This paper presents some of the current technology for the design of flexible mechanical power transmission systems for fixed-wing, V/STOL, GEM, and hydrofoil craft. Many vehicles now require long power transmission systems, between the engine and the propellers or fans, that are lightweight and highly efficient. When choosing the optimum mechanical system, three different basic arrangements of flexible couplings, pillow blocks, and interconnecting tubing should be considered. Each of these systems has advantages and disadvantages regarding angular and axial movement capabilities; therefore, some estimate of the required over-all system flexibility is necessary prior to making the tradeoff studies discussed herein.In general, mechanical systems have extremely high over-all efficiency. The most efficient lightweight mechanical design results when the system is operated at the highest practical speed and optimum use is made of all materials. This is accomplished by minimizing sliding or rolling surfaces that require lubrication, choosing sections and shapes that are highly and uniformly stressed, and attempting to use the resulting elastic deflections to advantage. One approach to making system tradeoff studies and a discussion of various intermediate supports, splines, and the contoured diaphragm-type flexible couplings are included in this paper. Nomenclature E = modulus of elasticity, psi / = shaft length, in. N c = first critical speed, rpm /S& = bending stress, psi S s = torsional shear stress, psi S s t = critical buckling shear stress, psi r = outside radius of shaft, in. Ti = inside radius of shaft, in. T = torque, Ib-in. t = shaft wall thickness, in. ju = Poisson's ratio y = total deflection at end point, in.