523. An Approximate Method of Estimating Weights of Rim-Loaded Rotating Disks

Paper

Abstract

In optimizing a gas turbine aircraft powerplant for a given mission, it is necessary to investigate the weight and performance characteristics for a number of engine cycles and configurations. While weight scaling procedures are comparatively easy to define for static structural components, the weight of a high speed rotating part such as the compressor or turbine disk is very sensitive to changes in rpm, temperature, and airfoil weight and is difficult to scale accurately from other designs. Since these disks represent a significant proportion of total engine weight, a quick and accurate method of estimating disk weight was necessary to establish total engine weights and to make parametric studies.
An optimum disk is designed by a iterative solution in which the disk profile is divided up into a large number of circumferential segments and each segment is analyzed and connected to the adjacent segment by compatibility equations. Computations are done rapidly by a digital computer and the result is a disk profile that has the desired stress distribution. A simple and more rapid method was needed for preliminary studies so a semi-graphical method was developed. A trapezoidal shape was assumed for the disk profile and equations were developed for this shape that defined the disk weight as a function of the applied load, disk radii, material weight density, and allowable average tangential stress. These equations were plotted in graphical form and eliminated a ‘trial and error’ process of successively assuming disk profiles and calculating the stresses until the desired stress level was obtained.
Disk weights estimated from these curves agree with final designs within 5%. Errors increased slightly on highly stressed disks where the bore thickness is large compared to the rim thickness. The trapezoidal profile assumed gives a conservative radial stress distribution which partially compensates for simplifying the solution by not considering the effect of loads applied by spacers or the reduced section around bolt holes.