FINITE ELEMENT CORRELATION OF THE U.S. ARMY/BHTI ACAP COMPOSITE AIRFRAME HELICOPTER

A cooperative effort between the Aviation Applied Technology Directorate (AATD) of the U.S. Army Aviation Systems Command (AVSCOM) and the Langley Research Center (LaRC) of the National Aeronautics and Space Administration (NASA) investigated the effects of difficult-to-model components on the vibration response of the Army/Bell Advanced Composite Airframe Program (ACAP) helicopter. A series of ground vibration tests of the Army/Bell ACAP were conducted by AATD using the AATD ground vibration testing facility. NASTRAN finite element correlations were performed for LaRC by Bell Helicopter Textron, Inc. (BHTI). This paper summarizes the results of the ground vibration testing and associated NASTRAN analysis used to identify component effects on overall airframe vibration response. Previous correlations of the Army/BHTI ACAP showed good agreement between NASTRAN and test through 10 to 20 Hz, but poor agreement in the higher frequency range of 20 to 35 Hz, which encompasses main rotor 4/rev and 6/rev operating frequencies. The objective of the program was to identify and resolve modeling discrepancies in order to enhance finite element model (FEM) fidelity beyond 10 to 20 Hz. To conduct the investigations, systematic testing and analysis of several airframe configurations, ranging from the "stripped-down” configuration (the airframe stripped of engines, landing gears, rotor isolation system, fuel, stabilizers, drive shafts, doors, cowlings, avionics, seats, etc.) to a "full-up” configuration, was performed to quantify the effects of each component on overall vibratory response of the airframe. The isolation of individual component effects, including the stripped-down structure, provided a unique database well-suited for finite element model checkout and improvement. The primary airframe and full-up helicopter configuration correlations were improved through 6/rev (35 Hz) without any significant changes to the original modeling philosophy.