Aircraft engines are classed as air-cooled or water-cooled, accordingly as they are cooled directly by contact with air or indirectly by air through the intermediary of the water which surrounds the cylinders. The method of cooling used has been a cause of wide divergence in design and arrangement between the two types. The air-cooled engine, except for the smaller powers, appears in the radial form in which exposure of each cylinder to the air stream is simplified at the expense, however, of a large frontal area while, with obviously greater latitude, the water-cooled engine can take a form having a smaller frontal surface.