Farhan Gandhi

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.

Auxetic honeycombs with lossy polymeric infills for high damping structural materials

Structural grade materials with Young’s Modulus in the gigapascal range generally have very low damping, leading to requirement for auxiliary damping treatments in many applications. This article examines whether a high Young’s Modulus and damping level can simultaneously be realized by introducing lossy polymeric infills into auxetic aluminum honeycombs. Using finite element analysis, auxetic honeycombs with varying cell geometry and infill stiffness, loaded in the x- and y-directions, are examined. From the results, it is observed that the best performance is achieved when auxetic honeycombs with shallow inclined walls and short vertical walls are loaded in the x-direction. Young’s Modulus in excess of 10 GPa is achievable in combination with a loss factor of 5%. If a higher loss factor of 10% is required, Young’s Modulus in the range of 5.5–6.5 GPa can still be realized. The high loss factors achieved are attributed to the very high local strain levels in the polymeric infill in the central region of the cell between the top and the bottom cell walls, which are in close proximity, further increased by the large negative Poisson’s ratios of the configuration.

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...

Design, fabrication, and benchtop testing of a helicopter rotor blade section with warp-induced spanwise camber variation

This article focuses on the design, fabrication, testing, modeling, and validation of a spanwise variable camber section of a helicopter rotor blade. The lower surface skin was slit aft of the trailing edge spar and subjected to a spanwise warping actuation input. A kinematic linkage facilitates a corresponding chordwise motion of the skin along the span length (from zero at the no-camber end to maximum at the maximum camber end). Input warping actuation of 0.18 in length produced an 18° camber variation over a 45 in span section of a modified CH-46 blade. Finite element model predictions of the active camber section showed good agreement with benchtop test data for both the output camber for a given warping actuation input and the corresponding actuation force required. Finite element results suggest that some geometry distortions in the active camber section could be experienced due to the presence of centrifugal and aerodynamic loads, but these distortions could be alleviated with the introduction of a shear-flexible (but stiff through the thickness) core.

Morphing Hull Implementation for Unmanned Underwater Vehicles

Benefits of aircraft morphing might be applicable to unmanned undersea vehicles (UUVs) as well. UUVs may perform under a wide variety of operational requirements involving slow speeds, moderate speed travel, and high speed attack/escape during execution of a single mission. Shape, size, and/or profile change, all represent morphing capabilities that could benefit the versatility and efficiency of UUVs by providing the ability to maximize the performance of each mission envelope. The primary goal of the research presented here was to construct, and test a scalable actuation system suitable for a morphing hull UUV, and to apply this actuation concept to a small scale demonstration UUV-type shell. Numerous designs were considered and one design was chosen based on its ability to achieve the desired goals, its ease of construction, and its applicability to more advanced types of morphing being considered for future work. The design chosen was a simple scissor mechanism that allows the easy conversion of a horizontal actuation force into vertical translation. The design requires low power for actuation via the use of a power screw. Increases in diameter of 80% were illustrated with the demonstrator. A natural latex skin was used to illustrate the concept of a flexible skin. This research intends to spur future ideas for UUV and submersible morphing and hence the ideas presented here represent a snapshot of morphing ideas that could be applicable to underwater vehicles. Shape morphing also has the ability to drastically change the operational regimes of UUVs, for instance making an efficient underwater vehicle capable of being an efficient surface vehicle as well. Shape morphing could also be useful in the areas of control. Typically, surface appendages, such as wings, or buoyancy control methods are used for control purposes. However, the ability to shape morph a UUV hull could make the entire UUV a control surface, eliminating the need for such appendages. For instance, if it was desired to reduce the depth of a UUV, the UUV hull could be morphed into a lifting body, such as an ellipse, that would force the vehicle to move towards the surface. Conversely, a diving body shape configuration could be assumed in the same manner. Furthermore, port and starboard controls could also be assumed using shape morphing. Sideways lifting bodies, again in the form of ellipses, could be used to steer the vehicle port or starboard.

