bluuebunny/crossref_metadata_2025 huggingface dataset:
Synthesis, Structure, and Light Absorption of Octanoyl Amino Acids
Authors: Bohnuud Aorigele , Naren Gerile , Qiu Zi Hao , Meng Rui , Tegus Ojiyed
Published: September 2024

Amino acid lanthanide complexes are a promising class of environmentally friendly and non-toxic optical organic materials that find extensive applications in biomedicine, molecular probes, and optical devices. In this study, three amino acid lanthanide complexes, octanoyl glycine gadolinium (Gd(oct-ala)3×H2O), octanoyl serine gadolinium (Gd(oct-ser)3×H2O), and octanoyl phenylalanine gadolinium (Gd(oct-phe)3×H2O), were synthesized by reacting octanoyl amino acid ligands with gadolinium chloride. The coordination structure of the gadolinium complexes was determined by elemental analysis and infrared spectroscopy. XRD and polarized light microscopy confirmed the amorphous structure of the complexes. DSC and TG-DTA were used to investigate the thermodynamic stability of the gadolinium complexes, and their solubility and UV-visible light absorption properties were also evaluated. Our results demonstrate that all three gadolinium complexes exhibit excellent UV-visible light absorption performance, with the Gd(oct-phe)3×H2O containing a phenyl ring showing the highest light absorption efficiency.
Performance Analysis of Galvanized Structures for an Earth-Air Heat Exchanger System
Authors: Domingues Ana Maria Bersch , Ramalho Jairo Valões de Alencar , Fernando Honório Joaquim , Rodrigues Michel Kepes
Published: September 2024

This study presents an investigation on Earth-Air Heat Exchangers (EAHEs), a sustainable and cost-effective alternative for improving thermal conditions in environments. The EAHE consists of one or more ducts buried at a certain depth, and a ventilation system, which allows air to flow through the ducts and exchange heat with the soil. The soil, being warmer than the ambient air during cold periods and cooler during hot periods, facilitates this heat exchange. The research aims to evaluate and compare the potential of the soil and EAHE in a system where a galvanized material, shaped like an ellipse, is attached around the duct. Given the high thermal conductivity of galvanization, this material helps enhance the thermal potential of the soil. Two tests were conducted by altering the horizontal and vertical dimensions of the ellipse while keeping the area constant. The results obtained with the galvanized structure were compared with those obtained without the use of this material. Moreover, the authors compared two different geometries of the structures: a circular one, which had been previously tested, and an ellipsoidal one. Additionally, the thermal potentials of the soil and the system improved as the horizontal length of the ellipse decreased.
Comprehensive Measurement and Simulation of Prototype Injection Moulds
Authors: Krizsma Szabolcs , Suplicz András
Published: September 2024

The injection moulding industry is dynamically developing. The growing demand for more customizable products can be served by low or middle volume production using prototype moulds and inserts. The conventional material of prototype moulds is aluminum because of its excellent machinability, acceptable strength and stiffness and outstanding thermal conductivity. Prototype moulds are gaining ground in the injection moulding industry, yet their operational behavior (including exact mechanical and thermal process parameters) is largely unknown. We created a comprehensive state monitoring system that measures the operational strain, cavity pressure and temperature of different prototype injection moulds. This way, all important process parameters can be measured and the relations between the moulding parameters and the operational pressure loads, deformations and temperatures can be quantified and analysed.
Generalized Aerodynamic Optimization of Hovering Coaxial Rotor Blades
Authors:   Singapore Technologies Aerospace Ltd, Chan Keen
Published: September 2024

This paper presents the development and application of a generalized approach to the aerodynamic optimization of hovering coaxial rotors. The generalization lifts the constraints of previous theories, and enables dis-similar rotor diameters, dis-similar rotational speeds and thrust-sharing percentage of the upper and lower rotor of a coaxial pair to be incorporated as design variables. This increased versatility was necessitated by the emergence of all-electric VTOL UAVs which feature independently driven rotors, and have unique operational requirements. The optimization methodology begins with the derivation of the generalized momentum theory for coaxial rotors, and then extending the formulation to become an optimization technique which determines optimal combinations of rotor diameters and rotational speeds for a specified thrust-sharing percentage. These results are next used as inputs to determine the optimal dis-similar geometries for the upper and lower rotor blades. The optimization is demonstrated on a multi-rotor VTOL UAV with three coaxial pairs of rotors. The results show that high Figure of Merit is achieved, and this is a contributing factor in achieving high hover endurance.
Experimental and Computational Studies to Understand Unsteady Aerodynamics of Cycloidal Rotors in Hover at Ultra-low Reynolds Numbers
Authors:   Texas A%M University, Walther Carolyn ,   NASA Ames Research Center, Lakshminarayan Vinod , Coleman David , Benedict Moble
Published: September 2024

