The United States Navy has successfully operated their Landing Craft Air Cushion (LCAC) with Vericor’s ETF40B engines since 2001. The engines interface with the craft drivetrain through sprag clutches, which engage when the engine output is greater than the drive shaft speed. Historically, sprag clutch failures have been observed on multiple LCACs, resulting in craft downtime and associated repair & replacement costs. Subsequently, a US Navy investigation revealed the presence of high frequency, low level fluctuations in the shaft rotational speed when operated at steady state conditions. The study also suggested that the speed governor fuel control could excite these fluctuations and induce sprag clutch failure. The original speed governor gains were tuned for maximum transient performance, but not necessarily steady state stability. The aggressive gain selection resulted in a governor response that could be characterized as “hyper-reactive” where the system was willing to respond to even the slightest disturbance, including common drivetrain noise. The US Navy requested that Vericor modify the Full Authority Digital Engine Controller (FADEC) speed governor logic to improve steady state stability while maintaining an acceptable transient response. This paper summarizes the basics of ETF40B operation on the LCAC and describes the effort that improved the governor response. As of the writing of this paper, two (2) years have passed in which no sprag clutch failures have been observed on US Navy LCACs operating with the optimized speed governor.

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