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

Control and Performance of a Reconfigurable Multicopter

This paper presents a concept of a multicopter that can be reconfigured between a quadcopter, hexacopter, octocopter, and decacopter. The controls for each of the configurations are identified, and for the configurations with control redundancy, the power optimal controls are presented. A dynamic simulation model is implemented and used to compare the various configurations. The maximum useful weights of the octocopter, hexacopter, and quadcopter were 76.5%, 53.1%, and 29.7% that of the decacopter, respectively. Over a range of useful weights, the decacopter required the least power when the useful weight was greater than around 23% of its maximum, due to lower induced and profile power requirements of the lighter-loaded, slower-spinning rotors. At lower useful weights, smaller configurations required less power due to their lower empty weight. Increasing the number of rotors increased the maximum hover endurance, cruise endurance, and maximum range. Maximum moments and accelerations produced by each airc...

Multirotor Controls, Trim, and Autonomous Flight Dynamics of Plus- and Cross-Quadcopters

This paper examines a quadcopter operating in the plus and cross configurations. The plus configuration generates a yaw moment when a pitch/roll control input is introduced; but, for the cross configuration, the pitch/roll control is decoupled from yaw. Although the collective revolutions-per-minute control requirement, pitch attitude, and power requirement versus flight speed are identical for both configurations, in forward flight, the plus configuration requires a larger pitch control input because it uses only two rotors and a compensatory yaw control input. Quadcopters display two oscillatory modes in hover, a longitudinal phugoid mode (coupling longitudinal translation and pitch), and a lateral phugoid mode (coupling lateral translation and roll). Both modes are stable, and their poles are coincident in hover. In forward flight, the modes are distinct, and the frequency and damping of both modes increase. The nature of the lateral phugoid mode in forward flight is very similar to hover, but the long...

Design studies on cellular structures with pneumatic artificial muscle inclusions for modulus variation

This article presents a variable modulus cellular structure based on a hexagonal unit cell with pneumatic artificial muscle inclusions. The cell is pin-jointed, loaded in the horizontal direction, with three pneumatic artificial muscles (one vertical pneumatic artificial muscle and two horizontal pneumatic artificial muscles) oriented in an “H” configuration between the vertices of the cell. A method for calculation of the hexagonal cell modulus is introduced, as is an expression for the balance of tensile forces between the horizontal and vertical pneumatic artificial muscles. Simulation is then compared to experimental measurement of the unit cell modulus in the horizontal direction over a pressure range of 682 kPa, and an increase in cell modulus of 200% is demonstrated experimentally. A design study considering parametric variation in cell angle, vertical to inclined wall length ratio, and pneumatic artificial muscle contraction ratios shows that changes in modulus of over 1000% are possible when the pneumatic artificial muscles are pressurized to 1992 kPa. This concept provides a way to create a structural unit cell whose in-plane modulus can be tuned based on the orientation of pneumatic artificial muscles within the cell and the pressure supplied to the individual muscles.

Design, development, and hover testing of a helicopter rotor blade chord extension morphing system

A rotor blade chord extension system was designed, fabricated and hover tested, using electromechanical and pneumatic actuation. A 1.5 in actuator stroke output in the spanwise direction was converted into chordwise motion of a trailing-edge plate (TEP), via a rigid link. On the hover stand, with a 20 V dc input, the electromechanical actuator was shown to fully extend and retract the plate at rotational speeds up to 385 RPM (which put the system at a centrifugal loading of 209.5 g, or 47.2% of that on a Black Hawk helicopter at 73% span). The configuration was changed to reduce the actuator force requirement for the pneumatic actuator. The rotor test facility allowed a maximum of 105 psi pressure input through the rotary union (significantly lower than the rating of the actuator). At these moderate pressure inputs, full TEP deployment was observed at 315 RPM (140.2 g, or 31.6% of that on a Black Hawk helicopter at 73% span). The model prediction of TEP displacement versus pressure showed good correlation with test results.