Rotorcraft Design Research Articles

Application of Causal Analysis based on Systems Theory (CAST) to Regulatory Decision-Making: A Case Study of the Sikorsky S92A

Ensuring aviation safety requires maintaining the integrity of product design and operations. The Federal Aviation Administration (FAA) regulates Transport Category rotorcraft design through 14 CFR Part 29, establishing Categories A and B of certification for multiengine rotorcraft, and requires aircraft to be operated according to the certified procedures in flight manuals. This paper presents a case study of the Sikorsky S92A, a Transport Category rotorcraft that is not certified for elevated helideck operations according to Part 29, but operates primarily in the offshore market through FAA-authorized exemptions from applicable regulation. To understand how this discrepancy between the aircraft's certification and operation came to be, a relatively new accident analysis methodology called Casual Analysis based on Systems Theory (CAST) is applied to a hypothetical accident involving an S92A, strongly based on a real incident described in a service difficulty report. The CAST results identify unsafe decisions on the part of flight crews, air operators, the aircraft manufacturer, and the FAA that contribute to the accident. We explain these contributions by identifying several systemic factors generalizable to the entire offshore rotorcraft industry that underlie unsafe decisions, and we propose a set of recommendations to address them.

Parametric structure optimization of a main rotor blade as an application for FSI analysis in main rotor optimization process

Purpose Modern warfare and modern battlefield are very demanding. The recent conflicts showed that the usage of the helicopters is very limited and only the best constructions are able to provide support for the operations. The purpose of this research is to show the possibilities of new design tool for main rotor aerostructural optimization. It is a next chapter of the research that is aimed at finding new solutions for rotorcraft constructions. Design/methodology/approach This work presents a method of preliminary structure optimization of the main rotor blade using parametric modeling. It is the next step in the main rotor optimization studies. It is the next step after preparing the parametric model for the external shape CFD analysis. As a basis for parametric blade structure calculations, the analytical model is provided in this paper. The equations of rigid blade loads and, as a consequence of the strength elements, stresses are shown. The parametric blade modeling is conducted using the Graphic Integrated Programming language. The parametric design method is shown to be used for various blade planform models and different section airfoils. The structure of a blade is generated automatically after the user enters the parameters. The code-inbuilt analysis systems provide a quick inertia examination of the generated geometry, which is the basis for further optimization. The program calculates the blade loads and verifies them with the given material conditions and proposed safety factors. In the analysis, composite materials for the strength elements were proposed. Findings The results of this research showed the application of parametrization into the main rotor blade design loop. It was presented that the main rotor blade structure can be enhanced using fluid-structure interaction (FSI) methods. The time saving with the implementation the process into design loop is shown. Practical implications This work can be practically used in the main rotor blade design process. It provides the possibilities to check various blade aerodynamic configuration in a structure strength aspect. Originality/value To the best of the authors’ knowledge, there were no published research that combines the main rotor FSI analysis. The method, which is presented in the work, provides a new approach to a rotorcraft design. The application of the parametrization and combining it with the FSI method gives a novel solution for helicopters construction enhancement.

Rotorcraft design influences noise impacts of multiple vehicles in flight

Rotorcraft design influences noise impacts of multiple vehicles in flight

Physics-Based Airframe Design Approach Expanded from the Conceptual Design of Coaxial Rotorcraft

Physics-Based Airframe Design Approach Expanded from the Conceptual Design of Coaxial Rotorcraft

Progress in Rotorcraft Design: 1974–2024

Fundamental advancements in aircraft design over the past 50 years have enabled a range of vertical takeoff and landing (VTOL) air vehicle configurations and have significantly enhanced aircraft performance, safety, and reliability. This summary paper chronicles the evolution of rotorcraft design from 1974 to 2024. It is segregated into three key pillars of aircraft design preceded by an aircraft design overview. The three pillars are processes and tools, technology, and air vehicle configuration. The first pillar, on design processes and tools, describes advancements in technology, methodologies, and computational capabilities such as the transition from design solely by wind tunnel testing, physical models, and hand calculations to computer-aided design/synthesis and simulation in a model-based engineering digital-twin environment. The second pillar, on technology, focuses on advances in disciplinary technologies and how they have been incorporated into aircraft design. Technologies discussed include composite materials; rotor systems; propulsion systems, ranging from advancements in turbine engines to all- and more-electric propulsion technologies utilizing various energy storage systems to convertible engines; fly-by-wire systems; avionics and cockpit architectures, including digital displays, navigation aids, and communication equipment; and autonomous and semiautonomous systems. The third pillar, on air vehicle configuration, focuses on platform architectures. Major architectures discussed are high-speed VTOLs such as the thrust-vectored aircraft, tiltrotors, lift/thrust compounded helicopters, and all- and more-electric aircraft.

