BASIC RESEARCH ON NOISE REDUCTION OF HOVERCRAFT

中小型气垫船将用于灾害地区的急救运输等一般用途。噪音是亟待解决的严重问题之一。特别是在平静的情况下，用于江河、湖泊巡逻的船只会产生较大的噪音。从操作人员工作环境的角度来看，降噪是必要的项目之一。采用高性能声级计对两种气垫船进行了野外噪声测量。为实现现场数据的快速处理，设计了测量台。利用摄像机和数码相机记录气垫船在水面高速行驶时的测量情况。由此得到的主要结果如下。首先，最大噪音水平超过110分贝。二是噪声由螺旋桨、升降机、发动机、排气、裙摆振动等噪声组成。其中螺旋桨噪声源由于其高阶、高次谐波数和噪声源机理复杂，噪声最为严重。作为一项基础研究，主要集中在螺旋桨噪声的分析上。试验螺旋桨是5种不同尺寸的大型飞机模型的6个螺旋桨。制作了一个螺旋桨支架来测量不同转速下的推力。主要结果如下。首先，推力与螺旋桨盘本身成正比，而不是与它的平方成正比。其次，噪声水平几乎与磁盘负载成正比。第三，大直径和小直径螺旋桨的组合可以在恒定推力下降低噪音。在不同组合角度下，推力和噪声没有显著差异。作者们现在正准备发表这一结果。

Hovercrafts of middle or small sizes are going to be used for general purposes including first-aid transportation in a disaster area. Noise is one of serious problems to be solved. Especially, a craft operated for patrol of rivers and lakes causes relatively big noise in a calm circumstance. From the point of view of working environment of operators noise reduction is one of necessary items.Noise measurements were carried out in open field using a sound level meter of high performance for two different hovercrafts. A measuring desk was made for speedy data processing in field. Video and digital cameras were used to record situations of measurements of high-speed runs of hovercrafts on water surface. Main result thus obtained is as follows. First, maximum noise level is over 110 dB. Second, the noise is composed of that of propeller, lift fun, engines, exhausts, vibration of skirt and other. Third, among them noise from the propeller is the most serious by the reason of its high level, its numbers of higher harmonics and complicated mechanism of noise source.As a basic research the main effort of work was concentrated to analyze noise from propeller. Test propellers were 6 propellers for large airplane model of 5 different sizes. A propeller stand was made to measure thrust for different rpm. Main result is as follows. First, thrust is proportional to the propeller disk itself instead of its square. Second, noise level is nearly proportional to the disk loading. Third, a combination of propellers of large and small diameters can reduce noise at a constant thrust. Fourth, thrust and noise do not show significant difference for different angle of combination. The authors are now preparing publication of the result.