Thesis (M. S.)--Aerospace Engineering, Georgia Institute of Technology, 2010. / Committee Chair: Hodges, Dewey; Committee Member: Bauchau, Olivier; Committee Member: Sankar, Lakshmi. Part of the SMARTech Electronic Thesis and Dissertation Collection.

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Thesis (PhD (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2006. / This thesis describes the design and incorporation of a twin-stream fan into the CIRSTEL tail boom. The Combined Infra-Red Suppression and Tail rotor Elimination (CIRSTEL) tail boom is a system designed to replace the tail rotor on a conventional helicopter. It relies on the Coanda effect to create circulation around the helicopter tail boom when exposed to the rotor downwash. This generates sideways-directed lift to counter the main rotor torque, and a tail thruster adds extra torque and directional control. A twin-stream fan supplies separate air streams to each of the Coanda and tail thruster sections. The first section of the study describes the experimental tests done on an 83% scale demonstrator of the twin-stream fan with the objective to verify the concept and determine the fan section efficiencies. Subsequent modifications done to the fan stator blades are also evaluated. The efficiencies of the design were shown to exceed the targets in both sections. The section concludes with design recommendations for a future fan, based on the findings of the experiments. A brief analysis of the CIRSTEL system is presented and by using optimisation techniques the predicted power demand of the system could be significantly reduced from a conventional tail rotor. The second section of the study details the conceptual design and CFD evaluation of air intakes for the fan that can be fitted to the helicopter. The objective here was to study the flow affecting helicopter intakes as well as to establish design considerations for a fan intake. A basic intake concept was developed for the Alouette III/CIRSTEL combination and modified according to results based on the CFD simulations. The intake design was evolved to the point were it was shown that the concept is feasible. These CFD simulations were an initial effort to design the fan intakes with the help of a simplified rotor flow field. The investigation was subsequently extended to investigate helicopter intake design considerations in the presence of a representative rotor, which was modelled as an actuator disk in the CFD simulations. In this investigation top and side mounted intake concepts were compared and analysed for suitability as a fan intake. Each intake concept showed its own advantages. Due to the proximity of the rotor hub to the intake, distortion and total pressure levels at the fan face are influenced negatively. The report is concluded with design recommendations for the intake as applied to the current Alouette III configuration, as well as for implementation on helicopters in general.

For optimal rotor performance in a tiltrotor aircraft the difference in the inflow and the rotor speeds between the hover and cruise flight modes suggests different blade twist and chord distributions. The blade twist rates in current tiltrotor applications are defined based upon a compromise between the figure of merit in hover and propeller efficiency in airplane mode. However, when each operation mode is considered separately the optimum blade distributions are found to be considerably different. Passive blade twist control, which uses the inherent variation in centrifugal forces on a rotor blade to achieve optimum blade twist distributions in each flight mode through the use of extension-twist coupled composite rotor blades, has been considered for performance improvement of tiltrotor aircraft over the last two decades. The challenge for this concept is to achieve the desired twisting deformations in the rotor blade without altering the aeroelastic characteristics of the vehicle. A concept referred to as the sliding mass concept is proposed in this work in order to increase the twist change with rotor speed for a closed-cell composite rotor blade cross-section to practical levels for performance improvement in a tiltrotor aircraft. The concept is based on load path changes for the centrifugal forces by utilizing non-structural masses readily available on a conventional blade, such as the leading edge balancing mass. A multilevel optimization technique based on the simulated annealing method is applied to improve the performance of the XV15 tiltrotor aircraft. A cross-sectional analysis tool, VABS together with a multibody dynamics code, DYMORE are integrated into the optimization process. The optimization results revealed significant improvements in the power requirement in hover while preserving cruise efficiency. It is also shown that about 21% of the improvement is provided through the sliding mass concept pointing to the additional flexibility the concept provides for tailoring of the structure without any additional weight penalty on the system.

