Discussion
In which sense is the phugoid mode described as an exchange of energy?
Answer: D
The phugoid is a swap between height and speed, that is between potential and kinetic energy, with total energy nearly conserved and drag removing a little on each cycle. That is why the mode is slow: the only restoring mechanism is the lift excess or deficit that follows a speed change at fixed incidence. Structural strain energy belongs to aeroelastic modes, which are orders of magnitude faster. Thrust and drag work balance in steady flight, but the phugoid persists with thrust fixed, so that is not the exchange in question. Rotational energy in pitch is negligible here, since the aeroplane barely rotates.
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