Unmanned Wingless Rotorcraft Scaling

The work addresses the allometric scaling of unmanned rotorcraft. Compound unmanned rotorcraft are not considered in this study. A population of 93 unmanned rotorcraft are compiled into a single dataset comprising size and performance parameters, including the maximum takeoff weight, maximum payload weight, battery mass, battery capacity, rotor diameter, endurance, range, maximum speed, and operational ceiling. Rotorcraft are grouped based on rotor configuration, propulsion type, and perceived application to study the clustering of the data points. An apparent performance limit for unmanned electric rotorcraft is found at 120 min of endurance and in the 70 km range; a higher range and endurance are currently only achievable using thermal propulsion systems. Furthermore, the regression analysis links size and performance parameters in easy-to-use expressions for the preliminary design of unmanned rotorcraft. It is determined that size parameters have a stronger correlation with the maximum takeoff weight than performance parameters. Comparative analysis suggests that unmanned rotorcraft cannot be treated as a subclass of conventional helicopters due to distinct scaling behaviors. A comparison with theoretical models based on first-order physics principles highlights the major role of low-Reynolds-number aerodynamic effects in rotorcraft scaling.

Invited Paper: Rotorcraft Design: The Crucial Influence of Safety from Concept to Fleet Support The 41st Alexander A. Nikolsky Honorary Lecture

It is an immense honor to have been selected to hold the prestigious 41st Nikolsky Lecture and to have the opportunity to synthesize my experiences with regards to the most important principle that permeates aeronautical engineering—"the concept of safety." Having worked in the rotary-wing field for 39 years, with growing levels of involvement and responsibilities, I have been involved in the design, development, and certification of many helicopter models at the Leonardo Helicopters Division (LHD; formerly Agusta and then AgustaWestland), such as A109, A119, EH101, A129, NH90, AW609. More recently, I had the full responsibility of design, development, certification, and entry into service of three new helicopter types within the "AW Family concept", specifically the AW139, AW189, and AW169. I am profoundly grateful for the mentors encountered in my professional life—Bruno Lovera and Santino Pancotti, both of whom were also honored with the Nikolsky Lectureship. In working with them, not a single day passed where the word "safety" was not mentioned. They taught me that "safety" shall be the mantra of every aeronautical engineer because it is our principal duty and responsibility, towards those who travel in, work on, and work with our products and entrust their lives to our work and professionalism daily. I have tried hard never to forget this lesson, and to convey this to the young engineers that I have had the chance and pleasure to work with. If I have been able to pass on this lesson successfully, through my work with others through this lectureship, it would be the greatest achievement of my life. In this vein, this paper is organized in three parts: (i) definitions and principles, along with some "philosophical" concepts; (ii) the application of these principles at Leonardo in the design of the latest generation of helicopters, and finally (iii) a discussion of emerging "safety technologies" that promise to improve the safety of future helicopters and operations.

Helicopter rotor blade vibration reduction with optimizing the structural distribution of composite layers

In helicopter rotorcraft design, two main approaches as passive and active control methods widely used to decline vibration. In this study passive control methods are used to reduce vibration in the helicopter rotor blade. The most common passive control devices are dynamic vibration dampers, isolators and structural distribution of the composite blades. A surrogate optimization formula is used as the objective function of vibration reduction which includes vibratory hub loads and bending moments. In optimization model, composite ply angles are design variables and spar frequency-placement, autorotation and stress conditions are constraints. As the optimization method, a hybrid solution is chosen. The gradient-based algorithms generate accurate results in trust region and heuristic methods scan very large area of solution space. Due to the aforementioned advantages, these algorithms are hybridized. As a result of the comparison of the optimization outcomes with the baseline UH-60 rotor blades, approximately 38 % vibration reduction is observed in the new design.