This paper provides a fundamental understanding of the unsteady aerodynamic phenomena on a cycloidal rotor blade operating at ultra-low Reynolds numbers (Re∼18,000) by utilizing a combination of experimental (force and flowfield measurements) and computational (CFD) studies. For the first time ever, the instantaneous blade fluid dynamic forces on a rotating cyclorotor blade were measured, which, along with PIV-based flowfield measurements revealed the key fluid dynamic mechanisms acting on the blade. A 2D CFD analysis of the cycloidal rotor was developed and systematically validated using both force and flowfield measurements. Studies were performed with both static and dynamic blade pitching. Direct comparison of the static and dynamic pitch experimental results helped isolate the unsteady phenomena (such as dynamic stall, unsteady virtual camber, etc.) from the steady effects. The dynamic blade force coefficients were almost double the static ones clearly indicating the role of unsteady mechanisms on force production on cyclorotor blades. For the dynamic case, the blade lift monotonically increased even up to ±45° pitch amplitude due to dynamic stall phenomenon; however, as expected, for the static case, the flow separated from the leading edge after around 15° with large laminar separation bubble (LSB) and eventually completely separating at higher pitch angles. For both static and dynamic pitching cases, there was significant asymmetry in the lift and drag coefficients between positive and negative pitch angles due to the flow curvature effects (virtual camber). CFD flow solution and PIV measured flowfield correlated well and both showed the formation and shedding of strong dynamic stall or leading edge vortices, especially at higher pitch amplitudes, which is the reason for the stall delay and force enhancement. Also, the dynamic stall process during the upper half of the trajectory was significantly different from the lower half even with symmetric blade pitch kinematics because of the reversal of dynamic virtual camber from the upper to the lower half. Even at such low Reynolds numbers the pressure forces, as opposed to viscous forces, were found to be dominant on the cyclorotor blade. The power required for rotation (rather than pitching power) was the domineering component of the total blade power for the dynamic pitching case.
Computational Analysis and Flow Physics of a Ducted Rotor in Edgewise Flight
Authors:   Rensselaer Polytechnic Institute, Misiorowski Matthew , Gandhi Farhan , Oberai Assad
Published: September 2024

This study examines the performance of the ducted rotor in hover and edgewise flight conditions. The flow over a three-dimensional model of a ducted rotor was simulated using the Spalart-Allmaras RANS model implemented in a stabilized finite element method. A sliding mesh was used to conveniently account for the large-scale motion associated with rotor revolutions. The simulation results were analyzed to understand the flow physics and quantify the contributions of the rotor and various sections of the duct interior surfaces on the total aerodynamic forces (thrust, drag and side force) and moments (pitching and rolling). In edgewise flight, freestream flow separates off the front of the duct inlet causing a region of recirculating flow and upwash in the rotor plane. The upwash region biases rotor thrust production to the front of the disk. The swirl velocity further biases the region of flow separation over the inlet and upwash at the front of the rotor towards the retreating side of the disk. The shift of thrust production on the rotor and duct towards the front produces a strong nose up pitching moment on the ducted rotor. The rear of the diffuser is a significant contributor to the total drag, this force as incudes a nose down pitch moment which partially negates the moment from the duct inlet. The rotor is the primary source of vertical vibratory forces as well as vibratory pitching and rolling moments. The small tip clearance of the rotor causes a local interaction between the blade tip and duct that is the dominant contributor to in-plane vibratory forces on the ducted rotor.
Assessment of Create-AV Helios for Complex Rotating Hub Wakes
Authors:   US Army AMRDEC, Potsdam Mark ,   Georgia Institute of Technology, Cross Philip ,   Bell Helicopter Textron Inc., Hill Matthew
Published: September 2024

Experimental data from high Reynolds number water tunnel experiments conducted at the Pennsylvania State University of a de-featured helicopter hub are compared with unsteady computational fluid dynamics (CFD) analyses using CREATE-AV Helios. CFD hub modeling employs a Spalart-Allmaras Detached Eddy Simulation (SA-DES) model. Different off-body wake treatments (baseline SA-DES, coarse off-body SA-DES, and inviscid) are computed. Total hub drag is predicted within 7%, while peak-to-peak loads, azimuthal variation and frequency content are accurately captured. Drag breakdown and frequency content by component is investigated. Mean and unsteady wake velocity characteristics from PIV and LDV measurements are compared with the calculations. The mean streamwise velocities are over-predicted. The frequency content in the CFD simulations is seen to be stronger and broader than experiment with excessive dissipation from near- to far-wake.
A New Actuator Surface Model with Improved Wake Model for CFD Simulations of Rotorcraft
Authors:   The University of Sydney, Linton Daniel ,   The University of Glasgow, Barakos George ,   The Defence Science and Technology Group, Widjaja Ronny , Thornber Ben
Published: September 2024

Simulations of rotorcraft operating in unsteady flow-fields, manoeuvring flight, or with complex rotor configurations pose a significant challenge to current simulation methods. Simplified rotor models lack the generality required for the diverse range of operating conditions that a rotor may be exposed to, while higher-fidelity Navier-Stokes CFD simulations with fully-resolved rotors are expensive in terms of computational resources, simulation time, and preprocessing time. Here we present a new rotor and wake model which is fully-coupled to a CFD solver and is based on the actuator surface model. This model is designed to reduce the cost of complex rotorcraft simulations in comparison with fully-resolved simulations and provide greater generality than other rotor models. Results from simulations using the new actuator surface and wake model provide validation of the concept for hover and forward flight. The spanwise loading distribution, thrust coefficient, and wake geometry are shown to be reasonable in comparison with data from experiments, fully-resolved simulations, and prescribed wake models.
Passive Flow Control of Dynamic Stall via Surface-based Trapped Vortex Generators
Authors:   Carleton University, Al-Jaburi Khider , Feszty Daniel
Published: September 2024

