Blade vortex interaction and its alleviation using passive and active control approaches

This dissertation describes the development of an aeroelastic response simulation capability suitable for modeling the effects of the phenomenon of blade-vortex interaction (BVI) on advanced geometry helicopter rotors. An actively controlled partial span trailing edge flap (ACF) has been incorporated in the aeroelastic model in order to be able to study its potential for alleviating BVI effects and reduce vibratory hub loads.; The structural model is capable of simulating composite helicopter blades with swept tips. It has provisions for generally anisotropic material behavior, arbitrary cross-sectional shape, transverse shear, out-of-plane warping and nonlinear effects due to moderate deflections. A free wake analysis is included in the aeroelastic model to represent the effects of blade-vortex interaction. The wake model in the aeroelastic simulation that was developed was tested and validated by comparing results with a different comprehensive rotor analysis code (CAMRAD/JA). The analysis capability has features that permit the computation of both quasi-steady and unsteady aerodynamic loads acting on the blades. The corresponding aeroelastic model is formulated both in the frequency and time domain. In the frequency domain analysis, the blade response and trim equations are solved simultaneously using the harmonic balance technique. In the time domain analysis, the coupled trim/response solution is obtained using direct numerical integration in combination with an autopilot type controller. A conventional combination of cyclic pitch inputs is used to investigate vibration reduction.; In the first part of the research, the effect of BVI on blades having swept tips (a swept tip implies here both sweep and anhedral) is studied so as to gain an improved understanding of the physical mechanism governing BVI. The use of tip sweep and anhedral as passive devices for the alleviation of blade-vortex interaction effects was also studied. The results of this endeavor were a precursor to the development of active control strategies.; In the second part of the study, the use of the actively controlled flap as an active control approach for the reduction of BVI induced vibration was studied. First, the effects on BVI of the trailing edge flap actuated in the open loop mode were considered, and a validation study was performed by comparing results with those from a different aeroelastic model including the ACE Subsequently, the actively controlled flap was employed in closed loop mode to reduce BVI induced vibration.; Next, the aeroelastic model was improved by including a new two-dimensional unsteady compressible aerodynamic model based on a rational function approximation (RFA) approach. Further studies on vibration reduction using the ACF with the improved aeroelastic model were conducted, and the effects of unsteady aerodynamics on BVI and its control were established by comparing the results using quasisteady aerodynamics and the new unsteady model. Results from the improved aeroelastic model were also compared with experimental data obtained at Ames Research Center on the effects of the ACF on blade response in forward flight. Simulations of phase sweeps corresponding to 2/rev, 3/rev, 4/rev and 5/rev flap motion were performed and compared with experimental results. Good correlation with experimental data was obtained for most cases.