Recent studies about rotary wings for next-generation rotorcraft, advanced air mobility, and Mars rotary aircraft are introduced. Due to recent technological innovations such as electrification, new applications of rotary-wing aircraft in aeronautics are expanding, and various new technologies are being researched and developed. This paper also surveys studies on aerodynamics, noise, and vibration of various kinds of rotorcraft, including the advanced air mobility vehicles, also known as eVTOL (electric Vertical Take-Off and Landing) aircraft, high-speed rotorcraft, and Mars helicopters. New airframe configurations such as multirotors and utilization in extraterrestrial environments may encounter new aerodynamic effects that were not previously well understood, which influence the performance and noise characteristics of the aircraft. To address these issues, research and developments have been conducted based on experimental and numerical simulation technologies. For multirotors, aerodynamic interferences between rotors and between rotor and fuselage are investigated to improve the flight performance and reduce noise. In addition, low drag technologies taking account of the interferences between aerodynamic components need to be explored further, as the required propulsion power is directly related to the airframe drag for the next-generation rotorcraft with complex configurations.
The propellant utilization efficiency of RAIJIN-66, a 2-kW class Hall thruster with anode layer, was investigated with varying anode temperature to evaluate ionization length, which is the length of the ionization region determined by the channel and magnetic field design. During RAIJIN-66’s thermally transient operation from the ignition, the ion beam current was monitored as a function of anode temperature using an ion collector plate biased negatively at 40 V. Two magnetic field profiles were examined with and without trim yokes. From the measured correlation between the propellant utilization efficiency and anode temperature, the ionization length was evaluated. As a result, the ionization length was 5.6 mm without the yokes, and 2.5 mm with the yokes. The ionization length of 2.5 mm was enough to achieve high propellant utilization efficiency with xenon, while it is too short to operate with argon because argon ionization mean free path is notably longer than xenon. This measured length was experimentally confirmed to be almost independent of discharge voltage and electron temperature. In conclusion, to achieve the high performance with argon propellant equivalent to that with xenon propellant, the ionization length as long as 14.5 mm is required in RAIJIN-66 size thrusters.
A challenging task is to explore a compliant morphing airfoil that allows for aerodynamically superior shape change without a predefined target shape. This study addresses this task and aims to develop an optimal design method that simultaneously explores the optimal morphing shape and internal structural configuration, to obtain a morphing airfoil structure that is superior in terms of the aerodynamic and structural performance. A panel method is adopted to evaluate the aerodynamic performance, which provides a close estimate of the aerodynamic characteristics of the airfoil profile with a low computational cost and low fidelity. The structure is modeled using the finite element method. A structural configuration that improves the aerodynamic performance evaluated by the panel method is obtained by topology optimization. Additionally, the sensitivity, which is a guideline for the optimal design search to improve the aerodynamic performance, is derived to efficiently perform the optimization. The proposed design method is then applied to the multi-objective optimal design problem of morphing airfoils, considering the aerodynamic and structural performances. Through numerical examples, the validity of the proposed method is discussed by investigating the aerodynamic and structural performances of the optimized structural configurations.
This study examines the methods for approximating the fast attitude maneuver of a spacecraft with flexible structures. Two methods have been proposed for attitude maneuvering around a single axis, in which the torque input is provided in the form of a time polynomial. In Method 1, the residual first-order vibration mode was eliminated by appropriately selecting the maneuver time. In Method 2, first-order residual vibration was suppressed by adding the boundary conditions for the vibration mode, and the maneuver time could be freely set. Furthermore, this study demonstrates that both methods are effective in reducing the residual vibration of higher-order modes. We also investigate the robustness of these inputs to uncertainties in the frequency and damping ratio of the first-order vibration modes. Simulations of the proposed methods are conducted using a spacecraft model. The results demonstrated that the spacecraft could be oriented to the target attitude with high accuracy, and at the same time, the vibration of the first-order mode could be suppressed regardless of modeling errors (such as frequency variations and damping effects). It was also shown that the residual vibrations of the second-order and higher-order vibration modes were suppressed to a permitted low value. Robustness to uncertainties in the vibration frequency and damping ratio was also demonstrated.
Formation flying (FF), which employs multiple satellites to achieve missions that are difficult to be realized with a single satellite, is becoming an important technology. Considering some FF missions such as exo-planet observation missions employing occulters, or X-ray telescope/interferometer missions, in which satellites have to be aligned in a straight line toward celestial targets, this paper studies the design of two satellites’ orbits and the schedule for observing multiple targets. In the proposed optimization method, the whole observation sequence will be divided into two phases: observation of the same target for several orbits, and the orbit maneuver to change observation targets. The relative orbits of two satellites for these two phases are optimized in terms of ΔV. Optimization results showed that along-track and cross-track formations require significantly small ΔV. By employing these results, the whole mission sequence to observe multiple targets is optimized by considering all the design parameters, including even target observation order. The results showed that a reasonable “Pareto-curve” was obtained in terms of total time and ΔV, including a better solution in both criteria than the ones obtained using the method in literature.
Scramjet engines are a promising airbreathing propulsion technology for high-speed transportation as well as access-to-space. The air intake plays a major role in determining the overall performance of scramjets. Designing high-performance intakes by taking both aerodynamic and thermodynamic characteristics into account represents a significant task. This paper presents new knowledge on scramjet intake performance along with underlying physical ground that has been obtained by comprehensive analytical formulations of global intake characteristics derived on a theoretical basis. Heat transfer across the intake surface has been found to be responsible for the trade-off between compression efficiency and drag under assumptions of a fixed contraction ratio or mean exit temperature, while the mean exit temperature and the contraction ratio are the decisive factors for the compression ratio and efficiency. Multi-objective design optimization has then been performed to verify these trade-offs and examine their influence on intake design. The resultant optimal solutions have been found to underpin the design strategy based on these relations. The new insights gained from this study conduce to rapid and reliable design of high-performance intakes.