A novel passive flow control concept - based on the local modification of an airfoil's surface - is proposed and examined via CFD for the mitigation of the negative effects of dynamic stall, i.e. for the reduction of peak negative pitching moment while not deteriorating significantly the original lift and drag characteristics. 2D CFD simulations of a NACA 0012 airfoil exposed to a freestream of Mach 0.3 and Re = 3.76×10⁶ and undergoing a 15°±10° pitch oscillation with a reduced frequency of 0.101 were conducted. The baseline airfoil simulations were carefully verified and validated, showing excellent agreement with wind tunnel data. Twenty-six different local geometry modifications were proposed and examined, all functioning as a trapped-vortex generator. The surface modifications were examined on both the upper and lower surfaces. In case of the upper surface modifications, the best geometries could reduce the peak negative pitching moment by as much as 37-63%, while sacrificing only 2-10% of peak lift and reducing drag by 14-38%. On the other hand, the lower surface modifications demonstrated the ability to increase lift by 4-16% with only minor penalty in pitching moment and drag.
Exploring the Power Balance of Main Rotor Primary Control
Authors:   ZF Luftfahrttechnik GmbH, Arnold Uwe ,   Institute of Helicopter Technology Technical University, Garre Willem , Hajek Manfred
Published: September 2024

In most cases a designer refers to the primary control system power consumption the discussion focuses on the rating of the hydraulic system i.e. pumps, boosters, auxiliary gearbox pickup, etc. In contrast, it is rarely acknowledged that this part is only one element and does not represent the total power which has to be invested for (primary) control. If we could trim the aircraft, clamp the swashplate und cut off any power flow to the primary control boosters we would have to accept that in practically all flight conditions power has to be applied to pitch the blades. This portion of power cannot be traced back to the motion of the boosters but is "stolen" via the swashplate from the rotor shaft torque. This paper uses simplified mechanical models which allow to apply measured loads and control motions to trace back these power paths from and to the rotor. The different power contributions are compared and related to the over-all rotor shaft power. When moving to more advanced swashplate-less architectures with individual pitch actuation at each blade root, the mechanical connections transferring power between the single blades or from/to the rotor shaft are lost. Therefore, any electric control system should in some way or the other replicate the inter-blade power exchange. Alternatively it has been proposed to use trailing edge servo flaps for primary control. While it might be possible to reduce the actuation power, it is obvious that such flaps add a considerable amount of aerodynamic drag to the blades. The resultant increase of rotor shaft power falls in the same category of an indirect, hidden component. Estimates of this portion are compared to the power balance of the conventional control system architecture. Finally, individual blade control (IBC) systems allow introducing of higher harmonic control (HHC) which in certain flight conditions can favorably change the blade torque - rate relations even to a point where the average power demand is reduced despite the additional blade pitch motions.
Benefits and Limitations of Reliance on an Open Architecture Technical Standard to Meet Expectations of an Open System
Authors:   The Boeing Company, DuBois Thomas ,   US Army AATD, Wigginton Scott ,   Lockheed Martin, Orlovsky Michael ,   Sikorsky a Lockheed Martin Company, Kinahan William
Published: September 2024

The US Army, Boeing, Sikorsky, Lockheed Martin and other partners conducted independent demonstrations on use of open architecture technical standards can be used to create an open system that meets the expectations identified in the Modular Open System Approach to achieve the benefits of the United States Department of Defense Better Buying Power initiative. This paper presents previously unreleased results and lessons learned from mission system experiments using the Future Airborne Capability Environment (FACETM) Technical Standard and the Army's Joint Common Architecture (JCA). The results show how to apply the FACE standard to address the expectations of an open system while identifying the potential to align with the open standard without achieving the expected benefits. This paper aims to inform acquisition authorities about the technical considerations of open architecture procurements, to provide guidance on how to apply emergent open architecture standards, and address actual or perceived gaps between meeting the requirements of an open standard and achieving the goals of procuring an open system.
Comparison of the Hybrid II, FAA Hybrid III, and THOR-NT in Vertical Impacts
Authors:   Federal Aviation Administration, Taylor Amanda , Moorcroft David , DeWeese Richard
Published: September 2024

