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The paper discusses the general lubrication problems associated with operation of high-output aircraft engines. Since the paper is concerned with two types of aircraft engines, namely, turbine and reciprocating, a natural division into two parts is made. Part I deals with the problems of turbine engines, and part II deals with the problems of reciprocating engines. In part I it is indicated that the choice of a lubricant is very difficult for the turbine engine particularly, because of the wide temperature range (from -67°F to approximately 400° F). Two solutions to the problem of proper choice of a lubricant are discussed, namely (1) the use of supplemental lubricants, and (2) the use of additive lubricants. Data are presented on supplemental lubricants including the various oxides of iron, molybdenum disulfide and graphite.

Synthetic lubricants are coming into prominence for lubrication of aircraft turbine engines, because of the stringent operating temperature requirements. Because many of the surfaces to be lubricated in the turbine engine operate under conditions of boundary or “thin-film” lubrication, the friction and surface-failure properties of the lubricant under these conditions are of extreme importance. In consequence, an investigation was made at the NACA laboratories of the friction properties of several classes of synthetic lubricants over a wide range of sliding velocities. Most synthetics including a diester, a polyether, a silicate ester and a phosphonate ester are more effective boundary lubricants at high sliding velocities than petroleum oils of comparable viscosity at 100° F. The breakdown of effective lubrication takes place at a much higher sliding velocity with these synthetic fluids than with the petroleum oils.

Research information on solid lubricants has been compiled for consideration in the possible use of such materials in aircraft electrical equipment. Solid lubricants are capable of lubricating at the maximum temperatures (600° F) for aircraft electrical equipment. Many solids that adhere well to metals may be useful lubricants; those with layer-lattice structure usually give low friction. Solid lubricants are most commonly used as bonded films but the use of fluid carriers and surface reaction products have considerable merit.

The feasibility of using presently available liquid lubricants in advanced, high speed aircraft engines has been investigated with a recirculating inerted lubrication system. Three fluids performed satisfactorily for short durations of 3-10 hr in a full-scale simulated aircraft bearing and seal assembly at an outer race bearing temperature of 700 F and with a bulk fluid temperature of 500 F. The principal problem has been with excessive leakage of the oil side bellows face seals which has accounted for a majority of the test terminations. With only one lubricant (a MIL-L-7808E oil) could bearing failure be blamed on a lack of lubricating ability. An inerted oil mist once-through system was also tested, but with less promising results with only one lubricant running successfully at 600 F bearing temperature. The mist system appears to be limited by its inability to maintain stable bearing temperatures and requires further development.