Floquet convergence analysis for periodic active rotor systems equipped with trailing edge flaps

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Floquet convergence analysis for periodic active rotor systems equipped with trailing edge flaps

Maurice, J.B. / King, F.A. / Fichter, W. / Dieterich, O. / Konstanzer, P.

Recent technological developments at Eurocopter in the field of actuation based on smart materials allow the secondary control of the rotor blade dynamics directly within the rotating frame by means of piezo-actuated trailing edge flaps. Among many advantages of this technology is the possibility to control different harmonics of the motion of the blade, and thus taking into account the inherent time-periodicity of the rotorcraft. This comes along however with the necessity during controller design phases to prove accurately the stability of the periodic system. In this paper, we highlight the periodic aspect of the considered CAMRAD II helicopter model equipped with trailing edge flaps in comparison to the time-invariant pendant. Floquet stability analysis is performed, rising convergence issues that can lead to false estimation of the plant stability, especially when considering the lowly damped first regressive lagging mode. An example of such deceptive conclusions is presented in the case of a closed-loop system meant to destabilize the first regressive lagging mode using the trailing edge flaps as actuators. The convergence analysis was proved efficient to reestablish trustful Floquet exponents and thus conclude accurately on the stability of the time-periodic system.