Review of Modern Helicopter Constructions and an Outline of Rotorcraft Design Parameters

This work contains the results of a modern helicopter construction analysis. It includes the comparison of almost seventy rotorcraft constructions in terms of size in line with EASA requirements – large and small helicopters. The helicopters are also divided because of a mission purpose. The proposed division for large aircrafts is: transport, multipurpose, attack and for small aircrafts: observation, training, and utility. The aircraft construction features are described. Average dimension values of airframes and rotors are shown. Helicopter rotor arrangements are presented in terms of an operational purpose. Next, the rotorcraft design inputs are described. The mathematical formulas for design inputs are given. The ratios are calculated and gathered for the compared aircrafts. Correlation between the analysed parameters is presented on charts. Design inputs are also presented in the paper as a function of MTOW. The function trends are determined to provide an evaluation tool for helicopter designers. In addition, the parameters are presented as possible optimisation variables.

Rotorcraft fuselage mass assessment in early design stages

Like the design of fixed-wing aircraft the design of rotorcraft is generally divided into the three consecutive phases of conceptual, preliminary and detailed design. During each phase the acquired results in turn serve as input for new calculations, thus increasing the detail level and information about the new concept, while uncertainties about the new design are reduced. An important aspect of the overall design process is the mass estimation in early design stages. The weight of the rotorcraft drives the design of many important components, such as the rotor(s), the propulsion system and, therefore, the required fuel. The fuselage is considered as the central structural part, since it connects all other components to each other and serves as protection of the occupants but in the past it often turned out to also be the heaviest part of all rotorcraft components. This paper shows an approach to estimate rotorcraft component masses using statistical methods based on existing rotorcraft but also an approach to use finite element methods that determine the structural airframe mass based on mission profiles, respectively, bearable load cases.

Simulation of step input in collective pitch for hovering rotor

Advanced CFD tools are nowadays used routinely for analysis and design of rotorcraft with the research community shifting towards simulations of rotorcraft during maneuvering flight. One of the impediments of this effort is the lack of data for validation, evaluation and thorough assessment of CFD methods when it comes to rotors with time-varying control inputs. This paper presents a first effort to validate CFD tools for step-inputs in rotor control angles against previously un-published experimental data. The experimental study was carried out at the Nanjing University of Aeronautics and Astronautics in China and consists of time varying control inputs for a hovering rotor. The agreement between the simulation and experimental study is good and quantify the overshoot in loads for this dynamic case. The results examine the lag in loads response to and the dynamic response of the wake with recommendations on spatial and temporal resolution to capture these effects.

Structural Sizing of a Rotorcraft Fuselage Using an Integrated Design Approach

For many years, the primary design objective of new helicopters was the design of the main rotor(s). Within the last couple of years, this approach has changed into an assessment of all helicopter components as an overall system, thus turning rotorcraft design into a highly interdisciplinary process. For instance, aerodynamics, flight mechanics, and the structural evaluation strongly affect each other, and these mutual influences are taken into account from the early phase of the conceptual design. Weight prediction in early design stages represents an essential part of the design process as it determines the basic properties of the rotorcraft. Owing to its function to carry crew and payload but also to serve as the central mounting for all components, the fuselage represents a major part of the rotorcraft. Therefore, the structural design of the fuselage airframe constitutes a significant factor of the rotorcraft design at the preliminary level.<br/> In this paper, an approach to include a higher fidelity method using finite elements for the structural analysis of rotorcraft fuselages within an integrated design environment is presented. Model generation and static analysis are conducted automatically. The helicopter is described using a common parametric data model during the complete design process, therefore providing a fast analysis of model changes. The generic finite element model presented in this paper was generated and structurally sized in about 2.5 min using a standard office computer, thus offering the integration of higher fidelity methods into early design sizing loops.

System identification of flybar-less rotorcraft UAV

Purpose This paper aims to build an accurate mathematical model which is necessary for control design and attitude estimation of a miniature unmanned rotorcraft and its subsequent conversion to an autonomous vehicle. Design/methodology/approach Frequency-domain system identification of a small-size flybar-less remote controlled helicopter is carried out based on the input–output data collected from flight tests of the instrumented vehicle. A complete six degrees of freedom quasi-steady dynamic model is derived for hover and cruise flight conditions. Findings The veracity of the developed model is ascertained by comparing the predicted model responses to the actual responses from flight experiments and from statistical measures. Dynamic stability analysis of the vehicle is carried out using eigenvalues and eigenvectors. The identified model represents the vehicle dynamics very well in the frequency range of interest. Research limitations/implications The model needs to be augmented with additional terms to represent the high-frequency dynamics of the vehicle. Practical implications Control algorithms developed using the first principles model can be easily reconfigured using the identified model, because the model structure is not altered during identification. Originality/value This paper gives a practical solution for model identification and stability analysis of a small-scale flybar-less helicopter. The estimated model can be easily used in developing control algorithms.

