Search

This paper discusses the development of a numerical method for the approximation of the nonlinear time-periodic rotorcraft flight dynamics with higher order linear time-invariant (LTI) models. The method relies on a per-rotor revolution perturbation scheme, which is of particular importance for the linearization of simulation models that do not allow for per-time-step perturbations, and for those output measures that necessitate the solution of partial differential equations and thus require several time steps to be computed. The paper demonstrates the application of the proposed methodology to obtain high-order LTI models capable of predicting vibrations for a generic utility helicopter. Simulations are used to validate the response of the linearized models against those from nonlinear simulations and from competing approaches in the literature. The proposed method is shown to predict accurately the nonlinear response for the case shown and for small amplitude maneuvers. Frequency-domain validation is also performed to compare the linear models derived with the proposed method with those obtained with harmonic decomposition, a competing approach based on a per-time-step perturbation scheme. Interestingly, the proposed algorithm yields nearly identical numerical results compared to harmonic decomposition, suggesting that the two methods are in fact equivalent but rely on different formulations.

The objective of this investigation is threefold. First, to assess the flight dynamics of an electric vertical take-off and landing (eVTOL) concept aircraft with a propeller-driven rotor. Second, to develop an automatic flight control system (AFCS) for this concept aircraft. Third, to verify the potential safety benefits of the concept aircraft by analyzing the autorotation performance following a total loss of power. The paper begins with an overview of the design of the aircraft and description of the simulation model, including a detailed discussion on the inflow model of the propellers that drive the main rotor. Next, the flight dynamics are assessed at hover and in forward flight. An AFCS based on dynamic inversion is developed to provide stability and desired response characteristics about the roll, pitch, yaw, and heave axes for speeds ranging from hover to 80 kt. Additionally, an RPM governor is implemented to hold the main rotor angular speed constant at its nominal value. Finally, simulations that make use of the AFCS are performed to analyze the autorotation performance following total loss of power.

To provide the government with independent control-system design, handling qualities analysis, and simulation research capabilities in support of Future Vertical Lift, the U. S. Army Combat Capabilities Development Command Aviation & Missile Center has developed generic high-fidelity flight-dynamics models of several advanced high-speed rotorcraft configurations including a tiltrotor. Full flight envelope explicit model following control laws were designed for the generic tiltrotor using a multiobjective optimization approach to meet a comprehensive set of stability, handling qualities, and performance specifications. Helicopter response types were used for hover/low-speed, while typical fixed-wing response types (normal acceleration and sideslip command) were used at high speed. The control laws were evaluated in a piloted simulation experiment at the NASA Ames Vertical Motion Simulator using a series of previously developed high-speed handling qualities demonstration maneuvers. This paper discusses the control laws and the results of the piloted handling qualities assessment which show overall assigned Level 1 handling qualities in high-speed and Level 2 in transition.

To provide the government with independent control-system design, handling qualities analysis, and simulation research capabilities in support of Future Vertical Lift, the U. S. Army Combat Capabilities Development Command Aviation & Missile Center has developed generic high-fidelity flight-dynamics models of several advanced high-speed rotorcraft configurations. One of the configurations modeled is a lift offset coaxial-compound helicopter with a pusher propeller. Full flight envelope explicit model following control laws were designed for the generic coaxial-compound helicopter using a multiobjective optimization approach to meet a comprehensive set of stability, handling qualities, and performance specifications. Helicopter response types were used for hover/low-speed, while typical fixed-wing response types (normal acceleration and sideslip command) were used at high speed. The control laws were evaluated in a piloted simulation experiment at the NASA Ames Vertical Motion Simulator using a series of previously developed high-speed handling qualities demonstration maneuvers. This paper discusses the control laws and the results of the piloted handling qualities assessment, which showed assigned Level 1 handling qualities for six of the seven maneuvers evaluated.

