| dc.description.abstract | Present generation of hover-capable micro air vehicles (MAVs) based on conventional rotors have shown poor performance in terms of endurance (<15 minutes), agility, and disturbance-rejection capability. Developing next generation of MAVs would require radical improvements in propulsion systems as well as control and guidance strategies. Cycloidal rotor is one such novel propulsion concept, which has huge potential due to its higher efficiency and maneuverability (instantaneous 360° thrust vectoring capability). Cycloidal rotor is a horizontal-axis rotary wing system which utilizes cyclic blade pitching to generate lift and thrust. A crucial step towards building efficient MAVs utilizing cycloidal rotor systems involves developing an aeroelastic framework and a coupled trim methodology, which could be utilized for design optimization and this is the main objective of the present dissertation. To obtain instantaneous blade aerodynamic forces and performance (cycle-averaged thrust and power) of cycloidal rotor, an unsteady aerodynamic model is developed. Towards this, aerodynamics of cycloidal rotor is investigated thoroughly and various underlying physical phenomena such as dynamic virtual camber, effects of near and shed wake, leading edge vortices are rigorously modeled. All these detail modeling helped the aerodynamic model to systematically validate with not only time averaged forces, but also time-history of aerodynamic forces obtained from in-house experiments. Once validated, the aerodynamic model is utilized to understand the physics behind the force production of cycloidal rotor. Through systematic investigation, it was observed that the dynamic virtual camber effect plays a very important role in this aspect. Dynamic virtual camber due to pitch rate creates asymmetry in side-force between the right and the left halves, which in turn causes net time averaged side force on a cycloidal propeller in hover even with zero phase offset. Moreover, it is found extremely crucial for a cycloidal rotor to rotate in opposite direction (back-spin) with respect to the incoming flow in order to produce an upward vertical force in forward flight. This is due to the dynamic nature of virtual camber effect. Although the above mentioned lower order model is computationally inexpensive and capable of predicting rotor performance with sufficient accuracy, it cannot accurately capture the complex flow-field of cycloidal rotor, specifically the blade vortex interaction and geometry of trailing vortices. For this reason, a high-fidelity model of cycloidal rotor based on free-wake is developed to further investigate aerodynamics of cycloidal rotor in more detail. The prediction of the developed free wake model shows even better correlation with in-house experimental data compared to that of a lower model. Although, wake model is much more expensive from computational point of view which limits its application for preliminary design optimization of cycloidal rotor. Experimental study shows that cycloidal rotor goes through large blade deflections mainly due to centrifugal force which decreases thrust production and increases power requirement of the rotor. To capture these deflections, a fully nonlinear geometrically exact model is developed which shows much better prediction compared to a traditional 2nd order nonlinear model. To investigate effect of blade deflections on cycloidal rotor performance an aeroelastic framework of cycloidal rotor is developed by coupling lower order unsteady aerodynamic model with the structural model. The experimental validation shows inclusion of geometrically exact model is crucial for accurate performance prediction of flexible cycloidal rotors. Through systematic investigation utilizing the aeroelastic model, it is observed that nonlinear moment, arising due to coupling of bending curvatures in two orthogonal directions, is the key reason behind performance drop of flexible rotors. To obtain performance of conventional nose rotor in low Reynolds number regime, a modified blade element momentum theory based model is developed which utilizes look-up table obtained from CFD study. Both CFD look-up table and model predictions are validated with previously published experimental data. Control strategy of a cycloidal rotor based MAV, known as ‘Cyclocopter’ is developed for different flight conditions. Based on that, a coupled trim analysis of cyclocopter is performed for by simultaneously solving blade response equations and vehicle trim equations. Once systematically validated with in-house experimental data, the coupled trim model is utilized to investigate effect of several design parameters on the control inputs of the vehicle | en |
