Fluid-Structural Dynamic Characterization of a Membrane Wing in a Gust Environment

Unmanned aerial vehicles are applicable in a lot of areas including weather condition monitoring, surveillance, and reconnaissance. They need further development in design, especially, for the turbulent atmospheric conditions. Smart materials are considered for wing manufacturing for gust alleviation whereas membranes are found suitable for such applications, and therefore, analyzing aerodynamic properties of the membrane is important. Wind gusts create an abrupt atmospheric situation for unmanned aerial vehicles during the flight. In this study, a continuous gust profile and two types of stochastic gust models, i.e., Dryden gust model and von Karman gust model are developed to study the effects of gust load on a flexible membrane wing. One of the promising ways to reduce the effects of the gust is by using an electroactive membrane wing. A fluid-structure-interaction model by coupling the finite element model of the membrane and computational fluid dynamics model of the surrounding airflow is generated. Aerodynamic coefficients are calculated from the forces found from the numerical results for different gust velocities. A wind-tunnel experimental setup is used to investigate the aerodynamic responses of the membrane wing. Dryden gust model and von Karman gust model are found comparable with a minimum variation of magnitude in the gust velocity profile. The coefficients of lift and drag fluctuate significantly with the change in velocity due to wind gust. A validation of the fluid-structure-interaction model is performed by comparing the numerical results for the lift and drag coefficients with the experimental results. The outcome of this study contributes to better understand the aerodynamics and maneuverability of unmanned aerial vehicles in the gust environment.

A Numerical Study Into the Influence of Leading Edge Tubercles on the Aerodynamic Performance of a Highly Cambered High Lift Airfoil Wing at Different Reynolds Numbers

Through the last two decades, many studies have demonstrated the ability of leading-edge protrusions (tubercles), inspired from the pectoral flippers of the humpback whale, to be an effective passive flow control method for the stall phase of an airfoil in some cases depending on the geometrical features and the flow regime. Nevertheless, there is a little work associated with revealing tubercles performance for the lifting surfaces with a highly cambered cross-section, used in numerous applications. The present work aims to investigate the effect of implementing leading edge tubercles on the performance of an infinite span rectangular wing with the highly cambered S1223 foil at different flow regimes. Two sets; baseline one and a modified with tubercles have been studied at Re = 0.1 × 106, 0.3 × 106 and 1.5 × 106 using computational fluid dynamics with a validated model. The numerical results demonstrated that Tubercles have the ability to entirely alter the flow structure over the airfoil, confining the separation to troughs, hence, softening the stall characteristics. However, the tubercle modification expedites the presence of the stalled flow over the suction side, lowering the stall angle for the three mentioned Reynolds numbers. While, no considerable difference occurs in lift and drag before the stall.

An Investigation of the Aerodynamic and Vibration Behavior of a Helicopter Rotor Blade

Rotary-wing aircrafts are the best-suited option in many cases for its vertical take-off and landing capacity, especially in any congested area, where a fixed-wing aircraft cannot perform. Rotor aerodynamic loading is the major reason behind helicopter vibration, therefore, determining the aerodynamic loadings are important. Coupling among aerodynamics and structural dynamics is involved in rotor blade design where the unsteady aerodynamic analysis is also imperative. In this study, a Bo 105 helicopter rotor blade is considered for computational aerodynamic analysis. A fluid-structure interaction model of the rotor blade with surrounding air is considered where the finite element model of the blade is coupled with the computational fluid dynamics model of the surrounding air. Aerodynamic coefficients, velocity profiles, and pressure profiles are analyzed from the fluid-structure interaction model. The resonance frequencies and mode shapes are also obtained by the computational method. A small-scale model of the rotor blade is manufactured, and experimental analysis of similar contemplation is conducted for the validation of the numerical results. Wind tunnel and vibration testing arrangements are used for the experimental validation of the aerodynamic and vibration characteristics by the small-scale rotor blade. The computational results show that the aerodynamic properties of the rotor blade vary with the change of angle of attack and natural frequency changes with mode number.