The Federal Aviation Administration (FAA) Civil Aerospace Medical Institute conducted a series of dynamic tests to compare the dynamic vertical response of typical civil aviation Anthropomorphic Test Dummies (ATDs) with the Test Device for Human Occupant Restraint (THOR-NT) at three loading rates 9 G, 14 G, and 19 G. The performance of the THOR in the vertical direction has not been well established, and no injury criteria have been developed to assess the spinal injury risk with the THOR. The lumbar load measured by the Hybrid II and FAA Hybrid III closely matched for all three loading conditions with a relative error of less than 10%. The change in lumbar load was not linear with respect to sled acceleration, suggesting that the ATD is rate sensitive at these loading rates. Compared with the FAA Hybrid III, the THOR-NT thoracic load was higher for the 9 G and 14 G tests and approximately equal at the 19 G condition. Based on the tests run in this series, the THOR-NT would not be considered an equivalent ATD to the Hybrid II for vertical testing.
Civil Rotorcraft Crash-Resistant Fuel System Performance
Authors:   BAE Systems, Richards Marvin
Published: September 2024

A detailed review of 14CFR-Part-27/29.952-certified rotorcraft accidents from 1996 through 2015 has been performed. 58 incidents were recorded in the NTSB accident database, 48 of which were considered crashes. 12 of the accidents were rated as severe or extreme, in which 29 out of the 31 occupants onboard the rotorcraft received fatal injuries. There were three extreme crashes that resulted in significant fuel leakage and post-crash fires. According to autopsy data and the NTSB reports, all of the fatalities aboard these three extreme crashes were a result of blunt-force trauma (not thermal caused). For all of the 45 remaining crashes, there were no recorded occupant thermal injuries. All of the results indicate that rotorcraft certified to 14CFR Part 27/29.952 are performing as intended, and that the specified design / performance levels are appropriate for civil rotorcraft. Other rotorcraft that have crash-resistant fuel systems installed in them, but which are not certified to 14CFR Part 27/29.952, were not included in the analysis.
A Closer Look at the Impact of Helicopter Vibrations on Ride Quality
Authors:   University of Stuttgart, Rath Tobias , Fichter Walter
Published: September 2024

The typical vibration level of helicopters is considerably higher than the levels found on fixed wing aircraft. The main reason for this are periodic airloads on the main rotor leading to elevated cabin vibrations at distinct frequencies equal to multiples of the main rotor speed. Thus, the development of technologies for vibration reduction mostly focuses on solutions that counteract the vibrations at rotor blade passage frequencies. As the reduction of those vibrations is performed efficiently, the question whether non-rotor induced vibrations are relevant for helicopter ride quality arises. The work presented in this paper addresses this subject. Existing vibration evaluation methods are assessed with respect to the capability of including non-rotor vibration sources. These evaluation methods are used for an enhanced comfort evaluation by isolation of vibration signals related to relevant discomfort sources. This approach enables comfort optimization considering arbitrary vibration sources. The application of the presented comfort evaluation procedure to flight test data reveals that non-rotor induced vibrations contribute a relevant amount to the discomfort of occupants of helicopters. This is especially the case if the vehicle is equipped with modern anti-vibration systems. Furthermore, the importance of non-rotor vibration sources is greatly increased on flights in atmospheric turbulences. Hence, existing vibration evaluation methods and procedures have to be evaluated as to how they should be extended accordingly. The provided comfort evaluation method can be used for detailed comfort analysis and the results can serve as a basis for the definition of comfort improvement means.
Nonlinear Aeroelastic Coupled Trim Analysis of a Cyclocopter in Hover
Authors:   Texas A&M University, Halder Atanu , Benedict Moble
Published: September 2024

In this paper, detailed development of a nonlinear aeroelastic coupled trim model of a twin-cyclocopter, consisting of two cycloidal rotors (also known as cyclorotors) as main rotors and a conventional horizontal tail-rotor for anti-pitch torque and control, is presented. Coupled trim analysis requires simultaneous computation of trim controls, vehicle orientation and blade structural responses so that both blade response equations and vehicle trim equations are satisfied. To obtain the blade structural response and the hub loads in the vehicle frame for the cyclorotors, a nonlinear aeroelastic model of cyclorotor is developed. For this purpose, a high-fidelity unsteady aerodynamic analysis of a cyclorotor is developed, which includes rigorous modeling of effects such as dynamic virtual camber, effects of near and shed wake, and leading edge vortices. To include effect of blade deformations on cyclorotor performance, a structural framework consisting of fully nonlinear geometrically exact beam model and an FEM based solver is developed. An aeroelastic framework of cyclorotor is developed by coupling the aerodynamic and structural models and the coupled aeroelastic model is validated with in-house experiments with flexible cyclorotors. To obtain the performance of the conventional horizontal tail rotor a modified BEMT based model with CFD-based airfoil lookup tables is developed and validated with test data. Once the complete aeroelastic framework of cyclocopter is developed, coupled trim analysis is performed by simultaneously solving blade response equations and vehicle trim equations until trim controls, blade response, inflow and circulation converge all together. Variation of control inputs required for hover trim is investigated with change in gross-weight and longitudinal center of gravity location of the vehicle.
Interpreting Time-Frequency Analyses of Pilot Control Activity in ADS-33E Mission Task Elements
Authors:   US Naval Test Pilot School, Tritschler John ,   Systems Technology Inc., Klyde David , O'Connor John , Holder John , Lampton Amanda
Published: September 2024

