Justin Manzo

Adaptive structural systems and compliant skin technology of morphing aircraft structures

Morphing aircraft design - the design of aircraft capable of macroscale shape change for drastic in-flight performance variation - is an extremely broad and underdefined field. Two primary means of developing new concepts in morphing exist at Cornell University: design of broad test platforms with generalized motions that can provide future insight into targeted ideas, and specifically adapted aircraft and shape change mechanisms attempting to accomplish a particular task, or hybridize two existing aircraft platforms. Working with both schools of thought, Cornell research has developed a number of useful concepts that are currently under independent analysis and experimentation, including three devices capable of drastically modifying wing structure on a testbed aircraft. Additional concerns that have arisen include the desire to implement ornithological concepts such as perching and wingtip control, as well as the necessity for a compliant aerodynamic skin for producing flight-worthy structural mechanisms.

Analysis and optimization of the active rigidity joint

The active rigidity joint is a composite mechanism using shape memory alloy and shape memory polymer to create a passively rigid joint with thermally activated deflection. A new model for the active rigidity joint relaxes constraints of earlier methods and allows for more accurate deflection predictions compared to finite element results. Using an iterative process to determine the strain distribution and deflection, the method demonstrates accurate results for both surface bonded and embedded actuators with and without external loading. Deflection capabilities are explored through simulated annealing heuristic optimization using a variety of cost functions to explore actuator performance. A family of responses presents actuator characteristics in terms of load bearing and deflection capabilities given material and thermal constraints. Optimization greatly expands the available workspace of the active rigidity joint from the initial configuration, demonstrating specific work capabilities comparable to those of muscle tissue.

Demonstration of an in situ morphing hyperelliptical cambered span wing mechanism

Research on efficient shore bird morphology inspired the hyperelliptical cambered span (HECS) wing, a crescent-shaped, aft-swept wing with vertically oriented wingtips. The wing reduces vorticity-induced circulation loss and outperforms an elliptical baseline when planar. Designed initially as a rigid wing, the HECS wing makes use of morphing to transition from a planar to a furled configuration, similar to that of a continuously curved winglet, in flight. A morphing wing concept mechanism is presented, employing shape memory alloy actuators to create a discretized curvature approximation. The aerodynamics for continuous wing shapes is validated quasi-statically through wind tunnel testing, showing enhanced planar HECS wing lift-to-drag performance over an elliptical wing, with the furled HECS wing showing minimal enhancements beyond this point. Wind tunnel tests of the active morphing wing prove the mechanism capable of overcoming realistic loading, while further testing may be required to establish aerodynamic merits of the HECS wing morphing maneuver.

Bat-Inspired Wing Aerodynamics and Optimization

A S ENGINEERS search to make micro air vehicles more stable, maneuverable, and efficient, many are turning toward biology for inspiration. Bats adapt to their environment by morphing their wings in flight. Depending on their niche, certain species soar and glide [1], whereas others perform barrel rolls in nature [2] and can pull up to 4.5g in obstacle courses [3].Morphological changes afford bats great agility at low speeds, and they are able tomaintain stability and control at low Reynolds numbers, in which viscous effects and leading edge laminar separation bubbles cause nonlinearities in lift [4–6]. Bats achieve these feats with fingerlike jointed bone structures and flexible wing membranes. These unique traits allow them to change camber and twist in flight, unlike avian span changes. Recent studies investigate replicating flapping bat flight [7–9], whereas others focus specifically on flexible, membrane wing benefits at low Reynolds numbers for improvements in gust alleviation and delayed stall characteristics [10–12]. These wings are passive elements, and it is difficult to attach control surfaces to them for flight authority. By actively controlling and morphing flexible wings, conventional controllers can be replaced. Aircraft morphing allows single vehicles to have multiple functions, ideally with continuous lifting surfaces to alleviate drag and vibration and increase efficiency. This is the focus of Garcia et al. [13], which proposes a nonflapping wing with twist capabilities. Morphing enables tailoring of wing shapes to multiple flight regimes, from takeoff through cruise to landing [14–16]. Many mechanisms have been proposed for morphing, such as the smart joint, an active rigidity composite suited to actuating a batlike membrane wing in flight [17]. This low-profile device can be embedded at joints in the fingerlike skeletal wing structure as a bimorph actuator [18]. Whereas this work focuses on static wing configurations rather than morphing behavior, results help specify actuator requirements used for a variable camber and twist wing. In this work, two key features of bat flight are studied, uniquely evolved bat wing planforms adapted to environments, and capabilities afforded through variable camber and twist, and are applied to rigid, fixed wing designs for small man-made craft. The goal of this study is not to mimic natural bat flight, but to understand how certain aspects of bat flight apply to the engineering problem of wing design for micro air vehicles. II. Background and Motivation

The Smart Joint: Model and Optimization of a Shape Memory Alloy/Shape Memory Polymer Composite Actuator

The Smart Joint, developed at Cornell University, is a composite device which functions as both a structural element and shape changing mechanism. Through resistive heating, the device will provide a tip deflection on the order of 5–20% of its undeflected length, with a high specific work capability. The joint possesses sufficient stiffness to function as a load-carrying element on a structure, inspired by the need to consume minimal energy through passive rigidity. An overview of Smart Joint operation is provided, followed by an improved model encompassing embedded actuators, applicable to many strain actuation systems. Previous work has developed a model that describes the shape change capability of the joint as a function of composition and layering structure, and the revised model is an extension of that work, agreeing well with finite element analysis. Benchmarking is conducted through a heuristic optimization study, providing a framework for selecting joint structure to match desired application by joint composition family. Implementation on a bat-like morphing wing is proposed that uses the Smart Joints as self-actuated hinge structures along the skeleton, capable of providing increased wing camber and tip deflections while in flight.Copyright

