Design, fabrication, and structural evaluation of an advanced composite hydrofoil control flap

A hydrofoil control flap was designed and fabricated by the Boeing Company under contract to the Naval Sea System Command. It consisted of two graphite epoxy-titanium clad skins bolted and bonded to a titanium crank-spar assembly. It was tested statically at the Boeing Company to calibrate the response of electrical-resistance strain gages to strip loads; installed on the aft inboard foil of the hydrofoil PCH-1 (HIGH POINT) to demonstrate the ability of the composite to withstand the hydrofoil environment and to obtain service loads information; and tested in the laboratory at the David W. Taylor Naval Ship Research and Development Center to demonstrate its compliance with a four times-design-life capability. After being instrumental with 52 strain gages and calibrated for gage response as a function of applied load, the flap was installed on PCH-1. The flap remained on the boat for 26 months, but only saw 18 hours of foilborne operations due to unrelated mechanical difficulty with PCH-1. The 18 hours were dedicated to calm-water trials. Although the flap performed satisfactorily, attempts to relate composite material strains to flap pressure loads proved unsuccessful due to the complex nature of the flap loading, the flap's structural geometry, and the loss of the majority of strain gages just prior to trails data collection. The flap was then prepared for cyclic testing in the Structural Evaluation Laboratory, but failed during the initial static test because of an overload caused by a malfunction of the control system.