Browse Topic: Rotary-wing aircraft
Regarding the external sling load system of heavy-lift helicopters, the influence of the law of lifting point position on flight control stability characteristics has not been distinctly explained. To address this challenge, this paper constructs a sling load flight simulation model based on multi-body dynamics. Overall, the proposed model consists of four parts, including the rotor aeroelastic coupling model, the fuselage rigid body dynamics model, the flexible sling model, and the slung object rigid body model. Furthermore, through the hub six-degree-of-freedom rigid model and the flexible sling model, this paper realizes the dynamic coupling between the components. On this basis, taking the CH-53E heavy-lift helicopter as the research object, this paper utilizes real flight test data to validate the multi-body dynamic model. Subsequently, this paper systematically analyzes the influence of different lifting points’ lateral position, sling load mode, load-mass ratio, and forward flying speed on helicopter control stability characteristics. Simulation results indicate that the lifting point location exerts a significant impact on the helicopter’s trim attitude angles and dynamic stability. Of them, the lifting point location of the front center of gravity is the optimal in terms of trim characteristics and eigenvalue distribution. Furthermore, within a certain flight speed range, the lifting point of the front center of gravity demonstrates superior speed adaptability and system robustness. Apart from providing a solid theoretical basis for the lifting point layout design of the external sling load system of heavy-lift helicopters, the research results have important engineering application value for improving the safety of sling load flight of heavy-lift helicopters.
This SAE Aerospace Recommended Practice (ARP) recommends a methodology to be used for the design, analysis and test evaluation of modern helicopter gas turbine propulsion system stability and transient response characteristics. This methodology utilizes the computational power of modern digital computers to more thoroughly analyze, simulate and bench-test the helicopter engine/rotor system speed control loop over the flight envelope. This up-front work results in significantly less effort expended during flight test and delivers a more effective system into service. The methodology presented herein is recommended for modern digital electronic propulsion control systems and also for traditional analog and hydromechanical systems.
This SAE Aerospace Information Report (AIR) outlines a recommended procedure for evaluation of the vibration environment to which the gas turbine engine powerplant is subjected in the helicopter installation. This analysis of engine vibration is normally demonstrated on a one-time basis upon initial certification, or after a major modification, of an engine/helicopter configuration. This AIR deals with linear vibration as measured on the basic case structure of the engine and not, for example, torsional vibration in drive shafting or vibration of a component within the engine such as a compressor or turbine airfoil. In summary, this AIR discusses the engine manufacturer’s "Installation Test Code" aspects of engine vibration and proposes an appropriate measurement method.
The U.S. ARMY Primary Helicopter Center/School, USAPHC/S, was activated at Fort Wolters on September 26, 1956. Located in north-central Texas, the school would train over 40,000 helicopter pilots during 17 years of operation, through the end of the Vietnam War in 1973. Approximately 95 percent of all helicopter pilots who flew in Vietnam would pass through Wolters. Students included active-duty Army Officers, Warrant Officer Candidates, and Officers representing 33 allied countries. They trained for 16 weeks at Wolters and then another 16 weeks of advanced training at Fort Rucker, Alabama before earning army aviator wings. At the peak of activity in 1968, Wolters was sending 608 pilots per month to Fort Rucker. Students flew a total of 1,285 piston-powered OH-13, OH-23D, and TH-55A training helicopters departing out of three different heliports. It is a mystical place that still lives in the history of Army Aviation through the helicopter pilots who trained there. This is their story.
Rotorcraft airfoils often feature a tab which aides in the manufacturing of composite rotor blades, but also has aerodynamic merits. This study performs a comprehensive analysis of the impact of this tab on the 2D airfoil performance, structural adjustments and 3D rotor performance. The aerodynamics are evaluated using CFD, with CFD/CSD coupled results for the rotor performance. The structural data is adjusted using an FEM based in-house process. The HART II model rotor has been taken as a baseline and modified according to the tab variation studies. These included the comparison of a sharp trailing edge versus a tabbed airfoil, various tab thicknesses, lengths, and angles. The studies showed a variation of peak Figure of Merit between 66% to 68% and peak rotor L/D from 4.2 to 4.6 The careful design of the airfoil tab is therefore advised, but similarly the structural design of rotor blades.
This paper extends a previously developed adaptive pilot model framework for inner-loop roll-attitude tracking [1] to outer-loop position tracking tasks. Pilot Model identification is performed for two command signal types - a discrete step-like signal and a continuous Sum-of-Sines (SOS) signal - yielding distinct parameter signatures that reflect the different anticipatory and tracking demands of each signal type. An adaptive pilot model for the outer-loop position tracking task is formulated using a model-reference neural network (MRNN) architecture with a linearly parameterized neural network updated by a Lyapunov-stable adaptive law. Simulation results for both discrete and continuous tasks demonstrate that the adaptive pilot model remains stable and maintains position tracking performance under both a doubling and a halving of the nominal control sensitivity. Preliminary results are also presented for a multi-axis maritime task, extending the framework to simultaneous lateral and vertical position tracking.
The FAA VR-HeliSTART (Virtual Reality-Helicopter Simulator Training for Airplane to Rotorcraft Transition) is a 15-week study conducted at Marshall University (WV) to determine the effectiveness of an H125 VR reduced-motion platform simulator in training fixed-wing pilots to fly helicopters. Eleven students received three four-week blocks of instruction in the flight simulator, each followed by a simulator evaluation and a helicopter evaluation. This paper presents results for eleven hovering maneuvers trained and evaluated in the study. The evaluation of the students relied on both an objective and a subjective evaluation: a flight parameter analysis against Airman Certification Standards criteria, and an assessment by certified flight instructors. A key finding is that simulator training enabled all pilots to perform most hover maneuvers on their first helicopter flight without intervention, although sometimes below standards. Overall, results also suggest that while the simulator provides a useful learning environment for basic hover control, the further refinement of the core hovering skills acquired in the simulator did not appear to transfer effectively to the actual helicopter within the time frame of the study. Therefore, this indicates that initial hover training in the simulator is beneficial, but additional improvements still seem to require practice in the actual helicopter.
