Rotorcraft Operations Research Articles

An overall evaluation toolchain for assessing helicopter ship deck landings

AbstractA compact toolchain is implemented for the control system design for helicopter ship deck landings in a simulated environment which provides a cost-efficient and safe environment to test different scenarios. The evaluation of the ship deck landing manoeuvres is mostly performed for Ship Helicopter Operation Limits (SHOL) evaluation and performance of the helicopter. In most of the assessments, a subjective assessment is performed using different pilot rating scales which is not sufficient and the objective assessment of the simulation data for the mission success itself is seldom performed. The developed toolchain evaluates the successful operation of ship-based rotorcraft by considering both ship and helicopter dynamics, environmental conditions, and the pilot’s responses. The toolchain evaluates various touchdown conditions like helicopter position, velocity and attitude relative to the ship deck. It also displays additional evaluation parameters for the entire approach. A preliminary pilot study was performed in a simulator within a complete maritime scenario design. The maritime simulation environment includes a nonlinear helicopter flight model with a model-based flight control system, a ship dynamic model with the simulated turbulent air wake and realistic wave and water effects. For a subjective assessment of the ship deck landings, subjective pilot ratings using different rating scales were recorded during the pilot study. Simulation results show that the toolchain is convenient and well suited for assessing helicopter ship deck landing performance in a simulation environment as it provides a complete objective and subjective evaluation.

Helicopter short-term collision avoidance with sampling-based model predictive control

AbstractAutomatic helicopter flight in uncertain surroundings remains a challenging task due to sudden changes in environment, requiring fast response to guarantee safe and collision-free guidance. Increasing numbers of small unmanned aerial vehicles, which are not covered by air traffic control, pose a potential threat to rotorcraft operating in lower airspace. To provide collision avoidance in this scenario, the capability of reacting immediately to appearing obstacles and guiding the rotorcraft along feasible evasive trajectories is required. This paper presents an approach to short-term collision avoidance based on model predictive techniques. The proposed method, originally developed for automotive applications, finds optimal control inputs by predicting a set of trajectories utilizing a model resembling the helicopter dynamics. Compared to model predictive control, no iterative optimization is adopted, resulting in deterministic execution time. The proposed method is evaluated by closed-loop simulations with a non-linear helicopter model. Additional hardware-in-the-loop simulations are conducted to examine the real-time capability of the approach.

Comparison between Performance Requirements for Certification and Performance Requirements for Operation of Transport Category Rotorcraft

In order to achieve aviation safety, integrity of two pillars is essential: product and operations. Regarding product integrity of transport category rotorcraft, its requirements are established in USA by the Federal Aviation Administration (FAA) Part 29, as Categories A and B. Regarding rotorcraft operations, the International Civil Aviation Organization (ICAO) has established Performance Classes 1, 2 and 3 in its Annex 6 Part III, determining they shall occur under a Code of Performance established by the State of the Operator. Should operations neglect certification requirements, particularly certification performance operating limitations, the intended protection (i.e., aviation safety) remains, therefore, unassured. This is possibly the case with offshore operations, pushed by a supposed commercial viability (increased payload) for oil and gas industry. Aiming to clarify it, the authors critically peruse through an exploratory and qualitative research of historical and bibliographic documentation of both rotorcraft certification and operation issued by ICAO and FAA and, in doing so, enhance understanding of the overlaps and complementarity (or otherwise) of certification and operational performance requirements for transport category rotorcraft.

Pilot workload investigation for rotorcraft operation in low-altitude atmospheric turbulence

This paper aims to investigate pilot workload for rotorcraft operations in low-altitude atmospheric turbulence. A novel low-altitude turbulence model derived through the pre-warped Tustin transformation is used to modify the classic distributed turbulence model. The distributed model is then validated against the von Kármán spectra and is integrated into a flight simulation environment that consists of a high-fidelity nonlinear flight dynamics model and a multi-loop compensatory pilot model. The simulation responses and pilot controls in turbulence have been compared against flight test data for operations in bluff-body and freestream turbulence conditions. The effect of terrain roughness on pilot workload is analyzed. The results show that low-altitude turbulence becomes more intense and has smaller length scales over rougher terrain for the same conditions of the reference wind speed and altitude. Moreover, the effect of terrain roughness can increase pilot workload with increasing airspeed and decreasing flight altitude.