Helicopters are versatile flying machines that have capabilities that are unparalleled by fixed wing aircraft, such as operating in hover, performing vertical take-off and landing on unprepared sites. However, modern helicopters still suffer from high levels of noise and vibration caused by the physical phenomena occurring in the vicinity of the rotor blades. Therefore, improvement in rotorcraft design to reduce the noise and vibration levels requires understanding of the underlying physical phenomena, and accurate prediction capabilities of the resulting rotorcraft aeromechanics. The goal of this research is to study the aeromechanics of rotors in steady and maneuvering flight using hybrid Computational Fluid Dynamics (CFD) methodology. The hybrid CFD methodology uses the Navier-Stokes equations to solve the flow near the blade surface but the effect of the far wake is computed through the wake model. The hybrid CFD methodology is computationally efficient and its wake modeling approach is non-dissipative making it an attractive tool to study rotorcraft aeromechanics. Several enhancements were made to the CFD methodology and it was coupled to a Computational Structural Dynamics (CSD) methodology to perform a trimmed aeroelastic analysis of a rotor in forward flight. The coupling analyses, both loose and tight were used to identify the key physical phenomena that affect rotors in different steady flight regimes. The modeling enhancements improved the airloads predictions for a variety of flight conditions. It was found that the tightly coupled method did not impact the loads significantly for steady flight conditions compared to the loosely coupled method. The coupling methodology was extended to maneuvering flight analysis and the flight test control angles were employed to enable the maneuvering flight analysis. The fully coupled model provided the presence of three dynamic stall cycles on the rotor in maneuver. Analysis of maneuvering flight requires knowledge of the pilot input control pitch settings, and the vehicle states. As the result, these computational tools cannot be used for analysis of loads in a maneuver that has not been duplicated in a real flight. This is a significant limitation if these tools are to be selected during the design phase of a helicopter where its handling qualities are evaluated in different trajectories. Therefore, a methodology was developed to couple the CFD/CSD simulation with an inverse flight mechanics simulation to perform the maneuver analysis without using the flight test control input. The methodology showed reasonable convergence in steady and maneuvering flight regimes and control angle predictions compared fairly well with test data. In the maneuvering flight regions, the convergence was slower due to relaxation techniques used for the numerical stability. Further, the enhancement of the rotor inflow computations in the inverse simulation through implementation of a Lagrangean wake model improved the convergence of the coupling methodology.

Dynamic stall deeply affects the response of helicopter rotor blades, making its modeling accuracy very important. Two commonly used dynamic stall models were implemented in a comprehensive code, validated, and contrasted to provide improved analysis accuracy and versatility. Next, computational fluid dynamics and computational structural dynamics loose coupling methodologies are reviewed, and a general tight coupling approach was implemented and tested. The tightly coupled computational fluid dynamics and computational structural dynamics methodology is then used to assess the stability characteristics of complex rotorcraft problems. An aeroelastic analysis of rotors must include an assessment of potential instabilities and the determination of damping ratios for all modes of interest. If the governing equations of motion of a system can be formulated as linear, ordinary differential equations with constant coefficients, classical stability evaluation methodologies based on the characteristic exponents of the system can rapidly and accurately provide the system's stability characteristics. For systems described by linear, ordinary differential equations with periodic coefficients, Floquet's theory is the preferred approach. While these methods provide excellent results for simplified linear models with a moderate number of degrees of freedom, they become quickly unwieldy as the number of degrees of freedom increases. Therefore, to accurately analyze rotorcraft aeroelastic periodic systems, a fully nonlinear, coupled simulation tool is used to determine the response of the system to perturbations about an equilibrium configuration and determine the presence of instabilities and damping ratios. The stability analysis is undertaken using an algorithm based on a Partial Floquet approach that has been successfully applied with computational structural dynamics tools on rotors and wind turbines. The stability analysis approach is computationally inexpensive and consists of post processing aeroelastic data, which can be used with any aeroelastic rotorcraft code or with experimental data.