A common-open data exchange standard for rotorcraft health and usage monitoring systems (CODEX-HUMS), SAE Aerospace Standard AS7140, was issued in September 2025. This standard provides a definition for the CODEX-HUMS open data format produced or used by an on-board or off-board system. The centerpiece of the standard is the data model. This paper describes how the two main data types, stream and batch data, are defined and modeled distinctly by AS7140. The batch data model, targeted at high-frequency, short-duration recorded data, features and delineates a "source", an "indicator", and a "status" element. The streaming data model, intended for lower-frequency, longer-duration HUMS data, covers events and parametric data. The data model is structured by defined data collections to describe the data collected or supporting metadata specifying details about the system or underlying data. In particular, there are definition-type entities and recorded data-type entities. This data model is designed to be flexible and efficient in order to accommodate existing HUMS as well as future HUMS development that support legacy and new rotorcraft platforms.
This paper presents enhancements to the supervisory controller developed for the National Research Council Canada's Bell 412 autonomous helicopter. Building on a Discrete Event System Specification (DEVS)-based framework, the updated Supervisor introduces two new operational modes-Knobs Mode and Sticks Mode-and a structured approach for managing transitions between them and the existing modes. Drawing inspiration from NASA's Flight Guidance System philosophy, the proposed design emphasizes consistency, scalability, and flexibility in handling multiple autonomy modes. Implementation results demonstrate the effectiveness of the updated architecture in supporting future expansion of autonomous mission operations in complex and dynamic environments.
The Enhanced Tiltrotor blade, also known as the RGF3 blade, represents a major milestone in Leonardo Helicopters Division's pursuit of advanced rotorcraft technology. Developed at the Yeovil facility in the United Kingdom as part of a dedicated program and in collaboration with the European Clean Sky 2 initiative, it is a key enabler for the Next Generation Civil Tiltrotor Technology Demonstrator. Leveraging the AW609 airframe, the NGCTR integrates a new lateral rotor control system and a V-tail with ruddervators to expand maneuverability and control authority. The RGF3 blade combines aerodynamic efficiency with manufacturability, cost effectiveness, and certification readiness. Innovations include advanced airfoil families, highly swept anhedral tips, dual-redundant anti-ice systems, and full compatibility with legacy components. A comprehensive test campaign—covering structural loads, lightning and bird strikes, icing, and wind tunnel validation—confirmed its robustness and performance. The RGF3 blade embodies Leonardo's vision for high-speed, sustainable, and reliable next-generation rotorcraft.
Rolling-element bearings in rotorcraft dynamic systems are critical components susceptible to rolling contact fatigue (RCF), a dominant degradation mechanism manifesting through subsurface-initiated spalling, surface micropitting, and fatigue fractures. Robust inspection strategies compliant with EASA and FAA requirements are therefore essential. Traditional methods are often invasive, requiring disassembly, and are susceptible to human-factor errors. Smart Duplex introduces a design-for-monitoring architecture integrating in-situ videoscopic and coherence scanning interferometry (CSI) for high-resolution 3D surface mapping, including under partial grease coverage. This paper details a repeatability and reproducibility (R&R) framework ensuring metric consistency; a maintainability assessment projecting significant man-hour reductions and high availability; certification rationale emphasizing airworthiness improvements via enhanced detectability, workload reduction, and digitized inspection records; and an airworthiness mapping supporting threat assessments, Airworthiness Limitations Section (ALS) entries, and usage-based maintenance credits. By embedding sensing capability and digitizing inspection records, Smart Duplex minimizes downtime, mitigates human-factor errors, and facilitates predictive maintenance, optimizing cost, enhancing performance, and ultimately improving safety.
This paper develops an engineering concept and research framework showing how Cherokee MC2 (Mobile Command Center) and MVP (Mobile Vertipad Platform) can individually, and then as a combined system, resolve key operational and infrastructure challenges facing rotorcraft, eVTOL, VTOL, and UAS missions across civil, commercial, and military contexts. The investigation synthesizes current vertiport / vertipad design guidance, UAM and UTM operational architectures, and recent research on rotor downwash and degraded visual environment hazards to derive a deployable "vertiport node" architecture for austere and time-critical operations. MC2 is treated as the digital and procedural core enabling command, control, communications, data fusion, and manned–unmanned teaming, while MVP is treated as the physical landing interface enabling rapid, load-bearing, illuminated vertical-lift operations without fixed infrastructure. The primary contribution is a traceable topic-to-capability mapping supported by standards and research, plus a modeling, simulation, and optimization workflow to validate safety zones, capacity, scheduling, and resilience. Conclusions identify practical deployment pathways and research gaps for certification-aligned operations.
Future military operations are expected to take place in highly dynamic, contested and multi-domain environment, where speed, flexibility and survivability are essential. Fast rotorcraft are emerging as critical asset to meet future operational requirements, offering a hybrid solution that bridges the gap between conventional helicopters and fixed-wing aircraft. Given the increasing complexity of both operational requirements and system architectures, a Model-Based System Engineering (MBSE) approach has become fundamental to support concept design. However, MBSE is often neglected in the earliest phases of aircraft concept definition and proposal process, when business and mission requirements are agreed between Contractor and Supplier, due to the fast pacing of Parties interactions with respect to the time and effort required to perform modelling activities. This prevents nurturing the benefits of MBSE in this crucial phase, and it generates omissions in the systems engineering data to perform design validation in future phases. This paper describes a tailored methodology to make MBSE feasible to support project requirements agreement and initial aircraft sizing.
