SHAPES FOR THE TIPS OF LIGHTLY-LOADED PROPELLER BLADES

Abstract

The considerations of this paper are relevant to maximizing the effiiciency of lightly-loaded propellers, particularly such as are used on man-powered aircraft. They are also relevant, in a more qualitative fashion, to more high-loaded propellers (e.g. for motor-gliders) where noise reduction is important. Prof. E. Larrabee has observed that there is an error in Glauert's analysis of the effect of a finite number of propeller blades. When the appropriate correction is made, it is found that special attention has to be paid to the development planform shape of the blade tips if the local axial and circumferential induced velocity factors are not to become infinite of indefinite. It is showm that if the tip shape is 'sharper' than a parabola, these factors become zero at the tip whilst a tip of suitably rounded form but 'blunter' than a parabola can lead to finite values of these factors. The former condition seems preferable both on grounds of improved efficiency and dimished noise.