The Behaviour of Oil Coolers: Obtaining Reliable Operation in Cold Weather, with Particular Reference to the Honeycomb Type

UNTIL the last few years, oil cooling on aircraft was not a very serious problem and could be dealt with by a relatively small and primitive cooler assisted by the casual cooling from pipes and tanks. Lately, how‐ever, the problem has become one of acute importance, for two reasons. The power developed by an engine of given size has increased, so that a large oil cooler is necessary. At the same time the general cleanness of aircraft design is now such that the drag of an inefficient oil cooler cannot be tolerated. The result is that a number of oil cooler designs have appeared in which the utmost attention has been paid to obtaining efficient heat transfer between the oil and metal by passing the oil through a multiplicity of narrow passages, groups of which must necessarily be in parallel to avoid excessive oil pressure drop. Although such coolers are successful in tropical conditions, a fresh crop of troubles often appears if they are used in wintry conditions, as some of the passages may become “frozen up” and there‐fore little cooling may be done. Trouble is also caused by the dual function sometimes required of the relief valve; a strong pressure may be necessary to ensure that the flow of oil is maintained through the cooler itself, whereas a valve set to a low pressure is desirable so that control of cooling may be effected. With a strong spring, overcooling and frothing of the oil occurs. In what follows, the laws governing the behaviour of coolers with relief valves are examined, and ways of avoiding the difficulties are given.