Drawing Insight From Nature: A Bat Wing for Morphing Aircraft

Being the only flying mammal, bats have evolved unique flight devices affording them high maneuverability and efficiency despite their low flight speeds. By selecting bats of three different ecological niches — a highly efficient fishing bat, a nimble insectivorous forager, and a large soaring bat of the ‘flying fox’ family — passive wing shapes can be demonstrated as capable of attaining very different aerodynamic performance characteristics. The aerodynamics of man-made equivalents to these wing shapes, using thin airfoils rather than skeleton and membrane construction, are studied both computationally through a lifting-line approach and experimentally with quasistatic wind tunnel data of ‘morphed’ and ‘unmorphed’ wing shapes. Results confirm that shape inspired by the larger soaring bat has higher lift-to-drag ratios, while that of the foraging bat maintains lift at higher angles of attack than the other wings. The advantages are more pronounced by morphing, increasing both lift coefficient and lift-to-drag ratios by up to 50% for certain wings. This is validated both numerically and in the Cornell University 4′ ×4′ wind tunnel. Analysis of these shapes provides the first phase of wing design for use on a morphing aircraft vehicle. In order to take greater advantage of vehicle morphing, wing shapes with camber and twist distributions unique from those found in nature will evolve to suit a man-made structure. In this way, a wing shape intended for cruise may extend its practicality into highly maneuverable operations through the use of wing morphing. Starting from the bat planform shapes, a series of optimizations determines the best camber and twist distributions for effective morphing. Given a fixed degree of shape change at any point along an airfoil based on mechanism constraints, improved morphing performance can be found compared to initial assumptions of the natural shape change. Heuristic optimization employing simulated annealing determines the required morphing shapes for increased performance, broadening the abilities of each wing shape by increasing parameters such as lift, rolling moment, and endurance.Copyright

Evolutionary flight and enabling smart actuator devices

Recent interest in morphing vehicles with multiple, optimized configurations has led to renewed research on biological flight. The flying vertebrates - birds, bats, and pterosaurs - all made or make use of various morphing devices to achieve lift to suit rapidly changing flight demands, including maneuvers as complex as perching and hovering. The first part of this paper will discuss these devices, with a focus on the morphing elements and structural strong suits of each creature. Modern flight correlations to these devices will be discussed and analyzed as valid adaptations of these evolutionary traits. The second part of the paper will focus on the use of active joint structures for use in morphing aircraft devices. Initial work on smart actuator devices focused on NASA Langleys Hyper-Elliptical Cambered Span (HECS) wing platform, which led to development of a discretized spanwise curvature effector. This mechanism uses shape memory alloy (SMA) as the sole morphing actuator, allowing fast rotation with lightweight components at the expense of energy inefficiency. Phase two of morphing actuator development will add an element of active rigidity to the morphing structure, in the form of shape memory polymer (SMP). Employing a composite structure of polymer and alloy, this joint will function as part of a biomimetic morphing actuator system in a more energetically efficient manner. The joint is thermally actuated to allow compliance on demand and rigidity in the nominal configuration. Analytical and experimental joint models are presented, and potential applications on a bat-wing aircraft structure are outlined.

Active Rigidity Smart Joint for a Bat-Wing Micro Air Vehicle

In order to maximize lift for use in turning and landing maneuvers, bats make use of continuous camber change along their fifth metacarpal more effectively than all modern-day aircraft flaps. This biological shape change produces lower drag than modern aircraft, allowing for greater flight efficiency and lower noise signatures. A mechanism to replicate this demands a seamless actuator to avoid gaps and discontinuities, and requires the use of morphing structures. However, a recurring problem in morphing aircraft design is inefficiency of both space and power consumption. Problems often stem from the replacement of rigid structural elements with actuator elements that must be powered in order to carry static loads. To resolve this issue, a ‘smart joint’ concept is proposed which allows rigidity in its passive state, and becomes compliant while serving as an actuator by way of a composite of smart materials. Using a network of shape memory alloy and shape memory polymer, the joint is capable of rotations on the order of 5 percent camber over an arbitrary length when placed along a skeletal element of a bat-like wing structure. An analytical model is used to predict the behavior of the joint as a function of resistive heating and external loading, and is used to examine the layer thicknesses and locations (i.e. bimorph vs. unimorph) and placement of rigid elastic members in order to maximize deflection under a given load. Validation of the joint using is conducted via finite element modeling, and expected airfoil data for a generic shape maneuver to be accomplished by this joint is shown.Copyright

A Bat-Wing Aircraft Using the Smart Joint Mechanism

A bat-like aircraft is proposed, using a smart joint mechanism to actuate the morphing of the wings. The smart joint stays in its deformed shape after cooling, which can be up to 5% of 25 mm length joint. The morphing of the wing shapes of three different bat species is evaluated using a planar lifting line analysis. The morphing improves the lift coefficient over 1000% and the lift to drag ratio over 300% at an angle of attack of 0.6°. The results compare well with what is expected from the type of flight and morphology that has been documented for the bats.

Active rigidity joint

Unimorph active rigidity joints, constructed from Shape Memory Alloy and Shape Memory Polymer and capable of bending actuation, are reported in this work. An embedded aluminum shim was added to each joint as a structural element to facilitate actuation. Joints were actuated using ohmic Tri-Phase and pulse heating processes with different results. It appeared that openloop position control could be achieved using pulse heating. Actuator improvements and future experiments are proposed.