This review focuses on summarizing current knowledge on how time-restricted feeding (TRF) and continuous caloric restriction (CR) affect central neuroendocrine systems involved in regulating satiety. Several interconnected regions of the hypothalamus, brainstem, and cortical areas of the brain are involved in the regulation of satiety. Following CR and TRF, the increase in hunger and reduction in satiety signals of the melanocortin system [neuropeptide Y (NPY), proopiomelanocortin (POMC), and agouti-related peptide (AgRP)] appear similar between CR and TRF protocols, as do the dopaminergic responses in the mesocorticolimbic circuit. However, ghrelin and leptin signaling via the melanocortin system appears to improve energy balance signals and reduce hyperphagia following TRF, which has not been reported in CR. In addition to satiety systems, CR and TRF also influence circadian rhythms. CR influences the suprachiasmatic nucleus (SCN) or the primary circadian clock as seen by increased clock gene expression. In contrast, TRF appears to affect both the SCN and the peripheral clocks, as seen by phasic changes in the non-SCN (potentially the elusive food entrainable oscillator) and metabolic clocks. The peripheral clocks are influenced by the primary circadian clock but are also entrained by food timing, sleep timing, and other lifestyle parameters, which can supersede the metabolic processes that are regulated by the primary circadian clock. Taken together, TRF influences hunger/satiety, energy balance systems, and circadian rhythms, suggesting a role for adherence to CR in the long run if implemented using the TRF approach. However, these suggestions are based on only a few studies, and future investigations that use standardized protocols for the evaluation of the effect of these diet patterns (time, duration, meal composition, sufficiently powered) are necessary to verify these preliminary observations.

Calorie restriction (CR) is a common approach to inducing negative energy balance. Recently, time-restricted feeding (TRF), which involves consuming food within specific time windows during a 24-h day, has become popular owing to its relative ease of practice and potential to aid in achieving and maintaining a negative energy balance. TRF can be implemented intentionally with CR, or TRF might induce CR simply because of the time restriction. This review focuses on summarizing our current knowledge on how TRF and continuous CR affect gut peptides that influence satiety. Based on peer-reviewed studies, in response to CR there is an increase in the orexigenic hormone ghrelin and a reduction in fasting leptin and insulin. There is likely a reduction in glucagon-like peptide-1 (GLP-1), peptide YY (PYY), and cholecystokinin (CCK), albeit the evidence for this is weak. After TRF, unlike CR, fasting ghrelin decreased in some TRF studies, whereas it showed no change in several others. Further, a reduction in fasting leptin, insulin, and GLP-1 has been observed. In conclusion, when other determinants of food intake are held equal, the peripheral satiety systems appear to be somewhat similarly affected by CR and TRF with regard to leptin, insulin, and GLP-1. But unlike CR, TRF did not appear to robustly increase ghrelin, suggesting different influences on appetite with a potential decrease of hunger after TRF when compared with CR. However, there are several established and novel gut peptides that have not been measured within the context of CR and TRF, and studies that have evaluated effects of TRF are often short-term, with nonuniform study designs and highly varying temporal eating patterns. More evidence and studies addressing these aspects are needed to draw definitive conclusions.

This paper presents a systematic investigation of high-speed rotorcraft pitch-axis response types, command models, and handling qualities specifications. The investigation was done using two future vertical lift-relevant rotorcraft configurations—a lift offset coaxial helicopter with a pusher propeller and a tiltrotor. Five response types were investigated, consisting of a pitch rate-command/attitude-hold response type typically used for rotorcraft, a pitch rate-command/attitude-hold response type using a higher-order command model based on the conventional airplane pitch rate transfer function, a normal acceleration command/angle-of-attack hold response type, a flight path rate command/flight path hold response type, and a “blended” flight path rate command response type which varies the command model bandwidth based on stick input size. Designs of varying levels of pitch attitude bandwidth, flight path bandwidth, control anticipation parameter, and pitch attitude dropback were evaluated in a piloted simulation experiment conducted at the Penn State Flight Simulator facility using two high-speed mission task elements. The results of the piloted simulation suggest that both the pitch attitude bandwidth and the pitch attitude dropback requirements must be met for Level 1 handling qualities. A set of recommended specifications and associated updated level boundaries is provided in the Appendix.

This article describes the implementation and linearization of free-vortex wake models in state-variable form as applied to rotary- and flapping-wing vehicles. More specifically, the wake models are implemented and tested for a UH-60 rotor in forward flight and for a hovering insect representative of a hawk moth. A periodic solution to each wake model is found by time marching the coupled rotor/wing and vortex wake dynamics. Next, linearized harmonic decomposition models are obtained and validated against nonlinear simulations. Order reduction methods are explored to guide the development of linearized wake models that provide increased runtime performance compared to the nonlinear and linearized harmonic decomposition wake models while guaranteeing satisfactory prediction of the periodic response of the wake. This constitutes a first attempt to extend free-vortex wake methods in state-variable form, originally developed for rotary-wing applications, to flapping-wing flight.