Evaluation of High-Fidelity Multidisciplinary Sensitivity-Analysis Framework for Multipoint Rotorcraft Optimization

A multidisciplinary gradient-based sensitivity-analysis methodology is evaluated for optimization of rotorcraft configurations. The tightly coupled discipline models include physics-based fluid dynamics and rotorcraft comprehensive analysis. A discretely consistent adjoint method accounts for sensitivities of the unsteady flow and unstructured, dynamic, overset grids; whereas sensitivities of structural responses to aerodynamic loads are computed using a complex-variable method. The methodology is applied to optimize the shape of UH-60A Blackhawk helicopter blades for hover and forward-flight conditions. The objective of the multipoint design is to simultaneously increase the rotorcraft figure of merit in hover flight and reduce the rotor power in forward flight. Trimmed loose-coupling solutions for the baseline configuration are used to initiate the tight-coupling multidisciplinary analysis. The target thrust, rolling moment, and pitching moment are enforced as optimization constraints. The optimized configuration improves the optimization metrics at both design points. The improved performance and all constraints are maintained over many revolutions beyond the optimization interval, satisfying the required flight conditions. The computational cost of the optimization cycle is assessed in a high-performance computing environment and found affordable for design of rotorcraft in general level-flight conditions.

High-Fidelity AeroAcoustic Optimization Tool for Flexible Rotors

A high-fidelity multidisciplinary analysis and gradient-based optimization tool for rotorcraft aero-acoustics is presented. Tightly coupled discipline models include physics-based computational fluid dynamics, rotorcraft comprehensive analysis, and noise prediction and propagation. A discretely consistent adjoint methodology accounts for sensitivities of unsteady flows and unstructured, dynamically deforming, overset grids. The sensitivities of structural responses to blade aerodynamic loads are computed using a complex-variable approach. Sensitivities of acoustic metrics are computed by chain-rule differentiation. Interfaces are developed for interactions between the discipline models for rotorcraft aeroacoustic analysis and the integrated sensitivity analysis. The multidisciplinary sensitivity analysis is verified through a complex-variable approach. To verify functionality of the multidisciplinary analysis and optimization tool, an optimization problem for a 40% Mach-scaled HART-II rotor-and-fuselage configuration is crafted with the objective of reducing thickness noise subject to aerodynamic and geometric constraints. The optimized configuration achieves a noticeable noise reduction, satisfies all required constraints, and produces thinner blades as expected. Computational cost of the optimization cycle is assessed in a high-performance computing environment and found to be acceptable for design of rotorcraft in general level-flight conditions.

Generation of periodic gusts with a pitching and plunging airfoil

The generation of uniform, periodic gust disturbances in an experimental context is demonstrated using a single oscillating airfoil. A pitching and heaving symmetric airfoil is suggested as a simpler alternative to existing gust-generation methods. The Theodorsen theory of unsteady aerodynamics is used as an analytical tool to dictate the kinematics necessary to produce well-defined sinusoidal gusts downstream of the airfoil. These analytic predictions improve the symmetry of fluctuations in the vertical velocity induced by the airfoil, as well as minimize the influence of vorticity shed by the oscillating airfoil. The apparatus is shown to produce smooth, repeatable gusts with high amplitudes and reduced frequencies compared to other gust-generation mechanisms in the literature. Furthermore, the control of downstream flow properties by airfoil motion kinematics has applications in experimental aerodynamics, the design of rotorcraft and light aerial vehicles, and biological propulsion.

