Stability and Control — Page 253, Lesson 303

Let's pick up right where the concept of the neutral point takes center stage. I want you to picture the CG, the centre of gravity, moving rearwards from a position where the aircraft already has static longitudinal stability. As that CG slides back, two things happen simultaneously. The tail arm, which we call 'y', will decrease, and the wing arm, 'x', will increase. Now, because of those arm changes, the negative tail moment will decrease, and the positive wing moment will increase. You can see this in Figure 10.9. Eventually, the CG reaches a very special position. It's the position where the tail moment is exactly the same as the wing moment. Now, imagine a vertical gust hits the aircraft and displaces the nose-up. At this exact CG position, the sum of the moments will be zero. That means there is no angular acceleration about the CG to return the aircraft towards its original position of equilibrium. Because there is no resultant moment, either nose-up or nose-down, the aircraft will simply remain in its new position of equilibrium. This is the definition of neutral static longitudinal stability. The position of the CG where the sum of the changes in the tail moment and wing moment caused by the gust is zero is what we call the neutral point. Now, let's move to the static margin. We've established that with the CG on the neutral point, the aircraft has neutral static longitudinal stability. But what if the CG is positioned just forward of the neutral point? Then the tail moment will be slightly greater than the wing moment, because arm 'y' has increased and arm 'x' has decreased. A vertical gust that increases the angle of attack will now generate a small nose-down angular acceleration about the CG. That gently returns the aircraft towards its original position of trim, its equilibrium. And here's the key relationship: the further forward the CG, the greater the nose-down angular acceleration about the CG, and therefore the greater the degree of static longitudinal stability. The neutral point is an important point of reference in the study of static longitudinal stability. In practice, the CG will never be allowed to move so far aft that it reaches the neutral point. If it did, the aircraft would be much too sensitive to the controls. So, we have the static margin. The distance the CG is forward of the neutral point gives a measure of the static longitudinal stability, and that distance is called the static margin. The greater the static margin, the greater the static longitudinal stability. A certain amount of static longitudinal stability is always required, so the aft CG limit is positioned some distance forward of the neutral point. The distance between the neutral point and the aft CG limit gives the required minimum static stability margin. You can see this clearly in Figure 10.10, which shows the static margin and the aft CG limit.