Helicopters suffer from severe vibration levels compared to fixed-wing aircraft. The main source of vibration in a helicopter is the main rotor which operates in a highly unsteady aerodynamic environment. Active vibration control methods are effective in helicopter vibration suppression since they can adapt to various flight conditions and often involve low weight penalty. One such method is the actively controlled flap (ACF) approach. In the ACF approach, a trailing-edge flap (TEF) located in each rotor blade is deflected at higher harmonics of rotor frequency to reduce vibratory loads at the rotor hub. The ACF approach is attractive because of its simplicity in practical implementation, low actuation power and enhanced airworthiness, since the flap control is independent of the primary control system. Multiple-flaps are better suited to modify the aerodynamic loading over the rotor blade and hence offer more flexibility compared to a single flap. They also provide the advantage of redundancy over single-flap configuration. However, issues like the number, location and size of these individual flaps need to be addressed based on logic and a suitable performance criteria. Preliminary studies on a 4-bladed hingeless rotor using simple aerodynamic and wake models predict that multiple-flaps are capable of 70-75 percent reduction in hub vibration levels. Numerical studies confirm that multiple-flaps require significantly less control effort as compared to single-flap configuration for obtaining similar reductions in hub vibration levels. Detailed studies include more accurate aerodynamic and wake models for the rotor with TEF’s. A simple and efficient flap control algorithm is chosen from literature and modified for use in multiple-flap configuration to actuate every flap near complete authority. The flap algorithm is computationally efficient and performs creditably at both high and low forward speeds. This algorithm works reasonably well in the presence of zero-mean Gaussian noise in hub load data. It is also fairly insensitive to small changes in plant parameters, such as, blade mass and stiffness properties. The optimal locations of multiple TEF’s for maximum reduction in hub vibration are determined using Response Surface methodology. Piezoelectric stack actuators are the most promising candidates for actuation of full-scale TEF’s on helicopter rotors. A major limitation of piezoelectric actuators is their lack of accuracy due to nonlinearity and hysteresis. The hysteresis in the actuators is modeled using the classical Preisach model (CPM). Experimental data from literature is used to estimate the Preisach distribution function. The hub vibration in this case is reduced by about 81-86 percent from baseline conditions. The performance of the ACF mechanism can be further improved by using an accurate hysteresis compensation scheme. However, using a linear model for the piezoelectric actuator or an inaccurate compensation scheme can lead to deterioration in ACF performance. Finally, bench-top experiments are conducted on a commercially available piezostack actuator (APA500L from CEDRAT Technologies) to study its dynamic hysteresis characteristics. A rate-dependent dynamic hysteresis model based on CPM is used to model the actuator. The unknown coefficients in the model are identified using experiments and validated. Numerical simulations show the importance of modeling actuator hysteresis in helicopter vibration control using TEF’s. A final configuration of multiple flaps is then proposed by including the effects of actuator hysteresis and using the response surface approach to determine the optimal flap locations. It is found that dynamic hysteresis not only affects the vibration reduction levels but also the optimal location of the TEF's.

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