Ron Barrett

Morphing Wing Flight Control Via Postbuckled Precompressed Piezoelectric Actuators

The design, modeling, and testing of a morphing wing for flight control of an uninhabited aerial vehicle is detailed. The design employed a new type of piezoelectric flight control mechanism which relied on axial precompression to magnify control deflections and forces simultaneously. This postbuckled precompressed bending actuator was oriented in the plane of the 12% thick wing and mounted between the end of a tapered D-spar at the 40% chord and a trailing-edge stiffener at the 98% chord. Axial precompression was generated in the piezoelectric elements by an elastic skin which covered the outside of the wing and served as the aerodynamic surface over the aft 70 % of the wing chord. A two-dimensional semi-analytical model based on the Rayleigh-Ritz method of assumed modes was used to predict the static and dynamic trailing-edge deflections as a function of the applied voltage and aerodynamic loading. It was shown that static trailing-edge deflections of ±3.1 deg could be attained statically and dynamically through 34 Hz, with excellent correlation between theory and experiment. Wind tunnel and flight tests showed that the postbuckled precompressed morphing wing increased roll control authority on a 1.4 meter span uninhabited aerial vehicle while reducing weight, slop, part-count, and power consumption.

Post-buckled precompressed elements: a new class of control actuators for morphing wing UAVs

This paper describes how post-buckled precompressed (PBP) piezoelectric bender actuators are employed in a deformable wing structure to manipulate its camber distribution and thereby induce roll control on a subscale UAV. By applying axial compression to piezoelectric bimorph bender actuators, significantly higher deflections can be achieved than for conventional piezoelectric bender actuators. Classical laminated plate theory is shown to capture the behavior of the unloaded elements. A Newtonian deflection model employing nonlinear structural relations is demonstrated to predict the behavior of the PBP elements accurately. A proof of concept 100 mm (3.94 �� ) span wing employing two outboard PBP actuator sets and a highly compliant latex skin was fabricated. Bench tests showed that, with a wing chord of 145 mm (5.8 �� ) and an axial compression of 70.7 gmf mm −1 , deflection levels increased by more than a factor of 2 to 15.25 ◦ peak-to-peak, with a corner frequency of 34 Hz (an order of magnitude higher than conventional subscale servoactuators). A 1.4 m span subscale UAV was equipped with two PBP morphing panels at the outboard stations, each measuring 230 mm

Missile flight control using active flexspar actuators

A new type of subsonic missile flight control surface using piezoelectric flexspar actuators is presented. The flexspar design uses an aerodynamic shell which is pivoted at the quarter-chord about a graphite main spar. The shell is pitched up and down by a piezoelectric bender element which is rigidly attached to a base mount and allowed to rotate freely at the tip. The element curvature, shell pitch deflection and torsional stiffness are modeled using laminated plate theory. A one-third scale TOW 2B missile model was used as a demonstration platform. A static wing of the missile was replaced with an active flexspar wing. The 1 in 2.7 in active flight control surface was powered by a bimorph bender with 5 mil PZT-5H sheets. Bench and wind tunnel testing showed good correlation between theory and experiment and static pitch deflections in excess of . A natural frequency of 78.5 rad with a break frequency of 157 rad was measured. Wind tunnel tests revealed no flutter or divergence tendencies. Maximum changes in lift coefficient were measured at which indicates that terminal and initial missile load factors may be increased by approximately 3.1 and 12.6 g respectively, leading to a greatly reduced turn radius of only 2400 ft.

Active aeroelastic tailoring of an adaptive Flexspar stabilator

The aeroservoelastic properties of a new class of adaptive aeronautical surfaces are detailed. These new active surfaces use the newly invented Flexspar configuration which employs a high-strength main spar around which an aerodynamic shell is pivoted. Within the aerodynamic shell, a piezoelectric actuator is mounted with one end bonded rigidly to the spar and the other attached to a point on the shell. As the piezoelectric element is energized, the pitch angle of the shell is changed. Adjacent to the piezoelectric element, a sensor is used to determine the position of the shell. A simple feedback loop connecting the sensor and actuator provides a high degree of stability. Inertial and aerodynamic coupling are minimized by collocating the pitch axis, aerodynamic center and center of gravity. Laminated plate theory estimations are used with basic kinematic expressions for relating piezoelectric flexure to shell pitch angle change. Wind tunnel test results demonstrate that stable deflections up to are possible. By using an adaptive positioning system, the aerodynamic shell may be moved with respect to the main spar. This modification lends aeroservoelastic characteristics to the system. Accordingly, as the quarter-chord of the shell is moved forward of the pitch axis, small pitch deflections are effectively magnified with increasing air speed. Experimental testing of an aeroservoelastically coupled wing specimen showed magnification of pitch deflections from to and good correlation with theory.

