Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
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Item Rotor Hover Performance and System Design of an Efficient Coaxial Rotary Wing Micro Air Vehicle(2007-03-02) Bohorquez, Felipe; Pines, Darryll J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Size restrictions force MAVs to operate in a low Reynolds number aerodynamic regime that results in poor aerodynamic performance of conventional airfoils and rotor configurations. A computerized hover test stand was used for the systematic testing of single and coaxial small-scale rotors. Thin circular arcs were chosen for blade manufacturing because of their good aerodynamic characteristics and simplified parameterization. Influence of airfoil geometry on single rotor hover performance was studied on untwisted rectangular blades. Non rectangular blades were used to study coupled airfoil and blade parameters. Performance gains were obtained by introducing large negative twist angles over short radial distances at the blade tips. A parametric study of the blade geometries resulted in maximum figures of merit of 0.65. Coaxial rotor performance at torque equilibrium was explored for different trims and operating conditions. It was found that the upper rotor was marginally affected by the lower one at spacings larger than 35% of the rotor radius, and that it produced about 60% of the total thrust. Experiments showed that power loading was maximized when higher collectives were used at the lower rotor, resulting in sizable differences in rotational speed between rotors. The CFD solver INS2d was used for a two-dimensional parametric aerodynamic study of circular arc airfoils. Lift, drag, and moment coefficients were explored over a range of Reynolds numbers. Lift predictions were satisfactory; however, drag was under-predicted at low angles of attack. The CFD database was integrated to a BEMT rotor model through a parameterization that coupled blade planform with twist distribution and airfoil shape. Thrust and maximum FM predictions were satisfactory for rectangular and non-rectangular blades with maximum cambers of 6% and below. The BEMT model was extended to the coaxial rotor case, producing good thrust and power predictions with errors within 5% of the experimental measurements. The approach validated the use of analytical and numerical tools commonly used in full-scale analysis, and proved to be a powerful system design tool. A fully functional coaxial MAV was developed based on the aerodynamic studies performed. It has been used as a testing platform for control system and algorithms.Item Low Reynolds Number Validation Using Computational Fluid Dynamics with Application to Micro Air Vehicles(2005-12-05) Schroeder, Eric Joseph; Baeder, James; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The flow physics involved in low Reynolds number flow is investigated computationally to examine the fundamental flow properties involved with Micro Air Vehicles (MAV). Computational Fluid Dynamics (CFD) is used to validate 2-D, 3-D static and hover experimental data at Reynolds numbers around 60,000, with particular attention paid to the prediction of laminar separation bubble (LSB) on the upper surface of the airfoil. The TURNS and OVERFLOW flow solvers are used with a low Mach preconditioner to accelerate convergence. CFD results show good agreement with experimental data for lift, moment, and drag for 2-D and static 3-D validations. However, 3-D hover thrust and Figure of Merit results show less agreement and are slightly overpredicted for all measured collectives. Areas of improvement in the hover model include better vortex resolution and wake capturing to ensure that all the flow physics are accurately resolved.