Overall Temperature-Dependent Elastic Properties of Carbon Fiber Polymer Matrix Composites at High Temperatures

In this paper we discuss the effect of volumetric ablation on the overall elastic properties of the carbon fiber reinforced polymer matrix composite. An Arrhenius type equation describing polymer decomposition was used to determine volume fractions of evolving polymer matrix phases (i.e. polymer, growing pores filled with pyrolysis gases, and char). The effect of the pressure exerted by pyrolysis gases trapped inside the pores was analyzed. Microstructures consisting of carbon fibers (circular inclusions) in the matrix and pores (elliptic inclusions) in the polymer were generated. Temperature dependency was addressed by generating microstructures with different volume fraction of pores, which were calculated from the mass loss model. Two-step numerical homogenization of representative volume elements (RVEs) was performed using finite element analysis (FEA). The developed procedures were applied to calculate temperature dependent (up to 700 K) effective elastic properties of the AS4/3501-6 composite. The results are compared to the existing experimental data and show good agreement.

Nanomechanical Characterization of Additive Manufactured GRCop-42 Alloy Developed by Directed Energy Deposition Methods

Propulsion applications such as combustion chambers and nozzles for liquid rocket engines require the use of unique materials with superior mechanical and thermal properties. GRCop-42 (Cu-4 wt.% Cr-2 wt.% Nb) is one such candidate material developed by NASA and is now being manufactured using additive manufacturing (AM) techniques. AM offers a unique processing environment different from traditional metal fabrication processes. This study characterized the mechanical and structural properties of as-deposited and heat-treated GRCop-42 manufactured with Blown Powder Directed Energy Deposition (BPD) based on preliminary testing. The materials were characterized using several techniques including surface profiling by laser microscopy, mechanical stiffness by instrumented indentation, crystal structure, residual stress measurements using XRD and residual XRD, and chemical composition and microstructural evaluation using SEM with EBSD. The results of the study are compared using the BPD techniques for samples in the as-deposited and heat-treated conditions.

Experimental Flow Visualization Study of Flow Separation Control With High-Frequency Translational Surface Actuation

Flow separation causes aircraft to experience an increase in drag degrading their aviation performance. The current study aims to delay flow separation on an airfoil by embedding a high-frequency translational piezoelectric actuator along the surface of the airfoil. The actuators with two actuation surfaces were embedded on the suction surface of an Eppler 862 airfoil model and placed in a low-speed wind tunnel. Consecutive pictures of the flow fields with dry ice fogs around the airfoil were taken using a high speed camera in order to observe the flow separation phenomenon before and after turning on the high-frequency translational surface actuation. The effects of the actuation on the flow separation were observed at various actuation displacements, angles of attack, and free stream velocities. The operating frequency of the surface actuation was 565 Hz. The measured actuation mean-to-peak displacement ranged up to 0.12 mm at the maximum applied voltage of 150 V. The angle of attack of the airfoil varied from 6° to 24°. The chord Reynolds number was increased up to around 262,000. It was confirmed that the actuation had a very strong influence on the flow separation even at a very small displacement of 0.024 mm remaining significantly reduced separation bubble compared to the one before activating the actuators at 4.3 m/s of velocity and 14° of angle of attack. The flow separation was completely suppressed when the actuation displacement reached around 0.082 mm under the same conditions of flow velocity and angle of attack. This implied that the actuation should generate a strong enough momentum relative to the free stream in order to completely suppress the flow separation. In summary, the study confirmed that the employed high-frequency translational surface actuation had the obvious control authority on delaying or suppressing the flow separation over the airfoil depending on the parameters changed.

Effects of Rapid Microwave-Curing on Mechanical and Piezoresistive Sensing Properties of Elastomeric Nanocomposites

Carbon nanotubes (CNTs) have the unique ability to absorb microwave radiation and efficiently transfer the energy into substantial heat. When adequately dispersed in a thermoset polymer, such as polydimethylsiloxane (PDMS), the nanocomposite can be fully cured in seconds in a microwave oven rather than in hours in a convection oven. In this paper, cylindrical PDMS nanocomposites containing well-dispersed CNTs are fabricated by either microwave-curing or conventional thermal-curing. The mechanical, electrical, and piezoresistive properties of the fabricated samples are compared to understand the effects of different curing methods. Microwave-cured nanocomposites exhibit a significantly reduced compressive modulus for different CNT loadings. In addition, the electrical conductivity of microwave-cured nanocomposites is significantly enhanced over the thermally-cured counterparts. Experimental results demonstrate that the one-step microwave-curing procedure can improve the electrical conductivity of 1 wt% nanocomposites by almost 150 % over thermal-curing. However, their piezoresistive sensitivity remains remarkably similar, showing the potential for microwave-curing to replace thermal-curing for the manufacturing of highly flexible CNT-based nanocomposites.

