Discussion
Balance between the lateral and directional axes.
| Balance | Dutch roll | Spiral mode |
|---|---|---|
| Dihedral effect much stronger | Poorly damped | Stable |
| Well balanced | Adequately damped | Slightly divergent |
| Directional stability much stronger | Well damped | Divergent |
| Both weak | Poorly damped | Divergent |
Why do designers often accept a slightly divergent spiral mode?
Answer: B
Correct answer: It is slow and easily controlled, whereas a bad Dutch roll is not
The table shows the trade: strengthening the dihedral effect stabilises the spiral but spoils the Dutch roll, and strengthening directional stability does the reverse. Something must be given up.
What makes the spiral the acceptable sacrifice is its timescale. A divergent spiral takes tens of seconds to double its bank angle and a pilot corrects it with a glance at the instruments. A poorly damped Dutch roll oscillates in seconds, is tiring and can be difficult to arrest - and it is the one certification scrutinises most closely.
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