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Refer to figure.
Which of these statements about the pitching moment coefficient versus angle of attack lines in the annex is correct?
  • A
    Static longitudinal stability is greater at line 4 when compared with line 3 at low and moderate angles of attack.
  • B
    In its curved part line 2 illustrates a decreasing static longitudinal stability at high angles of attack.
  • C
    The CG position is further forward at line 1 when compared with line 4.
  • D
    Static longitudinal stability is greater at line 3 compared with line 4 at low and moderate angles of attack.

Refer to figure.
Static longitudinal stability depends upon the relationship of angle of attack and pitching moment (CM).

A variation of CG position can cause large changes in the static longitudinal stability. In the conventional airplane configuration, the large changes in stability with CG variation are primarily due to the large changes in the wing contribution. If the incidence of all surfaces remains fixed, the effect of CG position on static longitudinal stability is typified by the chart in figure 1:

As the CG is gradually moved aft, the airplane static stability decreases, then becomes neutral then unstable. The CG position which produces zero slope and neutral static stability is referred to as the “neutral point”. The neutral point defines the most aft CG position without static instability.

Positive static stability is indicated by the curve with negative slope. Moving the CG forward, will increase static longitudinal stability and this is represented by a (steeper) negative slope.

Neutral static stability would be the result if the curve had zero slope. If neutral stability existed, the airplane could be disturbed to some higher or lower lift coefficient without change in pitching moment coefficient.

An airplane which demonstrates a positive slope of the CM versus CL curve would be unstable.

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