Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
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Item Analysis of Factors Affecting the Aerodynamics of Low Reynolds Number Rotating Wings(2013) Schlueter, Kristy Lynn; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A computational analysis was performed to address the effects of walls on wings rotating at a Reynolds number of 120. For rotation angles less than one revolution, a tip clearance of 0.5 chord-lengths is sufficient to approximate a wing rotating in an infinitely large volume of fluid. However, for a maximum rotation of two revolutions, a tip clearance of 5.0 chords is necessary. At the start of the second revolution, the wing encounters its wake, and the wake structure is significantly affected by low tip clearances. Lift and drag forces were measured experimentally for wings rotating at a Reynolds number of 10,000 while parameters including root cutout were varied. Root cutout significantly alters the lift and drag coefficients, including the location of a second local maximum in both lift and drag. The root-relative method of force non-dimensionalization provided the best comparison of force coefficients between cases with different root cutouts.Item A Continuous-Time Nonlinear Observer for Estimating Structure from Motion from Omnidirectional Optic Flow(2010) Conroy, Joseph Kim; Humbert, James S.; Pines, Darryll J.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Various insect species utilize certain types of self-motion to perceive structure in their local environment, a process known as active vision. This dissertation presents the development of a continuous-time formulated observer for estimating structure from motion that emulates the biological phenomenon of active vision. In an attempt to emulate the wide-field of view of compound eyes and neurophysiology of insects, the observer utilizes an omni-directional optic flow field. Exponential stability of the observer is assured provided the persistency of excitation condition is met. Persistency of excitation is assured by altering the direction of motion sufficiently quickly. An equal convergence rate on the entire viewable area can be achieved by executing certain prototypical maneuvers. Practical implementation of the observer is accomplished both in simulation and via an actual flying quadrotor testbed vehicle. Furthermore, this dissertation presents the vehicular implementation of a complimentary navigation methodology known as wide-field integration of the optic flow field. The implementation of the developed insect-inspired navigation methodologies on physical testbed vehicles utilized in this research required the development of many subsystems that comprise a control and navigation suite, including avionics development and state sensing, model development via system identification, feedback controller design, and state estimation strategies. These requisite subsystems and their development are discussed.Item Hover and Wind-Tunnel Testing of Shrouded Rotors for Improved Micro Air Vehicle Design(2008-08-29) Pereira, Jason Louie; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation describes an experimental investigation of the effects of varying the shroud profile shape on the performance of MAV (Micro Air Vehicle) -scale shrouded rotors. Hover tests were performed on seventeen models with a rotor diameter of 16 cm (6.3 in) and various values of diffuser expansion angle, diffuser length, inlet lip radius and blade tip clearance, at various rotor collective angles. Compared to the baseline open rotor, the shrouded rotors showed increases in thrust by up to 94%, at the same power consumption, or reductions in power by up to 62% at the same thrust. These improvements surpass those predicted by momentum theory, due to the additional effect of the shrouds in reducing the non-ideal power losses of the rotor. The uniformity of the rotor wake was improved by the presence of the shrouds and by decreasing the blade tip clearance, resulting in lower induced power losses. Strong suction pressures were observed on the shroud inlet surface, at the blade passage region; taking advantage of this phenomenon could enable further increases in thrust. However, trade studies showed that, for a given overall aircraft size limitation, and ignoring considerations of the safety benefits of a shroud, a larger-diameter open rotor is more likely to give better performance than a smaller-diameter shrouded rotor. A single shrouded-rotor model was subsequently tested in translational flight at various angles of attack. In axial flow, at the same collective, the net thrust and the power consumption of the shrouded rotor were lower than those of the open rotor; in edgewise flow, the shrouded rotor produced greater thrust than the open rotor, while consuming less power. Measurements of the shroud surface pressure distributions illustrated the extreme asymmetry of the flow around the shroud, with consequent pitch moments much greater than those experienced by the open rotor. Except at low airspeeds and high angles of attack, the static pressure in the wake did not reach ambient atmospheric values at the diffuser exit plane; this challenges the validity of the fundamental assumption of the simple-momentum-theory flow model for short-chord shrouds in translational flight.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.