With an increasing trend towards automatic flight control system applied to rotorcraft, the goal of the present paper is to understand the effects of rotor dynamics on the design of robust incremental non-linear controllers such as INDI (Incremental nonlinear Dynamic Inversion) and IBS (Incremental Backstepping Control). Nonlinear dynamic controllers are a desirable solution to helicopter flight control as it can solve its highly nonlinear dynamic behavior. However, conventional nonlinear controllers heavily rely on the availability of accurate model knowledge and this can be problematic for rotorcraft. Therefore, incremental control theory can solve the modelling errors sensitivity by relying on the information obtained from the sensors instead. The paper will demonstrate that for helicopters the incremental nonlinear controllers depend on the delays introduced in the controller by rotor dynamics. The paper will show how the residualization and synchronization methods need to be applied to an IBS controller in order to remove the effects of the flapping (disc-tilt) dynamics from the controller. This indicates that the incremental nonlinear controllers can have relatively small stability robustness margin when subjected to rotorcraft time delays and unmodelled dynamics that influence the feedback path and should be therefore carefully applied.