Autologous chondrocyte implantation (ACI) is a cell therapy to repair cartilage defects. In ACI a biopsy is taken from a non-load bearing area of the knee and expanded in-vitro. The expansion process provides the benefit of generating a large number of cells required for implantation; however, during the expansion these cells de-differentiate and lose their chondrocyte phenotype. In this review we focus on examining the de-differentiation phenotype from a mechanobiology and biophysical perspective, highlighting some of the nuclear mechanics and chromatin changes in chondrocytes seen during the expansion process and how this relates to the gene expression profile. We propose that manipulating chondrocyte nuclear architecture and chromatin organization will highlight mechanisms that will help to preserve the chondrocyte phenotype.

A marine hydrokinetic (MHK) cycloturbine is a renewable electric power generation system used in rivers or tidal environments to address the need for electricity in remote regions. MHK cycloturbines have hydrofoils oriented perpendicular to the flow in a paddlewheel configuration, and use lift generated from these foils to produce power. Due to the high cost associated with its operation and maintenance, an MHK system with four stacked counter-rotating turbines that can self-deploy and with propulsion and control mechanisms similar to a cycloturbine aircraft has been designed. A detailed turbine simulation model is necessary to understand the vehicle dynamics and assist in the design of the vehicle controllers. The simulation model solves the six degree-of-freedom rigid body equations of motion for the maneuvering MHK system subject to the hydrodynamic lift and drag forces, hydrostatic forces, and the propulsive forces from the turbines. The turbine propulsive force model is matched to computational fluid dynamics analysis and experimental data. Experimental work includes investigation at 1/5.56 scale of a single turbine rapid prototype device and a subscale demonstrator in a reverberant tank.

Two methods of multi-input multioutput (MIMO) command-model-following control design are compared for rotorcraft flight control. The first, explicit model following (EMF), employs single-input single-output inverse plants with a dynamic decoupling matrix, which is a purely feed-forward approach to inverting the plant. The second is dynamic inversion (DI), which involves both feed-forward and feedback path elements to invert the plant. The EMF design is purely linear, while the DI design has some nonlinear elements in vertical rate control. For each of these methods, an architecture is presented that provides angular rate model following with selectable vertical rate model following. Implementation challenges of both EMF and DI are covered, and methods of dealing with them are presented. These two MIMO model-following approaches are evaluated regarding (1) fidelity to the command model and (2) turbulence rejection. Both are found to provide good tracking of commands and reduction of cross coupling.

The nuclear envelope (NE) has emerged as a nexus for cellular organization, signaling, and survival. Beyond its role as a barrier to separate the nucleoplasm from the cytoplasm, the NE’s role in supporting and maintaining a myriad of other functions has made it a target of study in many cellular processes, including senescence. The nucleus undergoes dramatic changes in senescence, many of which are driven by changes in the NE. Indeed, Lamin B1, a key NE protein that is consistently downregulated in senescence, has become a marker for senescence. Other NE proteins have also been shown to play a role in senescence, including LINC (linker of nucleoskeleton and cytoskeleton) complex proteins. LINC complexes span the NE, forming physical connections between the cytoplasm to the nucleoplasm. In this way, they integrate nuclear and cytoplasmic mechanical signals and are essential not only for a variety of cellular functions but are needed for cell survival. However, LINC complex proteins have been shown to have a myriad of functions in addition to forming a LINC complex, often existing as nucleoplasmic or cytoplasmic soluble proteins in a variety of isoforms. Some of these proteins have now been shown to play important roles in DNA repair, cell signaling, and nuclear shape regulation, all of which are important in senescence. This review will focus on some of these roles and highlight the importance of LINC complex proteins in senescence.

Magnetodielectric materials have been recently attracting great attention owing to their significant advantages for miniaturization and wide bandwidth of antennas. A precise measurement of broadband permeability and permittivity is crucial to develop and evaluate advanced magnetodielectric materials. The Nicolson–Ross–Weir (NRW) method is widely used to extract electromagnetic constitutive parameters from measured scattering parameters. One of the drawbacks of this method is requiring direct measurement of the actual sample position in the measurement fixture, which leads to uncertainties in the measured permeability and permittivity. In this article, we improve the standard NRW method by removing the need for prior knowledge of sample position. For any isotropic material, the sample position in the measurement fixture can be precisely determined by matching the phasor angles of the reflection coefficients at the sample surfaces. Due to eliminating the error from the direct measurement of sample position, the improved NRW method effectively reduces the uncertainties in the calculated permeability and permittivity. Based on the results from two kinds of magnetodielectric materials measured in two sets of test fixtures of different geometries, this straightforward, explicit, and noniterative method is theoretically and experimentally proven to have a high and position-independent accuracy over a wide frequency range.

