An empirical study of hovercraft propeller noise

Abstract An assessment of the hovercraft noise problem shows that the noise arising from the use of air propellers is often the dominant factor. Of great concern at the time of writing is the external noise problem associated with hoverferry operations. Because of the high rate of working possessed by hovercraft, the area surrounding a ferry terminal will be subjected to a large number of noise peaks per day. From work carried out by the Wilson Committee on the Problem of Noise, tentative noise design targets can be established. The problem of accurate prediction then arises. Difficulty was experienced with the prediction of propeller noise at the low tip speeds, Mach tip 0·4 to 0·6, essential for any major reduction in propeller noise and so exploratory tests were run on a propeller of the Hovercraft Development Ltd. HD 1 hovercraft. The results from these tests showed the areas of discrepancy between theory and experiment and highlighted the uncertainty over the contribution of vortex noise. Rotational noise estimates using the Gutin method and vortex noise estimates using the Yudin method gave overall results of around 10 dB less than those given by experiment. As a result of these differences it was decided to attempt to derive an empirical method based on available test data. By means of experimentally determined correction curves for blade number it was found possible to achieve a correlation of noise (at an azimuth angle of 105°) with a parameter equal to tip Mach number ( HP ) 1 5 for lines of various diameter. Of the 86 test points involved in the correlation, 93 % fell within ± 2 1 2 dB while three of the remainder appeared to be rogue points, possibly due to blade flutter. This empirical method shows both tip Mach number and blade number effects to be considerably weaker than predicted by theory. The values given by the method are dB Linear, i.e. unweighted values. To obtain A weighted values, to compare with a design target of 7o dBA at 500 ft, some experimental results are given of dBLIN-dBA. For a four-blade propeller a value of (dBLIN-dBA) = 5 ·0 is suggested. Typical results derived from the method show that a single 16 ft diam. propeller absorbing 200o hp at a tip Mach number of 0·5 will generate 81·5 dBA at 500 ft; or a 10 ft diam. propeller absorbing 1000 hp at a tip Mach number of 0·5 will generate 75·5 dBA at 500 ft. Hence the design target of 7o dBA at 500 ft will be difficult to meet and with the above figures, will require the design of new low-tip-speed propellers.