynamic center, the lift and the weight have the
same application point and there is no moment created the aircraft is neutral
Hence the other name for the aerodynamic center: the Neutral Point.

Neutral Point : AFT limit (all phases)
b) Maneuverability and elevator efficiency
Considering the previous paragraph, the more forward the CG, the more stable the aircraft.
However, in the case of a very forward CG position, a significant elevator deflection will be
necessary to give the aircraft a pitch up attitude.
The CG position can be moved forward until it reaches the point where the aircraft is very stable
but cannot be maneuvered because the elevator has reached its maximum deflection.

In order to be able to fly the aircraft, a compromise has to be reached between stability and
maneuverability. The CG is shifted aft until an appropriate maneuverability is found.

Note : It is important to notice that the effects of the reduced maneuverability due to a forward
CG are emphasized at low speeds when the elevators have a reduced efficiency.
The elevator efficiency limit is to be considered for all flight phases and it gives more constraining
limit for take-off, approach, and landing than for flight phases.

Elevator maximum deflection + Maneuverability margin : FWD limit (all phases)

A. GENERALITIES

c) Longitudinal stability during maneuvers
During flight the aircraft must remain stable during steady flight but also during different maneuvers.
　
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