A novel method for robust and efficient prediction of ice shedding from rotorcraft blades

The safety of rotorcraft operating in cold environments is jeopardized by the possibility of ice accretion on the rotor blades. Eventually, ice can be shed from the blade due to the high centrifugal forces and can impact other parts of the rotorcraft or unbalance the rotor. To establish the shedding time and location for rotorcraft, a robust and efficient numerical multi-step icing simulations tool is presented here for predicting the shedding phenomenon. A volume-mesh based approach is used to allow for representing the ice shape. In order to increase the robustness of the method, an interpolation procedure is implemented which establishes the possible occurrence of the shedding event and restricts the search domain. Ice shapes along the blades are computed by means of two-dimensional ice accretion simulations: ice shapes are then interpolated over the blade span.Numerical results compares fairly well, in terms of shedding time and location, to the experimental ones obtained in the AERTS test facility, thus demonstrating the soundness of the present approach.

Maritime operation of an unmanned rotorcraft with tethered ship deck landing system

In security-critical situations in the maritime area, ship-based deployment of remotely piloted aircraft systems could enable automated acquisition of aerial images and other sensor data. If intervention is required, response times of federal institutions could be shortened significantly without endangering personnel. This paper gives an overview of research undertaken at DLR in cooperation with the German Federal Police for Maritime Tasks. For a safe ship deck landing of an unmanned rotorcraft a combination of precise navigation and tethered landing system has been developed and patented. Relative double-difference algorithms are used for precise navigation of the rotorcraft to a hover point over the ship deck. From there, an electromagnetic winch is lowered from the rotorcraft that connects to the ship deck. The resulting cable connection is used for aligning the rotorcraft during landing and securing it on the ship deck afterwards. First demonstrations on a real ship deck show the feasibility of the system.

Analysis of Ground Effect for Small-Scale UAVs in Forward Flight

The letter investigates how the behavior of small-scale unmanned aerial vehicles (UAVs) is influenced by the system's close proximity to the ground/rigid surfaces both at hover and in forward flight. We perform an extensive experimental study where a quadrotor is tasked with flying in forward velocities in the range of 0–8 m/s and at altitudes that range between 0.05–0.5 m. Experimental data are used to evaluate four existing ground effect models. Results suggest that existing models for helicopters and in-hover multi-rotors cannot fully describe the forward motion of a quadrotor when it operates close to ground. We introduce two new data-driven models for rotorcraft operating in ground effect both at hover and in forward flight, and evaluate the proposed models with another quadrotor of different size. The proposed models simultaneously consider several operating conditions, which are parameterized by the vehicle's forward velocity and altitude. The models link the thrust produced when operating in ground effect and hovering out of ground effect as forward velocities vary. This information can be incorporated into flight controllers for robust and adaptive UAV flights, and can benefit motion planners for safe and energy efficient near-ground trajectory planning.

Integrated methodology for the prediction of helicopter rotor noise at mission level

This paper presents an integrated approach for the aeroacoustic assessment of four-dimensional rotorcraft operations. A comprehensive rotorcraft code is utilized to model aircraft flight dynamics across complete missions. A free-wake aero-elastic rotor model is employed to predict high-resolution unsteady airloads, including blade-vortex interactions, at each mission element. A rotor aeroacoustics code is developed to calculate source noise and far-field ground acoustic impact. Time-domain acoustic formulations are used to evaluate near-field noise generation across designated acoustic spherical surfaces surrounding the helicopter main rotor. A numerical procedure is developed for the derivation of acoustic spheres on-the-fly, coupled with trajectory-adaptive ground observer grids. The individual analytical models are incorporated into a mission analysis numerical procedure. The applicability of the integrated method on “real-world” rotorcraft operations is demonstrated for two generic, four-dimensional missions, without the need of pre-stored noise data. The proposed approach provides insight into helicopter noise prediction at mission level, elaborating on the coupling of aeroelastic rotor response with rotorcraft flight dynamics and aeroacoustics.

