Cylinder Cooling and Drag of Radial Engine Installations 380180
REDUCTION in the baffle pressure drop required to cool promises a great reward from every standpoint, Mr. Campbell asserts, and further progress in this direction is theoretically possible through both fin and baffle development. Present aluminum-alloy head fins, he continues, are still at least twice too thick for their height, and the limiting factor in fin design is no longer engineering knowledge but the manufacturing art.
The cooling problem presents a “potential limitation” to the engine manufacturer, Mr. Campbell points out, explaining that two conflicting requirements must be met, each of which grows more severe as engine powers and airplane speeds increase. The first requirement dictates that, for the improved engine, the rate of waste-heat transfer must be increased whereas the second requires that, for the improved airplane, the cooling drag must be decreased.
The engine manufacturer can improve the rate of heat transfer either by increasing the velocity of the cooling air past the surfaces, or by increasing the area of the useful heat-transfer surface, the author states. He then shows that the latter method is far less costly, especially when done by adding to the height of existing fins and increasing the cross-sectional area of the heat-transfer passages.
Data are presented to enable the cowl designer to provide the effective cowl exit-slot width necessary at his particular indicated air speed to meet the baffle pressure drop requirements specified for the conditions.
Three main avenues of approach to the problem of increasing the low baffle pressure drops existing at very low airplane speeds in order to obtain the velocity of cooling air necessary for increased heat rejection, are studied:
Use of the propeller slipstream to induce a depression after the baffle passages.
Use of the propeller slipstream to build up a pressure before the baffle passages.
Use of a fan for either purpose.
The research data presented indicate that the progress to date in cylinder cooling of air-cooled radial aircraft engines, though considerable, has not yet reached the point of diminishing returns.