The “static margin” in aircraft design refers to the longitudinal stability of an aircraft, particularly its tendency to return to its trimmed or equilibrium position after being disturbed in pitch (nose-up or nose-down attitude). It’s a measure of how stable an aircraft is in maintaining its desired pitch attitude during flight.
The static margin is usually expressed as a dimensionless parameter called the “static margin ratio” (SMR), and it’s calculated using the following formula
 
- is the static margin ratio (dimensionless).
- XCGÂ is the location of the aircraft’s center of gravity (CG) from the aircraft’s reference point (typically in inches or meters).
- is the location of the aircraft’s neutral point (NP) from the same reference point (typically in inches or meters).
- is the mean aerodynamic chord of the wing (typically in inches or meters).