Longitudinal Stability: Pitch Plane (Pitch Moment, M)
Figure 12.3 depicts the conditions for aircraft longitudinal static stability. In the pitch plane, by definition, the angle of attack, a, is positive when an aircraft nose is above the direction of free-stream velocity. A nose-up pitching moment is considered a positive. Static-stability criteria require that the pitching-moment curve exhibit a negative slope, so that an increase in the angle of attack, a, causes a restoring negative (i. e., nose-down) pitching moment. At equilibrium, the pitching moment is equal to zero (Cm = 0) when it is in trimmed condition (atrim). The higher the static margin (see Figure 12.11), the greater is the slope of the curve (i. e., the greater is the restoring moment). Using the spring analogy, the stiffness is higher for the response.
The other requirement for static stability is that at a zero angle of attack, there should be a positive nose-up moment, providing an opportunity for equilibrium at a positive angle of attack (+atrim), typical in any normal flight segment.