Flowfield measurements of reverse flow on a high advance ratio rotor

One challenge that plagues the high-speed operation of rotorcraft is the inherent increase in size of the reverse flow region on the retreating side of the rotor disk. The present work experimentally investigated a portion of the reverse flow region on a 1.7- m-diameter sub-scale slowed rotor at advance ratios up to $$\mu =0.9$$ and three shaft tilt angles $${\alpha _{\text {s}}=-\ 4^\circ ,\,0^\circ ,\,4^\circ }$$ . Two-component time-resolved particle image velocimetry was used to characterize the flow field around a blade element in the reverse flow region, nominally positioned at $$\psi =270^\circ $$ and $${y/R=0.4}$$ . Four dominant flow structures were observed: the reverse flow entrance vortex, the blunt trailing edge wake sheet, the reverse flow dynamic stall vortex, and the tip vortex. Analyses are focused on the reverse flow dynamic stall vortex due to its influence on unsteady airloads, and similarities to that of a leading edge vortex over a rotating or surging wing at high angle of attack. The number of semi-chords traveled by the blade element through the reverse flow region directly affects the strength, trajectory, and estimated vortex-induced pitching moment of the reverse flow dynamic stall vortex. Shaft tilt angle also has a strong effect on the evolution of the vortex, with forward shaft tilt resulting in dramatically increased strength and size. The results of this characterization and sensitivity study serve as a reference for evaluation of numerical simulations and aid in the development of low-order aerodynamic models of the reverse flow region and resulting force histories. Collectively, these tools will be used to better predict airloads, wake interactions, and vibrations of rotors operating at high advance ratios.

Coupling of an Unsteady Aerodynamics Model with a Computational Fluid Dynamics Solver

Momentum source methods are an efficient means of representing airfoils in Navier–Stokes computational fluid dynamics (CFD) simulations. Momentum source terms are added to the Navier–Stokes equations instead of resolving the solid boundary of the airfoil with a mesh. These source terms are calculated using an aerodynamics model. This approach is useful where the overall performance and mid- to far-field influence of a wing or rotor are desired and details of the flowfield near the blade are not the objective of the simulation. One example is simulation of rotorcraft operations where the objective may be to assess an operation’s feasibility in terms of control margins, rather than to inform rotor design decisions. Coupling an aerodynamics model to a CFD solver is straightforward in cases where the airflow relative to the blade is steady. Unsteady conditions require an unsteady aerodynamics model, complicating the coupling with the CFD solver. A coupling method is proposed whereby the incident velocity is extracted from the CFD solution and corrected using a theory-based approximation for the unsteady induced velocity. The steady-state momentum source method is demonstrated for two- and three-dimensional simulations, and the unsteady coupling method is validated against experiments on a pitching airfoil and verified for blade–vortex interactions. The unsteady coupling method enables meaningful incident velocities to be extracted from unsteady flowfields, as shown by agreement with experiments and simulations using analytical expressions for the incident velocity in place of the CFD solver.

Various Types of The Rotorcraft Design in the Context of Aerospace Regulations

Abstract In the next article on synthesis of regulations, the authors presented the general legal requirements for rotorcraft, which is limited to US regulations, because the vast majority of world regulations are based on them. The Institute of Aviation in Warsaw has developed requirements concerning the construction and operation of rotorcraft in Poland. These requirements constitute an important contribution to the regulations of the Civil Aviation Authority which are in force in the air space of the Republic of Poland. This paper presents the legal situation of rotorcraft and compares the requirements for helicopters and gyroplane. The conclusions highlighted the differences that arise from regulations between helicopters and rotorcraft. The authors have suggested the necessity to separate provisions for the group of windmills, which is implemented in the regulations of the Civil Aviation Authority. The presented work is the second of a planned series of publications in which authors intend to bring some of the issues to the reader about the design aspects of aircraft in selected global aviation regulations.

Analysis of Rotor Start-Up and Shutdown on a Sea-Based Oil Rig

A combined rotor transient responses and analysis tools/computational fluid dynamics effort was completed to assess the rotor response in a complex flowfield. A generic helicopter on an oil-rig platform was chosen because the bluff nature of the rig presents unique challenges to rotorcraft operations. The rotor start-up and shutdown simulation model was validated with the test data. Comprehensive studies of rotor transient responses under various wind directions and wind speeds were conducted to establish the safe-operation envelope. The study found that horizontal wind velocities may be accelerated by the oil-rig building corner, and cause significant rotor flap motions. The upwash and downwash flow due to the blockage of the helicopter fuselage to the crosswind had great impact on rotor transient responses, and the peak blade flap responses can be 2.5 times higher than the simulation results when the effect of airframe on air wake was ignored. The influence of unsteady air wake on rotor transient responses was also investigated, and the results showed that it could increase the blade tip deflections by 70% if the unsteady-flow components were included. The sensitivity of the rotor-blade initial azimuth positions for rotor start-up operations was also examined.

Quieting a rib-framed honeycomb core sandwich panel for a rotorcraft roof.

