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The concept of variable rotor speed (VRS) has been recognized as an efficient means to improve rotorcraft operational performance and environmental impact, with electrification being a potential technology to further contribute to that. This paper explores the impact of optimal implementation and scheduling of VRS and power management strategy for conventional and hybrid-electric rotorcraft on energy, fuel, and emissions footprint. A multidisciplinary simulation framework for rotorcraft performance combined with models for engine performance and gaseous emissions estimation is deployed. A holistic optimization approach is developed for the derivation of globally optimal schedules for combined rotor speed and power split targeting minimum energy consumption. Application of the derived optimal schedules at mission level resulted to a 6% improvement in range capability for the VRS tilt-rotor relative to its conventional counterpart. For the hybrid-electric tilt-rotor, combined optimization of VRS and power management leads to an increase in range to 18.4% at 40% and 25% reduced payload for current (250 Wh/kg) and future (450 Wh/kg) battery technology, respectively. For representative urban air mobility (UAM) scenarios, it is demonstrated that the VRS concept resulted in up to 10% and 14% reductions in fuel burn and NOx relative to the nominal rotor speed case, respectively. The utilization of the combined optimum VRS and power split schedules can boost performance with reductions of the order of 20% and 25% in mission fuel/CO2 and NOx at a reduced payload relative to the conventional tilt-rotor.

In order to overcome the influence of internal and external disturbances caused by rotor tilt motion and gust disturbance on the full flight mode control of a tilt-rotor unmanned aerial vehicle (UAV), a design method using fuzzy backstepping control based on an extended-state observer (FBS-ESO) is proposed. In this paper, fuzzy control is used to tune the parameters of the backstepping control law online, and the extended-state observer estimates the total disturbance of the controlled system to improve the controller’s robustness and anti-disturbance capability. This paper designs the attitude control system of a tilt-rotor UAV based on an FBS-ESO controller. The control performance of the FBS-ESO controller is tested in a hardware-in-loop simulation of the attitude control system. The simulation results show that changing the rotor tilt angle will destroy the stability of the traditional backstepping controller and active disturbance rejection controller (ADRC). In contrast, the FBS-ESO controller maintains good control performance. In addition, the performance of the FBS-ESO controller is not be significantly affected by adding external gust disturbance or changing the UAV parameters in the simulation. These disturbances significantly impact the traditional backstepping controller and ADRC. Therefore, the FBS-ESO controller has better anti-disturbance capabilities and robustness, as well as higher attitude control accuracy.

This article presents the four-dimensional surfaces that guide the gait plan for a tilt-rotor. The previous gaits analyzed in the tilt-rotor research are inspired by animals; no theoretical base backs the robustness of these gaits. This research deduces the gaits by diminishing the adverse effect of the attitude of the tilt-rotor for the first time. Four-dimensional gait surfaces are subsequently found on which the gaits are expected to be robust to the attitude. These surfaces provide the region where the gait is suggested to be planned. However, a discontinuous region may hinder the gait plan process while utilizing the proposed gait surfaces. The ‘Two Color Map Theorem’ is then established to guarantee the continuity of each gait designed. The robustness of the typical gaits on the gait surface, obeying the Two Color Map Theorem, is demonstrated by comparing the singular curves in attitude with the gaits not on the gait surface. The result shows that the gaits on the gait surface receive wider regions of the acceptable attitudes.

The increasing demands for air-taxi operations together with the ambitious targets for reduced environmental impact have driven significant interest in alternative rotorcraft architectures and propulsion systems. The design of hybrid-electric propulsion systems (HEPSs) for rotorcraft is seen as being able to contribute to those goals. This work aims to conduct a comprehensive design and tradeoff analysis of hybrid powerplants for rotorcraft, targeting enhanced payload-range capability and fuel economy. An integrated methodology for the design, performance assessment, and optimal implementation of HEPSs for conceptual rotorcraft has been developed. A multidisciplinary approach is devised comprising models for rotor aerodynamics, flight dynamics, HEPS performance, and weight estimation. All models are validated using experimental or flight test data. The methodology is deployed for the assessment of a hybrid-electric tilt-rotor, modeled after the NASA XV-15. This work targets to provide new insight into the preliminary design and sizing of optimally designed HEPSs for novel tilt-rotor aircraft. The paper demonstrates that at present, current battery energy densities (250 Wh/kg) severely limit the degree of hybridization if a fixed useful payload and range are to be achieved. However, it is also shown that if advancements in battery energy density to 500 Wh/kg are realized, a significant increase in the level of hybridization and hence reduction of fuel burned and carbon output relative to the conventional configuration can be attained. The methodology presented is flexible enough to be applied to alternative rotorcraft configurations and propulsion systems.

