Discussion
Lateral roots measured on three aeroplanes at the same flight condition.
| Aeroplane | Dutch roll roots | Spiral root |
|---|---|---|
| P | -0.10 +/- 1.0j | -0.01 |
| Q | -0.30 +/- 1.5j | +0.04 |
| R | -0.05 +/- 0.8j | +0.01 |
Which aeroplane has both the best damped Dutch roll and a divergent spiral?
Answer: C
Aeroplane Q has the largest damping ratio, \( 0.30/1.53 = 0.20 \), and the only strongly positive spiral root, so it diverges in the spiral while being the best damped in the oscillation. That is exactly what the design trade predicts: whatever gave Q its damping - less dihedral effect, more fin, or a damper - cost it spiral stability. P is spirally stable and less well damped, and R is the worst damped of the three while only marginally divergent.
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