Stability and Control — Page 280, Lesson 338

Let's start with the core idea that ties this whole page together. An aeroplane needs two things to be satisfactory: adequate stability and adequate controllability. And here's the tension — they fight each other. High static longitudinal stability means the aeroplane strongly resists being displaced from its trimmed equilibrium. That's great for stability, but it's terrible for controllability, because to move that aeroplane away from equilibrium you need a lot of control power. So the most critical controllability conditions occur exactly when static stability is highest. In practical terms, the lower limits of controllability set the upper limits of static stability. That's why we have a forward CG limit — the forward centre of gravity position is the controllability limit, not a stability limit. Now, there are three principal flight conditions that set the critical requirements for longitudinal control power: manoeuvring, take-off, and landing. Any one of them, or a combination, can determine the overall longitudinal control power needed and therefore set the limit on how far forward the CG can go. Let's look at the manoeuvring control requirement first. The aeroplane must have enough longitudinal control power to reach the maximum usable lift coefficient, or the limit load factor, during manoeuvres. As the CG moves forward, longitudinal stability increases, and that means you need larger elevator deflections just to change the trim lift coefficient. In the example in Figure 10.43, the maximum effective elevator deflection simply cannot trim the aeroplane at CLMAX for CG positions ahead of 18 percent MAC. So that 18 percent MAC is the most forward CG position for manoeuvring controllability in that example. This manoeuvring requirement is most critical for supersonic flight. Why? Two reasons. Supersonic flight brings large increases in static longitudinal stability, because the centre of pressure moves aft, and simultaneously there's a reduction in control surface effectiveness. To cope with both trends, you need powerful all-moving surfaces — that's the all-moving tailplane — to attain the limit load factor or maximum usable CL in supersonic flight. And this requirement is so dominant that once you satisfy it, the supersonic configuration usually has enough longitudinal control power for every other flight condition. So the takeaway: controllability limits set the forward CG limit, and the manoeuvring case, especially in supersonic flight, is often the binding constraint.