Nowadays the origins and solutions for rotor-induced vibrations in helicopters are well-known. However, non-rotor induced low-frequency vibrations still remain a topic of research, although it has proven that these tend to become a dominant source of pilot discomfort, especially in case of turbulence. Therefore, this research aims to come to a conclusion on the effect of (auto-)pilot control inputs on the low-frequency vibrations of the helicopter, and how these influence the ride quality, in calm and turbulent air. This will be done by calculating the correlation between control inputs and low-frequency vibrations. For this, Airbus Helicopters provides flight test data and a non-linear helicopter model. This work adds knowledge on how much the control inputs and vibrations are correlated and in which frequencies region they are correlated. Furthermore, the causes for correlations are investigated, which assist in finding solutions to improve the pilot comfort. Finally, new methods that will help in the process of designing a better ride quality are introduced, which includes the assessment of the power of the control inputs and the correlation coefficient.