Discussion
Effect of design changes on directional stability.
| Change | Effect on the derivative | Reason |
|---|---|---|
| Enlarge the fin | Increases | More side force aft of the centre of gravity |
| Lengthen the fin arm | Increases | Greater moment for the same force |
| Move a tractor propeller forward | Decreases | Larger destabilising moment |
| Lengthen the forward fuselage | Decreases | More side area ahead of the centre of gravity |
What single principle underlies all four rows?
Answer: D
Correct answer: Aft area stabilises, forward destabilises
All four rows are the same statement applied in turn. Enlarging the fin and lengthening its arm both increase side force or moment acting aft of the centre of gravity, and both raise the derivative.
Moving a tractor propeller forward and lengthening the forward fuselage both increase side force or moment acting ahead of it, and both lower the derivative. Which side of the centre of gravity a side force acts on decides the sign of the moment it produces, and how far from it decides the size. Larger aeroplanes are not automatically more stable, and the third and fourth rows show plainly that the fin is not the only contributor.
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