Stability and Control — Page 265, Lesson 317

Let's start with the big picture. We're looking at longitudinal stability — that's stability about the lateral axis, the axis that runs wingtip to wingtip, which controls the aircraft's pitching motion, nose up and nose down. The key idea here is that the net pitching moment of the whole aeroplane is not produced by one surface. It's the sum of contributions from each component surface, each acting in its own flow field. So we have to study each one separately to understand how it affects static stability. Now, to compare these contributions, we need a common measure. That's where the pitching moment coefficient comes in. It's defined as: CM = M divided by (Q times S times MAC) Let me unpack that. M is the pitching moment itself. Q is dynamic pressure — the pressure due to the aircraft's motion through the air. S is the wing area. And MAC is the mean aerodynamic chord — the average chord of the wing, the reference length we use for these calculations. The beautiful thing about this definition is that every pitching moment coefficient, no matter where it comes from, has the same denominator: dynamic pressure, wing area, and mean aerodynamic chord. So we can directly compare the pitching moment contributions from the fuselage and nacelles, from the horizontal tail, from power effects, and from the wing itself. Let's focus on the wing first, because that's what this excerpt is about. The wing's contribution to stability depends primarily on where its aerodynamic centre is located relative to the aeroplane's centre of gravity. Now, what is the aerodynamic centre? It's the point on the wing's mean aerodynamic chord where the wing pitching moment coefficient does not vary with lift coefficient. In other words, all changes in lift coefficient effectively take place at the aerodynamic centre. So if the wing experiences a change in lift coefficient, the pitching moment that results is a direct function of the relative location of the aerodynamic centre and the centre of gravity. That relative position is everything. Now, there's an important note here. The degree of positive camber of the wing has no effect on longitudinal stability. The pitching moment about the aerodynamic centre is always negative, regardless of angle of attack. So camber doesn't change the stability picture. Stability comes from the development of restoring moments — moments that push the aircraft back toward its original attitude. And here's the crucial point: because the wing's aerodynamic centre is forward of the centre of gravity, the wing contributes an unstable pitching moment to the aircraft. That's shown in Figure 10.19. So the wing alone is destabilising. That's why we need the other components — the tail, the fuselage, the power effects — to provide the restoring moments that give the aircraft its overall stability. We'll look at those next.