The main purpose of the vertical fin is to provide yawing stability. As shown in Fig. 11.13, by placing the fin well aft of the centre of gravity, it tends to turn the aircraft towards the relative air flow direction. This is known as weathercock stability, for obvious reasons. The fin does not, as is often believed, tend to point the aircraft into the actual wind direction relative to the ground. The fin force merely tries to point the aircraft towards the relative wind direction. This means that it will try to turn the aircraft towards the direction of a gust, so excessive yawing stability can make the aircraft rather twitchy. Note, that since the aircraft tends to turn towards the direction of gusts, it will not maintain a constant heading.
Fig. 11.13 Yawing or ‘weathercock’ stability provided by the fin
The same principle has been used on weathercocks for centuries
LATERAL STABILITY 313
Fig. 11.14 Lateral dihedral
The dihedral angle is the angle made between one wing and the horizontal
Fig. 11.15 The effect of dihedral
The aircraft is shown sideslipping towards the observer. The near wing presents a higher effective angle of attack. The aircraft will, therefore, tend to roll back, away from the sideslip
The main difficulties with the yawing stability arise from the cross-coupling between yaw and roll that we mentioned in the previous chapter, and shall further describe under the heading of dynamic stability in the next chapter.