Discussion
An aeroplane's phugoid period is measured as \( 68 \ \mathrm{s} \). Using \( T \approx 0.45\,u_0 \), its trim speed was about
Answer: D
Inverting the shortcut gives \( u_0 \approx 68/0.45 = 151 \ \mathrm{m/s} \), so about 150 m/s, a plausible speed for a jet in a low-level cruise. The relation comes straight from \( T = \sqrt{2}\pi u_0/g \), which is worth carrying as \( T \approx 0.45\,u_0 \) in SI units. 68 m/s is the period read back as a speed. 31 m/s multiplies by 0.45 instead of dividing. 300 m/s would give a period near 135 s, twice what was measured.
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