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OIL cooling of aircraft powerplants is increasingly difficult. The weight and drag of the oil coolers necessary with the present maximum “Oil-in” temperature of 185 deg. fahr. (85 deg. cent.) are both decidedly objectionable. It appears possible to increase the “oil-in” temperature to about 220 deg. fahr. (104 deg. cent.) with oils which can be produced by the newer refining methods. The use of an “oil-in” temperature of 220 deg. fahr. would render possible a material reduction in weight, size and drag of oil coolers in comparison with present practice. Oils suitable for use at 220 deg. fahr. “oil-in” temperature would not be likely to cause a material increase of engine-starting difficulty, as they would only be used in summer when the shearing resistance of the oil has slight influence on engine starting. The approximate temperature cycle encountered by the oil in its passage through a modern aircraft-engine is discussed.

A STUDY has been made of materials available and suitable for the construction of transparent windows employed for observation inside the combustion-chamber. Materials that can be used include quartz, spinel, sapphire, periclase, and fluorite. The transparency of these materials in the infrared region increases in the order just mentioned. Mechanical strength varies greatly, being highest for sapphire and lowest for fluorite. Resistance to the chemical action of the combustion products varies widely. Sapphire is not attacked by any products of combustion, including the lead oxide present when leaded fuels are used. The choice of a suitable material for a given investigation will be based on a judicious balance of the four chief characteristics: mechanical resistance, chemical resistance, transparency, and cost.