Effect of Lag Damper Failure on Helicopter Ground Resonance

This paper examines ground resonance of a helicopter with a 4-bladed rotor with degradation in one of the lag dampers. The analysis is conducted with lag equations in individual blade coordinates solved using Floquet theory, lag equations in multi-blade coordinates solved using Floquet theory, and lag equations in multi-blade coordinates simplified using a constant coefficient approximation and then solved as an eigenvalue problem. From the study it was observed that regardless of whether the blade lag motions are in individual or multi-blade coordinates, the predicted stability levels are identical if the analysis is conducted using Floquet theory. In multi-blade coordinates, collective and differential lag needs to be retained in the analysis, unlike the case of a classical ground resonance analysis where only the cyclic lag modes and body motions are required. Using the constant coefficient approximation in multi-blade coordinates it is equivalent to smearing the damping loss of a single damper equally over all the damper. With the constant coefficient approximation predicts a smaller reduction in damping with damper degradation than the Floquet method, with the differences increasing as the level of degradation increases. For a completely failed damper, the loss in system damping predicted using the constant coefficient approximation was 46% of that from the Floquet analysis for an articulated rotor, and 55% for a hingeless rotor.