This paper evaluates the feasibility of Restricted Icing operations for light to medium helicopters, which typically lack Full Ice Protection Systems (FIPS). Current regulations normally prohibit these aircraft from flying in known icing conditions, leading to frequent mission cancellations for HEMS and SAR operators. To address this, Airbus conducted flight test campaigns in Norway (2023, 2025) to characterize a safe icing envelope for "cold blade" operations. Results demonstrate that the H145 was able to sustain continuous flight in icing conditions between 0°C and -3°C and perform time-limited operations (5–10 minutes) down to -6°C without compromising safety, handling, or structural integrity. Safe Restricted Icing operations require an operational framework that ensures proper planning, safe routing, briefing, in-flight decision making, and specialized crew training. The study concludes that a Restricted Icing Clearance could significantly enhance winter flight safety. By providing an IFR alternative to VFR flights in marginal weather within a clear operational framework, the introduction of a Restricted Icing Clearance could ensure the availability of critical life-saving missions in typical winter weather.
This study presents the development and evaluation of two multi-fidelity surrogate models for predicting the first blade-passage frequency acoustic directivity of a propeller-wing configuration across a parametric sweep of wing leading-edge positions. The configuration follows the experimental setup at NASA Langley Research Center, comprising a three-bladed Mejzlik propeller operating upstream of a NACA 632-215 MOD B wing at 40 discrete leading-edge positions spanning the horizontal and vertical parameter space. Medium-fidelity predictions from the Rotorcraft Comprehensive Analysis System (RCAS), using the Viscous Vortex Particle Method, serve as the low-fidelity input, while high-fidelity predictions from NASA's OVERFLOW solver coupled with PSU-WOPWOP, both serve as the training datasets. QR decomposition is employed in both frameworks to identify the most informative subset of wing positions for high-fidelity simulation. The first surrogate model, a linear regression model, requires 15 high-fidelity snapshots to outperform the RCAS predictions, achieving l∞ norm, l2 norm, and RMSEs below the RCAS baselines of 32.98%, 8.56%, and 5.67 dB, respectively, over the entire parameter space. The second surrogate model, a Co-Kriging surrogate model implemented via the Surrogate Modeling Toolbox 2.0, requires only seven high-fidelity snapshots and achieves an RMSE of 4.45 dB and an R² value of 0.67. Both surrogate models accurately reproduce the high-fidelity directivity at wing positions where the CFD and RCAS trends are in reasonable agreement, but struggle at positions where the two solvers predict substantially different trends, highlighting the sensitivity of multi-fidelity methods to solver discrepancies and the potential need for denser parameter space sampling in regions of high acoustic variability.
This paper examines the documented evolution of Kaman Aircraft Corporation's early helicopter development, specifically the progression from the K-225 evaluation aircraft to the groundbreaking HTK-1K drone helicopter. Through analysis of primary and secondary sources, this study establishes the technical and operational foundations that enabled the world's first remotely controlled helicopter. Additionally, this paper critically examines a hypothesis suggesting that 1st Lt. Donald M. Thompson may have been involved in preliminary remote-control helicopter experiments prior to the officially recognized HTK-1K program. While initially appearing speculative, this hypothesis gains substantial support from the discovery of a 1944 Army Air Forces memorandum documenting Thompson's position as Chief of Special Weapons Unit at Wright Field, with explicit responsibility for developing radio-controlled aircraft systems. This primary source evidence establishes Thompson as a documented historical figure with relevant expertise, though direct evidence of helicopter-specific work remains to be discovered. The paper outlines a methodological framework for continued archival investigation and examines the legacy of these early programs on modern unmanned aerial vehicle development.
The United Kingdom's convertible rotorcraft studies of the 1960s and early 1970s represent a systematic effort to combine vertical take-off and landing capability of helicopters with the speed and efficiency of fixed-wing transport aircraft. Conducted primarily by Westland Helicopters under the Short Range Transport (SRT) programme, these investigations explored both tiltrotor and tiltwing configurations for civil and military applications. Early work focused on the WE-01 tiltrotor, conceived as a research and demonstrator aircraft to investigate transition aerodynamics, control integration, and rotor–wing interactions, and subsequently scaled to the larger WE-02, intended for intercity and tactical transport missions. In parallel, Westland pursued the more ambitious WG.22 tiltwing, a 100-seat intercity VTOL transport incorporating high-incidence stall wing technology, large prop-rotors, and mechanically scheduled flight controls to ensure benign handling through transition. Although none of these aircraft were built, the studies addressed aerodynamics, structures, propulsion, flight controls, noise, and socio-economic viability with exceptional depth. Shaping the future through the past, this paper revisits Westland's early involvement in tiltrotor and tiltwing research, with the aim of assessing the technical maturity of these configurations and evaluating their lasting relevance to contemporary vertical lift aircraft design.
This paper presents the contributions of the Royal Netherlands Aerospace Centre (NLR) to a Leonardo-led consortium participating in NATO Next-Generation Rotorcraft Capability (NGRC) Study #5, which is aimed at the development of Integrated Platform Concepts (IPC) to fulfill the NATO NGRC 2035+ Capability Development needs. NLR contributed to this industry effort by providing an independent evaluation of the high-speed tiltrotor concept proposed by the consortium led by Leonardo, considering aspects such as handling qualities, rotorcraft-ship compatibility, as well as a broader operational Battle Lab assessment. The handling qualities assessment and related control law development considered predictive metrics from ADS-33E and MIL-1797A, and yielded Level 1 handling for the baseline tiltrotor concept along with an evaluation of design sensitivities. The ship compatibility assessment, based on in-airwake trim and simulated lateral approach procedures, offered early insight into tentative limits for tiltrotor shipboard operations on Frigate-class navy vessels. The Battle Lab simulations provided an early holistic evaluation of operational effectiveness of the proposed IPC in the Air Assault mission scenario that is part of the NGRC Concept of Operations (CONOPS).
