Discussion
Consider the following statements about the short-period approximation.
1. It retains the angle-of-attack and pitch-rate equations
2. It assumes the forward speed perturbation is zero
3. It requires the phugoid to be unstable
Which of the statements given above is or are correct?
Answer: B
The approximation keeps the two fast equations and freezes the slow one, so statements 1 and 2 are the whole of it: incidence and pitch rate are retained, the speed perturbation is set to zero. Statement 3 is not a requirement at all, and gets the logic backwards. The approximation works because the two modes are widely separated in frequency, whatever the phugoid's stability. It is just as valid for an aeroplane whose phugoid is slightly divergent, since the phugoid barely moves during the second or two that the short period takes to decay.
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