Stability and Control — Page 280, Lesson 337

Let’s pick this up right where the control-system devices start doing real work for you. We’ve already talked about stick force stability and the feel of the aeroplane through the controls. Now I want to walk you through two specific devices that are fitted to the control system to shape that feel: the down spring and the bobweight. Start with the down spring. Picture a spring attached to the control system in such a way that it’s always pulling the control column in one direction — typically pulling it forward, toward the nose-down position. That’s the “down” in down spring. Now, here’s the key idea: when you add this down spring and then re-trim the aeroplane for the original speed, something very specific happens to the stick force gradient. The airspeed stick force gradient is increased. In plain terms, that means for a given change in airspeed, the force you feel at the stick changes more than it did before. So the aeroplane gives you a stronger feel for airspeed — you can sense speed changes more positively through the controls. Why does that matter? Because some aeroplanes are deficient in airspeed stick force stability. That’s a fancy way of saying the aeroplane doesn’t naturally give you enough force feedback as speed changes, so it’s easy to let the speed drift without noticing. The down spring provides what we call a “synthetic” improvement to that deficiency. Synthetic, because it’s not coming from the aerodynamics of the aeroplane itself — it’s a mechanical device artificially adding that feel. Now here’s a crucial limitation you need to remember. The force increment from the down spring is unaffected by stick position, and it’s unaffected by normal acceleration. What does that mean? It means no matter where the stick is, and no matter how many ‘g’ you’re pulling, the down spring contributes the same fixed amount of force. Consequently, the manoeuvring stick force stability is unchanged. So the down spring helps you with airspeed feel, but it does absolutely nothing for the feel during manoeuvres — pulling ‘g’ in a turn, for example. That’s the down spring’s whole story: it’s a speed-feel device, not a manoeuvre-feel device. Now let’s move to the bobweight. This is a more sophisticated device, and it’s an effective one for improving stick force stability. Look at how it’s built: the bobweight consists of an eccentric mass attached to the control system. Eccentric just means the mass is offset from the pivot point, so it’s not centred on the axis of rotation. In unaccelerated flight — that is, straight and level, one ‘g’ — this mass contributes an increment of pull force that is identical to the down spring. In fact, I want you to note this carefully: a bobweight added to the control system produces an effect identical to the down spring in that unaccelerated condition. So it will increase the airspeed stick force gradient and increase the feel for airspeed, just like the down spring did. But here’s where the bobweight parts company with the down spring. The bobweight mass is subjected to the same acceleration as the aeroplane. Think about that — the mass is physically attached to the airframe, so when the aeroplane accelerates in a manoeuvre, the bobweight accelerates with it. That means the bobweight provides an increment of stick force in direct proportion to the manoeuvring acceleration of the aeroplane — which is the load factor applied. So unlike the down spring, the bobweight does affect the manoeuvring stick force gradient. What’s the practical benefit? It prevents the pilot from applying too much ‘g’ during manoeuvres. Here’s the feel of it: the more you pull back on the stick, the more resistance the bobweight adds to the control system. So as you tighten a turn and the load factor builds, the bobweight pushes back harder against your pull. It gives you a natural, physical cue that you’re loading the aeroplane up, and it discourages over-stressing the airframe. So let me summarise the contrast, because this is the heart of it. The down spring gives you a fixed, synthetic improvement to airspeed feel, but it leaves manoeuvring feel untouched. The bobweight gives you the same airspeed feel improvement, but it also adds manoeuvring feel that grows with load factor — because its mass feels the same acceleration the aeroplane does. That’s the fundamental difference: one is acceleration-insensitive, the other is acceleration-sensitive. You can see the down spring and the bobweight drawn out in the figures — the preloaded spring arrangement and the eccentric mass of the bobweight, and how each one feeds its force into the control system.