Discussion
Arrange these events after a gust yaws the nose of a directionally stable aeroplane:
1. A sideslip angle develops between the flow and the plane of symmetry
2. The fin generates a side force
3. That force acts behind the centre of gravity, producing a yawing moment
4. The nose turns back towards the relative wind
5. The sideslip reduces towards zero
Select the correct sequence.
Answer: C
Correct answer: 1, 2, 3, 4, 5
The chain is the definition of directional stability, and each step is the cause of the next. The gust yaws the nose, so the relative wind no longer lies in the plane of symmetry and a sideslip angle appears.
The fin, now meeting the flow at an angle, generates a side force. Because the fin is aft of the centre of gravity, that force produces a yawing moment; had it acted forward of the centre of gravity the moment would have been divergent instead. The moment turns the nose back towards the relative wind, and the sideslip that produced it decays to zero - the disturbance erases its own cause.
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