Stability and Control — Page 273, Lesson 331

Let’s pick this up right where the stick force stability picture gets interesting. We’re looking at what happens to the stick force gradient when you move the centre of gravity, and then what friction in the control system does to that whole relationship. First, the setup. Imagine you trim the aeroplane for a constant airspeed, and then you vary the CG position. What you’re really doing is testing stick force stability — that is, how much push or pull force you need on the control column to hold the aeroplane at a speed different from the trim speed. The graph in Figure 10.35 plots stick force against EAS, which is equivalent airspeed. You’ll see a line for each CG position, from 10% MAC up to 50% MAC. MAC is the mean aerodynamic chord, the reference line for CG position. Now, the key relationship. As you move the CG aft — that is, towards the tail, towards the higher percentages of MAC — the slope of that stick force versus airspeed line decreases. The slope is the stick force gradient. A smaller gradient means you need smaller stick forces to displace the aeroplane from the trim speed. So the aeroplane feels less stable in the sense that it takes less effort to change its speed. Here’s the critical threshold. When the stick force gradient becomes exactly zero, the CG is at the neutral point, and you have neutral stability. That means the stick force doesn’t change with airspeed at all. And if the CG goes aft of the neutral point, you get stick force instability. In that case, the aeroplane will require a push force at a lower speed, or a pull force at a higher speed. That’s the opposite of what you’d expect from a stable aeroplane — normally you’d pull to go faster and push to slow down, but here it’s reversed. There’s an important practical warning here. The stick force gradient is low at low airspeeds anyway. So when you’re at low speed, with high power, and the CG is near the aft limit, the “feel” for airspeed becomes weak. That’s a dangerous combination because the pilot loses the tactile cue that tells them how fast they’re going. Now let’s move to Figure 10.36, which is about control system friction. Friction in the control system — the cables, pulleys, hinges, all the mechanical bits — creates a very undesirable effect on control forces. Instead of a single clean line on the stick force versus airspeed graph, you get a band. The band is bounded by the friction force. So at any given airspeed, the stick force can be anywhere within that band, depending on whether you’re pulling or pushing against the friction. The problem is this. If the friction force band is wide, it can completely mask the stick force stability when the stick force stability is low. In other words, the friction can hide the very stability cue we just talked about. That’s why modern flight control systems require precise maintenance — to minimise the friction force band and preserve proper feel to the aeroplane. The pilot needs that clean, predictable stick force response, and friction destroys it. So to tie it together: stick force stability is governed by CG position relative to the neutral point, and it’s expressed as the gradient of stick force against airspeed. Move the CG aft, the gradient flattens, and eventually you cross into instability. And on top of that, control system friction turns that clean line into a band, which can mask the stability entirely if the friction is too high. Both of these are about one thing — giving the pilot a reliable, honest feel for the aeroplane’s speed.