Chapter 6: stability and control
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- Table 6.1. Static Margins for Several Aircraft
- Altering Stability
| Static Margin
The definition of neutral point leads to a very convenient and commonly used alternate criteria for static longitudinal stability. It is clear from (6.10) and (6.17) that locating the center of gravity at the neutral point gives the aircraft neutral stability, moving the center of gravity forward of the neutral point produces positive static stability, and moving the center of gravity aft of the neutral point makes the aircraft statically unstable. An alternate criterion for positive static longitudinal stability, therefore, is that the center of gravity is forward of the neutral point. This criterion is normally stated in terms of the aircraft’s static margin, S.M., which is defined as:  (6.18)
Stated in terms of static margin, the stability criterion becomes S.M. > 0. Static margin is a convenient non-dimensional measure of the aircraft’s stability. A large static margin suggests an aircraft which is very stable and not very maneuverable. A low positive static margin is normally associated with highly maneuverable aircraft. Aircraft with zero or negative static margin normally require a computer fly-by-wire flight control system in order to be safe to fly. Table 6.1 lists static margins for typical aircraft of various types. Table 6.1. Static Margins for Several Aircraft
As a final comment on static margin, it is interesting to note the relationship between static margin, lift curve slope, and moment curve slope. An inspection and comparison of (6.10), (6.17), and (6.18) reveals:  (6.19)
Altering Stability The discussion of neutral point began with considering how moving the center of gravity location would change an aircraft’s static longitudinal stability. Equation (6.10) can be used to predict how other changes in an aircraft configuration will alter its stability. For example, suppose the value of wing/strake/fuselage lift curve slope,  , is increased by increasing the wing aspect ratio, the strake size, or the wing’s span efficiency factor. If, as in most conventional aircraft, the aerodynamic center of the wing/strake/fuselage combination is forward of the aircraft center of gravity so that xcg - xac. > 0, then increasing makes the wing term in (6.10) more positive.  therefore becomes less negative, and the aircraft less stable. For an aircraft configuration where xcg - xac. < 0, on the other hand, (6.10) shows that increasing increases static stability. Table 6.2 lists several other common aircraft configuration changes and the effect they have on stability.
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