As a basic research for the active control concept applied to the supression of helicopter vibrations by the higher harmonic blade pitch vibration, the frequency response of a hovering rotor executing harmonic blade pitch variation is investigated with placing emphasis on its aeroelastic behavior. A modal analysis method to predict aeroelastic response characteristics of the hub vertical load is developed utilizing Loewy's two dimensional wake model. Experiments with a model rotor are intentionally planned to grasp quantitatively the effects of blade structural and geometrical properties as well as wake structure on the frequency response over a wide range up to 5th harmonic of rotor speed. Both numerical and experimental results showed clearly the unique dependence of the hub load response upon the blade collective pitch control frequency and amplitude, which are outstanding in rather narrow range near the integer multiples of blade passing frequency. Correlations between them are good within the scope of this model rotor testing and usefullness and validity of Loewy's model for estimating the response due to harmonic pitch variation are ascertained.
It is well recognized that the autorotation capability of a rotor plays a fundamental role in the flight safety of a helicopter following power failure. Although the vertical autorotation is a basic steady free flight mode of an isolated rotor descending in the air, there is little research to investigate its aerodynamic and dynamic behaviours by taking the wake deformation into account. In this research, to grasp quantitatively the wake structures of a rotor in the vertical autorotation and to develop a reasonable wake model to be consistent with steadiness of the descending motion, flow visualization studies for model rotors are conducted. Pictures obtained by visualization studies show clearly the facts that regular and steady flow fields are formulated around the rotor in the vertical autorotation state, and this aerodynamic working condition is different from the vortex ring or the turbulent wake state defined by the classical rotor aerodynamics. Based on flow visualization results, a simplified wake model which represents dominant tip and root helical vortices by definite numbers of the circular ring vortices is proposed and several numerical analyses of the flow fields are carried out. Numerical results show good correration with those of visualization and wake survey by a hot wire anemometer.
A closed-type strand bomb was developed to trap all species participating in metalized solid propellant combustion. Combustion efficiencies (η) of aluminum (Al) and boron (B) were measured using titration techniques. Variations of ηAl and ηB with pressure and mutual effects of Al and B combustion were investigated. Al particles burn faster than B particles. However, it was found that burning of Al is hindered by B burning when Al coexists with B in propellants. On the other hand B burning is not affected by Al. These experimental results could be explained using a simplified analytical model describing agglomeration, ignition delay and burning processes of particles on the burning surface.
This paper describes an experimental investigation on the blockage effect of an upper wall of wind-tunnel on the flow past sphere placed on a ground plane. The experiment was carried out in an N. P. L. blow down type wind-tunnel having a working section of 400mm×400mm×2, 000mm in size at the Reynolds number Re=4.72×104. The surface pressure distribution on a sphere, the frequency of arch vortices shedding from sphere and the length of recirculation region were measured, and the drag and lift coefficients were determined by integration of surface pressure. The experimental results were discussed in comparison with the corrected results by the existing formulae. From those results, it was found that the drag and lift of the sphere increase as the blockage ratio (S/C) increases, and that the critical value of blockage ratio of wind-tunnel wall which can be ignored, is S/C=0.05.