2036. The Effect of Composite Material Allowable Changes on VTOL Airframe Weights

Paper

Abstract

This paper describes and demonstrates a method of predicting the weight change of any VTOL aircraft structure as a result of construction material design allowable changes (with emphasis on composite materials). This analysis is needed because the bulk of the composite airframe preliminary design experience and data base was accumulated at a time when less conservative allowables were used (and the same allowables were applied to all load cases). The weight savings reported in earlier composite program have generally eroded in the decade of the 80’s as a result of the application of hot/wet property reductions, open hole allowables, impact damage reduction factors, and new failure and design criteria. Also, improvements in the specific properties of light alloys have reduced composite weight savings potential (by reducing metal aircraft component baseline weights). The analysis assumes small changes in the design allowables which do not affect vehicle configuration, structural concepts, or degree of dependence on bonding or mechanical fastening. The selection and application of specific materials throughout the structure remain unchanged. The design criteria for each structural element is held constant. Also, no reduction in safety margins is permitted and internal load levels are presumed to remain constant These assumptions make the analysis most appropriate where material properties change incrementally. Gross changes in material properties ultimately result in structural design changes to the vehicle. These types of cascading effects are not addressed by this analysis.