Wall-climbing drones have many applications, including structural health monitoring of civil structures, such as bridges and high-rise buildings, cleaning of solar panels to improve power generation efficiency, and airplane visual inspections. For these applications, the drone requires a high-payload capacity, and consequently the size and weight of the drone increase. The drone also should not damage the target structures considering the purpose of its mission. Our previous versions of a wall-climbing drone could have high-impact force on the surface where the drone perches and on the platform itself because of the impact caused by a fast pose change and landing speed. In order to overcome this potential risk, a mechanism and a control algorithm for perching on a vertical surface through low-speed pose change are proposed in this paper. The drone platform is based on an X-configuration quadcopter, and a tilt-rotor mechanism is incorporated into the two axes, such that the front thrusters and the rear thrusters are paired. The vertical soft landing mechanism using the tilt-rotors is validated by the experimental tests of the prototype.

This paper introduces a novel gimballed rotor mathematical model for real-time flight simulation of tilt-rotor aircraft and other vertical take-off and landing (VTOL) concepts, which improves the previous version of a multi-purpose rotor mathematical model developed by ZHAW and Politecnico di Torino as part of a comprehensive flight simulation model of a tilt-rotor aircraft currently implemented in the Research and Didactics Simulator of ZHAW and used for research activities such as handling qualities studies and flight control systems development. In the novel model, a new formulation of the flapping dynamics is indroduced to account for the gimballed rotor and better suit current tilt-rotor designs (XV-15, V-22, AW-609). This paper describes the mathematical model and provides a generic formulation as well as a specific one for 3-blades proprotors. The method expresses the gimbal attitude but also considers the variation of each blade&rsquo, s flapping due to the elasticity of the blades, so that the rotor coning angle can be represented. A validation of the mathematical model is performed against the available literature on the XV-15 Tilt-rotor aircraft and a comparison between the previous model is provided to show the improvements achieved. The results show a good correlation between the model and the reference data and the registered performance allow real-time flight simulation with pilot and hardware in the loop.

A nonlinear unsteady aerodynamics model is coupled with a three degree of freedom quadplane to control the forward and backward transition between hover and steady level flight. The unsteady lift and drag forces are modeled using a lumped vortex model for flat plates. Two variants for the quadplane are considered: (i) a pusher and (ii) a tilt-rotor configuration in the absence of control surfaces to assess the controllability for altitude, attitude and forward speed. Conventional PID control is applied to generate the control inputs. The simulation results conclude that the pusher quadplane configuration is effortless to control as all the selected states are controllable, whereas for the tilt-rotor configuration, even though the vehicle is stable, altitude control is significantly more challenging due to one less control input when compared to the pusher configuration.

In order to calculate the unsteady aerodynamic characteristics of a tilt-rotor in a conversion mode, a virtual blade model (VBM) and an real blade model (RBM) are established respectively. A new multi-layer moving-embedded grid technique is proposed to reduce the numerical dissipation of the tilt-rotor wake in a conversion mode. In this method, a grid system generated abound the rotor accounts for rigid blade motions, and a new searching scheme named adaptive inverse map (AIM) is established to search corresponding donor elements in the present moving-embedded grid system to translate information among the different computational zones. A dual-time method is employed to fulfill unsteady calculations on the flowfield of the tilt-rotor, and a second-order centered difference scheme considering artificial viscosity is used to calculate the flux. In order to improve the computing efficiency, the single program multiple data (SPMD) model parallel acceleration technology is adopted, according to the characteristic of the current grid system. The lift and drag coefficients of an NACA0012 airfoil, the dynamic pressure distributions below a typical rotor plane, and the sectional pressure distributions on a three-bladed Branum–Tung tilt-rotor in hover flight are calculated respectively, and the present VBM and RBM are validated by comparing the calculated results with available experimental data. Then, unsteady aerodynamic forces and flowfields of an XV-15 tilt-rotor in different modes, such as a fixed conversion mode at different tilt angles (15°, 30°, 60°) and a whole conversion mode which converses from 0° to 90°, are numerically simulated by the VBM and RBM respectively. By analyses and comparisons on the simulated results of unsteady aerodynamic forces of the tilt-rotor in different modes, some meaningful conclusions about distorted blade-tip vortex distribution and unsteady aerodynamic force variation in a conversion mode are obtained, and these investigation results could provide a good foundation for tilt-rotor aircraft design in the future.

Multi-rotor autonomous aerial vehicles have been proposed for several missions where the capability of controlled flight in small areas is required. Unfortunately, small-size electrically powered rotary wings UAVs have usually very short endurance, because of inherent limitation in battery energy density and constraints on maximum weight of the battery pack. At the same time, maneuverability can be improved by a variation of rotor thrust orientation. This paper presents a novel quad—rotor configuration where the use of a combustion engine and variable pitch propeller will allow for increasing endurance performance, whereas tilting of rotor disks improves maneuverability. A preliminary design of simple control laws is also discussed for this particular configuration, tested on a few basic maneuvers.