The present work presents results from a continuing research investigation into the use of multiple methods for analyzing pilot control (inceptor) activity in order to better characterize handling qualities. The investigation involved flight test data that were collected for an OH-58C Kiowa helicopter performing the pirouette mission task element from ADS-33E. Subjective handling qualities ratings were assigned, pilot comments were recorded, and multiple post-test analysis methods were also applied, including the (i) time-varying cutoff frequency, (ii) time-varying power frequency, and (iii) multi-frequency. The interpretation of each of these analysis methods is compared and contrasted. The results demonstrate that analysis methods based on time-frequency representations can provide unique insight into handling qualities evaluations by quantifying the character of the pilot inputs, though these insights must be considered within the context of the mathematical limits of the techniques. These methods may be used to supplement classical (i.e., subjective) techniques in providing added comprehension to assigned handling qualities ratings, particularly for comparative analyses.
Neural Network and Machine Learning Allocation of Redundant Controls for Power Optimization on a Compound Helicopter
Authors:   Rensselaer Polytechnic Institute, Reddinger Jean-Paul , Gandhi Farhan
Published: September 2024

For a compound helicopter with control of main rotor speed, auxiliary propeller thrust, and stabilator pitch; a predictive neural network is trained to estimate power as a function of the redundant control settings for a range of flight speeds using a comprehensive database of 2,335 prior Rotorcraft Comprehensive Analysis System (RCAS) simulations. This neural network can be used as a surrogate model for a gradient based optimization to find the redundant control settings that produce a trim state with a minimized power requirement. The study highlights the importance of modeling relevant constraints (main rotor flapping and control limits), as well as having training data in the same locality that the predictions are being made. For producing accurate results with sparse initial datasets (20 trim states), a process of machine learning is demonstrated, wherein the neural network is iteratively updated using the resulting power from each prior optimization. When used appropriately, both the machine learning and fixed models demonstrate ability to allocate redundant controls such that the power requirement is within 1% - 3% of the true minimum.
Red China's First Helicopter: "Yan-An 2"
Authors:   AHS History Committee, Virasak Jacques
Published: September 2024

In the Summer of 1958, nine years after the establishment of Red China, a group of students and faculty of the Northwestern Polytechnical University (NPU) adventured into the indigenous design and development of socialist China's first helicopter: "Yan-An 2" (Y-2, Yan' an No. 2). In span of one decade, two helicopter models of the Y-2 were developed: Y-2A and Y-2B. The obscurely known Y-2A helicopter model design project was carried out at NPU in 1958 and then halted in 1960. The better known Y-2B helicopter model development was launched in 1965 and executed by the same faculty and student team of NPU. Although both helicopter models share the identical designated name of "Yan-An 2" (Y-2), their design configurations are basically different. Despite the fact that most information about the helicopter is still sealed and classified, this paper will present the tortuous rotorcraft design and development history of the Y-2A and Y-2B.
Topology Optimization in Additive Manufacturing
Authors:   PZL Mielec, Wojtuszewski Radoslaw , Banas Aleksander
Published: September 2024

One of the many advantages of additive manufacturing is that it doesn't have geometric constraints. It is very common knowledge that additive technology is appropriate to manufacturing parts of very complex shapes, additive manufacturing also gives the designer opportunity to create parts which are optimal in terms of mechanical strength, stiffness, and functionality. For this reason during the design process of components intended to additive manufacturing it is worth using of various shape optimization methods. Methods based on finite element analysis are most prevalent. The connection of additive technology and topology optimization allows creation of optimized parts. Optimal design gives opportunity to reduce the mass of components, increase stiffness and make parts much more cost effective. Additive manufacturing can be also an option for sheet metal parts. It allows creation of thin wall components fitted with integrated stiffeners such as ribs and stringers. Reducing the number of sheet metal parts (consolidation of parts) and increasing stiffness contribute to manufacture of less mass and cost effective aircraft structures.
Portable Bonding System
Authors:   Sikorsky Aircraft, Tuscano Mark
Published: September 2024

The current method used for bonding liners onto dynamic components requires the use of spring-loaded clamps, vacuum bags, and ovens. This process works well for our smaller articles although has posed problems for our larger bonding requirements. The primary problem for our large bonded components is our ovens would take up to 3.5 hrs. to heat the thickest areas up to bonding temperature. For the adhesives that we use the recommended temperature ramp-up time is 20-60 minutes. In addition, the thinner areas reach temperature sooner and would exceed the recommended maximum curing time of 60 min. A minimum of 25 pounds per square inch (PSI) is required to achieve a sound bondline. The clamps used during cure cycles rely on springs to apply pressure to the liner being bonded. The pressure they exert is not exact, and tends to vary. A vacuum bag can only apply the pressure that is supplied by shop air, 14.7 psi. The solution to achieving uniform and accurate pressure during bonding operations is to have a flexible zoned system that can bring all the areas, thick and thin, to temperature simultaneously, apply even amounts of pressure to the liners, and record all zones of temperature and pressure during the bonding process. Through the entire process, the zones are monitored, recorded, and graphed. This system can be moved anywhere within the factory because the heating elements and the air bladders are incorporated into the fixture.
ACT/FHS System Identification Including Rotor and Engine Dynamics
Authors:   German Aerospace Center (DLR), Seher-Weiß Susanne
Published: September 2024

