What does static margin numerically represent in relation to stability?

What does static margin numerically represent in relation to stability?

• A. Static margin is defined as (NP - CG)/MAC, and a positive value indicates static stability.
• B. Static margin is defined as (CG - NP)/MAC, and a positive value indicates stability.
• C. Static margin measures wing twist.
• D. Static margin has no relation to stability.

Answer: (A)

Static margin expresses how far the center of gravity is forward of the neutral point, scaled by the mean aerodynamic chord. It’s defined as (NP − CG) divided by MAC. When the CG is ahead of the NP, NP − CG is positive, giving a positive static margin, which means the aircraft is statically stable: a small disturbance produces restoring moments that return the airplane toward equilibrium. The larger this positive margin, the stronger the stability, up to practical limits. If the margin were zero, the aircraft would be neutrally stable, and if negative, it would be statically unstable. The MAC in the denominator makes the value a dimensionless, size-normalized measure. This concept doesn’t relate to wing twist, and using CG − NP would invert the sign, misrepresenting stability.

Static margin expresses how far the center of gravity is forward of the neutral point, scaled by the mean aerodynamic chord. It’s defined as (NP − CG) divided by MAC. When the CG is ahead of the NP, NP − CG is positive, giving a positive static margin, which means the aircraft is statically stable: a small disturbance produces restoring moments that return the airplane toward equilibrium. The larger this positive margin, the stronger the stability, up to practical limits. If the margin were zero, the aircraft would be neutrally stable, and if negative, it would be statically unstable. The MAC in the denominator makes the value a dimensionless, size-normalized measure.

This concept doesn’t relate to wing twist, and using CG − NP would invert the sign, misrepresenting stability.