Fuel-Optimal Trajectory Planning for Lunar Vertical Landing

In this paper we consider a trajectory planning problem arising from a lunar vertical landing with minimum fuel consumption.The vertical landing requirement is written as a final steering angle constraint,and a nonnegative regularization term is proposed to modify the cost functional.In this way,thefinal steering angle constraint will be inherently satisfied according to Pontryagin's Minimum Principle.As a result,the modified optimal steering angle has to be determined by solving a transcendental equation.To this end,a transforming procedure is employed,which allows forfinding the desired optimal steering angle by a simple bisection method.Consequently,the vertical landing trajectory can be found offline by the indirect shooting method.Finally,some numerical examples are presented to demonstrate and verify the developments of the paper.

Monitoring absolute vertical land motions and absolute sea-level changes from GPS and tide gauges data over French Polynesia

In this study,we estimate the absolute vertical land motions at three tidal stations with collocated Global Navigation Satellite System(GNSS)receivers over French Polynesia during the period 2007-2020,and obtain,as ancillary results,estimates of the absolute changes in sea level at the same locations.To verify our processing approach to determining vertical motion,we first modeled vertical motion at the International GNSS Service(IGS)THTI station located in the capital island of Tahiti and compared our estimate with previous independent determinations,with a good agreement.We obtained the following estimates for the vertical land motions at the tide gauges:Tubuai island,Austral Archipelago-0.92±0.17 mm/yr,Vairao village,Tahiti Iti:-0.49±0.39 mm/yr,Rikitea,Gambier Archipelago-0.43±0.17 mm/yr.The absolute variations of the sea level are:Tubuai island,Austral Archipelago 5.25±0.60 mm/yr,Vairao village,Tahiti Iti:3.62±0.52 mm/yr,Rikitea,Gambier Archipelago 1.52±0.23 mm/yr.We discuss these absolute values in light of the values obtained from altimetric measurements and other means in French Polynesia.

Hover performance estimation and validation of battery powered vertical takeoff and landing aircraft

Battery powered vertical takeoff and landing(VTOL) aircraft attracts more and more interests from public, while limited hover endurance hinders many prospective applications. Based on the weight models of battery, motor and electronic speed controller, the power consumption model of propeller and the constant power discharge model of battery, an efficient method to estimate the hover endurance of battery powered VTOL aircraft was presented. In order to understand the mechanism of performance improvement, the impacts of propulsion system parameters on hover endurance were analyzed by simulations, including the motor power density, the battery capacity, specific energy and Peukert coefficient. Ground experiment platform was established and validation experiments were carried out, the results of which showed a well agreement with the simulations. The estimation method and the analysis results could be used for optimization design and hover performance evaluation of battery powered VTOL aircraft.

Space range estimate for battery-powered vertical take-off and landing aircraft

A novel method for estimating the space range of battery-powered vertical take-off and landing(VTOL) aircraft is presented. The method is based on flight parameter optimization and numerical iteration. Subsystem models including required thrust, required power and battery discharge models are presented. The problem to be optimized is formulated, and then case study simulation is conducted using the established method for quantitative analysis. Simulation results show that the space range of battery-powered VTOL aircraft in a vertical plane is an oblate curve, which appears horizontally long but vertically short, and the peak point is not located on the vertical climb path. The method and results are confirmed by parameter analysis and validations.

Prognostics for Lithium-ion batteries for electric Vertical Take-off andLanding aircraft using data-driven machine learning

The health management of batteries is a key enabler for the adoption of Electric Vertical Take-off and Landingvehicles (eVTOLs). Currently, few studies consider the health management of eVTOL batteries. One distinctcharacteristic of batteries for eVTOLs is that the discharge rates are significantly larger during take-off andlanding, compared with the battery discharge rates needed for automotives. Such discharge protocols areexpected to impact the long-run health of batteries. This paper proposes a data-driven machine learningframework to estimate the state-of-health and remaining-useful-lifetime of eVTOL batteries under varying flightconditions and taking into account the entire flight profile of the eVTOLs. Three main features are consideredfor the assessment of the health of the batteries: charge, discharge and temperature. The importance of thesefeatures is also quantified. Considering battery charging before flight, a selection of missions for state-ofhealth and remaining-useful-lifetime prediction is performed. The results show that indeed, discharge-relatedfeatures 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-lifeare well estimated using Random Forest regression and Extreme Gradient Boosting, respectively.