At the DLR Institute of Flight Systems models of the ACT/FHS, an EC135 with a fly-by-wire/light flight control system, are needed for control law development and simulation. Thus, models are sought that cover the whole flight envelope and are valid over a broad range of frequencies. Furthermore, if the models are to be used in the feedforward loop of the model following control system, they have to be invertible and thus must not have any positive transmission zeros. Maximum likelihood system identification in the frequency domain was used to derive the desired models. For rotor flapping the explicit formulation with flapping angles was modified slightly to avoid positive transmission zeros. For the regressive lead-lag a simple model formulation was found that needs only one dipole with two states. The engine dynamics were first modeled separately and then coupled to the body/rotor model. The final integrated model has seventeen states and yields a good match for frequencies up to 30 rad/s.
Closed-Loop Rotorcraft System Identification Using Generalized Binary Noise
Authors:   German Aerospace Center (DLR), Wartmann Johannes
Published: September 2024

Current frequency-domain system identification methods require an open-loop experiment design to identify the bare airframe dynamics of rotorcraft. Trained test pilots are needed to fly these open-loop flight tests on an unstable air vehicle and thus open-loop rotorcraft system identification is very challenging. In this paper, a novel closed-loop system identification approach is presented. This approach does not require pilot controls but applies Generalized Binary Noise as automatic test signals to all helicopter controls simultaneously. The bare airframe dynamics of the ACT/FHS rotorcraft simulator are identified by the optimized Predictor-Based Subspace Identification method in the time domain using these automatic maneuvers. Robust settings for the frequency range of interest of the chosen test signal and the overall experiment time are determined. Furthermore, the approach is evaluated in representative conditions including measurement noise and weak turbulence. Very promising results are obtained in comparison to classical frequency responses.
Aeroelastic Analysis for Highly Flexible Flapping Wing in Hover
Authors:   Texas A&M University, Yang Xuan , Sudhir Aswathi , Halder Atanu , Benedict Moble
Published: September 2024

Aeromechanics of a flexible flapping wing is a complex nonlinear fluid-structure interaction problem and therefore, cannot be analyzed using conventional linear aeroelasticity methods. This paper presents a stand-alone coupled aeroelastic framework for highly flexible flapping wings in hover for Micro Aerial Vehicle (MAVs) application. A realistic hover-capable flapping-wing MAV utilizes highly flexible wings operating at high frequencies/amplitudes, which causes extreme wing deflections and highly unsteady, vortical flows. The strong fluid-structure coupling and the complicated flow physics make the performance of such a system extremely difficult to predict. The MAV-scale flapping wing is modeled using fully nonlinear beam and shell theories. A potential-flow-based unsteady aerodynamic model is then coupled with the structural model to generate high fidelity coupled aeroelastic framework. Both the structural and aerodynamic models are validated independently before coupling. Direct numerical transient analysis is performed using the Newmark-b method. The instantaneous lift force and wing deflection predictions from the coupled aeroelastic model correlated well with the force and deflection measurements (using Digital Image Correlation, DIC) obtained from in-house flapping wing experiments at both moderate (13 Hz) and high (20 Hz) flapping frequencies.
Development and Validation of an Engineering Simulation Model in FLIGHTLAB with Customized Modeling Enhancements
Authors:   Sikorsky a Lockheed Martin Company, Zhang Chi , Xin Hong , Driscoll Joseph
Published: September 2024

A high fidelity engineering simulation model has been developed in FLIGHTLAB for a Sikorsky production helicopter to support future design modifications. The simulation model consists of major subsystems for main rotor, tail rotor, fuselage, empennage, landing gear, flight control system, and propulsion system. As the manufacturer of the aircraft, Sikorsky is able to provide a complete and validated set of model data required for a high quality engineering model. A large database of flight test records was used to conduct a comprehensive validation effort to ensure the model quality and fidelity. The trim tests include hover (both in and out-of-ground effect), lateral and longitudinal low-speed flight, level flight, vertical climb, forward climb and descent, and autorotation. The dynamic response tests include longitudinal, lateral, collective, and pedal step and doublet control responses in both hover and cruise. The model predictions satisfactorily correlate with the test data in most of the test cases covering a broad range of configurations and flight conditions. For the most challenging low-speed flight regime, a set of customized modeling enhancements were implemented via the "user-defined subsystems" in FLIGHTLAB. These enhancements allow for more sophisticated adjustment of some semi-empirical corrections to address certain model limitations in the main rotor ground effect, the main rotor inflow and interference, and the tail rotor inflow and interference. The model-data correlation in low-speed flight was significantly improved by implementing these model enhancements, resulting in a high fidelity engineering simulation model.
Helicopter Turbulence Modeling with Accurate Spatial Correlations for Handling- Quality Analysis
Authors:   Nanjing University of Aeronautics and Astronautics, Ji Honglei , Chen Renliang
Published: September 2024