Design, construction and characterization of a flightworthy piezoelectric solid state adaptive rotor

The development of a new type of flightworthy adaptive rotor system is presented. By building upon earlier adaptive rotor work, a new miniature solid state adaptive rotor (SSAR) was built using directionally attached piezoelectric (DAP) torque-plates controlling Hiller servopaddles. These servopaddles change the rotor disk tilt and thereby induce changes in forces and moments for flight control. To demonstrate the concept, a 23.5 in diameter helicopter rotor was built using DAP servopaddles at the hub. The servopaddles were constructed from PZT-5H piezoceramic actuator sheets bonded symmetrically at . An aluminum substrate and a high temperature cure was used to provide precompression. Analytical modeling was accomplished by laminated plate theory along with strip theory aerodynamics and inertial relations. Because propeller moments are proportional to servopaddle deflections at a fixed rotational speed, it was possible to cancel them out by balancing an aeroelastic coupling between the center of mass, aerodynamic center and elastic axis. Bench testing of the SSAR showed that the rotor system could produce static servopaddle deflections in excess of with good agreement between theory and experiment. With the spinning rotor, the servopaddles demonstrated dynamic capability in excess of . As the rotor speed was increased, deviations between linear theory and experiment also increased. Nonetheless, the rotor still demonstrated servopaddle deflections at full rotor speed (1600 RPM). A detailed weight statement of the conventional and SSAR systems shows that the SSAR helicopter experienced a 40% reduction in flight control system weight, which resulted in an 8% cut in total aircraft gross weight, a 26% drop in parasite drag and a drop in flight control system part count from 94 components down to five.

All-moving active aerodynamic surface research

The structural and aerodynamic characteristics of a new class of active flight control surface are presented. This new type of surface uses a symmetric, subsonic aerodynamic shell which is supported at the quarter-chord by a main spar and actively pitched by an adaptive torque-plate. The structural mechanics of the torque-plate and several actuator elements are detailed, including newly invented interdigitated electrode (IDE) and constrained directionally attached piezoelectric (CDAP) elements. Laminated plate models demonstrate that both generate similar deflections with comparable torsional stiffness. An experimental torque-plate specimen constructed from PSI-5A-S2 piezoceramic shows high torsional deflections and stiffness as well as excellent correlation with theory. The constrained torque-plate was integrated into a 12.5 cm span *5 cm chord adaptive missile fin which was designed for Mach 0.6 flight under standard conditions. The specimen showed static pitch deflections up to +or-8.1 degrees and dynamic deflections of +or-19 degrees at resonance. The active surface was also wind tunnel tested up to 40 m s-1 and demonstrated invariant pitch deflections as a function of airspeed, a steady break frequency of 50 Hz, no flutter, buffet or divergence tendencies and steady lift coefficient changes up to +or-0.51.

Active plate and wing research using EDAP elements

The deflection characteristics of structures using directionally attached piezoelectric (DAP) and enhanced DAP (EDAP) elements are explored. Tests demonstrate that piezoceramic elements, which are isotropic, exhibit orthotropic behavior when directionally attached using any of three methods: (i) partial attachment, (ii) transverse shear lag, and (iii) differential stiffness bonding. Test results demonstrate that directional enhancement through transverse stiffening can increase DAP element strain from 5 to 25%. Closed form expressions of DAP/EDAP strains based on classical laminated plate theory are presented. The models demonstrate that DAP/EDAP elements generate any in-plane strain (extensions and shear) or out-of-plane curvature (bending in either direction and twist) independent of other strains or curvatures. Test results show that fiberglass and aluminium DAP/EDAP beams produce torsional and bending deflections in excess of 30° m-1 with theory and experiment in close agreement. The deflections of DAP/EDAP and conventional piezoelectric active structures are compared. Tests show that DAP/EDAP elements can produce up to 16 times more twist than conventionally attached piezoceramic elements. Two wings were constructed with DAP and EDAP elements. EDAP elements were laminated into the skin of a graphite/epoxy supersonic wing that had a 9% thick diamond airfoil section and an aspect ratio of 3. DAP elements were also laminated to a torsion beam of a subsonic wing that had an NACA 0012 profile and an aspect ratio of 1.4. The supersonic wing demonstrated static twist deflections in excess of 2°. The subsonic wing demonstrated static pitch deflections of 9°. The lifting capability of the DAP/EDAP wings are compared to piezo-ailerons. The DAP/EDAP wings are shown to produce much larger changes in lift coefficient and greater deflection stability with increasing airspeed than the piezo-aileron configuration.

