Stability and Control — Page 280, Lesson 332

Let’s pick this up with the idea of manoeuvre stability. When you pitch the aircraft, it rotates about the centre of gravity, and the tailplane suddenly finds itself moving through the air with a pitching velocity — in our example, downwards. That pitching motion changes the airflow the tail sees, and it turns out the aeroplane is actually slightly more stable in manoeuvring flight than it is in steady, straight-and-level flight. Here’s the mechanism. Because the tail is pitching downwards, it experiences an upwards component of airflow relative to itself. Think of the tail’s true airspeed, the TAS, as the horizontal flow, and the pitching velocity as adding a vertical component on top of it. When you add those two vectors together, the effective angle of attack of the tail increases. A higher angle of attack on the tail means more tail lift, and that extra lift opposes the nose-up pitch displacement you started with. So the tail is actively fighting the pitch change. Now, here’s the subtle part. That negative pitching moment — the one opposing the nose-up displacement — is itself caused by the nose-up pitching motion. Because the moment is generated by the motion and acts against it, we call this damping in pitch, or aerodynamic damping. It’s a stabilising effect that only exists while the aircraft is actually rotating. There’s a relationship with speed worth noting. For a given pitching velocity, if you increase the TAS, the angle of attack change due to that pitching velocity decreases. The vertical component from the pitching velocity becomes a smaller fraction of the total airflow, so the tail’s effective angle of attack change is smaller. Higher speed, less damping effect from the same rotation rate. Now let’s bring in the manoeuvre point. Because aerodynamic damping gives extra stability in manoeuvres, the aeroplane behaves as if it’s more stable than it appears in steady flight. The centre of gravity position at which the tail moment would equal the wing moment during manoeuvring is called the manoeuvre point. And that manoeuvre point sits further aft than the neutral point for 1g flight — the neutral point you’d find in steady, level flight. So the manoeuvre margin, the distance between the CG and the manoeuvre point, is larger than the static margin you’d measure in 1g conditions. In practice, the manoeuvre point is rarely a critical design item. If the aeroplane demonstrates static stability in 1g flight, it will definitely have stability in manoeuvring flight. The manoeuvre point being further aft just means you have more margin than the steady-flight analysis suggests. Finally, let’s talk about stick force per ‘g’. This is the most direct way to appreciate manoeuvring stability. You plot stick force against load factor — load factor being the ‘g’ the aircraft is pulling. A positively stable aeroplane should show a steady increase in stick force as load factor increases. That gradient, the manoeuvring stick force gradient, or stick force per ‘g’, must be positive. But it also has to be the right magnitude. If it’s excessively high, the aeroplane is difficult and tiring to manoeuvre — you’re fighting the stick the whole time. If it’s too low, you have light control forces, and the risk is that the pilot inadvertently overstresses the airframe because a tiny input produces a large ‘g’. One last point, and it’s a limitation to remember. Increasing altitude at a constant indicated airspeed decreases aerodynamic damping. The damping depends on the actual dynamic pressure the tail sees, and at altitude, for the same IAS, the true airspeed is higher, which — as we discussed — reduces the angle of attack change from a given pitching velocity. So the damping effect weakens as you climb.