The objective of this research effort is to assess the impact of load alleviation control on the quantitative handling-qualities specifications (also known as predicted handling qualities) of both ...

The present study considers two notional rotorcraft models: a conventional utility helicopter, representative of an H-60, and a wing-only compound utility rotorcraft, representative of an H-60 with a wing similar to the X-49A wing. An explicit model following (EMF) control scheme is designed to achieve stability and desired rate command / attitude hold response around the roll, pitch, and yaw axes, while alleviating vibratory loads through both feed-forward and feedback compensation. The harmonic decomposition methodology is extended to enable optimization of primary flight control laws that mitigate vibratory loads. Specifically, linear time-periodic systems representative of the periodic rotorcraft dynamics are approximated by linear time-invariant (LTI) models. The LTI models are subsequently reduced and used in linear quadratic regulator (LQR) design to constrain the harmonics of the vibratory loads. Both fuselage state feedback and rotor state feedback are considered. A pseudo-inverse strategy is incorporated into the EMF scheme for redundant control allocation on the compound configuration. Simulations of the load alleviating controllers are compared to results from a baseline controller. Finally, an analysis is performed to assess the impact that load alleviating control action, rotor state feedback, and pseudo-inverse have on handling qualities in terms of ADS-33E specifications.

Updates to the military rotorcraft handling qualities specification are currently being considered that address the high-speed flight regime envisioned for the Future Vertical Lift platform of the U. S. Army. A team that features industry and academia has developed and evaluated a set of mission task elements (MTEs) that are defined to address vertical takeoff and landing (VTOL) high-speed handling qualities. Following the mission-oriented approach upon which ADS-33E-PRF is based, the MTEs were designed to meet different levels of precision and aggressiveness. Tracking MTEs based on a sum-of-sines (SOS) command signal were defined for precision, aggressive, and precision, nonaggressive applications. The command signals were derived from fixed-wing analogs that have long been used to evaluate aircraft handling qualities. While the precision, aggressive SOS tracking tasks, the primary subject of this paper, are surrogates for air-to-air tracking and nap-of-the-earth tracking, the known forcing function allows for complete open- and closed-loop pilot—vehicle system identification. The MTE objectives, descriptions, and performance criteria were assessed and refined via several checkout piloted simulation sessions. Formal evaluations were then conducted by Army test pilots at four simulator facilities, each featuring a unique high-speed platform including a generic winged-compound helicopter, two tiltrotor configurations, and a compound helicopter with coaxial rotors. To aid in the MTE evaluation process, baseline VTOL configurations were varied to achieve different handling qualities levels. Quantitative measures based on task performance and qualitative measures based on pilot ratings, comments, and debrief questionnaires were used to assess MTE effectiveness. The piloted simulation results demonstrated that the SOS tracking MTEs provided an effective means to discern precision, aggressive handling qualities in high-speed flight.

The objectives of this paper are to advance dynamic inversion (DI) and explicit model following (EMF) flight control laws for quadrotor unmanned aerial systems (UAS) and to develop an efficient strategy to compute the stability and performance robustness statistics of such control laws given parametric model uncertainty. For this purpose, a parametric model of a quadrotor is identified from flight-test data. The identified model is validated both in frequency and time domains. Next, DI and EMF flight control laws are designed for both inner attitude and outer velocity loops. Finally, a novel approach based on an unscented transform is used to evaluate the statistics of the controller's performance based on the statistics of the uncertain model parameters.

In the present work, unsteady effects present in a ship airwake are further analyzed. The effort is performed in the context of simulation of helicopter launch and recovery operations under a reali...

The use of an active cargo hook for stabilizing external loads during high-speed flight is demonstrated in simulation. A CONEX cargo container with two rear-mounted stabilization fins is used as the subject load. Significant nonlinearities in the dynamics of the external load result in multiple equilibria and limit cycle oscillations. A full-state feedback linear quadratic controller is developed assuming an isolated load in a wind tunnel model and shown to be successful in stabilizing the originally unstable load at a target airspeed of 100 kt. The design is then completed to cover the target carriage envelope from hover to high-speed flight. Simulations of a coupled system incorporating a UH-60 Black Hawk helicopter with an actuated cargo hook and the external load show that the controller is successful in providing system stability throughout the target flight speed envelope.