Stability and Control — Page 260, Lesson 311

I want to walk you through the heart of longitudinal stability now — the balance between the wing moment and the tail moment, and how that balance decides whether your aircraft returns to its original attitude after a disturbance. Let me set the scene. Every aircraft in flight has moments acting around its centre of gravity. The wing produces a moment, and the tail produces a moment. If the tail moment is greater than the wing moment, then the sum of those two moments will not be zero. Instead, you get a resultant nose-down moment, and that nose-down moment gives an angular acceleration about the centre of gravity — about the CG. So the aircraft pitches nose-down. Now here's the key relationship: the greater the tail moment is relative to the wing moment, the greater the rate of return to the original trimmed position. Think about it — a stronger restoring moment means the aircraft snaps back to its trimmed attitude more quickly. How do we increase the tail moment? We move the aircraft's CG forwards. Moving the CG forward increases the tail arm — that's the lever arm from the CG to the tail — and it decreases the wing arm, the lever arm from the CG to the wing's centre of pressure. So the tail gets more leverage and the wing gets less. Now, if that nose-down tail moment is greater than the nose-up wing moment, the aircraft will have positive static stability. That's the condition we want — the aircraft naturally wants to return to its trimmed attitude. There's a special position of the CG where something important happens: when changes in the sum of the tail moment and wing moment due to a disturbance are zero. That position is called the neutral point. At the neutral point, a disturbance produces no net restoring moment — the aircraft is neutrally stable. Here's the crucial geometry: the further forward the CG is from the neutral point, the greater the nose-down angular acceleration about the CG — and therefore the greater the degree of positive static stability. So the distance the CG sits forward of the neutral point gives you a measure of the longitudinal stability. That distance is called the static margin. The greater the static margin, the greater the positive longitudinal stability. So a bigger static margin means a more stable aircraft. Now, because of this, the forward CG limit will be positioned some distance aft of the neutral point. Wait — let me re-read that carefully. The forward CG limit is positioned some distance aft of the neutral point. That means the forward limit is behind the neutral point, and the distance between the forward CG limit and the neutral point gives the required positive static stability margin. So the forward limit is set to guarantee a minimum static margin — you never want the CG to reach the neutral point, because that would mean zero stability. Let me now move to trim. An aircraft is said to be trimmed if all moments in pitch, roll, and yaw are equal to zero. Trim — which is also called balance — is the function of the tailplane, and it may be accomplished in several ways. First, by pilot effort — you physically holding the controls. Second, by trim tabs. Third, by moving fuel between the wing tanks and an aft located fuel tank — shifting weight to change the CG. And fourth, by bias of a surface — that's the bias of a surface, which applies to irreversible flying controls. Now I want to contrast two terms. The term controllability refers to the ability of the aircraft to respond to control surface displacement and achieve the desired attitude of flight. So controllability is about responsiveness — how well the aircraft obeys your control inputs. Here's the trade-off you must remember: a high degree of stability tends to reduce the controllability of the aircraft. The stable tendency of an aircraft resists displacement from trim equally, whether that displacement is caused by pilot effort on the controls — that's a stick force — or by a gust. So the aircraft doesn't care whether you or the wind pushed it off trim — it resists either way. Now let's trace what happens when the CG moves. If the CG moves forward, static longitudinal stability increases, and controllability decreases — and the stick force increases. You need more physical effort to move the controls. If the CG moves aft, the opposite happens: static longitudinal stability decreases, and controllability increases — and the stick force decreases. So you get a lighter, more responsive aircraft, but one that's less stable. That's the fundamental stability–controllability trade-off, and it's governed entirely by where the CG sits relative to the neutral point.