With increasing size of wind turbines, new approaches to load control are required to reduce the stresses in blades. Experimental and numerical studies in the fields of helicopter and wind turbine blade research have shown the potential of shape morphing in reducing blade loads. However, because of the large size of modern wind turbine blades, more similarities can be found with wing morphing research than with helicopter blades. Morphing technologies are currently receiving significant interest from the wind turbine community because of their potential high aerodynamic efficiency, simple construction and low weight. However, for actuator forces to be kept low, a compliant structure is needed. This is in apparent contradiction to the requirement for the blade to be load carrying and stiff. This highlights the key challenge for morphing structures in replacing the stiff and strong design of current blades with more compliant structures. Although not comprehensive, this review gives a concise list of the most relevant concepts for morphing structures and materials that achieve compliant shape adaptation for wind turbine blades.

Review of morphing concepts and materials for wind turbine blade applications

This report provides an overview on the current state of wind turbine control and introduces a number of active techniques that could be potentially used for control of wind turbine blades. The focus is on research regarding active flow control (AFC) as it applies to wind turbine performance and loads. The techniques and concepts described here are often described as 'smart structures' or 'smart rotor control'. This field is rapidly growing and there are numerous concepts currently being investigated around the world; some concepts already are focused on the wind energy industry and others are intended for use in other fields, but have the potential for wind turbine control. An AFC system can be broken into three categories: controls and sensors, actuators and devices, and the flow phenomena. This report focuses on the research involved with the actuators and devices and the generated flow phenomena caused by each device.

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Active load control techniques for wind turbines.

Broad-scale area change of a non-porous surface while maintaining resistance to aerodynamic loading was demonstrated through the development of a passive elastomeric matrix composite morphing skin. The combined system includes an elastomer-fiber-composite surface layer that is supported by a flexible honeycomb structure, each of which exhibit a near-zero in-plane Poisson’s ratio. A number of elastomers, composite arrangements, and substructure configurations were evaluated and characterization testing led to the selection of the most appropriate components for prototype development. The complete prototype morphing skin demonstrated 100% uniaxial extension accompanied by a 100% increase in surface area. Results from out-of-plane pressure loading showed that out-of-plane deflection of less than 0.1 in. (2.5 mm) can be maintained at various levels of area change under pressures of up to 200 psf (9.58 kPa). Applications to wing span morphing UAVs are also discussed.

Design and Fabrication of a Passive 1D Morphing Aircraft Skin

Research on aircraft morphing has exploded in recent years. The motivation and driving force behind this has been to find new and novel ways to increase the capabilities of aircraft. Materials advancements have helped to increase possibilities with respect to actuation and, hence, a diversity of concepts and unimagined capabilities. The expanded role of unmanned aerial vehicles (UAVs) has provided an ideal platform for exploring these emergent morphing concepts since at this scale a greater amount of risk can be taken, as well as having more manageable fabrication and cost requirements. This review focuses on presenting the role UAVs have in morphing research by giving an overview of the UAV morphing concepts, designs, and technologies described in the literature. A presentation of quantitative information as well as a discussion of technical issues is given where possible to begin gaining some insight into the overall assessment and performance of these technologies.

https://iopscience.iop.org/article/10.1088/0964-1726/20/10/103001/pdf

Morphing unmanned aerial vehicles

This paper outlines the benefits and challenges of utilizing active flow control (AFC) for wind turbines. The goal of AFC is to mitigate damaging loads and control the aeroelastic response of wind turbine blades. This can be accomplished by sensing changes in turbine operation and activating devices to adjust the sectional lift coefficient and/or local angle of attack. Fifteen AFC devices are introduced, and four are described in more detail. Non-traditional trailing-edge flaps, plasma actuators, vortex generator jets and microtabs are examples of devices that hold promise for wind turbine control. The microtab system is discussed in further detail including recent experimental results demonstrating its effectiveness in a three-dimensional environment. Wind tunnel tests indicated that a nearly constant change in CL over a wide range of angles of attack is possible with microtab control. Using an angle of attack of 5 degrees as a reference, microtabs with a height of 1.5%c were capable of increasing CL by +0.21 (37%) and decreasing CL by −0.23 (−40%). The results are consistent with findings from past two-dimensional experiments and numerical efforts. Through comparisons to other load control studies, the controllable range of this microtab system is determined to be suitable for smart blade applications. Copyright © 2009 John Wiley & Sons, Ltd.

