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An approach to reduce certification development cost can be accomplished by testing the important strength allowables that make up the primary design drivers. Through a process referred to as a “modified building block approach,” these primary design drivers are weighted and selected based on criticality. This approach was used to minimize the development cost of a prototype composite tailboom being considered for a light model helicopter (similar in size to a Bell Model 407). The material system of choice (AS4/APC-2 thermoplastic) and the tailboom's susceptibility to impact damage drove the need to understand impact damage and its effect on strength. Compression after impact (CAI) at barely visible impact damage (BVID) was therefore selected as the critical design parameter. The cost benefit is realized by focusing on the critical design parameters; thus the number of coupon tests necessary to support full aircraft development can be significantly reduced because only limited design aspects have to be considered. The critical design drivers selected would not necessarily be applicable to the fuselage or rotor blades, but they do provide a means to optimize the tailboom design while minimizing the development cost. Certification would still be verified by full-scale testing.
compression after impact, barely visible impact damage, composite tailboom, modified building block, thermoplastic, certification
Principal engineer, Airframe Structures, Bell Helicopter Textron Inc., Fort Worth, TX