The high level of vibration experienced on the helicopter has been an unfortunate feature of the vehicle throughout its existence, and considerable efforts have been expended over many years in attempts to reduce vibration to levels typical of other forms of transport. Much has been achieved, but as the forward speed increases some components of rotor generated vibration increase very rapidly and consequently, at the increased cruise speed of modern helicopters, vibration can still be high. Also, the requirements have become more stringent, due partly to what the passengers or crew are prepared to tolerate, and partly to the complexity of the tasks required of the crews, especially on military machines.