Page 139 - Aeronautical Engineer Data Book

114 Aeronautical Engineer’s Data Book
7.7 Stability
Stability is about the nature of motion of an
aircraft after a disturbance. When limited by
the assumptions of the linearized equations of
motion it is restricted to the study of the motion
after a small disturbance about the trim condi­
tion. Under linear system assumptions, stability
is independent of the character of the disturb­
ing force. In practice, many aircraft display
distinctly non-linear characteristics. Some
useful definitions are given in Table 7.2, see
also Figures 7.5 and 7.6

0.2
Pitching moment coefficient C m –0.1 0 0 0.5 1.0 Lift coefficient C L 2.0
0.1

0.0
1.5

–0.2
Fig. 7.5 Stability reversal at high lift coefficient

Nose
up
1
Pitching moment coefficient C m O 2 point Incidence

Trim
3
e
4

2 Stable
3 Neutral stability
Nose 1 Very stable
down 4 Unstable
Fig. 7.6 Degree of stability (static, longitudinal)