This paper presents a turbulence model with accurate spatial correlations for helicopter flight simulation and handling-quality analysis. First, digital filters with longitudinal correlations of the von Karman turbulence are developed to generate discrete turbulence velocity components. Turbulence transverse correlations are considered by relating the filters in different positions with spatial correlations of the von Karman theory. Then, the distributions of both the related filters in front of helicopter and their velocity components in the longitudinal direction of airspeed, as well as turbulence models for helicopter aerodynamic surfaces are established. On this basis, a flight dynamics model coupled with the turbulence model is developed and validated against the flight test data. The contribution of each aerodynamic surface to the helicopter handling qualities is analyzed. Finally, the helicopter handling qualities in turbulent atmospheric environment are discussed. The results show that turbulence transverse correlations have important impact on the handling qualities of both the roll and heave motions, while there is a little impact on the handling quality of pitch motion and nearly no impact on that of yaw motion.
Aircraft Accidents: Investigating Human Error
Authors:   Sikorsky Aircraft, Braman Gary
Published: September 2024

On August 15, 1995, at approximately 2000 hours, a flight of two (2) United States (US) Army UH-60A BLACKHAWK helicopters departed from Royal Air Force (RAF) Akrotiri (in southern Cyprus) for the US Embassy in Beirut, Lebanon utilizing Night Vision Goggles (NVG). Not long after departure, with both aircraft at approximately 800 feet above ground level (AGL) over the Mediterranean Sea, the second aircraft radioed the first aircraft informing them there "#1engine was on fire." Approximately 45 seconds later, the lead aircraft impacted the water at over 200 knots killing all four crewmembers and destroying the aircraft. After learning of the mishap, the US Army Safety Center (USASC) dispatched a team to the site. The accident investigation process had process. The Accident Investigation Board would utilize its 3W approach to answer three questions: "what happened?", "why did this happen?", and "what can we do to prevent it from happening again?"1 The investigation would be conducted in four phases and focus on three areas: material factors, environmental factors, and human factors. It was determined that the aircraft crashed as a result of human error. But why the human error occurred was not determined. In order to answer this question, two exhaustive studies were initiated.
Effects of Nanosilica on the Ballistic Impact Tolerance of Carbon/Epoxy Tubes Loaded in Torsion
Authors:   The Pennsylvania State University, Vashisth Aniruddh ,   NASA Glenn Research Center, Ruggeri Charles ,   US Army Research Laboratory, Henry Todd , Bakis Charles , Roberts Gary
Published: September 2024

Fiber reinforced polymer composites can save weight in rotorcraft structures, but have not been widely used in driveshafts due in part to their limited impact tolerance. The objective of the current investigation is to evaluate the effects of incorporating variable amounts of nanosilica (NS) particles in the matrix on the ballistic impact tolerance of carbon/epoxy tubes loaded in torsion. Tubes manufactured with these matrix materials were ballistically impacted using a round steel projectile aimed at normal incidence across the major diameter. After impact, the tubes were nondestructively inspected and subjected to mechanical tests to determine the axial and shear stiffness and the residual shear strength in torsion. In the best material formulations, which were 15 and 25 weight percent NS in epoxy, the use of NS decreased the impact damage area by 50%, increased the residual shear strength by 38%, and increased the energy absorbed per unit damage area by 120% versus the control material with no NS. Overall, the addition of NS significantly improved the impact tolerance of carbon/epoxy tubes loaded in torsion, with little change in the mass density, glass transition temperature, and elastic modulus.
Equivalent Level of Safety Methodology for Structural Usage Credit
Authors:   Bell Helicopter, Tucker Brian , Muniz Rick , Neus Michael , Green Paul , Altman Leigh , Barbarin Alejandro
Published: September 2024

Usage credits may be used to extend retirement lives for structural components. However, any credit substantiation must account for the contribution of conservative usage assumptions to the current level of safety. Structural reliability methods have been proposed as a means to achieve this end. Herein a new, relative method to determine a practically equivalent reliability (and safety) for aircraft fleets is developed using system reliability theory. Simple mathematical examples are used to illustrate the basic principles. A more realistic example based on the AHS Fatigue and Damage Tolerance subcommittee Round Robin problem is presented. These examples show that, even if only a few aircraft in a fleet operate in a severe manner, these aircraft drive the overall fleet reliability. This means that many aircraft may be able to receive credit without having any appreciable change on fleet reliability. A generalized procedure to apply the method to real world problems is developed. Use of the method as part of a certification methodology is presented along with a justification of what change in reliability would be practically equivalent. Application of this method may allow for safe extension of component lives based on usage.
Flight Testing of a Coaxial Ultralight Rotorcraft
Authors:   Technical University of Munich, Feil Roland , Rinker Markus , Hajek Manfred
Published: September 2024