We present a nonlinear topology optimization framework for designing crash--tolerant rotorcraft substructures by maximizing plastic work under prescribed crush displacement and volume constraints. The quasi-static response is modeled using a rate-independent elastoplastic formulation to capture path-dependent inelastic deformation of metallic components. A path-dependent adjoint method is developed to efficiently compute sensitivities of accumulated plastic work, revealing a mechanistic decomposition into elastic stiffness, deviatoric response, and yield surface contributions. Optimized 2D and 3D subfloor structures develop emergent plastic hinge networks and distributed deformation paths, significantly enhancing energy absorption compared to uniform designs. The results demonstrate that topology optimization can directly embed energy-dissipating mechanisms into primary rotorcraft structures, providing a practical framework for crashworthy rotorcraft and eVTOL airframe designs.
The Vertical Flight Society (VFS) was founded as the American Helicopter Society, Incorporated (AHS) in 1943. During the first decade, several means were used to communicate Society news and industry news to its members. Until 1952, the management of the Society was primarily through elected volunteers, and executives from the leading helicopter companies typically served as the AHS president. Letters to the Society's members were typed and mailed by the companies to the few hundred early members. In 1950, AHS also partnered with the broader aviation professional society, the Institute of Aeronautical Sciences (IAS), to typeset and publish a column on Society news in its membership magazine and which was then mailed to AHS members. It wasn't until 1953 that the first regular publication, the monthly "NEWS LETTER" was sent to members. This newsletter continued to increase in its length and depth of industry news -- it was renamed Vertiflite in 1963. Although some of this was covered in previous histories of VFS, this paper publishes the details of this period for the first time.
A generalized turbulence model for rotorcraft, analogous to the Dryden or von Kármán models commonly used for fixed-wing aircraft, does not yet exist. The closest available formulations are Control-Equivalent Turbulence Input (CETI) models, which reproduce the portion of the vehicle response attributable to atmospheric disturbances through equivalent control inputs applied at the inceptor or effector level. While the underlying concept is broadly applicable, these models are highly configuration dependent, making their broader generalizability uncertain. This paper adopts a recently developed methodology to extract CETI models directly from simulation and extends it to the identification of State-Equivalent Turbulence Input (STETI) models, which are Dryden-like in form and inject turbulence-equivalent excitations directly into the state dynamics. The approach is applied to six conventional main-tail rotor helicopters spanning trainer, light, intermediate, medium, heavy, and ultra-heavy classes, from the Robinson R-22 to the Mil Mi-26, and to four tiltrotors spanning the current range of tiltrotor weight classes, from the Bell XV-15 to the Bell Boeing V-22. The resulting CETI and STETI models are used to examine cross-vehicle trends, assess the extent to which they generalize across configurations, and evaluate whether they can be parameterized in terms of fundamental rotorcraft properties, including gross weight and rotor radius, as well as derived parameters such as disk loading.
This paper presents the flight-test evaluation of a velocity-aided navigation solution that integrates inertial measurements with line-of-sight (LOS) Doppler velocity observations from the Psionic Navigation Doppler Lidar (PNDL) prototype to support navigation in GPS-denied environments. LOS velocity measurements collected during a helicopter flight-test campaign were first compared with velocities derived from an Applanix reference navigation system to assess measurement accuracy. The navigation solution was then developed and evaluated under simulated GPS-denied conditions by removing GPS aiding and continuing operation using LOS velocity measurements alone for extended periods. Results show that Doppler lidar velocity aiding effectively constrains inertial navigation error growth and maintains a stable navigation solution during prolonged GPS outages. These flight-test results demonstrate the utility of FMCW Doppler lidar velocity measurements as an enabling technology for Assured Positioning and Navigation (APN) and underscore its applicability to Contested Logistics operations, where resilient, GPS-independent navigation is essential for mission continuity.
This study evaluates the capability of Simcenter™ Flightstream™, a viscous surface-vorticity compressible-flow panel method, for predicting aerodynamic performance of rotorcraft configurations. Simulations are performed on the ROBIN-mod7 fuselage, PSP rotor, and combined rotor-fuselage system under conditions consistent with available experimental data. Results are compared against experiments and high-fidelity CFD methods, including DES, URANS, and IBM-ASM. For the isolated fuselage, Simcenter Flightstream accurately captures surface pressure distributions, particularly in attached flow regions. For the isolated rotor, thrust, torque, and figure of merit trends show strong agreement with reference data. In the rotor-fuselage configuration, the solver successfully captures interaction effects and predicts performance within experimental uncertainty. Notably, Simcenter Flightstream achieves these results with one to two orders of magnitude lower computational cost compared to high-fidelity approaches. The findings demonstrate its effectiveness as a rapid, cost-efficient tool for early-stage rotorcraft aerodynamic design and analysis.