Dynamic Simulation and Ground Testing of the Autonomous Vertical Air-Launch Sled Concept

A unique towed air-launch concept called the vertical air-launch sled is studied using a 1/32nd scale vehicle. The sled allows a launch vehicle carried by it to be towed to an altitude by a mother aircraft using a standard towline. The focus was to design and test autonomous tow taxi and takeoff phase of flight using simulations and flight tests. In order to study the scaling validity, experimental flight test data was compared to the theoretically predicted dynamic response of the scaled system, as well as compared to a theoretically predicted dynamic response of the full-scale system. The towing aircraft is a 1/11th scale T-28 remotely piloted aircraft. Lateral stability, runway centerline following ability as well as tautness of the tow line are studied as measures of performance of the autonomous system designed. Test results are reported showing that the tow aircraft was able to tow the vehicle along the correct path. It is identified that the rudder servos were saturated in adjusting the trajectory and future flight tests will include implementing front wheel steering into the system to alleviate the rudder servos saturation. Duration of the flight tests will be increased to better evaluate the automatic controller.

Tensile and Fatigue Properties of Ti-6Al-4V Alloy Fabricated by Laser Powder-Bed Fusion Process

The tensile and fatigue properties of laser-powder-bed-fusion (L-PBF) processed Ti-6Al-4V specimens are investigated at different loading conditions. Two types of as-built and post-machined L-PBF processed dogbone specimens are considered for the study, one is an ASTME8M round specimen and the other one is a customized small-scale flat structure. The tensile and fatigue behavior of the specimens are investigated numerically using the finite element (FE) method. The FE modeling considers both low cycle fatigue (LCF) and high cycle fatigue (HCF) test conditions by applying cyclic loads in fully-reversed and stress ratio R = 0.1 conditions. The FE results for the von Mises stress, strain, total deformation, fatigue life, factor of safety, and fatigue limit of the Ti-6Al-4V specimens are obtained at room temperature (295 K). Results obtained from the model show that the fatigue life decreases as the load increases. It is also found that fatigue life does not vary with the change of the test frequency under a specific fatigue load. The comparison of mechanical properties of the L-PBF processed specimens with conventionally manufactured Ti-6Al-4V parts is also shown to understand the differences in the tensile and fatigue behavior. The validation of the FE model is performed by comparing the numerical results for the yield stress and fatigue limit with the experimental results found from the literature. The overall study contains a detailed analysis of the tensile and fatigue behavior of additively manufactured Ti-6Al-4V parts and provides a guide to investigating the similar properties for other functional materials used in the L-PBF process.

Rapid Configuration and Launch of a UAS Swarm for Post-Nuclear Blast Response

A multi-disciplinary team of West Point cadets developed an Unmanned Aircraft System (UAS) swarm to provide automated remote sensing of radiation sources and hazards following a nuclear blast. The swarm employs autonomous and adaptive distributed control algorithms to survey (1) a radiation fallout plume to map its contour, and (2) a road network to visually identify obstacles to ground travel. To effectively conduct these missions USMA designed modifications to a fleet of Tarot 650 drones to interface with an existing swarming infrastructure, and to enable rapid tool-free integration of the required sensor suite. In addition, a rapid deployment launcher was designed to safely transport, power, and remotely launch the UAS from the back of a HMMWV. This paper describes considerations and analyses that supported the integrated mechanical and electrical designs, as well as a description of the resulting product. Flight testing was conducted to demonstrate the functionality of most of the designed components. Following the COVID-19 shutdown, the team pivoted to remote subcomponent testing, and computer simulation to demonstrate the remaining functions.