Coupling between yaw and roll

As the aircraft yaws to the right, the left-hand wing will move slightly faster than the right-hand wing. The faster-moving left-hand wing will, therefore, generate more lift, and the aircraft will tend to roll clockwise (left-wing up). However, the fin and rudder are normally mounted on top of the fuselage, and hence, above the centre of gravity. As the aircraft yaws, the sideforce on the fin therefore exerts an anti-clockwise rolling moment. The overall result of these two opposing tendencies depends on the aircraft design. The cross­coupling of yaw and roll movements is an important feature in aircraft stabil­ity and control.

Pitch control

Figure 10.4 also shows the pitch-control surfaces of a conventional aircraft. In the traditional arrangement, the rear portion on the tailplane (horizontal stabiliser) is hinged to form an elevator. The same arrangement may be seen on the old Auster in Fig. 10.5. By deflecting the rear of the elevator upwards, the tailplane is given a negative camber, resulting in a downward (negative lift) force. As the tail is pulled down, the angle of attack of the wing is increased, so that the final result of up-elevator is to cause a nose-up pitching moment, and an increase in overall lift.

Before the aircraft has had time to respond to the pitching moment, the initial effect is to produce a temporary reduction in lift, as a consequence of the downforce on the tail. On small aircraft with low inertia, the reduction may be so short-lived, as to be hardly noticeable. On large aircraft, and particularly on tailless types, the effect can be quite severe, and the aircraft may drop some distance before the increased wing angle of attack takes effect. On Concorde, the elevator control is linked to the throttle to alleviate this problem in low speed flight.

As the tailplane is required to produce both downward and upward forces, with little force during cruise, it is normally given a near symmetrical or un­cambered section.