PERIODIC PROPELLER FORCES IN NON-UNIFORM FLOW,
MASSACHUSETTS INST OF TECH CAMBRIDGE
Pagination or Media Count:
Ship screws experience vibration inducing periodic forces at blade frequency and its multiples because of their operation in the circumferentially non-uniform flow field of the wake. These forces are presently computed on the basis of either quasi-steady analyses or stripwise application of the Sears function for the unsteady lift of a two-dimensional airfoil subjected to sinusoidal gusts. The quasi-steady analyses are correct only at zero frequency while the unsteady strip theory is presumed correct only at high frequencies because of the neglect of three-dimensional effects in the latter. Using a vortex sheet model, the linearized integral equation for the bound vorticity of the blades is derived. This is simplified by approximations analagous to those made by Reissner for the plane wing. The difference between the downwash in the three and twodimensional flows is determined in a vortex line model of the propeller. The analysis proceeds on the basis of computing a correction to the two-dimensional unsteady strip theory to account for three-dimensional effects.