The results of a full-scale flight test on an ultralight counter-rotating coaxial rotorcraft, the CoAX 2D, are presented. The CoAX 2D was developed and manufactured by edm-aerotec in Geisleden, Germany. It can be operated up to a maximum take-off weight of 450 kg. The helicopter features 2-bladed, 6.5 m diameter upper and lower rotors in teeter configuration with uniform airfoil sections and linearly twisted blades. Fundamental measurements included air data, data from an inertial measurement unit, control forces and deflections, and tail-boom loads in the nonrotating system as well as upper- and lower-rotor teeter angles and data from bladeand hub-load instrumentation. Flight data was synchronized by a common time stamp that was recorded in each system and an additional rotor position pickup sensor. A semiautomated, steady-state recognition algorithm facilitated the evaluation of the results within chosen limitations, accounting for the rotorcraft's position, speed, and attitude in space. This paper explains how flight test data of a coaxial ultralight rotorcraft were processed and it presents results from extensive flight testing. The current focus is on hover and forward flight tests during which the CoAX 2D's power consumption, control angles, tail loads, hub loads, and teeter angles for the whole range of flight speeds were analyzed and evaluated. Upper- and lower-rotor thrust-sharing characteristics as a function of flight speed were analyzed and the resulting rotor–rotor interferences were identified. Hover was dominated by 2/rev characteristics from rotor-fuselage interference.
Characterization of Endevco Piezoresistive Sensors for Blade Pressure Measurements
Authors:   University of Maryland, Grizzle Andrew , Hrishikeshavan Vikram , Saxena Anand , Chopra Inderjit
Published: September 2024

The use of extremely miniature silicon piezoresistive pressure sensors have come into wide use in recent years. The sensors realized by microelectronic techniques, along with a Wheatstone bridge configuration, present advantages in size, sensitivity, and linearity of electrical response. As a result, these qualities are valuable for pressure measurements on a rotor blade. In-house rotor testing involves the study of Mach scaled rotor blades operating up to high advance ratios (μ=1.5). The surface pressure data from the sensors at specific radial and chord-wise locations are integrated to obtain pitching moment, lift and chord force. Currently, two pressure transducers, Kulite and Endevco, are used in blades for testing on hover stand and in wind tunnel experiments. Through systematic bench top calibration, it was shown that the Endevco and Kulite sensors performed identical in their phase and magnitude response up to a bandwidth of 250 Hz. Because Endevco sensors do not come pre-calibrated to compensate for temperature effects, simple passive methods were explored to conduct thermal calibration and remove temperature sensitivity. In this manner, cost-effective pressure sensor solutions were achieved using the Endevco sensors that are about 1/20th the price of a Kulite sensor. Finally, the Endevco sensors were subjected to centrifugal loading in a vacuum chamber and it was determined that the centrifugal effects on pressure output were negligible. Experiments are being conducted to quantify the electrical response of doped p-type silicon resistors to heating and cooling, and consequently the gauge factor of the Endevco piezoresistor pressure transducer are being determined in an effort to streamline the temperature calibration process.
An Investigation of Emergency Flight Operations with Vertical Flight Aircraft in Class B Airspace
Authors:   Praxis Technologies Inc., Stanzione Kaydon ,   ATH Group Inc., Baiada Michael ,   Boston Area HAAO, Ruff Richard
Published: September 2024

This paper investigates how vertical flight aircraft operations can more efficiently mesh with airline arrival and departure flows in both emergency and non-emergency conditions within Class B airspace. Specifically, Helicopter Air Ambulance Operations (HAAO) are merged with the results of FAA studies addressing Aircraft Arrival Management Systems. The unique flight capabilities of vertical flight aircraft must be optimized to provide rapid emergency response while permitting queued airlines to land with minimal to no delay. FAA's NextGen and specific HAAO procedural and equipment mandates offer improved aircrew situational assessment that results in improved safety. However, wide-area emergencies place high demands on multi-jurisdictional military, police, and helicopter air ambulance interoperability and integrated Command and Control. The ever-increasing threat of terrorism and natural disasters combined with escalating passenger travel increases the risk of near misses and accidents in Class B. During emergencies HAAO, which generally operate under airline take-off and landing approach paths, should be queued to still allow airlines to safely land, particularly if they are unable to reroute or have a minimum fuel issue. Class B airspace can become quickly congested and hazardous given multi-jurisdictional operators who may not be intimately familiar with the airport, hospitals, landing zones, and airspace procedures, ultimately presenting unique challenges to ATC and aircraft operators. Affordable solutions such as the Airborne Flight Reporting System (AFRS) can be integrated with Aircraft Arrival Management Systems to provide real-time Command and Control, interoperability, and situational assessment resulting in a common operating picture for multi-jurisdictional first responders, ATC and airlines.
Sound Diffraction Modeling of Rotorcraft Noise Around Terrain
Authors:   US Army, Stephenson James ,   Wave CPC Inc., Chitta Subhashini , Sim Ben , Steinhoff John
Published: September 2024

A new computational technique, Wave Confinement (WC), is extended here to account for sound diffraction around arbitrary terrain. While diffraction around elementary scattering objects, such as a knife edge, single slit, disc, sphere, etc. has been studied for several decades, realistic environments still pose significant problems. This new technique is first validated against Sommerfeld's classical problem of diffraction due to a knife edge. This is followed by comparisons with diffraction over three-dimensional smooth obstacles, such as a disc and Gaussian hill. Finally, comparisons with flight test acoustics data measured behind a hill are also shown. Comparison between experiment and Wave Confinement prediction demonstrates that a Poisson spot occurred behind the isolated hill, resulting in significantly increased sound intensity near the center of the shadowed region.