Analysis of cooling limitations and effect of engine-cooling improvements on level-flight cruising performance of four-engine heavy bomber

摘要

The NACA has developed means, including an injection impeller and ducted head baffles, to improve the cooling characteristics of the 33500-cubic-inch-displacement radial engines installed in a four-engine heavy bomber. The improvements afforded proper cooling of the rear-row exhaust-valve seats for a wide range of cowl-flap angles, mixture strength, and airplane speeds. The results of flight tests with this airplane are used as a basis for a study to determine the manner and the extent to which the airplane performance was limited by engine cooling. By means of this analysis for both the standard airplane and the airplane with engine-cooling modifications, comparison of the specific range at particular conditions and comparison of the cruising-performance limitations were made. ududThe analysis of level-flight cruising performance of the airplane with both the standard- and the modified-engine installations indicated that the maximum cruising economy is attained at the minimum brake specific fuel consumption when engine cooling under these conditions is possible. Operation at lean mixtures, high altitudes, and large gross weights was limited for the standard airplane by engine cooling at the point where larger cowl-gap openings increase the power required for level flight at such a rate that the additional cooling air available is insufficient to cool the engine when developing the additional power. When cooling becomes impossible at the minimum brake specific fuel consumption, the maximum cruising economy is obtained with a cowl-flap angle of approximately 6[degrees] and with the leanest mixture (above the stoichiometric value) giving satisfactoryudengine cooling. ududComparison of the calculated perfomance of the standard and the modified airplane indicated that cooling improvements increased the maximum specfic range as much as 38 percentudfor operation where wide cowl-flap angles and enriched mixtures are required to cool the standard airplane. Corresponding increases in cruising range were calculated for flights in which conditions allowing large increases in cruising economy were encountered. The cooling improvements allow either an increase of more than 10,000 feet in operating altitude at a given airplane weight or a gross-weight increase of from 10,000 pounds at sea lerel to 35,000 pounds at all operating altitudes above 10,000 feet.