Discussion
Five complete phugoid cycles are timed at 450 seconds. The phugoid period and, by Lanchester's relation, the approximate trim speed are
Answer: B
The period is \( 450/5 = 90 \ \mathrm{s} \), and inverting \( T \approx 0.45\,u_0 \) gives \( u_0 \approx 90/0.45 = 200 \ \mathrm{m/s} \), so 90 s and 200 m/s. Timing several cycles and dividing is the standard flight-test technique, since it averages away the difficulty of picking a single peak on a noisy trace. The second option forgets to divide by the number of cycles. The third reads the period as a speed. The fourth halves the period, which would follow from counting half cycles as whole ones.
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