Modelling and estimation of aerodynamic disturbances acting on a hovering micro helicopter in close proximity to planar surfaces
thesisposted on 02.03.2017 by Robinson, David
In order to distinguish essays and pre-prints from academic theses, we have a separate category. These are often much longer text based documents than a paper.
Micro rotor-craft, and micro helicopters in particular, are a class of micro aerial vehicle (MAV) that utilise rotary wings to generate lift. Such aircraft are capable of hovering and performing precise manoeuvres, making them particularly well suited to carrying out missions within confined indoor or urban environments. Operation within a confined environment will, by definition, place a micro rotor-craft in close proximity to obstacles. Fluid interactions between rotors and adjacent obstacles will induce aerodynamic disturbances that can have adverse effects on stability and performance. At a micro scale, viscous effects are more dominant within the flow due to the low Reynolds number regime that micro rotors operate within. With the exception of a hovering micro rotor near a ground plane, fluid interactions between low Reynolds number micro rotors and adjacent obstacles are poorly understood. The research outlined in this thesis extends the existing body of micro rotor research by investigating the physical nature of fluid interactions between a hovering micro rotor and nearby planar surfaces. This thesis uses an unsteady computational fluid dynamics (CFD) method to investigate fluid interactions between a micro rotor and ceiling and wall planes. A Reynolds-averaged Navier-Stokes (RANS) method is used to resolve the flow field and a Spalart-Allmaras turbulence model is used to account for turbulent effects. An immersed boundary technique is used to model the effect of the planar surfaces near the rotor. The CFD method is comprehensively validated by comparison against experimental measurements, two-dimensional fluid dynamics theory and results from the existing literature. The CFD results show that a hovering micro rotor in close proximity to a ceiling plane will experience an increase in thrust and efficiency as rotor-ceiling gap decreases. This effect is driven by an increase in local effective angle of attack along the blade which is caused by the ceiling plane reducing the induced velocity through the rotor disk. For a hovering micro rotor in close proximity to a wall plane, asymmetry in the flow field induces a thrust imbalance across the rotor disk. This generates a torque that will rotate the rotor disk toward the wall plane. Additionally, as a first step toward implementing control systems for stabilising micro helicopters in close proximity to obstacles, this thesis postulates a novel linear simultaneous state and disturbance estimator for estimating disturbances induced by aerodynamic proximity effects during flight. The estimator relies only on measurements that are provided by a helicopter's on-board inertial sensors and considers the effect of direct feed-through of force disturbances to measured accelerations. Flight test results show that this scheme can provide a fair in-flight estimate of aerodynamic disturbances that are induced by fluid interactions with ground, ceiling and wall planes. Finally, it is shown that the disturbance estimate, along with a priori knowledge of how the disturbance varies with proximity to a ground/ceiling/wall plane, can be used to passively estimate proximity. This is a significant contribution of this thesis as the passive proximity estimate can serve as the basis for future development of robust helicopter control and collision avoidance systems.