Silvestro Barbarino

Design of Extendable Chord Sections for Morphing Helicopter Rotor Blades

Chord extension morphing of helicopter rotors has recently been shown to be highly beneficial for stall alleviation, with the ability to reduce power near the envelope boundaries and increase maximum gross weight, altitude, and speed capability of the aircraft. This article presents a morphing mechanism to extend the chord of a section of the helicopter rotor blade. The region aft of the leading-edge spar contains a morphing cellular structure. In the compact state, the edge of the cellular structure aligns with the trailing edge of the rest of the blade. When the morphing cellular structure is in the extended state, the chord of that section of the blade is increased by 30%. In transitioning from compact to extended states, the cellular structure slides along the ribs which define the boundaries of the morphing section in the spanwise direction. The cellular section has mini-spars running along the spanwise direction to attach the flexible skin and provide stiffness against camber-like deformations due to aerodynamic loads. This article presents a finite element analysis and design of the morphing cellular structure, ensuring that the local strains in the elastic ligaments of the cellular structure do not exceed the maximum allowable, even as the section undergoes a large global strain. The morphing cellular structure itself is designed to be stiff enough to support the pre-stretched skin attached to its surfaces. Various methods of flexible skin attachment to the morphing substructure, and their ramifications, are considered. A model of a blade section is fabricated and shown to undergo chord morphing, as designed.

Bistable arches for morphing applications

This article examines the bistable behavior of an arch for morphing applications. The arch has a cosine profile, is clamped at both ends, and is restrained axially by a spring at one end. Fabrication and testing of several Delrin and NiTiNOL arch specimens (with varying arch height, thickness, and spring stiffness) were followed by ANSYS finite element modeling, and the ANSYS simulation results showed good overall agreement with the test results. A parametric study was conducted using the ANSYS model to assess the influence of arch thickness, height, and spring stiffness on the bistable behavior. The results indicated that lower arch thickness, larger arch height, and higher spring stiffness tend to promote bistability; lower arch thickness and height reduce peak strains as the arch moves between equilibrium states, but increasing spring stiffness has a smaller effect; and higher arch thickness, height, and spring stiffness increase the snap-through force, which in turn increases the actuation force requirement as well as load carrying capability of the bistable morphing arch. If the arch slenderness ratio is unchanged, change in arch span (size) does not change the maximum stress while increasing the peak snap-through force proportionally.

Helicopter Rotor-Blade Chord Extension Morphing Using a Centrifugally Actuated Von Mises Truss

Previous studies have shown that chord extension morphing over a spanwise section of helicopter rotor blades can reduce main rotor power requirements in stall-dominant flight conditions while being able to increase the maximum gross weight, altitude, and flight speed capability of the aircraft. This study examines a centrifugally driven, fully passive chord morphing mechanism for helicopter rotor blades. It is based on a von Mises truss connected to a rigid extension plate that deploys through a slit in the trailing edge. When the rotor revolutions per minute increases beyond a critical value, the chordwise component of centrifugal force on the assembly results in the deployment of the plate beyond the slit in the trailing edge, effectively increasing chord length. On reducing the revolutions per minute, a retraction spring pulls the plate back within the confines of the blade. This study presents the design process, iterations, and final design solution for a configuration that undergoes 20% chord extens...

Energy Dissipation of a Bi-Stable von-Mises Truss under Harmonic Excitation

This paper examines the energy absorption and dissipation properties of a von-Mises truss capable of snap-through, driving a viscous damper attached to its vertex. The proposed system exhibits almost opposite behavior if under displacement or force harmonic input, being able only to reproduce the negative stiffness or the snap-through in each loading condition, respectively. The loading method also greatly influences the system performance, and the effects of varying forcing frequency and damping coefficient on hysteresis loop shape and area are investigated. It is shown that the proposed system is able to combine stiffness, tailored to the application almost independently, and damping capabilities with performance above traditional materials and structures, and loss factors in excess of 0.6. The occurrence of snap-through is responsible for this performance, increasing the velocity in the damper and, therefore, energy dissipated.

Cellular Honeycomb-Like Structures With Internal Buckling and Viscous Elements for Simultaneous Load-Bearing and Dissipative Capability

Cellular structures with hexagonal unit cells show a high degree of flexibility in design. Based on the geometry of the unit cells, highly orthotropic structures, structures with negative Poisson’s ratios, structures with high strain capability in a particular direction, or other desirable characteristics may be designed. Much of the prior work on cellular structures is based on hexagonal honeycomb-like unit cells, without any inclusions. A companion paper to the current paper presented a vision of cellular honeycomb-like structures with diverse inclusions or internal features within the unit cells (such as contact elements resulting in stiffening behavior, buckling beams resulting in softening behavior, bi-stable elements producing negative stiffness or viscous dashpots producing dissipative behavior). That paper further went into details on linear springs as the most fundamental of inclusions.In the present paper, a buckling beam and viscous dashpots are used as inclusions in the basic pin-jointed rigid-walled hexagonal unit cell. The buckling beam provides the cell with a high initial stiffness and load carrying capability. At loads beyond the critical buckling load, the unit cell softens (while still retaining the ability to carry a “design” load), and undergoes large deformation under incremental load. The viscous dampers undergo a correspondingly large stroke resulting in high dissipative capability and loss factor under harmonic or transient disturbance beyond the design load. In the paper, an analysis and design study of the cell behavior with variation in unit cell geometric parameters, buckling beam parameters and viscous dashpot parameters is presented. The analytical results in the paper are validated against ANSYS Finite Element results. Further, a prototype unit cell with an aluminum internal buckling beam and viscous dashpots is fabricated and tested under static and dynamic loads in an Instron machine. Good correlation is observed between the tests, the FE results and the analytical simulations when accounting for the non-linear behavior of the viscous dashpot used in the tests.Copyright

Energy Dissipation of a Bi-stable von-Mises Truss under Impulsive Excitation

This paper examines the energy absorption and dissipation properties of a bi-stable system due to large deformation capability and bi-stability. The considered bi-stable system is a von-Mises truss, driving a damper attached to the truss vertex, used as a shock absorber. Performance of the proposed system is compared against a traditional mass-spring-damper assembly. The VMT system undergoes larger displacements due to snap-through and can dissipate larger amounts of energy. In addition, snap-through in the VMT limits the max velocity at which the system oscillate.