A rotorcraft roof composite sandwich panel has been redesigned to optimize sound power transmission loss (TL) and minimize structure-borne sound for frequencies between 1 and 4 kHz where gear meshing noise from the transmission has the most impact on speech intelligibility. The roof section, framed by a grid of ribs, was originally constructed of a single honeycomb core/composite facesheet sandwich panel. The original panel has acoustic coincidence frequencies near 600 Hz, leading to poor TL across the frequency range of 1 to 4 kHz. To quiet the panel, the cross section was split into two thinner sandwich subpanels separated by an air gap. The air gap was sized to target the fundamental mass-spring-mass resonance of the panel system to less than 500 Hz, well below the frequency range of interest. The panels were designed to withstand structural loading from normal rotorcraft operation, as well as 'man-on-the-roof' static loads experienced during maintenance operations. Thin layers of viscoelastomer were included in the facesheet ply layups, increasing panel damping loss factors from about 0.01 to 0.05. Transmission loss measurements show the optimized panel provides 6-11 dB of acoustic transmission loss improvement, and 6-15 dB of structure-borne sound reduction at critical rotorcraft transmission tonal frequencies. Analytic panel TL theory simulates the measured performance within 3 dB over most frequencies. Detailed finite element (FE)/boundary element (BE) modeling simulates TL slightly more accurately, within 2 dB for frequencies up to 4 kHz, and also simulates structure-borne sound well, generally within 3 dB.

Improvement of Rotorcraft Safety Metrics Using Performance Models and Data Integration

Historically, rotorcraft have featured higher accident and incident rates compared to fixed-wing air carrier and general aviation aircraft. Safety improvements are currently sought through the adoption of flight data monitoring, a voluntary and proactive safety program where retrospective analysis of flight data records identifies precursors of safety-critical events and tracks improvements pursuant of corrective measures. This paper provides a review of flight data monitoring and elaborates on the unique and significant challenges of its implementation for rotorcraft operations. Two enhancements to current practice are presented in the form of physics-based model integration for improved safety event definition and detection and integration with external data sources to evaluate meaningful safety metrics supplemental to a priori operational event detection. For the former, it is demonstrated that a model-based approach to helicopter autorotation significantly improves characterization of operational safety boundaries and misidentification of safety events. For the latter, it is shown that the integration of obstacles data with flight data records is key to evaluate collision risk, a critical safety issue in low-altitude rotorcraft operations, latent in a given rotorcraft flight. A qualitative evaluation was conducted by eliciting input from subject-matter experts who agreed that the proposed metrics are effective at conveying information about the associated risk.

The effects of military training vibration on rock art

This presentation will provide an overview of potential impacts of vibration due to military training on Native American rock art. Military training is often conducted in areas near culturally sensitive landmarks or structures, and information regarding the effects of vibration on these types of structures is limited. An extensive literature review was conducted to determine the current state of information regarding vibration effects on rock art and other cultural resources. The condition of rock art can vary from the extremely stable to fragile; and based on the existing studies, thresholds for effects were defined in term of stability class for different rock art structures. Vibrations and peak sound levels from maneuver and live fire training activities were estimated and compared to these effects thresholds. Both airborne and groundborne vibrations decreased with distance and at some point attenuate below the threshold of adverse effects. Critical distances for effects were estimated for a variety of training activities—including demolition activities, ground maneuvers, mortar fire, and rotorcraft operations. Results will be provided in both peak particle velocity and peak sound levels. These results as well as potential best management practices will be discussed.

A vision based ensemble approach to velocity estimation for miniature rotorcraft

Successful operation of a miniature rotorcraft relies on capabilities including automated guidance, trajectory following, and teleoperation; all of which require accurate estimates of the vehicle's body velocities and Euler angles. For larger rotorcraft that operate outdoors, the traditional approach is to combine a highly accurate IMU with GPS measurements. However, for small scale rotorcraft that operate indoors, lower quality MEMS IMUs are used because of limited payload. In indoor applications GPS is usually not available, and state estimates based on IMU measurements drift over time. In this paper, we propose a novel framework for state estimation that combines a dynamic flight model, IMU measurements, and 3D velocity estimates computed from an onboard monocular camera using computer vision. Our work differs from existing approaches in that, rather than using a single vision algorithm to update the vehicle's state, we capitalize on the strengths of multiple vision algorithms by integrating them into a meta-algorithm for 3D motion estimation. Experiments are conducted on two real helicopter platforms in a laboratory environment for different motion types to demonstrate and evaluate the effectiveness of our approach.

