Rotorcraft technology is living in a moment of intensive development. Alongside traditional disciplines—aeroacoustics, aerodynamics, dynamics, flight dynamics, human factors, structures and materials, to name a few, other aspects are emerging. Uncrewed air vehicles and their autonomous operation, automation in support of crewed ones, crew management and coordination, special operations with a particular focus on those on ship decks, and lots, lots of interdisciplinarity surfaced as the topics most addressed in the papers selected for this issue. All selected papers have been extensively edited by the authors and peer-reviewed according to the standards of the CEAS Aeronautical Journal. [...]@en

This paper aims to explore the development of a flight dynamics model for a small-scale side-by-side helicopter and describe its trim and stability characteristics. The helicopter is considered a suitable candidate for Urban Air Mobility (UAM) solutions, because of its reliable design and low noise characteristics, but still very small knowledge is present on the mathematical modeling approaches and dynamic properties. A 14 degrees of freedom nonlinear mathematical model is developed and semi-analytical models are employed to account for the presence of the shrouds. An iterative trim routine is developed and applied with a suitable control mix that allows the use of classic helicopter controls. To control the vertical speed and roll rate, the paper assumes an equal collective pitch and lateral cyclic in the two rotors, while a uniform plus a differential longitudinal cyclic is adopted for pitch and yaw maneuvers. The paper discusses unique characteristics of the side-by-side configuration as obtained from the stability analysis: an unstable high-frequency mode, governed by the vertical velocity and pitch angle arises when the center of gravity (CG) of the vehicle is aligned or placed in front of the two main rotors. Similarly, both lateral phugoid and roll subsidence modes are sensitive to the CG location. The side-by-side configuration presents also a stable spiral mode which needs to be carefully designed.

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In the present paper, a flight dynamics model is adopted to represent the trim and stability characteristics of a side-by-side helicopter in hovering conditions. This paper develops a numerical representation of the rotorcraft behavior and proposes a set of guidelines for trimming and linearizing the highly coupled rotor dynamics derived by the modeling approach. The trim algorithm presents two nested loops to compute a solution of the steady-state conditions averaged around one blade’s revolution. On the other hand, a 38-state-space linear representation of the helicopter and rotor dynamics is obtained to study the effects of flap, lead–lag, and inflow on the overall stability. The results are compared with an analytical framework developed to validate the rotorcraft stability and compare different modeling approaches. The analysis showed that non-uniform inflow modeling led to a coupled longitudinal inflow–phugoid mode which made the vehicle prone to dangerous instabilities. The flap and lead–lag dynamics introduced damping in the system and can be considered beneficial for rotor dynamics@en

This paper addresses the flight dynamics modelling, trim, and dynamic analysis of an intermeshing-rotor helicopter, indicated as synchropter. This configuration has gained a great interest for its suitability within heavy load lifting and transportation in extreme high temperature and altitude, and other harsh environments. The paper presents some relevant features related to synchropter's flight dynamics modelling of the interference between its two tilted main rotors. Trim results show the advantage of the synchropter in forward flight where the yawing moment is naturally balanced at almost all speeds and no lateral-directional compensation is needed. The synchropter's dynamic stability shows similarity to a conventional helicopter in the longitudinal phugoid. However, in the lateral phugoid, the synchropter is unstable at all flying speeds and therefore its vertical fin needs to be carefully designed.

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This paper analyzes the effects of the helicopter dynamics on pilots' learning process and transfer of learned skills during autorotation training. A quasi-transfer-of-training experiment was performed with 10 experienced helicopter pilots in the SIMONA moving-base flight simulator at Delft University of Technology. Pilots had to control an in-house flight dynamics model setup to simulate two types of helicopter dynamics: (1) a "hard"dynamics characterized by a low autorotative flare index requiring high pilot control compensation and (2) a "easy"dynamics characterized by a high autorotative flare index with low pilot control compensation required. Two groups of pilots tested these types of dynamics in a different training sequence: hard-easy-hard (HEH group) and easy-hard-easy (EHE group). The main conclusion of this study proved that simulator training for autorotation can best start with pilots training in the most resource demanding condition. A more challenging helicopter's dynamics will require higher pilot agility and more rapid responses to his/her perceptual changes. This will result in pilots developing more robust and adaptable flying skills. Indeed, a clear positive transfer of training effect was observed in the experiment presented in this paper in terms of acquired pilot skills in the HEH group, but not the EHE group. Positive transfer was especially observed in terms of reduced rate of descent at touchdown. The two groups differed in the control strategy applied, with the HEH group having developed a control technique mimicking more closely the one adopted in a real helicopter.

