Effect of lubricant on helicopter transmission efficiency

Efficiency tests were conducted using eleven different lubricants in the NASA Lewis Research Center's 500 hp torque regenerative helicopter transmission test stand. The test transmission was the OH58A helicopter main transmission. The mechanical power input to the test transmission was 224 kW (300 hp) at 6060 r/min. Tests were run at oil-in temperatures of 335°K (180°F) and 372°K (210°F). The efficiency was calculated from a heat balance on the water running through an oil-to-water heat exchanger while the transmission was heavily insulated. The test lubricants were analysed for their physical and chemical properties. Data on pressure-viscosity characteristics as well as friction data from LFW-1 type testing were examined for possible correlation with the efficiency data. The following results were obtained. (1) Among the eleven different lubricants, the efficiency ranged from 98.3 to 98.8%, which is a 50% variation relative to the losses associated with the maximum efficiency measured. (2) for a given lubricant, the efficiency increased as temperature increased and thus as viscosity decreased. There were two exceptions which could not be explained on the basis of available data. (3) There was a reasonable correlation of efficiency with absolute viscosity (corrected for temperature and pressure in the contact). (4) Between lubricants, efficiency did not correlate well with absolute viscosity at atmospheric pressure. (5) Between lubricants, efficiency did not correlate well with calculated lubricant film forming capacity. (6) Bench type (LFW-1) sliding friction and wear measurements could not be correlated to transmission efficiency and component wear. (7) The lubricants had no significant effect on the vibration signature of the transmission.