The Sikorsky S-92® helicopter fleet, representing more than 300 aircraft and 2.6 million flight hours, is relied upon to support a large range of important missions across the globe. In previous efforts, a high-fidelity CFD-CSD based full-aircraft simulation methodology, co-simulated with production FCS, was developed and applied to model both coaxial aircraft and single main/tail rotor configurations (Refs. 1-5). The CFD solver is based on the CREATE™-AV HELIOS toolset (Ref. 6) and the CSD solver is based on Rotorcraft Comprehensive Analysis System (RCAS) (Ref. 7). The current paper further correlated the CoSim methodology (Ref. 1) with the S-92® helicopter flight-test database at both hover, cruise and edge-of-envelope maneuver flight conditions. The consistent correlations for flight dynamics, static and fatigue component loads at conditions across the flight envelope demonstrate the reliable predictive capability of the high-fidelity CoSim methodology to be-used as a virtual digital flight test and to support advanced design at early stage.
This paper presents several methods for measuring large flow fields in the wake of a helicopter, including wind vanes ("flags"), tufts, BOS (Background Oriented Schlieren), and BOS-velocimetry. The motivation is to develop methods that can map the outwash distribution of VTOL aircraft with strongly asymmetric wakes and identify jets that can be particularly dangerous for bystanders. Each measurement technique is shown to have specific advantages, and the resulting flow fields are demonstrated for the BO105 helicopter in hovering flight in ground effect above a tarmac.
The bird strike performance of the flight critical components of a rotorcraft is to be proved. The study investigates the bird strike performance of the cowling structure through experiments and simulations by considering a Building Block Approach. Based on this approach, bird impact tests on a rigid plate and composite panels are performed to validate Smoothed Particle Hydrodynamics method (SPH) bird model and composite material model in LS-DYNA. The composite material properties are obtained from the coupon level test results. After the composite material model is calibrated and validated, the bird strike performance of the cowling structure at critical locations is assessed. A good correlation between the experimental and numerical results was obtained at coupon, sub-component and component levels. The developed composite material modeling technique and validated bird models may be used in showing bird resistances of other airframe components of similar structure of the rotorcraft.
In this work the authors present a method for estimating the modal properties (natural frequencies, damping and mode shapes) of a helicopter fuselage using available flight data. The identification process is based on Operational Modal Analysis (OMA). The modal parameters obtained from OMA are subsequently used in conjunction with Ground Vibration Testing (GVT) results and the Finite Element Model (FEM) of the helicopter structure to extract scaled mode shapes. Since in-flight vibration testing typically employs a limited number of measuring points, significantly less than the number of points represented in the FEM, the authors propose a method of expanding the OMA identified mode shapes onto the FE model geometry, enabling engineers to regulate the influence of local displacement and achieve improved representation of the structural dynamics.
Full state feedback offers theoretically guaranteed multi-axis stability, making it superior to conventional PID controllers. There is however one drawback, a full state controller has a mathematical difficulty if the B matrix is not square and thus not invertible. This is the case for helicopters with 6 degrees of freedom and 4 inceptors. Variations of linear quadratic regulators are a work around, however complexity dramatically increases. Best would be a direct solution to the original problem. This is the breakthrough result of this paper. This paper documents an approach which removes the analysis roadblock by partitioning the 6 x 6 system "A" matrix into two groups of 4 x 4 matrices. The 4x4 matrices are individually stabilized with full state gain matrices. One matrix is designated “Driver Matrix” which provides actuator commands. The other matrix is designated "Reference Matrix" which provides references. The two matrices are coupled together by requiring that the driver matrix follow references generated by the reference matrix. With each matrix individually stabilized, the coupled combination is also stabilized. Computation of flight dynamics states (u, v, w, p, q, r) is shared between the matrices. Initial results are very encouraging, showing an originally sluggish, heavy lift helicopter having now concise decoupled responses to pitch and roll commands. Stability derivatives are recomputed during flight allowing coverage over the whole flight envelope. A handling qualities task has been defined to relocate a 40 ft standard seaborne container directed by a pilot in a ground control station. Cooper Harper ratings of this task have demonstrated favorable Level 1 handling qualities if use is made of an automated lateral repositioning command.
The present study aims to investigate the dynamic behavior of composite drive shafts operating in the supercritical rotational speed regime, with a particular focus on the mode crossing and the potential emergence of vibratory instabilities. Composite shafts offer significant advantages in terms of mass reduction and mechanical properties, making them attractive for high-performance transmission systems such as helicopter drive lines. However, their operation beyond the first critical speed raises specific challenges related to stability, damping, and sensitivity to mechanical and operational parameters. To address these issues, an experimental and analytical framework was developed to explore a wide design space involving parameters that are known or suspected to influence supercritical behavior. These parameters include unbalance levels, support characteristics, flexible coupling properties, tightening conditions of the damper, and rotational speed. Particular attention was given to the analysis of the mode crossing phenomenon in composite supercritical shafts, as well as to the identification of the most influential contributors to the vibratory response within the defined experimental perimeter. Harmonic- and subharmonic-based analyses were conducted to assess the relevance of each parameter across different frequency regimes. Overall, the results indicate that, despite the wide parametric exploration, no dynamic instability was observed in the supercritical regime, thereby demonstrating the robust vibratory behavior of composite shafts and supporting their suitability for advanced helicopter transmission applications.
This study investigates the dynamics and associated vibratory loads of an underactuated swashplate-less rotor and its impact on the flight dynamics of a small-scale helicopter powered by this rotor via a combined experimental and computational approach. Unlike prescribing cyclic pitch using a swashplate, here the pitch is a response to the 1/rev cyclic rotor speed input. This is enabled on the current two-bladed rotor using a skewed lag hinge that utilizes the cyclic speed variation to produce lagging motion and subsequently pitching the blades in a cyclic fashion (ƍ4 coupling) for generating the pitch and roll control moments. One of the key dynamic characteristics that distinguishes this rotor from a conventional swashplate-controlled rotor is that the two blades have dissimilar pitch and flap responses leading to high fixed-frame vibratory loads. Results show that a large 1/rev vertical shear force is transferred to the fuselage resulting in half-peak-to-peak loads of +/−0.67g. The flap responses of the two blades being out of phase caused a net inertial force, which was the dominant contributor to the vibratory vertical shear force. Upon removal of the flap hinge, the vertical shear force was reduced by almost 85% while increasing the control moment authority by 50%. The inertial moment about the rotor axis generated by cyclic lagging nearly balances the moment due to the angular acceleration (Ω ) of the rotor, which significantly reduced the dynamic torque requirement from the motor.
