Rapid estimation of bending frequencies of rotating beams

A procedure is presented in the form of charts which permits the rapid estimation of the natural bending frequencies of helicopter rotor blades, both rotating and nonrotating. Since the approach is based on Southwell's equation, an evaluation of the method with regard to such things as higher modes, blade offset, and variable mass and stiffness distributions is also given. The evaluation shows that when nonrotating beam bending modes are used, Southwell's equation yields reasonably accurate bending frequencies for rotating helicopter blades. Example comparisons of frequencies estimated using the charts with values given by the manufacturer for several actual blades show that the simplified procedure yields good practical results.