NOTECHS이 안전운항(安全運航)에 미치는 영향(影響)

Rotorcraft operating in the domestic aviation safety techniques are applied CRM training is conducted but, aircraft accidents caused by human factors has not shown a declining trend. Thus, knowledge of aviation safety and human factors for the spread of awareness of improved rotorcraft flight operations department managers to understand the complexity of nature and culture, and to perform high-risk mission helicopter pilot study of local activation and enhance safety awareness research was conducted in order to. In this study, the development direction of CRM training studies in order to identify the leading NOTECHS (Non-Technical Skills; non-technical pilot skills) of the four categories as the independent variable and the dependent variable corresponding to the resulting effect on the key variables awareness of the differences were studied. In addition, the direction and strength of the relationship were analyzed to determine the relationship of each independent variable to assess the impact on the dependent variable regression analysis was performed. Pilot training and evaluation of non-technical skills related to the teaching reflected in the CRM training and assessment must be carried out with 5 star rating scale was preferred. Therefore, to meet our country rotorcraft operating environment 'NOTECHS' aviation safety by developing training programs reflected in the educational process, implementation, and periodic training and assessment is done in future research on this analysis and feedback is done to reflect the specific performance expect.

Rotorcraft Engine Cycle Optimization at Mission Level

This work investigates the potential to reduce fuel consumption associated with civil rotorcraft operations at mission level, through optimization of the engine design point cycle parameters. An integrated simulation framework, comprising models applicable to rotorcraft flight dynamics, rotor blade aeroelasticity, and gas turbine performance, has been deployed. A comprehensive and computationally efficient optimization strategy, utilizing a novel particle-swarm method, has been structured. The developed methodology has been applied on a twin-engine light and a twin-engine medium rotorcraft configuration. The potential reduction in fuel consumption has been evaluated in the context of designated missions, representative of modern rotorcraft operations. Optimal engine design point cycle parameters, in terms of total mission fuel consumption, have been obtained. Pareto front models have been structured, describing the optimum interrelationship between maximum shaft power and mission fuel consumption. The acquired results suggest that, with respect to technological limitations, mission fuel economy can be improved with the deployment of design specifications leading to increased thermal efficiency, while simultaneously catering for sufficient performance to satisfy airworthiness certification requirements. The developed methodology enables the identification of optimum engine design specifications using a single design criterion; the respective trade-off between fuel economy and payload–range capacity, through maximum contingency shaft power, that the designer is prepared to accept.

A practical biodynamic feedthrough model for helicopters

Biodynamic feedthrough (BDFT) occurs when vehicle accelerations feed through the pilot’s body and cause involuntary motions of limbs, resulting in involuntary control inputs. BDFT can severely reduce ride comfort, control accuracy and, above all, safety during the operation of rotorcraft. Furthermore, BDFT can cause and sustain rotorcraft-pilot couplings. Despite many different studies conducted in past decades—both within and outside of the rotorcraft community—BDFT is still a poorly understood phenomenon. The complexities involved in BDFT have kept researchers and manufacturers in the rotorcraft domain from developing robust ways of dealing with its effects. A practical BDFT pilot model, describing the amount of involuntary control inputs as a function of accelerations, could pave the way to account for adverse BDFT effects. In the current paper, such a model is proposed. Its structure is based on the model proposed by Mayo (15th European Rotorcraft Forum, Amsterdam, pp. 81-001–81-012 1989), and its accuracy and usability are improved by incorporating insights from recently obtained experimental data. An evaluation of the model performance shows that the model describes the measured data well and that it provides a considerable improvement to the original Mayo model. Furthermore, the results indicate that the neuromuscular dynamics have an important influence on the BDFT model parameters.

Multidisciplinary optimisation framework for minimum rotorcraft fuel and air pollutants at mission level

AbstractHelicopters play a unique role in modern aviation providing a varied range of benefits to society and satisfying the need for fast mobility. However, environmental concerns associated with the operation of rotorcraft have increased due to envisaged growth of helicopter operations. New rotorcraft designs, innovative aero engines and all-electrical systems, which may take decades to be in service, are being developed in order to diminish rotorcraft footprint on environment. However, since there is a large number of polluting rotorcraft that are in use and will only gradually be replaced, in the near-term, improvements to minimise air quality degradation may also be possible from better use of existing rotorcraft by focusing on mission profile management. A multidisciplinary framework, intended to generate outputs for estimating rotorcraft block fuel burn and emissions, was developed. Outcomes generated with this tool were, subsequently, the basis to carry out a parametric study for assessment of light single-engine rotorcraft environmental impact, in terms of fuel burn and emissions. Single and multi-objective optimisation for minimum fuel consumption and air pollutant emissions was part of this research as well.