969. Cost-Weight Interface in Aircraft Design

Paper

Abstract

This paper presents the results of a study sponsored by the U.S. Air Force Flight Dynamics Laboratory aimed at developing preliminary design level techniques to estimate the cost of flight vehicle basic structure, identifying sensitivity to the structural concepts and materials used. The study is intended to provide improved and more systematic cost estimating techniques in support of cost as a design parameter for military aircraft. Such usage is currently being recognized as a major design consideration. The term design-to-cost has been used to describe this objective, although the term has been given a number of different interpretations.
Two estimating techniques have resulted. One is concerned with supporting trade studies and predicts the relative cost of airframe structures that use different types of materials and constructions. The second estimates aircraft subsystem costs and adds these to the results of the first technique, providing total airframe system cost in terms of man-hours and materials, while retaining sensitivity to types of material and construction.
The trade study estimating technique involves using the output of automated structural design synthesis programs as cost-related variables and derived cost estimating factors as dummy variables in estimating relationships covering detailed basic structure elements (e.g., ribs, spars, skin panels, etc.). Weight is the primary synthesis program output used. The cost-weight interface is modified by cost estimating factors derived from an analysis of the relative complexity of the manufacturing process associated with typical types of construction and material. Relative complexity is referenced to benchmark cases based on historical cost data. The horizontal stabilizer was used as a demonstration case. The method is currently being extended to the entire basic structure. Wing and fuselage cost data are being collected, structural synthesis programs are being suitably modified, cost estimating relationships are being formulated, and a full range of complexity factors are being derived. The second technique, system costing, uses weight and subsystem characteristics as the primary cost-related variables. It produces estimates structured so as to be comparable to traditional estimating and is modularized for compatibility with integrated design procedures.
These estimating techniques are particularly useful for preliminary design phase cost estimating in support of design trade studies involving choices in structural concepts where vehicle and structural synthesis programs are available. They also appear to be particularly suited to integrated computer-aided design procedures. The primary application to be made by AFFDLI will be in advanced development programs, in structural research and development independent of systems acquisition, in assessments of the impact of various technologies, and in providing an alternative source of estimates against which weapons systems proposals can be evaluated.