Impurity of primary response

The helicopter rotor is sensitive to velocity perturbations in all directions and there is very little the rotor designer can do about this that doesn’t compromise control response. Early attempts to build in natural dynamic couplings that neutralized the rotor from external disturbances (Refs 2.40, 2.41) resulted in complex rotor mechanisms that only partially succeeded in performing well, but, for better or worse, were never pursued to fruition and production. In reality, such endeavours were soon overtaken by the advances in ‘electronic’ stabilization. All motion axes of a helicopter have natural damping that resists the motion, providing a basic rate command control response in the very short term. However, soon after a control input is applied, the changes in incidence and sideslip give rise to velocity variations that alter the natural rate response characteristics in all axes. This can occur within a very short time (0(1 s)) as for the pitch axis response in high-speed flight, or longer (O(several seconds)) as for the yaw response in hover. The impurities require the pilot to stay in the loop to apply compensatory control inputs, as any manoeuvre develops. Apart from the main rotor sensitivity, the tail rotor and empennage sensitivities to main rotor wake effects can also introduce strong impurities into the control response. The size, location and incidence of the horizontal stabilizer can have a profound effect on the pilot’s ability to establish trims in low-speed flight. Likewise, the tail rotor position, direction of rotation and proximity to the vertical stabilizer can significantly affect the pilot’s ability to maintain heading in low-speed flight (Ref. 2.42). Both horizontal and vertical tail surfaces are practically redundant in hover and low-speed flight but provide natural stiffness and damping in high-speed flight to compensate for the unstable rotor and fuselage. The modelling of the interactional flowfields is clearly important for predicting response impurities and will be discussed further in Chapter 3.