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Evaluation of the power required in level flight is essential to any new or modified helicopter performance flight-testing effort. The conventional flight-test method is based on an overly simplification of the induced and profile power components required for a helicopter in level flight. This simplistic approach incorporates several drawbacks that not only make execution of flight sorties inefficient and time consuming, but also compromise the level of accuracy achieved. This paper proposes an alternative flight-test method for evaluating the level-flight performance of a conventional helicopter while addressing and rectifying all identified deficiencies of the conventional method. The proposed method, referred to as the corrected-variables screening using dimensionality reduction (CVSDR), uses an original list of 36 corrected variables derived from basic dimensional analysis principles. This list of 36 corrected variables is reduced using tools of dimensionality reduction to keep only the most effective level-flight predictors. The CVSDR method is demonstrated and tested in this paper using flight-test data from a MBB BO-105 helicopter. It is shown that the CVSDR method predicts the power required for level flight about 21% more accurately than the conventional method while reducing the required flight time by an estimate of at least 60%. Unlike the conventional method, the CVSDR is not bounded by the high-speed approximation associated with the induced power estimation, therefore it is also relevant to the low airspeed regime. This low-airspeed relevancy allows the CVSDR method to bridge between the level-flight regime and the hover. Although demonstrated in this paper for a specific type of helicopter, the CVSDR method is applicable for level-flight performance flight testing of any type of conventional helicopter.

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In the case of complicated, non-linear problems where simulations in the time-domain are needed to understand systems' behavior, Hamiltonian formulation can be used to obtain insight into system evolution in time. Hamiltonian dynamics has two advantages: 1) there is no need to write down the complete equations of motion explicity and thus help to solve the problem much quicker and 2) it can help understanding and designing controllers using the energy flow, Hamiltonian phase space and port-Hamiltonian representation for system evolution. The present paper highlights the importance of using the Hamiltonian dynamics for helicopter flight dynamics, exemplifying it for the helicopter pitch motion and for a 6-DOF nonlinear model. The paper shows that, using Hamiltonian formulation, one can define energy stagnations areas in the Hamiltonian phase plane and dissipative non-passive terms in the equations of motion that need to be restrained when designing a helicopter controller. The extension of the Hamiltonian to the port-Hamiltonian formulation can be used to design nonlinear controllers robust to system nonlinearities.

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The health management of batteries is a key enabler for the adoption of Electric Vertical Take-off and Landing vehicles (eVTOLs). Currently, few studies consider the health management of eVTOL batteries. One distinct characteristic of batteries for eVTOLs is that the discharge rates are significantly larger during take-off and landing, compared with the battery discharge rates needed for automotives. Such discharge protocols are expected to impact the long-run health of batteries. This paper proposes a data-driven machine learning framework to estimate the state-of-health and remaining-useful-lifetime of eVTOL batteries under varying flight conditions and taking into account the entire flight profile of the eVTOLs. Three main features are considered for the assessment of the health of the batteries: charge, discharge and temperature. The importance of these features is also quantified. Considering battery charging before flight, a selection of missions for state-of-health and remaining-useful-lifetime prediction is performed. The results show that indeed, discharge-related features have the highest importance when predicting battery state-of-health and remaining-useful-lifetime. Using several machine learning algorithms, it is shown that the battery state-of-health and remaining-useful-life are well estimated using Random Forest regression and Extreme Gradient Boosting, respectively.@en

This paper investigates the effects of different automation design philosophies for a helicopter navigation task. A baseline navigation display is compared with two more advanced systems: an advisory display, which provides a discrete trajectory suggestion; and a constraint-based display, which provides information about the set of possible trajectory solutions. The results of a human-in-the-loop experiment with eight pilot participants show a significant negative impact of the advisory display on pilot trajectory decision-making: out of the 16 encountered off-nominal situations across the experiment, only 6 were solved optimally. The baseline and constraint-based display both lead to better decisions, with 14 out of 16 being optimal. However, pilots still preferred the advisory display, in particular in off-nominal situations. These results highlight that even when a support system is preferred by pilots, it can have strong inadvertent negative effects on their decision-making.