Journey to the Typhoon

Application of UAVs (unmanned aerial vehicles) for tropical cyclone missions is an emerging area of research and recent advances include the concept of spinsonde for multi-cycle measurement of vertical wind profile within the storm. This work proposes the design of a typhoon UAV as part of a cost-effective approach for acquiring atmospheric data to improve prediction and refine models. Land- and carrier-based flight schemes are proposed in this study and computer simulations are carried out to investigate the flight performance. Results suggest that the UAV achieves a maximum cruising speed in excess of 350 km·h-1 with excellent spinsonde performance. Furthermore, the UAV is capable of performing high-alpha maneuvers as well as vertical landing, thus rendering it suitable for space-efficient operation whether on land or aircraft carrier.

A multi-strategy pigeon-inspired optimization approach to active disturbance rejection control parameters tuning for vertical take-off and landing fixed-wing UAV

In this paper.Active Disturbance Rejection Control(ADRC)is utilized in the pitch control of a vertical take-off and landing fixed-wing Unmanned Aerial Vehicle(UAV)to address the problem of height fluctuation during the transition from hover to level flight.Considering the difficulty of parameter tuning of ADRC as well as the requirement of accuracy and rapidity of the controller,a Multi-Strategy Pigeon-Inspired Optimization(MSPIO)algorithm is employed.Particle Swarm Optimization(PSO),Genetic Algorithm(GA),the basic Pigeon-Inspired Optimization(PIO),and an improved PIO algorithm CMPIO are compared.In addition,the optimized ADRC control system is compared with the pure Proportional-Integral-Derivative(PID)control system and the non-optimized ADRC control system.The effectiveness of the designed control strategy for forward transition is verified and the faster convergence speed and better exploitation ability of the proposed MSPIO algorithm are confirmed by simulation results.

Spatial Distribution of Land Cover and Vegetation Activity along Topographic Gradient in an Arid River Valley, SW China

Anthropogenic activities have become more and more important in characterizing the landscape, but their impacts are still restricted by natural environments. This paper discusses the interactions of anthropogenic activity, vegetation activity and topography through describing the spatial distribution of land cover and vegetation activity (represented by Normalized Difference Vegetation Index, NDVI) along topographic gradient in a mountainous area of southwestern China. Our results indicate that the existing landscape pattern is controlled by anthropogenic activities as well as topographic factors. Intensive anthropogenic activities mainly occur in areas with relatively low elevation, gentle and concave slopes, as these areas are easy and convenient to attain for human. Because of the destruction by human, some land cover types (mainly grassland and shrub) are only found in relatively harsher environments. This study also finds that topographic wetness index (W) used in other places only reflects runoff generation capacity, but not indicate the real spatial pattern of soil water content in this area. The relationships between NDVI and W, and NDVI and length slope factor (LSF) show that runoff and erosion have complex effects on vegetation activity. Greater values of W and LSF will lead to stronger capacity to produce runoff and transport sediment, and thereby increase soil water content and soil deposition, whereas beyond a certain threshold runoff and erosion are so strong that they would destruct vegetation growth. This study provides information needed to successfully restore native vegetation, improve land management, and promote sustainable development in mountainous areas, especially for developing regions.

