Secondary Effects of Control

1.4 Secondary Effects of Control

With multi-rotors, there are four primary control inputs, throttle, pitch, roll and yaw. These inputs are controlled by the pilot using the control sticks and as such, they are the primary control actions applied by the pilot.

Secondary effects of control occur in addition to the primary actions that are input by the pilot. They occur as a consequence of the aircraft executing the primary commands input by the pilot. The simplest secondary effect occurs when a pilot applies pitch to move a multi-rotor forward.

When the aircraft is hovering the thrust vector is vertical, exactly counteracting the weight of the aircraft such as in Figure 1.13a. Pitching the aircraft forward means that the thrust vector is angled, such that the thrust is supplying forward force, as well as upward force such as in Figure 1.13b. So if nothing else changes, the aircraft will lose altitude as some of the force that was holding it up, is now being used to push it forward. So the primary effect of pitching the aircraft is forward motion and the secondary effect is a loss of altitude.

As a consequence of the primary action, the pilot will have to apply more throttle to overcome the secondary effect and prevent the aircraft from descending. The same effect can be observed when rolling to create sideways movement. When in automated modes, the controller may compensate for this secondary effect and prevent the aircraft from descending. Much more complex secondary effects can occur when the aircraft is yawed during forward movement in order to perform a turn. If you think of a multi-rotor moving forward at speed and then being quickly yawed 90 degrees, rotating about its axis would mean that it was now rolled outwards from the turn. This is an over-simplification to demonstrate that pure yaw will result in a roll in a moving multi-rotor. In reality, the interactions are much more complex as the thrust vector swings around and the controller tries to compensate for the roll. This will usually result in a skidding turn, exaggerated by the tendency of the aircraft to roll outward. 

When executing a controlled turn while moving forward, it is necessary to apply roll, throttle and pitch as well as yaw to make the multi-rotor turn smoothly with the nose of the craft always pointing in the same direction as it is heading. In this manoeuvre, the aircraft goes through a complex series of commands from the pilot. At various times, both pitch and roll are being applied simultaneously. In order to achieve this, three of the four rotors will need to be providing more lift than the remaining rotor — this will result in a secondary effect of uneven torque as the rotor pairs are no longer balanced. So rolling and pitching at the same time can also induce additional yaw in the multi-rotor. This usually manifests itself in some degree of adverse yaw when executing controlled turns — making it more difficult to fly a straight course out of a controlled turn. The pilot may notice that the course of the multi-rotor may curve outwards from the turn when trying to fly straight.

In short, in a controlled turn, you may need to use more roll than you think — and hence more throttle — and you may need to turn further than you think. Different controller modes can influence the final results of secondary effects. In manual mode, secondary effects will be greatest. The aircraft will drop noticeably when pitched or rolled and the more complex effects will be more apparent. In ’atti’ mode (attitude control), the controller will to a greater extent compensate for the secondary effects but will cause different results. The RPA may not roll as much but may skid more or yaw more. In GPS mode, most of the effects are compensated for as the controller detects deviations from the attitude, altitude and course requested by the pilot, though they may still be evident during more rapid manoeuvres.