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This paper presents a functional description of a versatile computer program developed to provide rapid response to spectrum generation requirements. The program, when supplied with flight profiles and the distribution of those profiles during the aircraft lifetime, will create a randomly sequenced flight-by-flight spectrum in terms of load factors, loads, or stress at a specific structural location.
The heart of the program is a data base containing the frequency definition of load factor occurrences representing the gust and maneuver load cycles for various flight segments. This paper discusses the establishment of the data base from data published in MIL-A-87221 and the FALSTAFF reports, from analytically derived load factor data and from flight load records from specific aircraft. The various forms of load factor data acceptable to the program, namely, exceedance curves, range/mean tables, and the FALSTAFF methodology, are discussed as to advantages and disadvantages.
The program will accept multiple flight profiles each with multiple flight segments defined by segment title, segment time, and selected significant flight parameters such as gross weight, Mach number, altitude, and aircraft geometry. For each profile, the procedure creates a series of individual flight load factor spectra each of which differs from all others in load sequence and magnitude while maintaining the sequence of the flight segments. The spectra include the infrequent high loads randomly selected from within the statistics of the data base. Load-factor spectra to stress spectra conversion is briefly discussed in terms of the program capability.
The output spectrum is a series of flights representing a defined period of usage with the flights representing the various profiles randomly sequenced within the supplied distribution statistics. The spectrum form is suitable for direct input to fatigue or crack growth programs or can be output to tape for submittal to a fatigue testing machine.
load spectra, fatigue loads, load sequence, fatigue (materials), testing
Senior engineering specialist, Rockwell International, Los Angeles, CA