Helicopter Rotor Blade Chord Extension Morphing Using a Centrifugally Actuated von-Mises Truss

Previous studies have shown that chord extension morphing over a spanwise section of helicopter rotor blades can reduce main rotor power requirement in stall-dominant flight conditions while at the same time being able to increase the maximum gross weight, altitude, and flight speed capability of the aircraft. This study examines a centrifugally driven, fully passive chord morphing mechanism for helicopter rotor blades. It is based on a von-Mises truss situated aft of the leading-edge spar, connected to a rigid extension plate which deploys through a slit in the trailing-edge. When the rotor RPM increases beyond a critical value the chordwise component of centrifugal (CF) force on the von-Mises truss and plate assembly results in the deployment of the plate beyond the slit in the trailing edge, effectively increasing chord length. On reducing the RPM, a retraction spring pulls the plate back within the confines of the blade. This study presents the design process, iterations and the final design solution for a configuration that undergoes 20% chord extension. A prototype was fabricated and tested on the bench-top as well as on a rotor test stand at rotational speeds simulating 70% full-scale CF loads. The test results demonstrate that the concept works. However, effects such as friction lead to higher force (or RPM) requirements for deployment than predicted by simulation, and are present during retraction as well. The effects are more pronounced in the high CF field in the rotor test.Copyright

A Novel Structural Element Combining Load Carrying and Energy Dissipation Capability

This paper reports on the design, fabrication, testing and simulation of a novel structural element that combines load carrying and energy dissipation capability. The principal components comprise post-buckling elements (PBEs), and von-Mises trusses (VMTs) coupled to a viscous dashpot, all integrated in a compact panel-like element. Load carrying capability of the unit comes from the PBEs which provide a high initial stiffness and very little deformation up to the critical buckling load. Energy dissipation is obtained through the deformation of VMTs at the top and bottom of a hexagonal cell, connected to the ends of the viscous dashpot. Under harmonic excitation the VMTs undergo large displacement, stroking the damper in the process. The paper explains the design procedure of the structural element in detail, and describes a prototype which is fabricated and tested under harmonic excitation. Under harmonic displacement input the energy dissipated increased over the frequency range (with loss factor increasing from 0.84 at 0.1 Hz, to 2.69 at 4 Hz). Although simulation predicted both top and bottom VMTs moving simultaneously while the tests showed one transitioning before the other, the experimental and simulation hysteresis loop areas compared well. Under harmonic force input, data was obtained only at 0.1 Hz and 0.5 Hz, as the testing machine could not move at the high-speeds associated with VMT snap-through. The energy dissipated (hysteresis loop areas) in these tests was slightly lower than that in the displacement controlled tests at the same frequencies, with loss factors of 0.61 at 0.1 Hz and 0.97 at 0.5 Hz calculated from the measured hysteresis cycles. The experimental and simulation work done to date establish the basis feasibility of developing novel structural designs which can carry high static load and also dissipate energy from undesirable dynamic inputs. Such designs have broad potential applications in aerospace, marine and ground structures.

A Three-Dimensional Multi-Stable Unit Cell for Energy Dissipation

This paper presents a three-dimensional version of a planar hexagonal cell whose top and bottom inclined members act as a three-dimensional von-Mises truss (VMT), displaying negative stiffness or snap-through behavior. A damper connected between the top and bottom vertices of the 3-D cell facilitates large energy dissipation under harmonic excitation. Tests conducted under a harmonic displacement input showed the top and bottom units transitioning sequentially through their negative stiffness regions, and Simscape simulations showed excellent correlation with tests at 0.5, 1, and 2 Hz. At the lower frequencies, the effect of the 3-D VMTs at the top and the bottom of the cell was evident in the hysteresis loops, but at the higher frequency, the damper was the dominating influence. A special test designed to capture snap-through allowed the rapid motion of the vertex of the three-dimensional cell at velocities greater than the capability of the Instron machine. The occurrence of snap-through was observed in both test and simulation, and results showed that the hysteresis loop area was larger by about 25%, compared to the case of a harmonic displacement input at the cell vertex which does not produce snap-through. Thus, snap-through increases the energy dissipation capability of the 3-D cell.

Shape Memory Alloy Actuated Morphing Cellular Frame using Bi-stable von-Mises Trusses with Variable Length Links

This study focuses on a morphing hexagonal frame actuated by Shape Memory Alloy (SMA) wires. The frame comprises of two bi-stable von-Mises trusses (VMTs), and as the trusses snap-through from one stable equilibrium position to the other, the frame morphs from a regular hexagonal to a bowtie configuration. To avoid large transverse (Poisson’s) deformation during snap-through, variable-length links are used in the VMTs. The study describes how the SMA wires need to be configured relative to the VMT links and the integrated design process. A prototype is fabricated and tested, and it is demonstrated that resistive heating of one set of SMA wires can indeed produce snap-through of the VMTs to morph the frame, while heating of another set of wires can produce snap-back. Analysis, based on numerical integration of the coupled VMT and SMA constitutive equations, was compared to test and showed reasonable overall correlation of the fundamental phenomena in the study.