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Longitudinal and lateral stability
In the previous chapter, in Fig. 10.1, we defined the three turning motions; pitch, yaw and roll. Pitching stability (nose-up/nose-down motion) is known as longitudinal stability.
Lateral stability is a term used rather loosely to refer to both rolling and yawing. These two motions are very closely interconnected, as we noted when describing control surfaces.
Fortunately, the coupling between longitudinal and lateral static stability is normally weak, and for the purposes of our simple introduction, it is convenient to treat them separately. This again, was part of the traditional approach. It should be noted, however, that in highly manoeuvrable aircraft, the crosscoupling can be significant.
Longitudinal static stability
Aerofoil centre of pressure and aerodynamic centre
For an aerofoil, the point along the chordline through which the resultant lift force is acting, is known as the centre of lift, or centre of pressure. On a cambered aerofoil, the centre of pressure moves forward with increasing angle of attack, as shown in Fig. 11.2(a).
When a cambered aerofoil is set at an angle of attack where it produces no lift, we find that it still gives a nose-down pitching moment. Since there is no force, this moment must be a pure couple. Figure 11.3 shows how this arises physically. The downforce on the front of the aerofoil is balanced by an upward force at the rear, so there is no net force, but a couple is produced.
It is a surprising feature of aerofoils that there is one position on the chord line where the magnitude of this pitching moment does not change significantly with varying angle of attack. Therefore, as illustrated in Fig. 11.2(b), we can represent the forces on an aerofoil as being a combination of a couple and a lift force (L) acting through that position. The position is known as the aerodynamic centre. It is useful to have such a fixed reference point, because, as the angle of attack reduces towards zero, the centre of pressure moves further and further aft, eventually disappearing off towards infinity.