Land cover change in different altitudes of Guizhou-Guangxi karst mountain area, China： patterns and drivers

Topography, especially altitude, will influence the way, process and characteristics of land cover changes in mountainous area, simultaneously, the vertical difference of land cover changes will affect soil quality and regional ecological environment. Therefore, the gradient relationship analysis between land cover changes and altitude is very important for regional sustainability. This study investigated land cover dynamics based on land cover data from a typical mountainous area in the Guizhou-Guangxi karst mountain area, China, in 2000 and 2010, then explored the relationship between altitude and land cover change and analyzed different drivers of land cover change at different altitudes. Our findings are as follows. 1) From 2000 to 2010, the total area of land cover transition was 7167.04 km^2 or 2.8% of the region. The increasing area of build-up land(926.23 km^2) was larger than that of forest(859.38 km^2), suggesting that the urban construction speed was higher than that of reforestation. 2) Intensity of land cover transition in northwestern Guizhou-Guangxi karst mountain area was much larger than that of southeast part and their transition trend was also significantly different, which was consistent with regional population and economy. 3) Human activity was the most dramatic at altitudes between 0–500 m. For 500–1000 m, grassland mainly converted to forest and build-up land. Area of land cover transition was the greatest between 1000–1500 m, while above 1500 m, the transition of grassland was the most obvious. 4) The drivers of land cover change varied. Land cover change was positively correlated with gross domestic product and population density but was inversely related to relief amplitude. There were correlations between land cover change and distance to roads and rivers, and their correlations varied with altitude. By revealing patterns and causes of land cover changes in different altitudes, we hope to understand the vertical dependence of land cover changes, so as to improve land productivity and protect land ecological environment scientifically.

A Nonlinear Optimal Control Approach for the Vertical Take-off and Landing Aircraft

The paper proposes a nonlinear optimal control approach for the model of the vertical take off and landing(VTOL)aircraft.This aerial drone receives as control input a directed thrust,as well as forces acting on its wing tips.The latter forces are not perpendicular to the body axis of the drone but are tilted by a small angle.The dynamic model of the VTOL undergoes ap-proximate linearization with the use of Taylor series expansion around a temporary operating point which is recomputed at each iteration of the control method.For the approximately linearized model,an H-infinity feedback controller is designed.The linearization procedure relies on the computation of the Jacobian matrices of the state-space model of the VTOL aircraft.The proposed control method stands for the solution of the optimal control problem for the nonlinear and multivariable dynamics of the aerial drone,under model uncertainties and external per-turbations.For the computation of the contollr's feedback gains,an algebraic Riccati equation is solved at each time-step of the control method.The new nonlinear optimal control approach achieves fast and accurate tracking for all state variables of the VTOL aircnaft,under moderate variations of the control inputs.The stability properties of the control scheme are proven through Lyapunov analysis.

Historical Change and Future Scenarios of Sea Level Rise in Macao and Adjacent Waters

Against a background of climate change, Macao is very exposed to sea level rise （SLR） because of its low elevation, small size, and ongoing land reclamation. Therefore, we evaluate sea level changes in Macao, both historical and, especially, possible future scenarios, aiming to provide knowledge and a framework to help accommodate and protect against future SLR. Sea level in Macao is now rising at an accelerated rate： 1.35 mm yr-1 over 1925-2010 and jumping to 4.2 mm yr I over 1970-2010, which outpaces the rise in global mean sea level. In addition, vertical land movement in Macao contributes little to local sea level change. In the future, the rate of SLR in Macao will be about 20% higher than the global average, as a consequence of a greater local warming tendency and strengthened northward winds. Specifically, the sea level is projected to rise 8-12, 22-51 and 35-118 cm by 2020, 2060 and 2100, respectively, depending on the emissions scenario and climate sensitivity. Under the --8.5 W m 2 Representative Concentration Pathway （RCP8.5） scenario the increase in sea level by 2100 will reach 65 118 cm--double that under RCP2.6. Moreover, the SLR will accelerate under RCP6.0 and RCP8.5, while remaining at a moderate and steady rate under RCP4.5 and RCP2.6. The key source of uncertainty stems from the emissions scenario and climate sensitivity, among which the discrepancies in SLR are small during the first half of the 21st century but begin to diverge thereafter.

New stabilization design for planar vertical take-off and landing aircrafts

This paper presents a new stabilizing control law for a planar vertical take-off and landing aircraft.The model is first transformed into an equivalent form,and then a control law consisting of a linear term and a saturated term is given for a related subsystem,with the saturation levels being assigned as large as possible.Compared to the existing saturation scheme in which all states are restricted by saturations,the design brings about a relatively fast convergence.The effectiveness and advantage of the design are validated by numerical simulations.