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The research presented in this paper focuses on the development of a quasi-Linear Parameter Varying (qLPV) model for the XV-15 tiltrotor aircraft. The specific category of qLPV modeling technique, known as the model stitching technique, is employed to model the time-varying dynamics of XV-15 tiltrotor aircraft over the entire flight envelope. In this modeling approach, discrete linear state-space models are interpolated through lookup tables as function of scheduling parameters with the implementation of nonlinear equations of motion. The XV-15 qLPV model is configured with four scheduling parameters: altitude, nacelle incidence angle, wing flap angle and velocity. Additionally, a computational complexity analysis is presented. In particular, computational sensitivity of qLPV models configured with lookup tables to number of states and number of scheduling parameters is demonstrated. This is done to show the feasibility of real-time implementation of qLPV models with increasing fidelity (number of states) and expanding dynamic flight envelope (number of scheduling parameters).

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Training has the potential to inject a “safety vaccination” into the rotorcraft community by reducing the number of accidents. The term training should not be intended only in a strict sense, i.e., as pilot technical skills training, but more broadly as risk avoidance and safety culture training. As in the case of vaccination, where immunity is created only when applied on a large scale, helicopter accidents will not be eradicated until every player in the rotorcraft community is involved in the safety enhancement process. In particular, as outlined by accident and safety reports, a reduction in the helicopter accident rate cannot be accomplished disregarding pilots’ training and the contribution that _ight simulators can provide to both training and certi_cation. This paper provides an overview of the research into simulator training for helicopter pilots conducted as part of the European Joint Doctorate NITROS (Network for Innovative Training on Rotorcraft Safety). An approach that requires an in-depth analysis of the actual training task is adopted for two different maneuvers, namely hover and autorotation. This approach enables the training developer to understand what are the aspects of the actual training situation that should be reproduced in the simulated training situation to avoid ineffective training and negative transfer of skills. Moreover, such an approach allows to identify differences in terms of requirements between the training of basic and advanced maneuvers and between initial and recurrent training. The results of three different pilot-in-theloop experiments, performed to explicitly con_rm the effectiveness of developed training programs and to understand whether certain elements of the simulation can foster the development of superior _ying skills, are summarized in this paper. @en

Rotorcraft flight dynamics simulation models require high levels of fidelity to be suitable as prime items in support of life cycle practices, particularly vehicle and control design and development, and system and trainer certification. On the civil side, both the FAA (US) and EASA (Europe) have documented criteria (metrics and practices) for assessing model and simulator fidelity as compared to flight-test data, although these have not been updated for several decades. On the military side, the related practices in NATO nations are not harmonised and often only developed for specific applications. Methods to update the models for improved fidelity are mostly ad hoc and lack a rational and methodical approach. Modern rotorcraft System Identification (SID) and inverse simulation methods have been developed in recent years that provide new approaches well suited to pilot-in-the-loop fidelity assessment and systematic techniques for updating simulation models to achieve the needed level of fidelity. To coordinate efforts and improve the knowledge in this area, STO Applied Vehicle Technology Panel Research Task Group (STO AVT-296 RTG) was constituted to evaluate update methods used by member nations to find best practices and suitability for different applications including advanced rotorcraft configurations. This report presents the findings of the AVT-296 RTG. An overview of previous rotorcraft simulation fidelity Working Groups is presented, followed by a review of the metrics that have been used in previous studies to quantify the fidelity of a flight model or the overall perceptual fidelity of a simulator. The theoretical foundations of the seven different update methods and a description of the eight flight databases (Bell 412, UH-60, IRIS+, EC135, CH-47, AW139, AW109, and X2, provided by the National Research Council of Canada, US Army, Airbus Helicopters, Boeing, Leonardo Helicopter Division, and Sikorsky) used by the RTG is presented. Both time- and frequency-domain fidelity assessment methods are considered, including those in current use by simulator qualification authorities and those used in the research community. Case studies are used to show the application, utility, and limitations of the update and assessment methods to the flight-test data. The work of the RTG has shown that time- and frequency-domain SID based metrics are suitable for use for assessing the model fidelity across a wide range of rotorcraft configurations. Gain and time delay update methods work well for well-developed flight dynamics models and can be used for flight control system design, but do not provide physical insights into the sources of errors in a model. Deriving stability and control derivatives from flight-test data using SID and nonlinear simulation models using perturbation extraction methods provides insight into the missing dynamics of the simulation model, which can subsequently be updated using additional forces and moments to significantly improve the fidelity of the model and can be used to update models for flight simulation training application methods. Reduced order model and physics-based correction methods provide large benefits when extrapolating to other flight conditions but does require detailed flight-test data. SID can quickly provide accurate point models, if detailed flight-test data are available, which can be ‘stitched’ together to produce models suitable for real-time piloted simulation and control design applications. However, the dependency on flight-test data means that this method is not suitable for early aircraft development activities.@en