Mechanics of pressure-adaptive honeycomb and its application to wing morphing

Current, highly active classes of adaptive materials have been considered for use in many different aerospace applications. From adaptive flight control surfaces to wing surfaces, shape-memory alloy (SMA), piezoelectric and electrorheological fluids are making their way into wings, stabilizers and rotor blades. Despite the benefits which can be seen in many classes of aircraft, some profound challenges are ever present, including low power and energy density, high power consumption, high development and installation costs and outright programmatic blockages due to a lack of a materials certification database on FAR 23/25 and 27/29 certified aircraft. Three years ago, a class of adaptive structure was developed to skirt these daunting challenges. This pressure-adaptive honeycomb (PAH) is capable of extremely high performance and is FAA/EASA certifiable because it employs well characterized materials arranged in ways that lend a high level of adaptivity to the structure. This study is centered on laying out the mechanics, analytical models and experimental test data describing this new form of adaptive material. A directionally biased PAH system using an external (spring) force acting on the PAH bending structure was examined. The paper discusses the mechanics of pressure adaptive honeycomb and describes a simple reduced order model that can be used to simplify the geometric model in a finite element environment. The model assumes that a variable stiffness honeycomb results in an overall deformation of the honeycomb. Strains in excess of 50% can be generated through this mechanism without encountering local material (yield) limits. It was also shown that the energy density of pressure-adaptive honeycomb is akin to that of shape-memory alloy, while exhibiting strains that are an order of magnitude greater with an energy efficiency close to 100%. Excellent correlation between theory and experiment is demonstrated in a number of tests. A proof-of-concept wing section test was conducted on a 12% thick wing section representative of a modern commercial aircraft winglet or flight control surface with a 35% PAH trailing edge. It was shown that camber variations in excess of 5% can be generated by a pressure differential of 40 kPa. Results of subsequent wind tunnel test show an increase in lift coefficient of 0.3 at 23 m s − 1 through an angle of attack from − 6° to + 20°.

Super-active shape-memory alloy composites

A new type of very-low-stiffness super-active composite material is presented. This laminate uses shape-memory alloy (SMA) filaments which are embedded within a low-hardness silicone matrix. The purpose is to develop an active composite in which the local strains within the SMA actuator material will be approximately 1%, while the laminate strains will be at least an order of magnitude larger. This type of laminate will be useful for biomimetic, biomedical, surgical and prosthetic applications in which the very high stiffness and actuation strength of conventional SMA filaments are too great for biological tissues. A modified form of moment and force-balance analysis is used to model the performance of the super-active shape-memory alloy composite (SASMAC). The analytical models are used to predict the performance of a SASMAC pull - pull actuator which uses 10 mil diameter Tinel alloy K actuators embedded in a 0.10 thick, 25 Durometer silicone matrix. The results of testing demonstrate that the laminate is capable of straining up to 10% with theory and experiment in good agreement. Fatigue testing was conducted on the actuator for 1 000 cycles. Because the local strains within the SMA were kept to less than 1%, the element showed no degradation in performance.

Aeroservoelastic DAP missile fin development

The development of an active aeroservoelastic missile fin using directionally attached piezoelectric (DAP) actuator elements is detailed. Several different types of actuator elements are examined, including piezoelectric polymers, piezoelectric fiber composites and conventionally attached piezoelectric (CAP) and DAP elements. These actuator elements are bonded to the substrate of a torque plate. The root of the torque plate is attached to a fuselage hard point or folding pivot. The tip of the plate is bonded to an aerodynamic shell which undergoes a pitch change as the plate twists. The design procedures used on the plate are discussed. These include an optimization of the actuator element orientation, substrate material type and thickness, as well as a determination of the optimum elastic axis location. A comparison of the various actuator elements shows that DAP elements provide the highest deflections with the highest torsional stiffness. A torque plate was constructed from 0.2032mm thick DAP elements bonded to a 0.127mm thick AISI 1010 steel substrate. The torque plate produced static twist deflections in excess of ±3o. An aerodynamic shell with a modified NACA 0012 profile was added to the torque plate. This fin was tested in a wind tunnel at speeds up to 50ms-1. The static deflection of the fin was predicted to within 6% of the experimental data.