Thermoplastic Simulated Tailboom Program - Final Test Results

The objective of this program was to demonstrate the application of advanced thermoplastic composite structural design concepts for helicopter tailbooms which are tolerant to High Explosive Incendiary (HEI), and Slew Beam Laser (SBL) threats. To accomplish this objective, two basic tailboom concepts were considered using a soft and hard skin approach. A soft skin tailboom concept utilizes the skins as a pressure relief mechanism against HEI rounds to minimize damage to the surrounding structure due to over-pressure. The hard skin tailboom incorporates ample design margins so the structure can sustain the over-pressure and the fragmentation. In this program, RAH-66 Comanche structural loads were used, and threat considerations typically found in military rotary wing requirements. In arriving at these concepts, design drivers were identified and they are shown in Figure 1. The program was divided into two phases: Phase I - Design & Analysis, and Phase II - Fabrication & Laboratory Testing. Both design concepts, the hard and soft skin concepts, were addressed in the early part of the program. These concepts included features, such as, crack arrestors hoop winds (straps) for the hard skin, and blow-out panels for the soft skin design. As the design phase progressed, it was determined from analysis and previous testing that in this case the HEI ballistic event would occur too rapidly for the soft skin blow off panels to react quickly enough to relieve the pressure build-up. As a result, two tailboom configurations were decided upon; one with 16-plies skin, and another with 10-plies skin and crack arrestor hoop winds. In both configurations the frames and longerons were identical. The crack arrestors, in addition to providing damage mitigation, help react hoop stresses thus reducing skin thickness without compromising structural integrity. Previous papers were given on the design, analysis, and fabrication portions of the program. A brief description of the fabrication process is given herein, however, this paper will focus on the testing activities. Four tailboom specimens (two 10-plies skin, and two 16-plies skin) were fabricated under this contract using the IM7/PEEK material system otherwise known as “military grade”. An additional 16- plies skin tailboom was fabricated under Boeing IR&D funding using the AS4/PPS material system, otherwise known as “commercial grade”. The AS4/PPS (polyphenylenesulphide) fiber/resin system is typically less expensive and easier to process (lower temperature and pressures) than the IM7/PEEK system. The disadvantage to this material is its lower mechanical properties and less favorable environmental and chemical tolerance. All tailbooms were subjected to a variety of testing, and they all successfully met or surpassed all test requirements. The military grade tailbooms exhibited better performance than the commercial grade tailboom. A short video will be shown highlighting the tests.