Retarded Harrier Maneuver as a New and Efficient Approach for Fixed-Wing Aircraft to Achieve S/VTOL

Modern day VTOL fixed-wing aircraft based on quadplane design is relatively simple and reliable due to lack of complex mechanical components compared to tilt-wings or tilt-rotors in the pre-80’s era. Radio-controlled aerobatic airplanes have thrust-to-weight ratio of greater than unity and are capable of performing a range of impressive maneuvers including the so-called harrier maneuver. We hereby present a new maneuver known as the retarded harrier that is applicable to un/manned fixed-wing aircraft for achieving VTOL flight with a better forward flight performance than a quadplane in terms of weight, speed and esthetics. An airplane with tandem roto-stabilizers is also presented as an efficient airframe to achieve VTOL via retarded harrier maneuver, and detailed analysis is given for hovering at 45° and 60° and comparison is made against the widely adopted quadplane. This work also includes experimental demonstration of retarded harrier maneuver using a small remotely pilot airplane of wingspan 650 mm.

Turbulent Energy Budgets of a Ground Vortex Flow

Turbulent kinetic energy budgets are presented for a highly curved flow generated by the collision of plane wall turbulent jet with a low-velocity boundary layer. The different terms are obtained in the vertical plane of symmetry by quadratic interpolation of the LDV （Laser Doppler Velocimetry） measurements, for a wall jet-to-boundary layer velocity ratio of 2. The results, which have relevance to flows encountered in powered-lift aircraft operating in ground effect, quantify the structure of the complex ground vortex flow. The analysis of turbulent energy equation terms using the measured data revealed that production by normal and shear stresses are both very important to the turbulent structure of the impact zone of the ground vortex. This is an indication that the modeling of turbulence of a ground vortex requires a good representation of the production by normal stresses which is most important in the collision zone.

Flight Transient Estimation of VTOL Aircraft with Propellers

Methodological issues associated with the determination of the vertical take-off and landing aerodynamic parameters equipped with two rotary propellers during take-off and hovering, descent and landing are studied in the proposed article. During the computer simulation process, kinematics parameters diagrams were made, aerodynamic coefficients and propellers thrust components at all stages of aircraft take-off were estimated. That numerical data can be used in a preliminary stage of aerodynamic design for the vertical take-off and landing aircraft and electric drones at the determination of control and equalization elements geometric and kinematic parameters.

Overall eVTOL aircraft design for urban air mobility

Electric vertical takeoff and landing(eVTOL)aircraft have emerged as a potential alternative to the existing transportation system,offering a transition from two-dimensional commuting and logistics to three-dimensional mobility.As a groundbreaking innovation in both the automotive and aviation sectors,eVTOL holds significant promise but also presents notable challenges.This paper aims to address the overall aircraft design(OAD)approach specifically tailored for eVTOL in the context of Urban Air Mobility(UAM).In contrast to traditional OAD methods,this study introduces and integrates disciplinary methodologies specifically catered to eVTOL aircraft design.A case study is conducted on a tilt-duct eVTOL aircraft with a typical UAM mission,and the disciplinary performance,including initial sizing,aerodynamics,electric propulsion systems,stability and control,weight,mission analysis and noise,is examined using the OAD methodologies.The findings demonstrate that the current approach effectively evaluates the fundamental aircraft-level performance of eVTOL,albeit further high-fidelity disciplinary analysis and optimization methods are required for future MDO-based eVTOL overall aircraft design.