Flight simulations are critical for aerial firefighting training, but realistic modelling of aircraft-atmosphere interactions within fire scenarios is particularly challenging. To this end, a two-way-coupled flight simulation system, the Daedalus I framework, has been developed at the University of Glasgow for helicopter firefighting research applications. This paper presents the initial results from flight experiments conducted with different coupling schemes between the rotorcraft model and the GPU-accelerated Lattice Boltzmann atmosphere model within the system. The two-way coupling scheme was first validated using an isolated, transient rotor case. To quantify differences in pilot control and strategy between the two-way, fully-coupled rotor-atmosphere method and two (2) one-way, superposition-based coupling methods, a series of flight experiments were conducted using the bimodal modification of the McRuer pilot model representing human pilot controls, in conjunction with objective performance metrics. The results highlighted noticeable changes in helicopter behaviours and pilot responses across the three tested coupling methods, with the two-way coupling method showing the most resistance to the generated fire disturbance.
An internal layout design framework for a medium-class rotorcraft fuselage is attempted to build based on the idea of the energy-based load-transfer index. Load-transfer index will quantify the way in which the flight loads are distributed among the fuselage internal structural members. The static load-transfer analysis will identify an inefficient transfer region in the baseline fuselage configuration, and the resulting layout refinement will lead to a more unified load-transfer pattern and allow an additional weight reduction in the subsequent thickness-optimization stage. For a UH-60A aircraft, the existing literature provides well-established information for an airframe layout, finite-element modeling guideline, and ground vibration test correlation.
Traditional safe-life methodologies for rotorcraft structural components rely on deterministic safety factors to account for uncertainty in loads, material properties, and operational usage. While effective for ensuring safety, these approaches lead to early retirement lives and reduced aircraft availability. This paper presents an updated digital twin-based probabilistic framework for rotorcraft component fatigue life assessment that integrates a probabilistic stress–life (S-N) material model, machine learning-based load estimation from flight data, and Monte Carlo uncertainty propagation. The approach is demonstrated for a critical location on the CH-146 Griffon main rotor yoke. Compared with earlier work, the present study advances the framework through independent validation of the load-estimation model and application to available in-service flight data from multiple mission categories. A probabilistic sensitivity analysis is used to examine the separate and combined effects of material variability and load-estimation uncertainty on fatigue life, cumulative probability of failure, and hazard rate. For the CH-146 demonstration case, the results indicate that the material fatigue strength uncertainty has a major impact on the lower tail of the life distribution and the corresponding reliability-based life, whereas load-estimation accuracy uncertainty has a secondary influence on risk metrics. The application of the digital twin framework to operational, search and rescue, and training mission data further shows that mission-specific usage variability plays an important role in the evolution of fatigue damage accumulation and structural risk. Overall, the proposed framework provides a more informative basis for risk-based rotorcraft life assessment by explicitly quantifying uncertainty and incorporating aircraft-specific operational data. The study is intended as a step toward validation of the framework rather than a completed operational deployment.
Ground resonance, a self-excited instability typical of helicopters, that can lead to catastrophic failure. Its analysis becomes particularly complex in non-symmetric rotor configurations, such as those with an inoperative damper, which give rise to Linear Time-Periodic (LTP) systems. The stability investigation of the equations of motion can be handled through the Floquet method of characteristic exponents, or with the more recent linear time-invariant (LTI) harmonic decomposition (HD) method. The two methods are presented and their relationship is explored through Hill's infinite matrix and its modal solutions. Numerical results are given for rotors with different blade counts and two damper arrangements—blade-to-hub (BH) and interblade (IB)—under single and multiple damper failure conditions. A minimum-harmonics rule is derived that links the required number of harmonic terms directly to the multi-blade coordinate (MBC) structure of the rotor. Finally, the LTI state-space system produced by the HD is coupled with a describing-function framework to extend the stability analysis to rotors equipped with nonlinear dampers, enabling efficient limit-cycle prediction without recourse to time integration.
This paper introduces a robust supervised machine learning framework for estimating helicopter gross weight during the takeoff phase. The methodology leverages high-fidelity datasets from Airbus's global in-service fleet to ensure a reliable training foundation. At the core of the approach is a long short-term memory recurrent neural network, supported by a patented data-curation pipeline designed to maintain high data integrity. To align with rigorous aviation safety standards, the study outlines a learning assurance process compliant with EASA guidelines, specifically addressing safety assessment objectives for machine learning. A central innovation is the characterization and monitoring of the model's operational design domain through multidimensional functional principal component analysis. By projecting high-dimensional, non-linear sensor data into a manageable tabular subspace, this approach enables the definition of safety envelopes using explainable and efficient classical methods. Validated against diverse real-world flight profiles, the framework demonstrates high predictive accuracy, marking a significant milestone toward deploying the model on airborne targets for safety-critical functions such as condition-based maintenance.
