System for generating sequences of phased gust or taxi loadings

Abstract

A method is presented for Monte Carlo generation of phased load conditions for multiple loading durability testing. These load conditions are consistent with the gust and taxi environmental-dynamic model used in aircraft design. Time histories of individual loads from sequences of load conditions are consistent with the load sequences used in standard single load point durability and damage tolerance testing and analysis. Truncation can be applied to a sequence of load conditions so that only the most severe are retained. The truncation process is consistent with the mission analysis approach to design and to durability and damage tolerance. The load conditions can be modified using an induced autocorrelation approach to give variation in the number of zero crossings of the load time histories. OAD sequences generated for durability and damage tolerance analysis are usually applied at a single point. However, the stresses in a component of structure often arise from combinations of multiple loads. Selection of a single application point and direction is often a difficult task. This is particularly true for aircraft structures flying in turbulence. In that case, loads arise from complex combinations of ex- ternal and internal forces. At McDonnell Douglas the full-scale durability test of the C-17 aircraft requires the simultaneous application of test loads which are properly phased by some rational definition of phasing. The following criteria was developed for satisfying this requirement: 1) Time histories for each individual load (e.g., wing root normal bending) must be consistent with load sequences used in durability and damage tolerance testing and analysis. 2) The phasing of the simultaneously applied loads must be consistent with the dynamic gust and taxi models used to design the aircraft. A method is presented here that meets this criteria. The method generates load conditions, that is, multiple loads for simultaneous application at several points on an aircraft struc- ture. The multiple loads are correlated in conformance with the mathematical model of the gust environment and the structure. In this model the magnitude, frequency response, and correlation of loads are based on a flexible linear aircraft response to a forcing function having the characteristics of a stationary Gaussian random process (see Fig. 1). The specifics of the mathematical model of an aircraft flying in turbulence have been summarized by Hoblit in Ref. 1. This method as implemented at Douglas Aircraft Corpo- ration is called the Phased Loading Sequence Generator Sys- tem (PLSGS). This system will be used to generate gust and taxi loadings for the C-17 full-scale durability test. For the C-17 application the taxi forcing function (the runway) was treated as a stationary Gaussian random process, and the