THE PROBLEM of determining bending moments in rotor blades is a fundamental one in the design of rotating wing aircraft. Several methods for solving the problem have been proposed, both empirical and theoretical. In general, the theoretical methods have been so involved that most blade designers have had to take recourse to empirical or semiempirical methods. Recently, Owen has shown that, in several cases for which he obtained more or less precise solutions of the differential equation for blade bending, Cierva's empirical formula gave excellent agreement with the ' "exact" solutions for the bending moment. Owen's method is not, however, adaptable to the practical case of a blade of varying section. This has made it extremely worth while to determine the assumptions and limitations inherent in Cierva's formula, in order that it may be used with confidence in its range of applicability. I t will be the purpose of this paper to review the fundamental equations that govern blade bending; to indicate several possible methods of solution, together with the simplifying assumptions necessary for each; and to discuss the analytical basis for various approximate and empirical methods, particularly Cierva's formula. Finally, an extension to hinged rotor blades of the Stuart-Myklestad tabular method of propeller blade stress analysis' 10 will be made, together with some notes on numerical calculation. The question of aerodynamic loading on the blade will not be discussed here, since it has been treated in considerable detail in a number of other papers.