Design and Optimization of Rotorcraft Morphing Structures

In order to meet civilian and military design requirements, helicopters must be able to safely complete their missions across a diverse range of operating conditions. For example, rotorcraft are expected to perform critical tasks anywhere between sea level and high altitude, across a potential 100 deg F temperature range, and with their lifting surfaces experiencing an array of varied flow conditions. However, rotor performance is a highly sensitive function of the operating environment. As an example, a rotor blade intended primarily for hover will ideally feature significant built-in geometric blade twist. On the other hand, such a highly twisted blade will perform poorly in high-speed forward flight. Due to the presence of these competing objective functions, many compromises are made in the design and manufacture of traditional rotorcraft components. Looking to the future, modern technologies such as tailored composites, shape memory alloys, compliant structures, and advanced manufacturing techniques permit realistic investigation and analysis of morphing rotorcraft structures. When properly implemented, these adaptive blade structures can provide a particular camber, twist, chord, span, or airfoil shape distribution specifically tailored to the given operating condition. And ideally, as the operating condition evolves throughout the mission, so too will the blade geometry morph to improve the local performance. In this work, several types of adaptive blade structures are designed, optimized, and manufactured. Some of these concepts leverage characteristics of the operating environment to autonomously morph without any dedicated energy input, while others require an actuation mechanism to reconfigure the system. Despite diversity in underlying principles, they all seek to minimize the performance penalty associated with operating a fixed rotor at disparate locations across the flight envelope. In Chapter 1, a brief introduction of rotorcraft adaptive morphing is presented. In Chapter 2, a rotor blade twist-morphing system is designed and optimized which leverages changes in rotor RPM to induce varying degrees of blade twist. In doing so, an extension-twist coupled composite blade was developed that can passively associate a 20% reduction in main rotor RPM with a 7.2 deg reduction in nose-down tip twist, without exceeding material failure constraints. This permits the blade to passively tailor itself in order to significantly improve vibrations and efficiency in hover and high-speed forward flight. In Chapter 3, a novel camber-morphing blade system is designed and tuned which leverages changes in ambient temperature to induce significant airfoil camber through the use of Shape Memory Alloys. In doing so, a series of UH-60A-derived morphing sections are developed that passively associate 80 deg F shifts in ambient temperature with 12-13 deg of downward camber morphing. This permits the rotor blade to become highly cambered in hot conditions, which helps to recover the lift lost due to the reduced air density. In Chapter 4, an internal blade mechanism is designed, optimized, and manufactured which permits airfoil camber morphing when necessary, while passively resisting aerodynamic loads that tend to reflex the airfoil during normal operation. This passive internal cantilever structure can provide a upward-to-downward bidirectional stiffness ratio of 13.82 for a maximum camber angle of 10 deg, 5.58 for a maximum camber angle of 19 deg, and 2.31 without a maximum desired camber angle. In Chapter 5, an active rotor blade shape-morphing system is designed and optimized which quasi-statically reconfigures the blade between a standard airfoil geometry designed for low-speed operation and an elliptical variant tailored for high-speed flight. The auxetic lattice core and actuation systems engineered for this application are able to move the airfoil skin between the two desired geometries with a high degree of accuracy, while resisting aerodynamic forces