Discussion
Consider the following statements about directional stability:
1. A positive yawing moment derivative with sideslip indicates stability.
2. The fin is the principal stabilising contribution.
3. The fuselage is also stabilising.
Which of the statements given above is or are correct?
Answer: B
Correct answer: 1 and 2 only
The first two are correct. A positive \( C_{n\beta} \) means a sideslip produces a moment yawing the nose back into the relative wind, which is the definition of directional stability; and the fin is by far the largest stabilising contribution, sized precisely for that role.
The third is wrong. A conventional fuselage carries most of its side area ahead of the centre of gravity, so its side force in sideslip produces a divergent moment - destabilising. Overcoming it is one of the reasons the fin has to be as large as it is.
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