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No description available.

Advances in electric drive technology have meant that power to weight ratios and efficiencies have increased to a point where the practical implementation of an electrically driven Vertical Takeoff and Landing (VTOL) Unmanned Air Vehicle (UAV) is viable. The replacement of the existing gearbox, tail-rotor assembly and tail-rotor drive shaft with electrical drives on the main and tail rotors offers many potential benefits.

The thesis builds and presents a feasibility study for the use of an electrical transmission to power the tail rotor of a helicopter; the electric tail rotor drive concept. The contributions to knowledge are: • Research into the state-of-the-art in electrical transmission technology and architectures in the context of application as a helicopter tail rotor transmission. Analysis of the best performing and most suitable technologies and the design trade-offs involved. • Development of an electrical transmission sizing model and process for estimating figures for critical factors determining feasibility and initial design optimisation. • Application of research and sizing model to estimate the feasibility of an electric tail rotor drive architecture, the impacts of variations of the architecture and potential improvements brought by further development. The thesis begins with an introduction to more-electric vehicle research, the tail rotor transmission application and the electric tail rotor drive concept. The most critical factors affecting the determination of feasibility are outlined and explained, along with a representation of effectiveness developed through systems thinking. Technical research into electrical transmission technology and architectures is conducted, presenting the state-of-the-art and most suitable for use in the electric tail rotor drive concept. A baseline electric tail rotor drive architecture is presented and explained. An electrical system sizing approach and model developed from technical research is used to calculate values for feasibility factors for the baseline architecture and several variations. Safety cases are developed for triplex and quadruplex architecture. The results of feasibility analyses are presented and compared with the existing mechanical tail rotor drive. Finally, conclusions are drawn about the current feasibility of the electric tail rotor drive, the important technical considerations and the future potential of the concept. The appendices include the derivation of the electrical system sizing model and diagrams generated during the systems thinking exercises.

The unique capabilities of the tilt-rotor configuration are generally accepted to provide significant potential when applied to numerous civil and military operations. A vital stage in the development of any tilt-rotor design is the simulation of its basic flying qualities which are essentially defined by the vehicle's response to a range of control inputs and the trim states it adopts. In order to carry out this simulation satisfactorily, an accurate generic mathematical model is required, however, the author is unaware of any existing tilt-rotor simulations which utilise the latest modelling techniques. A generic tilt-rotor simulation model (GTILT) which includes individual blade modelling to describe the behaviour of the rotor has been developed during this research. One of the most significant attributes of individual blade models is that they portray the oscillatory nature of the forces and moments produced by a lifting rotor. The resulting trimmed flight path of the vehicle is periodic rather than steady in nature and consequently existing trimming algorithms, formulated for use with rotor disc representations, are inappropriate when applied to individual blade simulations. A specialised trimming algorithm capable of rapidly trimming rotorcraft simulations to a specified periodic trim state has been developed and incorporated into the GTILT model. Individual blade modelling provides a higher level of fidelity than is possible when using a rotor disc representation but this benefit is obtained at the expense of computational burden. Hence, most sequential computing facilities are unable to provide the performance necessary to make such models practical. In order to reduce computational run-times to an acceptable level GTILT has been parallelised and implemented on a custom designed transputer network. GTILT has been configured using XV-IS data in order to investigate the fidelity of its predicted trim states and vehicle response to a range of control inputs. During the course of this investigation, the trim algorithm is shown to be robust and capable of producing rapid convergence to a wide range of trim states. Longitudinal trims predicted by GTILT are verified against those of the similarly configured Bell C81 for a range of nacelle incidences and good correlation obtained in all cases. A qualitative verification of the trim states adopted in turning flight reveals no anomalies and the results obtained are very encouraging. The dynamic response of the vehicle is demonstrated to be qualitatively valid when a range of control inputs are applied at various nacelle incidences with the behaviour of the vehicle being explicable in all cases.

The Active Control of Structural Response (ACSR) vibration suppression system, where hydraulic actuators located between the gearbox and the fuselage are used to cancel vibration in large helicopters, has been used successfully for many years. However the power consumed by the actuators can be high, and using hydraulic actuation for smaller rotorcraft has not been seen as practical. In contrast to active vibration reduction systems, passive vibration isolation systems require no external power. Passive vibration isolation systems however have the disadvantage of being limited to working at one specific frequency which will not be acceptable as slowed rotor flight becomes more common for fuel efficiency and noise legislation reasons. In this thesis two electrically powered actuation concepts, one piezoelectric, and one electromagnetic were initially evaluated. An electrically powered actively augmented passive, or hybrid, vibration reduction system based on an electro hydrostatic actuator (EHA) concept was proposed to be developed further. This hybrid actuator will have a wider range of operating frequencies than a purely passive system, and have lower power consumption than a purely active system. The design is termed a “Resonant EHA”; in that the resonant frequency of the coupled fluid, pump and electric motor rotor inertia matches the fundamental vibration frequency. The hydraulic cylinder, fluid and pump act as a single stage gear ratio, and the. brushless electric motor’s inertia is the main resonating mass as in a Dynamic Antiresonant Vibration Isolator (DAVI) passive vibration reduction system. The electrical power is used to compensate for friction in the actuator and other losses, and if needed can shift the operating point away from the resonant frequency. Simulation results indicated that a hydraulic circuit in which the pump leakage is fed back into the low pressure line would introduce unacceptable disturbances in the flows to and from the cylinder. To eliminate the source of the disturbances, a fully integrated electric motor and pump circuit design was chosen in which the electric motor is immersed in hydraulic fluid. An EHA demonstrator was built sized for a 1.5 tonne rotorcraft. For sizing comparison purposes the frameless brushless D.C motor for each strut of 1.5 tonne rotorcraft has a rotor and stator mass of approximately 1 kg, and can produce a continuous stall torque of 2 Nm. The bidirectional pump has a displacement of 1.5 cm3/rev, the mean system pressure was taken as 90 bar, and the double ended hydraulic cylinder has a 32 mm diameter bore, and 18 mm rod. Initial test results for the proof of concept EHA showed highly significant free play with a reversal of torque direction, resulting in unacceptable loss in transmission stiffness. The free play was traced to the gear pump and a hypothesis for the origin of the free play was put forward. To avoid torque reversals the EHA was further tested with a constant offset torque bias which proved successful in restoring a sufficient stiffness to the transmission. The sizing of the electric motor and power consumed with a non-zero offset torque is greater than a torque reversing motor, which limits the immediate application of the device in the present form. Future research investigating the use of other transmission elements, such as a piston pump, to obtain a more linear stiffness is recommended. As a hybrid vibration isolation system a Root Mean Square (RMS) reduction by a factor of four and near elimination of the fundamental frequency vibrations was achieved for the frequency range of 10 to 20 hertz.