Use of two-dimensional unsteady flow theory with low aspect-ratio blades

Abstract

It is known that if two-dimensional aerofoil theory is used to calculate the mean forces on a marine propeller blade, the answer obtained is likely to over-estimate the true answer by a factor of typically three. It is shown that when unsteady forces (at blade-rate frequency) are being calculated, the use of unsteady two-dimensional aerofoil theory is much more acceptable, being associated with errors of about 25%. It is pointed out that the more accurate unsteady lifting-surface theory is prohibitively expensive for use regularly as a design tool. and further, that the accuracy of the answer may be limited in any case by the accuracy of the measured wake data.