Manfred Hajek

Helicopter and VTOL structures are heavily influenced by vibrations and complex interaction effects. One possible solution to minimize design risks and to enhance cabin comfort is to increase the damping in the vehicle's structure, for which natural flax fibers offer a promising approach. The presented study investigated the hybridization of conventional composites with flax fibers of sub-structural components, including double-T beams and airfoil structures. The frequency dependent dynamic properties of the structures, damping and stiffness, are measured using experimental modal analysis. Local flax rib reinforcements on shells showed a significant increase in stiffness-damping factor but did not improve damping in flax laminates. According to the investigation, hybridization of conventional composites can lead to a significant increase in damping for both, double-T beams and airfoils. The damping can be increased by three to four times, and there can be a significant improvement in the damping-to-stiffness factor.

With the aim of investigating whether sustainable and environmentally friendly flax fiber reinforced composites can meet the high technical requirements of aeronautical structures, this paper presents the influence of temperature and humidity on their structural-mechanical properties using protective coatings. Four coatings, two of which are partially bio-based, are tested on unidirectional flax fibers reinforced with bio-epoxy and polyfurfuryl alcohol resin. To simulate environmental conditions, coated and uncoated test panels are conditioned for two weeks in a climatic chamber at different temperature and humidity levels before their tensile specimens are tested and evaluated for moisture absorption, tensile strength, Young's modulus and fracture strain. The results show that higher temperatures lead to an increased moisture absorption rate and a lower maximum uptake. Higher relative humidity results in an increased maximum moisture uptake. A fossil-based coating could significantly reduce and retard moisture uptake at 30° C and 95 % relative humidity.

Continued safe flight under all foreseeable scenarios will be crucial to assure airworthiness and build trust in future vertical takeoff and landing (VTOL) urban air mobility (UAM) concepts. Successful pilot intervention will be especially important for handling emergency procedures. This paper demonstrates the use of human-centric flight guidance concepts to mitigate the effects of non-catastrophic failures in future VTOL UAM aircraft. Specifically, a single failure in the lateral steering function of an automatic flight control system is considered. A new waypoint guidance law is proposed to generate the required roll command to manually steer the aircraft toward a predefined safe landing zone. Two display concepts are proposed to aid the manual execution of the roll steering command: a head-down display (HDD), and a head-mounted display (HMD). The guidance law and the display concepts were integrated into a high-fidelity, wide field-of-view rotorcraft simulation environment. Human-in-the-loop experiments were performed with non-pilot test subjects. The effectiveness of the guidance law and display concepts along with the associated piloting performance and human factor impacts were assessed. The results indicated that all test subjects were able to reach a close vicinity of the final waypoint. The HMD induced lower workload whereas the HDD resulted in better tracking performance.

View Video Presentation: https://doi.org/10.2514/6.2022-3751.vid This paper presents a modular and extensible CFD/CSD loose coupling approach that was developed for passive and active rotor blades. The aerodynamic loads were computed with the CFD solver TAU. The blade motion during the CFD simulation was determined by the CSD solver CAMRAD II, including a trim calculation and blade dynamics calculation. The capability of this CFD/CSD coupling approach was demonstrated by means of a passive and an active single-bladed isolated rotor operated at an advance ratio of μ=0.15. The active section was integrated between 25% and 95% of the rotor radius, also comprising a linear transition region from passive to fully active section of the rotor blade inboard and outboard,respectively. In chordwise direction, the dynamic camber morphing extended from 75% of the chord to the trailing edge. A non-harmonic actuation schedule was applied which yielded a reduction in required rotor power in a previous comprehensive analysis study. The results obtained with the CFD/CSD coupling approach were verified with stand alone comprehensive analysis results(CAMRAD II) using free wake and linear inflow models. The results from CFD/CSD and free wake simulations were in good agreement when comparing the thrust distribution for both passive and active rotor blades. The linear inflow based computations resulted in greater peak loads near the blade tip on the retreating side. The dynamic morphing led to a redistribution of the thrust over the rotor disk and, in particular, an offloading of the blade tip and higher loading in the region where high camber deflections were applied. The developed CFD/CSD coupling framework was demonstrated to be suitable for the investigation of active camber morphing on helicopter rotors.

View Video Presentation: https://doi.org/10.2514/6.2022-0513.vid The present work reports the design, integration, and piloted evaluation of a visual and control augmented pilot assistance system (PAS) for helicopter near-ship operations in degraded visual environments. The PAS consists of an automatic cluttered-scene-linked 2D and 3D symbology concept, visualized on a helmet-mounted display (HMD). The design was applied by combining operational needs, standard naval requirements, and procedures to support the pilot’s direct perception of critical helicopter operations within the limitations of safety and performance constraints, such as a degraded visual range and high ship motions. An additional advanced flight control mode, optimized together with maritime test pilots, focused on rendering pilots’ control inputs more precisely, while simultaneously decreasing subjective workload. The present approach of the PAS combines a precise and dynamic shiprelative approach and landing enhanced visual augmentation with scene-linked visual clutter modes designed for an HMD with see-through capabilities. For evaluation, traditional subjective workload, visual cues, and handling-qualities ratings were supplemented with analysis techniques, including the approach profile and pilot control activity observations. The results of two simulated flight test campaigns together with 10 maritime test and fleet pilots from the armed forces, industry, and public authorities, completing a total of over 120 simulated flights show that subjective workload and the corresponding handling-qualities ratings kept almost level 1, even when degraded visual environment (DVE) conditions increased. Subjective assessments indicate that most of the pilots preferred higher degrees of helicopter stabilization during the helicopter ship approach and landing maneuvers. Timeseries and frequency domain analysis of pilot control inputs and head motions offer the capability to associate subjective ratings of pilot workload and handling qualities with control inputs observations.

This paper synthesizes a continuous, multivariable, finite-time-convergent, super-twisting attitude and rate controller for rotorcraft with the objective of providing desired handling qualities and robustness characteristics. A sliding manifold is defined in the system state space to represent ideal attitude and rate command response dynamics of relative degree one with respect to the command input. Subsequently, robust command tracking is achieved via the synthesis of a multivariable super-twisting flight controller, which renders the plant states convergent on to the defined sliding manifold in finite-time and in the presence of matched external disturbance input. To validate the efficacy of the controller, simulation results are presented based on a nonlinear, higher-order rotorcraft model operating in turbulence. True system convergence to the sliding manifold from an untrimmed state is shown to lie within the theoretically predicted finite-time convergence bound. Furthermore, simulations with a linear quadratic flight controller are also presented for performance comparison with the proposed super-twisting flight controller.