Flexible Composite Hydrofoils and Propeller Blades

Shape-adaptive structures have properties that allow them to adapt to their working conditions, and have applications in a number of fields e.g. the INCOMPRO (Intelligent Composite Products) project designed and produced shape-adaptive structures as diverse as yacht booms, marine propellers, floor panels and pump impellers [ 1]. The design of shapeadaptive structures involving fluid-structure interaction is also called hydroelastic tailoring. A field of particular interest in the present day is that of flexible composite marine propellers and hydrofoils [ 2- 14]. Propeller hydroelastic tailoring aims to achieve adaptive blade pitching whereby the pitch changes with loading in a specified manner, resulting in increased efficiency over a range of operating conditions. The use of composite materials makes it possible to design a laminate to achieve the required pitch changes. There are several existing approaches to implementing hydroelastic tailoring of composite propellers: 1) The first approach sets performance goals for the flexible propeller at two operating conditions, and optimizes the composite laminate to achieve those goals (e.g. Lee and Lin [ 4], Lin et al. [ 13]). The first performance goal is that the flexible propeller should maintain the same performance in terms of torque, thrust and efficiency as a baseline rigid propeller at the designed operating condition. The second performance goal is that the pitch must reduce as much as possible as the inflow angle is reduced (i.e. inflow velocity reduced or rotational speed increased) to keep the torque within the narrowest possible range. 2) The second approach assumes that the propeller operates in a spatially varying wake, and designs the blades to adapt to the changing flow conditions encountered during each revolution (e.g. Chen et al. [ 5], Motley et al. [ 14]). This recognizes that while on average the propeller is operating at the design condition, an individual blade's incoming flow velocity varies throughout each revolution so that effectively the operating condition of the blade fluctuates about the design condition. To minimize efficiency losses and potentially benefit from the changing flow conditions, the efficiency of the flexible composite propeller should match that of its rigid counterpart at the design condition, and be higher at off-design conditions. 3) A third approach, developed by Blasques et al. [ 12], is applicable to controllable pitch flexible composite propellers. This method determines the optimal composite lay-upand pitch angle to reduce the weighted average fuel consumption for two conditions (cruising and maximum speeds). While the reasoning behind the first two approaches is different, the outcome is the same. The resulting flexible propeller will have the same shape and performance as a baseline rigid propeller at the design operating condition, the pitch reduces with increasing load, and the flexible propeller is more efficient than the baseline rigid propeller at off-design conditions. A common feature of the existing hydroelastic tailoring methods is that the flexible propeller is based on an existing rigid propeller. The method to be outlined and applied in the present paper assumes that the flexible propeller is to adapt to spatially varying wake flow, but with the knowledge that this will also mean that the propeller will have improved off-design efficiency. It will be shown that for optimum efficiency, the efficiency curve of the flexible propeller must be tangential to that of the baseline rigid propeller at the design condition, and that therefore the design condition cannot be arbitrarily specified. The remaining sections of the paper contain: (i) an outline of the proposed hydroelastic tailoring method, (ii) two design examples (hydrofoil and propeller), (iii) an outline of a method for estimating efficiency gain of a flexible propeller in a spatially varying wake, and (iv) conclusions. This paper does not explore the potential benefits that could be achieved from a composite flexible propeller in terms of reduced maintenance, weight, vibration and acoustic response, although these areas are the subject of current and planned work.