Stability and Control — Page 246, Lesson 298

We're starting a new chapter now — Stability and Control. Before we talk about whether an aeroplane is stable, we have to agree on a common language for describing its motion. So I want to walk you through the aeroplane reference axes first, because everything in this chapter hangs on them. To visualize the forces and moments on the aircraft, we establish a set of reference axes passing through the centre of gravity — the CG. Figure 10.4 shows a conventional right-hand axis system. Let's take the three axes one at a time. The longitudinal axis passes through the CG from nose to tail. A moment about this axis is a rolling moment, given the symbol L. And a roll to the right is a positive rolling moment. So if the right wing drops, that's a positive roll. The normal axis passes vertically through the CG, at 90 degrees to the longitudinal axis. A moment about the normal axis is a yawing moment, symbol N. A positive yawing moment would yaw the aircraft to the right — nose swinging right. The lateral axis is a line passing through the CG, parallel to a line passing through the wing tips. A moment about the lateral axis is a pitching moment, symbol M. A positive pitching moment is nose-up. Now, here's a trap that catches students all the time. The names of the stabilities don't match the axes they act about. Longitudinal stability is motion about the lateral axis — that's pitching. Lateral stability is about the longitudinal axis — that's rolling. Directional stability is about the normal axis — that's yawing. So the trick is to think about the axis about which the motion takes place, not the name of the stability. We study static longitudinal stability first, and there's a good reason: it can be studied in isolation. In general, it does not interact with motions about the other two axes. Lateral and directional stability, by contrast, tend to interact — that's coupled motion — and we'll study those later. So what does static longitudinal stability actually mean? An aircraft exhibits it if it tends to return towards the trim angle of attack when displaced by a gust OR a control input. Two sources of disturbance there — a gust, or the pilot moving the controls. It is essential that an aircraft has positive static longitudinal stability. Why? Because if it's stable, the aeroplane is safe and easy to fly — it seeks and tends to maintain a trimmed condition of flight. And it follows that control deflections and control "feel" — that's stick force — must be logical, both in direction and magnitude. In other words, the aeroplane behaves the way the pilot expects. Now the boundary cases. If the aircraft is neutrally stable, it tends to remain at any displacement to which it is disturbed. It doesn't come back, but it doesn't get worse either. Neutral static longitudinal stability usually defines the lower limit of aeroplane stability, because it's the boundary between stability and instability. A neutrally stable aeroplane may be excessively responsive to controls, and it has no tendency to return to trim following a disturbance. Generally, that would not be acceptable. And then the unstable case. An aircraft that is unstable will continue to pitch in the disturbed direction until the displacement is resisted by opposing control forces. And an aeroplane with negative static longitudinal stability is inherently divergent from any intended trim condition. If it is at all possible to fly such an aircraft, it cannot be trimmed, and illogical control forces and deflections are required to provide equilibrium with a change of attitude and airspeed. Clearly, that would be totally unacceptable. So the picture you should hold: positive stability — returns to trim. Neutral — stays where it's put, the lower limit. Negative — diverges, unacceptable. That's the foundation for everything that follows in this chapter.