The FAA VR-HeliSTART (Virtual Reality-Helicopter Simulator Training for Airplane to Rotorcraft Transition) is a 15-week study conducted at Marshall University (WV) to determine the effectiveness of an H125 VR reduced-motion platform simulator in training fixed-wing pilots to fly helicopters. 11 students received three four-week blocks of instruction from certified flight instructors in the flight simulator, each followed by evaluations in both the simulator and an actual H125 helicopter, covering 36 maneuvers drawn from the commercial helicopter Airman Certification Standards. A mixed-methods approach combined objective flight parameter analysis with subjective assessments from evaluators, instructors, and students. Results indicate broadly positive transfer of training, with students demonstrating at least private pilot level performance on 70% or more of maneuvers on their first helicopter flight, and consistent improvement across subsequent evaluations. However, specific areas of negative or limited transfer were identified, most notably Vortex Ring State recovery, approaches, and hover work, driven by limitations of the head-mounted display and motion platform. This paper presents the methodology and overall results, with future papers addressing individual maneuver groups in greater detail.
The current work presents a methodology to estimate the mission and performance capabilities of a generic rotorcraft configuration, to satisfy the need of evaluating the integration of a full electric powertrain in the aircraft design. To include all the design steps, two different approaches are proposed. For the preliminary phase, the "Analytic Method" is considered, which exploits a purely resistive model. Conversely, a method based on look-up tables called "Table Method" is intended to be used in more advanced phase, when the battery pack is defined. Both approaches are tested by evaluating a reference mission and a hover chart. Finally, a verification of the presented methodology is carried out by comparing the mission results with a commercial software, specialized in the evaluation of the cell discharge when a given power spectrum is provided.
This paper investigates the impact of aerodynamic interactions on the whirl flutter boundary of wing-twin-propeller configurations. A coupled wing-pylon-propeller model is developed in the Rotorcraft Comprehensive Analysis System (RCAS), where the wing is modeled using uniform inflow and the propeller wake is modeled using the viscous vortex particle method (VVPM). The study examines the effects of spanwise propeller placement and rotation direction by first analyzing a single-propeller configuration and subsequently extending the analysis to twin-propeller configurations. The analyses are performed for both rigid and flexible wings, with the latter designed such that whirl flutter governs the instability boundary. Results show that spanwise propeller placement strongly influences whirl flutter stability, with outboard locations exhibiting higher flutter speeds. Aerodynamic interactions between the wing and the propeller are found to be generally destabilizing, reducing the whirl flutter speed across all spanwise locations, with the largest reduction observed at outboard positions. In twin-propeller configurations, amplitude modulation is observed due to coupling between closely spaced modes enabled by wing flexibility and aerodynamic interactions. The inboard propeller is found to govern the instability boundary across all spanwise arrangements. Inter-propeller aerodynamic interactions are generally destabilizing, particularly for closely spaced configurations, while contra-rotating propeller configurations exhibit slightly higher flutter speeds due to the reduced impact of interaction effects. These results provide insight into the directional nature of aerodynamic interaction effects on whirl flutter in wing-twin-propeller configurations, highlighting their configuration-dependent behavior and generally modest influence on the whirl flutter boundary.
This paper presents the design, development, and successful demonstration of the first-ever ballistically tube-launched tailsitter unmanned aerial system. The vehicle expands upon the capabilities of existing tube-launched systems by simultaneously integrating the hovering capability of a rotary-wing aircraft with the efficiency and speed of a fixed-wing aircraft. To achieve this, the platform's design incorporates a novel coaxial thrust-vectoring propeller system for control in vertical flight and a unique foldable wing design for ultra-compact storage in the launch tube. The aeromechanics of the foldable wings during deployment are studied through a combination of wind tunnel experiments and flight dynamics model simulations, and the results are used to formulate a methodology for executing the ballistic launch. Simulations are also performed to characterize the robustness of the system against asynchronous deployment of the left and right wings. Experimental data, collected from flying a prototype in vertical, horizontal, and transitioning flight, demonstrate the aircraft’s flying performance. The study ultimately culminates with a demonstration of the prototype being rapidly launched from a tube at 25 m/s (56 mph) and autonomously unfolding, stabilizing, and transitioning into self-powered cruising flight.
This paper explores the potential of three different hybridization solutions for a medium-sized rotorcraft: an electric tail rotor, an "eco-mode", and a "boost-mode". The solutions were evaluated as a retrofit to a generalized medium lift rotorcraft and the impact on performance across five mission types, representative of the typical use cases for a military rotorcraft, was assessed. Two separate rotorcraft performance modelling tools were used to carry out the assessment, allowing for the results to be cross-examined. The models predicted performance gains for the eco-mode configuration when utilizing the single engine cruise capability for low-speed applications. Likewise, the models predicted improved performance for the boost-mode configuration when operating at hot and high (6,000 ft, 95°F) conditions due to the increased power provided by the battery system. However, all three solutions suffered from increased platform empty weight which negatively impacted performance at certain flight states.
Autorotation is an emergency flight maneuver in which a helicopter descends safely without engine power by using rotor energy. This paper investigates the use of reinforcement learning (RL) for autorotation trajectory generation and systematically evaluates it against optimal control problem (OCP) solutions. A one-degree-of-freedom powered descent problem is first solved as a surrogate to identify robust hyperparameter settings. The surrogate case results demonstrate that the RL policy closely matches the OCP solution in terms of landing time, confirming its effectiveness. The autorotation problem is then solved under both frameworks, and the resulting Height-Velocity diagrams are compared, with crash behavior in the deadman zone analyzed for each. The RL framework is shown to produce autorotation trajectories comparable to OCP, establishing it as a viable real-time alternative. Warm-starting the OCP with RL-derived solutions improves convergence compared to conventional initialization. Finally, the RL policy's versatility is discussed with an example of varying initial helicopter weight, capturing different fuel states at engine failure.