Eigenmode distortion is a novel quantitative methodology developed to objectively evaluate motion cueing fidelity in flight simulation. It relies on an explicit coupling of linearized vehicle and Motion Cueing Algorithm dynamics. Modal analysis subsequently performed on this coupled system reveals the degree of distortion imposed by the Motion Cueing Algorithm on to the dynamics of the simulated vehicle. Eigenmode distortion thereby provides unprecedented insight into the combined dynamics of the two systems along modal coordinates. Compared with existing methods for motion cueing fidelity assessment, the eigenmode distortion method enables a systematic analysis of the coupled vehicle and Motion Cueing Algorithm dynamics. This is mainly because it does not consider the Motion Cueing Algorithm in isolation and does not inherently rely on assumptions regarding the excitation of the simulated vehicle dynamics. This paper outlines the theoretical foundation of the eigenmode distortion method and includes a case study on helicopter longitudinal dynamics and a sensitivity analysis to demonstrate its utility. The results presented in this paper shown that the eigenmode distortion method can reveal interactions between the Motion Cueing Algorithm and the vehicle dynamics that are currently not captured by other established methods, such as the Sinacori–Schroeder criteria and the Objective Motion Cueing Test.@en

This paper investigates the effect of employing different display design principles for human–machine interaction in helicopters. Two obstacle avoidance support displays are evaluated during low-altitude forward flight. A baseline head-up display is complemented either by a conventional advisory display or a constraint-based display inspired by ecological interface design. The latter design philosophy has only been sparsely applied in the helicopter domain. Twelve helicopter pilots participated in an experiment in a research flight simulator. We found no significant effects of the displays on objective performance measures. However, there was a trend of decreasing pilot workload and increasing situation awareness when employing the support displays, as compared to the baseline display. The constraint-based display had the largest positive effect and increased the resilience of the pilot–vehicle system toward unexpected events when considering the safety of the flown trajectories. Pilots preferred the advisory display in nominal situations and the constraint-based display in off-nominal situations, reproducing similar findings from research in the fixed-wing domain. This experiment showed the potential of the developed constraint-based display to improve subjective pilot ratings, pilot preference, and safety during unexpected events. Future research will investigate more complex scenarios with longer time frames, possibly eliciting more divergent effects of different display design principles.

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This paper analyzes the effects of the helicopter dynamics on pilots’ learning process and transfer of learned skills during autorotation training. A quasi-transfer-of-training experiment was performed with 14 experienced helicopter pilots in a moving-base flight simulator. Two types of helicopter dynamics, characterized by a different autorotative index, were considered: “hard,” with high pilot compensation required, and “easy,” with low compensation required. Two groups of pilots tested the two types of dynamics in a different training sequence: hard-easy-hard (HEH group) and easy-hard-easy (EHE group). Participants of both groups were able to attain adequate performance at touchdown in most of the landings with both types of dynamics. However, a clear positive transfer effect in terms of acquired skills is found in both groups from the hard to the easy dynamics, but not from the easy to the hard dynamics, confirming previous experimental evidence. Positive transfer is especially observed for the rate of descent at touchdown. The two groups differed in the control strategy applied, with the HEH group having developed a more robust control technique. During the last training phase the EHE group aligned its control strategy with that of the HEH group.@en

