IT may be necessary to compromise among the ring-sticking, sludge-forming, and corrosion properties of oil for civil aircraft engines, the authors suggest. No laboratory tests are yet able to predict the performance of an oil in an aircraft engine, they contend, and therefore, full-scale engine tests are necessary for final judgment. However, they explain, much preliminary work can be carried out in suitable small units.
To illustrate the complexity of the problem the authors set forth five requirements for an aircraft-engine lubricant:
It must not cause ring-sticking under the full-throttle detonating conditions of take-off.
It must not cause ring-sticking under weak-mixture cruising conditions.
It must give freedom from sludging so that there is no ring-jamming, so that the oil scrapers are kept free, and so that the overhaul periods are not limited.
It must provide protection from cold corrosion.
It must not be corrosive to special alloys, such as cadmium-base bearings.
Test data which are treated in the appendix of the paper, include specifications of the single-cylinder 250-cc. engines used for ring-sticking tests; and of the 500-cc. single-cylinder engine for sludge-testing. Results of these tests are correlated with single-cylinder aircraft engine, full-scale bench, and flight-test results.
In a discussion of engine results, the paper takes up the influence of mechanical conditions; carbon formation; sludging; effect of lead content of the fuel; ring-sticking and dopes; and bearing corrosion.