Stability and Control — Page 253, Lesson 306

Let’s start with the idea of trim, because it’s the foundation for everything else here. An aircraft is said to be trimmed — or in trim — when all moments in pitch, roll, and yaw are equal to zero. A moment is a turning force, so what that means is the nose isn’t trying to pitch up or down, the wings aren’t trying to roll, and the rudder isn’t trying to yaw. The aircraft is in perfect balance. Now, how do we actually establish that trim at different flight conditions? The excerpt gives us five ways. First, pilot effort — the pilot simply holds the controls to maintain the condition. Second, trim tabs — those small hinged surfaces on the trailing edge of the control surfaces that relieve the pilot’s load. Third, a variable incidence trimming tailplane — that’s a tailplane whose angle can be changed in flight to trim the aircraft. Fourth, moving fuel between the wing tanks and an aft located trim tank — shifting weight to adjust the balance. And fifth, bias of a surface actuator, which applies to powered flying controls — the actuator itself holds a small offset to trim the surface. Now, controllability. That term refers to the ability of the aircraft to respond to control surface displacement and achieve the desired condition of flight. In plain terms, when you move the controls, the aircraft must respond and do what you want. And the excerpt stresses that adequate controllability must be available to perform take-off and landing and to accomplish the various manoeuvres in flight. Here’s the key tension: a contradiction exists between stability and controllability. A high degree of stability gives reduced controllability. Think about that — the more the aircraft wants to return to its original state, the harder it is to make it do something different. The relationship between static stability and controllability is demonstrated by four illustrations, and we’re going to walk through them. The first is positive static stability, shown by a ball in a trough. If the ball is displaced from equilibrium at the bottom of the trough, there is an initial tendency to return to equilibrium. That’s positive static stability — the initial tendency to return. Now here’s the controllability point: if you want to control the ball and maintain it in the displaced position, you must supply a force in the direction of displacement to balance the inherent tendency to return to equilibrium. So you’re fighting the stability just to hold it off-centre. And the excerpt makes a crucial link: this same stable tendency in an aircraft resists displacement from trim equally, whether by pilot effort on the controls — that’s stick force — or by atmospheric disturbance. So a gust tries to displace the aircraft, and the stability resists it; but equally, when you pull on the stick, the stability resists you too. Now the second illustration: increased positive static stability. This is illustrated by the ball in a steeper trough. The effect of increased static stability — and note the parenthetical, forward CG movement — on controllability is that a greater force is required to control the ball to the same position of displacement. So the steeper the trough, the harder you have to push to hold the ball off-centre. In this manner, a large degree of static stability tends to make the aircraft less controllable. And that brings us to the design requirement. It is necessary to achieve the proper proportion between static stability and controllability during the design of an aircraft, because too much static stability — that is, a forward CG position — reduces controllability. So the forward CG limit is set to ensure minimum controllability. That’s a critical point: the forward CG limit isn’t arbitrary, it’s there to guarantee you always have at least a minimum level of controllability. Let me bring in the figure that ties this together — Figure 10.13. It shows the aircraft with the lift at the wing, labelled L, acting at the aerodynamic centre, AC, and the tailplane lift, labelled Lt, acting at its own aerodynamic centre. There’s a distance x between the wing AC and the tailplane AC, and a distance y from the centre of gravity to the neutral point. The neutral point is marked, and the static margin is the distance between the CG and the neutral point. Then we have the aft CG limit and the forward CG limit. And along the bottom, we see high stick force at one end and low stick force at the other. So the picture is this: as the CG moves forward, the static margin increases, stability increases, and the stick force required to displace the aircraft increases — that’s the high stick force end. As the CG moves aft toward the neutral point, the static margin decreases, stability decreases, and the stick force required is low. The forward CG limit is set to ensure you never have so much stability that you lose minimum controllability. That’s the whole balancing act — stability on one side, controllability on the other, and the CG limits define the acceptable range. So to summarise what we’ve covered: trim is all moments equal to zero, achieved by five methods; controllability is the response to control displacement; and the contradiction is that more stability means less controllability, demonstrated by the ball in the trough, with the forward CG limit set to guarantee minimum controllability.