Vibration Testing of LCH Auxiliary Fuel Tank

Hindustan Aeronautics Limited (HAL) approached Council of Scientific and Industrial Research (CSIR) – Structural Engineering Research Centre (SERC) for carrying out the vibration tests on auxiliary fuel tank of Light Combat Helicopter (LCH). The LCH fuel system consists of three main tanks and two supply tanks. The jettissonable external auxiliary fuel tanks are installed to carry additional fuel of 300Kgs (150kg in each tank) required for a ferry mission of 750 kms as per ASR requirement. The test specimen i.e., external auxiliary fuel tank, is made up of Aluminium alloy of 1.5mm thick sheet without self-sealing and crashworthy feature. It consists of three sections viz. Front Section Assembly, Center Section Assembly and Rear Section Assembly. All three sections are welded together to form auxiliary fuel tank. The design and installation of auxiliary fuel tank is as per MIL-T-7378 (class 2). The tank assembly was mounted on the slip table using suitably designed fixtures for vibration testing. The whole system was instrumented with a total of five uniaxial accelerometers - three on the tank top (in XX-, YY- and ZZ-directions), one on the fixture and one the shaking table to measure the input motion profile. The resonance search tests are carried out from 5 to 500 Hz at constant acceleration of 0.1g-pk with logarithmic sweep rate not exceeding 1oct/min in each axis. For endurance test, the tank was subjected to a Sine-on-Random (SoR) signal with a spectral density as mentioned in the MIL-STD-810G for qualification. The first resonance frequency of the system was found to be around 38 Hz. The vibration qualification testing of the LCH fuel tank was successfully completed by CSIR-SERC.