THE DEVELOPMENT OF AN ADVANCED COMPOSITE TAIL ROTOR DRIVESHAFT

This paper describes the development of an advanced composite tail rotor driveshaft for a "next-generation" helicopter. Boron-epoxy was the material chosen for the initial program, and a follow-on program to develop a carbon-epoxy driveshaft is progressing. The high specific modulus of boron-epoxy permits the use of a longer shaft for a specified critical speed than is possible with conventional aluminum alloy. The configuration of the advanced composite shaft was optimized, using the unsupported length of the driveshaft as the independent variable and the weight of the drive system as the dependent variable. From the results of this study, detailed design parameters for the system were established. The optimum configuration is 18.0 pounds, 28.3 percent lighter than the conventional system with an aluminum alloy shaft. An unconventional, anisotropic laminate, with plies oriented at 0, ±45, and 90 degrees, was required to satisfy the requirements for critical speed, torsional stability (buckling), and torsional strength. Fabrication techniques and the results of tests of element and subsized torquetube specimens are discussed. Two full-sized driveshafts were fabricated and subjected to static and dynamic tests. Both bending and torsional moduli were in agreement with predicted values. One shaft was tested to failure to verify the predicted torsional buckling stress. The correlation between analytical and test results was good.