We have learnt the three main flight controls – elevator, ailerons, and rudder – each have the primary effects of pitch, roll, and yaw respectively. However, the controls also have further effects.
The elevator is said to have no further effects.
When a pilot rolls the aircraft, the lift force created by the wings is tilted and a sideways component of lift is created. This pulls the aircraft sideways in the direction of bank – known as a sideslip. The result is the relative airflow no longer flowing directly over the nose but instead being at a slight angle. In a sideslip to the right, as in this image, the relative airflow is impacting the right side of the fuselage and the large surface area of the tail is pushed to the left. Similar to a weathervane, the force on the tail causes the nose to yaw in the direction of the bank – in this case to the right. Thus, we say the primary effect of the ailerons is roll but the further effect of the ailerons is yaw.
When a pilot yaws the aircraft, the outside wing moves through a greater distance and so is at a higher speed. The faster speed creates more lift on the outside wing, forcing it to rise. This creates roll in the direction of the yaw. Thus, we say the primary effect of the rudder is yaw but the further effect of the rudder is roll.
We can see that once any roll or yaw has been initiated, the situation will compound itself if no input is made by the pilot. For example, an initial roll will result in a sideslip and cause yaw in the same direction. This yaw will cause further roll, resulting in further yaw and further roll and so on. The increasing bank will yaw the nose to a lower and lower attitude, causing rapidly increasing airspeed – this is known as a spiral dive. Recovery from a spiral dive is straightforward – simply level the wings then pitch to an appropriate attitude.