A multi-objective optimization problem research for amphibious operational mission of shipboard helicopters

Airborne landing with shipboard helicopters gradually replaces surface landing to dominate joint amphibious operations.A problem with shipboard helicopter mission planning is conducted in the context of amphibious operations.First,the typical missions of shipborne helicopters in amphibious operations are analyzed.An Amphibious Operational Mission Planning Model for Shipboard Helicopters(AOMPMSH)is established,with the objectives of minimizing the completion time of the amphibious campaign and minimizing troop and helicopter losses,taking the available operational resources and the order of the mission sub-phases into account.Then,a simulationbased amphibious operations effectiveness assessment model is constructed to calculate the optimization objectives of AOMPMSH by simulating the campaign development with an amphibious objective area situation transfer model and simulating the engagement process with a modified Lanchester model.A reference point based multi-objective optimization algorithm is designed to solve the proposed AOMPMSH.The population iteration mechanism employs an initial population generation method and a local search method to solve the problem of vast definition space.The population ranking selection mechanism employs a population distribution based reference point generation method to solve the problem of population irregular distribution.Finally,a simulation case with the background of a battalion-scaled amphibious campaign is presented.The calculation results verify the rationality of the proposed model and the superiority of the designed algorithm and have some reference value for the operational applications of shipboard helicopters in amphibious operations.

Comprehensive sizing and optimization method for series-hybrid unmanned convertiplane

This paper presents a novel sizing and optimization approach for the emerging serieshybrid unmanned convertiplane,which can be used to translate the top-level design requirements into the design parameters corresponding to the optimal power supply strategy and minimum total takeoff weight.The method comprehensively considers the design constraints in the rotor,fixedwing,and transition modes,and pays special attention to the characteristic response of Series Hybrid Electric System(S-HES)in complex application scenarios,especially the coupling of battery power,energy,and state-of-charge under high-power discharge conditions,the variation of fuel economy,and the adjustment of power supply strategy.With proposed method,it's possible to rapidly explore the design space in the initial design stage and find out the optimal design results with high confidence.A case study was proposed to verify the approach.The results reveal the particularity of convertiplane in terms of power requirements,and prove the necessity to consider detailed S-HES characteristic responses during parameter determination.The optimal design parameters were obtained through the hybrid control parameter optimization,which verified the effectiveness of proposed method.Possible errors and corresponding correction methods were also presented.

Future sea level rise along the coast of China and adjacent region under 1.5℃ and 2.0℃ global warming

Although future sea level rise along the China coast has been projected by various studies for different representative concentration pathways(RCPs),the projections for different warming thresholds,e.g.1.5℃ and 2.0℃,have not been done specifically for this region,to the best of our knowledge.We provide such a projection here based on the climate projections of Coupled Model Intercomparison Project Phase 5(CMIP5).The projections are given for 20 tide-gauge stations along the coast of China,Korea,Japan,and Vietnam.Vertical land motion(VLM)is also estimated for stations that have tide gauge records and satellite altimetry both covering the period of 1993-2018.Local land motion(LLM)is then estimated by subtracting the land motion due to glacial isostatic adjustment(GIA)from VLM.Without considering LLM,sea level rise by 2100 at median probability is projected to be 38-49 cm relative to the average sea level over 1986-2005 under warming of 1.5℃,and increase to 46-57 cm when the warming threshold is increased to 2.0℃.The steric component is the main contributor to this increase in sea level.Inclusion of LLM will not affect the sea level increase between the two warming thresholds,but it will make the local sea level rise by 2100 at certain locations substantially higher(up to 36 cm)or lower(up to 13 cm).

Robust Optimal Higher-order-observer-based Dynamic Sliding Mode Control for VTOL Unmanned Aerial Vehicles

This paper investigates the precise trajectory tracking of unmanned aerial vehicles(UAV) capable of vertical take-off and landing(VTOL) subjected to external disturbances. For this reason, a robust higher-order-observer-based dynamic sliding mode controller(HOB-DSMC) is developed and optimized using the fractional-order firefly algorithm(FOFA). In the proposed scheme, the sliding surface is defined as a function of output variables, and the higher-order observer is utilized to estimate the unmeasured variables,which effectively alleviate the undesirable effects of the chattering phenomenon. A neighboring point close to the sliding surface is considered, and as the tracking error approaches this point, the second control is activated to reduce the control input. The stability analysis of the closed-loop system is studied based on Lyapunov stability theorem. For a better study of the proposed scheme, various trajectory tracking tests are provided, where accurate tracking and strong robustness can be simultaneously ensured. Comparative simulation results validate the proposed control strategy′s effectiveness and its superiorities over conventional sliding mode controller(SMC) and integral SMC approaches.