Discussion
According to the standard estimate for phugoid damping, an aeroplane with a very high lift-to-drag ratio, such as a sailplane, will have
Answer: D
Since \( \zeta_{ph} \approx 1/(\sqrt{2}\,L/D) \), a large lift-to-drag ratio puts the damping ratio close to zero, so the sailplane has a poorly damped phugoid that can be seen wandering for minutes. It is one of the genuine penalties of aerodynamic efficiency: the only thing draining energy from the oscillation is drag, and an efficient aeroplane has very little of it. Heavy damping would need a high drag aeroplane. The period is set by speed, not by efficiency, and a sailplane's low speed makes its phugoid period short rather than long. The mode certainly still exists.
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