Fidelity is a fundamental concept for crucial decision-making processes during engineering design, test and evaluation of aircraft and rotorcraft. Fidelity, as applied to rotorcraft, proves to be a prerequisite for most human factors research and simulator development programs. This paper reviews the concept of fidelity, its metrics and methods concentrating on rotorcraft applications. The paper stems from the Applied Vehicle Technology (AVT) Panel of the NATO Science and Technology Organization (STO) work of NATO AV-296 concentrating on recent advancements on rotorcraft flight simulation model fidelity prediction and assessment. Starting from the question of what is meant with the term simulation fidelity, the paper will demonstrate that even though the term fidelity can be loosely translated as simulation goodness or faithfulness to reality, when it comes to fidelity assessment, this concept has a multidimensional and multifaceted character depending on the configuration considered. A key message of the paper is that simulation fidelity should be decomposed in different dimensions and attributes with their own metrics. Using multiple fidelity metrics is therefore meaningful for assessing simulation fidelity in general and in particular for the case of rotorcraft.

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The aim of the present paper is to present an understanding of the control characteristics for representative eVTOL (Electric Vertical Takeoff and Landing) quadcopters having the same properties of interest as the eVTOL configurations developed presently in Urban Air Mobility. In this context, the paper concentrates on eVTOL quadcopters with pure rpm control, pure propeller pitch control and both rpm and propeller pitch control. Dynamic effects of electric motors can potentially have significant effects on the flight characteristics of these vehicles. The paper will demonstrate for a pitch manoeuvre that while varying purely the rpm of the rotors results in a system that behaves as an acceleration-control system, varying purely the propeller pitch corresponds more to a velocity-control system. The rotor pitch control is more sensitive to the coupling rotor-motor and its transients dynamics. Varying both the rpm and the rotor pitch for controlling the pitch manoeuvre is the best option as this brings the system more towards velocity-control behaviour and dampens the transients due to the motor dynamics.

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Reinforcement learning is an appealing approach for adaptive, fault-tolerant flight control, but is generally plagued by its need for accurate system models and lengthy offline training phases. The novel Incremental Dual Heuristic Programming (IDHP) method removes these dependencies by using an online-identified local system model. A recent implementation has shown to be capable of reliably learning near-optimal control policies for a fixed-wing aircraft in cruise by using outer loop PID and inner-loop IDHP rate controllers. However, fixed wing aircraft are inherently stable, enabling a trade-off between learning speed and learning stability which is not trivially extended to a physically unstable system. This paper presents an implementation of IDHP for control of a non-linear, six-degree-of-freedom simulation of an MBB Bo-105 helicopter. The proposed system uses two separate IDHP controllers for direct pitch angle and altitude control combined with outer loop and lateral PID controllers. After a short online training phase, the agent is shown to be able to fly a modified ADS-33 acceleration-deceleration manoeuvre as well as a one-engine-inoperative continued landing with high success rates.@en

The process of empirical models evaluation is at the core business of experimental flight-testing data analysis. Accurate and convenient flight-testing of helicopter engine(s) available power is crucial for predicting the total helicopter performance. Common practice in estimation of in-flight helicopter gas turbine engine power consists of a reduction of flight-test data into simplistic single-variable analysis approach. While such an approach is convenient for practical use, it often results in unrealistic predictions of the available engine(s) power. A novel approach for the helicopter available power problem is the so-called Multivariable Polynomial Optimization under Constraints method. In this method, 18 regressors, constructed from the engine non-dimensional parameters, are used to define empirical polynomial models. This paper is intended to complement the Multivariable Polynomial Optimization under Constraints method and answer the question of which multivariable polynomial can be generally used in representing helicopter gas-turbine engine performance? In this sense, a variety of seven gas-turbine engines installed on different helicopters are analyzed, each one giving 512 possible polynomial models to be used for available-power calculations. While conventional statistical methods of hypothesis-testing failed in providing the answer to the question stated above of which the best general empirical model for representing engine performance is, an alternative approach based on the Singular-Value-Decomposition theorem, was proven successful in providing the answer. Moreover, this approach presented in the paper yielded a short list of 10 simple and convenient multivariable polynomials, best representing the performance of all seven engines analyzed as a group.

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