Stability and Control — Page 293, Lesson 349

We're moving into the dynamic side of longitudinal stability now. Static stability tells us whether the aeroplane wants to return to equilibrium; dynamic stability tells us how it actually gets there over time. And the first mode we meet is the long period oscillation, which has a special name: the phugoid. Let me define that precisely. The phugoid is a long period oscillation, and it's the first mode of dynamic longitudinal stability. What does it look like in the cockpit? It involves noticeable variations in three things: pitch attitude, altitude, and airspeed. But here's the key contrast — through all of that, the angle of attack stays nearly constant. That means there's not much change in load factor either. Load factor is the ratio of lift to weight, the 'g' you feel; if angle of attack isn't changing, the lift isn't changing much, so the 'g' stays roughly at one. Think of the phugoid as a gradual interchange of potential energy and kinetic energy about some equilibrium airspeed and altitude. The aeroplane trades height for speed, then speed for height, in a slow, gentle seesaw. The period of this oscillation is between 1 and 2 minutes — that's a long, slow cycle. Because the pitch rate is quite low and only negligible changes in angle of attack take place, the damping of the phugoid is weak. Damping is what kills the oscillation over time; here, it barely does. But here's the reassuring part: weak damping doesn't necessarily have great consequence. Because the period is so long, the phugoid is easily controlled by the pilot — you have plenty of time to correct it. And because of its nature, we don't need any specific aerodynamic provisions to counteract it. The aeroplane is simply allowed to have this gentle, weakly-damped mode. Now, the second mode of dynamic longitudinal stability is the short period oscillation — and this one is a completely different beast. Here we have significant changes in angle of attack and load factor, but with approximately constant speed, height, and pitch attitude. So it's the exact inverse of the phugoid: the phugoid changed speed and height with constant angle of attack; the short period changes angle of attack with constant speed and height. It consists of rapid pitch oscillations. During these, the aeroplane is constantly being restored towards equilibrium by its static stability — that's the restoring tendency we studied earlier — and the amplitude of the oscillations is decreased by pitch damping. So static stability pulls it back, pitch damping shrinks the swings. Now, the danger. Short period oscillation at high dynamic pressures — that means high speed, high energy airflow — with large changes in angle of attack, could produce severe 'g' loads, large changes in load factor. That's structural stress on the airframe and on the occupants. Here's the critical operational point. The short period has relatively short periods that correspond closely with the normal pilot response lag time — we're talking 1 or 2 seconds or less. That's the trap. Because the oscillation is so fast, an attempt by the pilot to forcibly damp it may actually reinforce the oscillation. We call that PIO — pilot-induced oscillation — and it can produce instability. The pilot's input arrives at the wrong phase of the cycle and adds energy instead of removing it. So the rule is blunt: short period oscillation is not easily controlled by the pilot. If it occurs, the instruction is to release the controls. The aeroplane is designed to demonstrate the necessary damping on its own. Even holding the controls stationary while the aeroplane oscillates can inject a small unstable input into the control system, reinforcing the oscillation and potentially producing failing flight loads — loads that can overstress the structure. And that's why modern, large, high-speed jet transport aircraft are fitted with pitch dampers — automatic systems that compensate for any dynamic longitudinal instability. The machine handles what the human cannot reliably handle at that timescale. So hold these two side by side: the phugoid, slow, weakly damped, easily controlled, no special provisions needed; and the short period, fast, dangerous, not pilot-controllable, requiring release of controls and automatic pitch damping. That contrast is the heart of dynamic longitudinal stability.