A steady-state, applied-field, rectangular magnetoplasmadynamics (MPD) thruster with permanent magnets has been developed. The thrust and discharge voltage were measured by varying propellant mass flow rates, discharge current and inter-electrode distance. Argon was used as the basic propellant. The effective energy consumption rates were evaluated from the effective exhaust velocity and discharge voltage characteristics. The discharge current distribution was the key to improve the thrust performance. When mixing hydrogen to the propellant of an equal volume flow rate, the discharge current distribution shifted downstream in the discharge channel and the thrust increased to 1.7 times of that with an equal total volume flow rate only of argon.
This paper investigates helicopter rotor instability coupled with its support structure. The modal analysis is executed for a system of four rotor blades which permit a hub motion, a lagging motion, and a flapping motion. Then, an approximate formula is derived for estimating the rotor speed where the ground resonance modes become unstable. The possibility of instability is also proved by taking account of the aerodynamic coupling between the whirl modes and the scissors modes, where this possibility is not trivial because each mode is neutrally stable.
In a previous study, the authors proposed slit injection as a jet noise suppressor for the Pre-cooled Turbojet Engine, and the noise reduction performance was examined experimentally. Using the slit injection, the noise radiation decreased. It was implied that, by using the slit injection, the instability of the jet was suppressed due to the deformation of the velocity profile of the jet. In this study, the effect of the deformation of the velocity profile of the jet by using the slit injection on the instability of the jet was investigated numerically. The velocity profiles were given by the results of a Pitot tube measurement. The stability of the jet was investigated using the linear stability analysis. The numerical results were compared with the jet noise spectra obtained experimentally. It was found that, by using the slit injection, the instability of the main jet oscillation mode, which may contribute to the noise radiation, was suppressed due to the mitigation of the steep velocity gradient of the main jet shear layer.
Helicopter rotor instability coupled with its support structure in forward flight is investigated. The linearized equations of hub and blade motions around the rotor periodic trim conditions are obtained, and the system instability is evaluated based on the Floquet theorem. By taking account of blade pitch angle variations due to hub motions, simulation results show that instability similar to the SH-60K case occurs at high speed condition, and predicted stability boundaries show the fair agreement with the SH-60K flight test results.
Unmanned helicopters are widely used in various industrial applications. Conventional unmanned helicopters equip stabilizer-bar on their rotor system, and it is effective to slow the gust response of the helicopter. However, the stabilizer-bar has brought about increase of complexity and inefficiency. The purpose of our work is to develop unmanned helicopters without stabilizer-bar system. In this paper, a model of flapping motion of the stabilizer-bar is derived. A model of flapping motion of the rotor blades and interactive dynamical model of the helicopter is derived, and a lower-dimensional model is also discussed. It was found that the lower-dimensional model was good approximation of the original model in the frequency domain which is important for the unmanned helicopter's flight. Flight experiments were carried out for the evaluation and the collected data were consistent with the proposed model. In conclusion, the model of the functions of mechanical feedback of the stabilizer-bar is developed. Moreover, the drawback of the stabilizer-bar is also elucidated. Our results shows that electrical control system can replace the stabilizer-bar as the stability augment system and it is possible to improve not only gust response but maneuverability of unmanned helicopters.