An advanced coupling framework was leveraged to assemble analytic sensitivities of lifting line theory aerodynamic loads with respect to externally-defined blade geometry parameters for optimization of main rotor performance of conventional helicopter configurations. Three vehicle weights and two flat-plate-equivalent drag configurations were examined across the flight envelope from hover to an advance ratio of 0.3. Two types of twist controls were investigated: quasi-static and fully active. Power savings were strongly correlated to the forward flight to hover power, ranging between 1.5 and 3.5% for quasi-static geometries and 2.0 and 4.5% for fully active controls when the installed power is twice of that required in hover. Blade twists optimized at higher power ratios were observed to favor high shaft tilt angles. Optimal twist deformation relative to hover-optimized designs is nonlinear across the blade span. Minimal penalties to aerodynamic vibrations were incurred through the use of either quasi-static or fully active twist controls as measured with a vibration intrusion index.
QuesTek is advancing a suite of emerging alloy technologies to address modern rotorcraft engineering challenges. Current initiatives prioritize the optimization of "print-to-use" materials, such as 17-4PH and other specialized steels designed to minimize or eliminate post-processing requirements in additive manufacturing. These innovations represent a strategic shift toward materials that are not only high-performing but are also specifically tailored for next-generation manufacturing workflows. The catalyst for these advancements is QuesTek’s mastery of Integrated Computational Materials Engineering (ICME). These core capabilities are now deployed through QuesTek's ICMD® software platform, which empowers engineering teams with predictive simulation tools that eliminate the bottlenecks of traditional trial-and-error methodologies. By integrating these physics-based models into a centralized digital environment, QuesTek enables the rotorcraft industry to design, test, and implement advanced materials with unprecedented speed, reduced costs, and increased technical confidence.
Accurate monitoring of helicopter operational usage relies heavily on robust regime recognition algorithms. How-ever, evaluating these approaches is challenging when they operate as opaque, "black boxes", as in the case of machine learning-based models. This paper introduces a comprehensive evaluation framework designed to assess regime recog-nition models from a number of perspectives and investigate anomalies in the predicted regimes. Centered around a high-fidelity data set derived from scripted flight tests covering a complete usage spectrum, the developed method-ology provides a comparative baseline. The analytical suite includes 3D spatial visualization tools for flight path mapping, sequential anomaly detection, and confusion matrix metrics. While applying the labeled data set to other platforms presents inherent limitations in terms of mapping features and regimes appropriately, the integrated toolset successfully exposes weaknesses in the model and highlights gaps in training data. Ultimately, this evaluation frame-work enhances the interpretability of model outputs and builds confidence in the use of regime recognition algorithms.
Efforts to increase lift and range capabilities of Mars rotorcraft have determined through comprehensive analysis of chord-scaled rotors that a 6-bladed rotor with a thrust-weighted solidity of 0.3 (high solidity) offers significantly improved thrust and efficiency in a Martian environment. However, while the optimal blade number and thrust-weighted solidity configuration is important, optimization of chord and twist distributions as well as airfoil shape is necessary to fully optimize a rotor. This study utilized the Evolutionary aLgorithm for Iterative Studies of Aeromechanics (ELISA) genetic algorithm to optimize chord and twist distributions in conjunction with Comprehensive Analytical Model of Rotorcraft Aerodynamics and Dynamics (CAMRADII) analysis and optimized airfoil shape in conjunction with OVERFLOW analysis. This work was conducted under the Mars Exploration Program's High Solidity Testing task and supports both scientific and exploration concepts, such as the Chopper platform, as well as future Mars heavy‑lift vehicle concepts that could enable logistics for sustained human presence on Mars.
The aeromechanics of a full-wing lift-compounded slowed-rotor rotorcraft were investigated experimentally at the Glenn L. Martin Wind Tunnel, characterizing the effects of rotor shaft tilt, wing configuration, and advance ratio on performance, blade structural loads, and hub vibratory loads. Measurements were obtained across advance ratios up to μ=0.7, three shaft tilt angles (-4°, 0°, and 4°), and three wing configurations, including an asymmetric wing arrangement. The results were used to validate the University of Maryland Advanced Rotorcraft Code (UMARC) coupled rotor-wing analysis. Rearward shaft tilt and increased wing lift sharing improved lift-to-drag ratio, reduced blade structural loads, and decreased hub vibratory loads due to the rotor being placed in a descent state and being partially unloaded. Rearward shaft tilt alone yielded a 5% improvement in lift-to-drag ratio and a 32% reduction in steady rotor flap bending moment relative to the forward tilt configuration at an advance ratio of 0.5 and 4° symmetric wing incidence. A peak combined rotor and wing lift-to-drag ratio of 9.6 was achieved at an advance ratio of 0.7 at rearward shaft tilt and with an asymmetric wing incidence configuration, demonstrating the potential of lift compounding for efficient high-speed edgewise rotorcraft flight.
This paper presents the implementation of a fully automated Health and Usage Monitoring System (HUMS) data chain designed to accelerate installed engine performance diagnostics during the pre-delivery phase of new-generation helicopters. Ensuring that engine performance remains consistent with original engine manufacturer (OEM) baseline data is a critical step in the final assembly process, yet traditionally time-consuming. The developed system automates data offloading and integrates three distinct streams: OEM engine performance characteristics, in-flight Engine Power Checks (EPC), and high-frequency continuous recordings. The core innovation lies in a multi-source data fusion methodology combined with a physics-based model to differentiate between genuine installation discrepancies and sensor anomalies through temperature deviation analysis. Results from the production environment demonstrate that this automated approach significantly reduces troubleshooting lead times and ensures on-time aircraft delivery. By shifting advanced monitoring from in-service operations to manufacturing, this system establishes a new digital benchmark for quality control in helicopter production.
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