An overview of active load control techniques for wind turbines with an emphasis on microtabs

Morphing aircraft structures can significantly enhance air vehicle performance. This paper highlights ongoing work to design novel compliant mechanisms that efficiently morph aircraft structures in order to exploit aerodynamic benefits. Computational tools are being developed to design structures that deform into specified shapes given simple actuator inputs. In addition, these synthesis methods seek to optimize the stiffness of the structure to minimize actuator effort and maximize the stiffness with respect to the environment (external loading). These tools have been used to study two different types of morphing systems: (i) variable geometry wings and (ii) high-frequency vortex generators for active flow control. Several case studies are presented which highlight the design approach and computational and experimental results of these morphing aircraft systems.

Design and application of compliant mechanisms for morphing aircraft structures

This paper describes flight test results of a “Mission Adaptive Compliant Wing” (MACWing) variable geometry trailing edge flap in conjunction with a natural laminar flow airfoil. The MAC-Wing technology provides lightweight, low power, variable geometry reshaping of the upper and lower flap surface with no seams or discontinuities. In this particular program, the airfoil-flap system is optimized to maximize the laminar boundary layer extent over a broad lift coefficient range for endurance aircraft applications. The expanded “laminar bucket” capability allows the endurance aircraft to significantly extend their range (15% or more) by continuously optimizing the wing L/D throughout the mission. The wing was tested at full-scale dynamic pressure, full scale Mach, and reduced-scale Reynolds Numbers on Scaled Composites’ White Knight aircraft. Test results confirmed laminar flow regime up to approximately 60% chord for much of the lift range. Analysis and test results suggest significant fuel savings, weight savings and a higher control authority. Preliminary drag results, future aerodynamic applications and vehicle performance projections are discussed.

Flight testing of Mission Adaptive Compliant Wing

Flight testing of the FlexFloil™ adaptive compliant trailing edge

SensorCraft is an Air Force Research Lab (AFRL) concept for a high flying vehicle that will be capable of providing greatly increased Intelligence, Surveillance, and Reconnaissance (ISR) capabilities. Part of the technology SensorCraft will require is high aerodynamic and structural efficiency to accomplish its mission goal of long range and sustained presence. To achieve a long loiter time, it will have a light weight structure with a high aspect ratio wing that carries much of the fuel mass. Hence, the structural modes will be in the same frequency range as the rigid body modes and will be strongly coupled with them. Wing stresses will need to be reduced and the gust load contribution will need to be minimized to keep the structural bending loads low along the span of the wing. This paper documents the results of a wind tunnel test at NASA Langley in the Transonic Dynamics Tunnel of a gust load alleviation system for a SensorCraft concept. The wing was fixed to the wall of the tunnel and the 4 trailing edge and 1 leading edge control effectors were used to simultaneously control first and second bending as well as control the pitching moment at the balance attachment.

Structural Modal Control and Gust Load Alleviation for a SensorCraft Concept

A closed-loop, stall sense and control system was demonstrated on a morphing airfoil. The FlexSys, Inc. Mission Adaptive Compliant Wing was modified to accept a Boeing Co. dielectric barrier discharge actuator panel in a location immediately upstream of the trailing-edge morphing flap, and hot-film sensors were installed on the model surface. A signal analysis algorithm, developed by Tao Systems, Inc., was applied to the hot-film signals to detect separation and trigger activation of the dielectric barrier discharge actuators. The system was successfully demonstrated in the U.S. Air Force Research Laboratory Phillip P. Antonatos Subsonic Aerodynamics Research Laboratory wind-tunnel facility, and an improvement in lift of about 10% was observed at Mach 0.05 (chord Reynolds number 9 x 10 5 ) under closed-loop control and a turbulent boundary-layer state. Actuator effectiveness was demonstrated up to Mach 0.1, but must be extended to Mach 0.2-0.3 to enable a practical stall control system for takeoff and approach of large aircraft. It may be possible to obtain that level of performance by optimizing the actuator locations and input waveforms.

Peter Flick

Papers

Design and application of compliant mechanisms for morphing aircraft structures

Flight testing of Mission Adaptive Compliant Wing

Flight testing of the FlexFloil™ adaptive compliant trailing edge

Structural Modal Control and Gust Load Alleviation for a SensorCraft Concept

Closed-Loop Stall Control System