A multidisciplinary process for integrated rotorcraft design

This paper presents a new integrated design process for rotorcraft developed by German Aerospace Center (DLR). The fundamental features of this process are distributed computation on the servers of the different institutes, analysis tools with increasing physical fidelity along the progress of the design process and a high modularity inside the software framework. A reliable data exchange between the tools is provided by an extended version of DLRs Common Parametric Aircraft Configuration Schema. The tools cover the phases of conceptual and largely preliminary design. The design process is initialized by a statistical concept study, providing the first configuration for the following sizing and optimization task. The methods of the tools applied range from blade element theory over vortex panel theory, to finite element methods for structural sizing. The high modularity allows an easy integration of new abilities into the toolbox. The process and design environment presented here are the results of two DLR internal projects carried out by the Institute of Flight Systems, the Institute of Aerodynamics and Flow Technology and the Institute of Structures and Design.

Prediction of Rotorcraft Broadband Trailing-Edge Noise and Parameter Sensitivity Study

This paper investigates the effects of rotorcraft design and operating parameters on trailing-edge noise. A rotor trailing-edge noise prediction method is first developed where the aerodynamics and the turbulence wall pressure spectrum near the trailing edge on airfoils are predicted by a combination of the standard blade element momentum theory , a viscous boundary-layer panel method, and a recently developed empirical wall pressure spectrum model. The coordinate transformations are combined with the Amiet model to predict far-field noise. Compared to experimental data, the validation of this method demonstrates its advantages and validity for airfoil and rotorcraft broadband noise predictions. Then, this method is used to study the effects of rotorcraft design and operating parameters on rotor trailing-edge noise. It is found that helicopter broadband noise scales with the 4.5th to 5.0th power of the tip Mach number in which the range is determined by the typical helicopter collective pitch angle in operation. Detailed trend analyses of noise levels as a function of frequency are presented in terms of the collective pitch angle, twist angle, rotor solidity, rotor radius, disk loading, and number of blades. It is found that the collective pitch angle, twist angle, and chord length make noticeable impacts on low- and midfrequency noise. Finally, a semianalytic model is presented to predict the directivity and geometric attenuation of rotor trailing-edge noise.

Biodynamic Modeling Techniques for Rotorcraft Comfort Evaluation

This work shows how different occupant biodynamic modeling techniques are integrated in a rotorcraft design environment and discusses the resulting differences in comfort assessment. Three modeling techniques, that are used for biodynamic characterization, are considered: lumped parameter, finite element and multibody dynamics. These models are identified for the same gender, age, weight and height, and then integrated into a virtual helicopter environment with a seat–cushion interface. A generic helicopter model is used to demonstrate the approach. For each of the three techniques, the vertical acceleration levels at the human–helicopter interface, as required by vibration regulations, and at the head are evaluated up to 30 Hz. At a first glance, it is observed that the lumped parameter is the easiest to implement in terms of model set-up. However, the use of lumped parameter models is limited to the population groups that they are identified from, and thus are not as flexible as the finite element and multibody ones in developing biodynamic models for individuals of an arbitrary population percentile. Furthermore, through numerical analysis, it is found that the differences are not very significant in terms of accelerations at the human–seat interface. Therefore, for comfort-related issues, the use of more complex models is not justified, unless complicated comfort assessments other than human interface accelerations are required. On the other hand, it is observed that the spine dynamics can play a significant role in estimating the acceleration of head; therefore, the sophisticated finite element and multibody dynamics models redeem their higher modeling cost and computation time when the head–neck health of occupants is considered.

Design and aeromechanics investigation of compound helicopters

This paper reviews lessons learned from the compound helicopter studies performed by NASA and the US Army Aviation Development Directorate to support the NASA Heavy Lift Rotorcraft Systems Investigation and the US Army's Joint Heavy Lift (JHL), Joint Multi-Role Technology Demonstrator (JMR-TD), and Future Vertical Lift (FVL) programs. These studies explored performance potential of advanced rotorcraft and investigated the impact of key modern-technologies in performance, weight, and aerodynamics on rotorcraft. The compound helicopter configurations considered in this paper represent a wide range of sizes, gross weight, rotor systems, and operating conditions. A brief description of design and aeromechanics analysis tools and methodologies is provided. Rotor performance correlation results at high advance ratio, which are critical for the accurate design and analysis of high-speed rotorcraft, are shown. Detailed aeromechanics analysis results, such as the effects of various compounding methods, lift share between rotor and wing, rotor rotational speed, blade twist, aircraft drag, and rotor/wing interference on aircraft performance, are presented.