The most serious difficulty in theoretical analyses of airplane performance arises from a lack of reliable data on the variation of the horsepower of the propulsive system with altitude. An empirical-theoretical solution of this problem has been given in McCook Field Serial No. 1221 and should serve as a close approximation. In this report an engine-propeller factor is derived which expresses the ration of the horsepower available from the propulsive system at any altitude to the horsepower obtained from the engine at the ground with a propeller of 100 per cent efficiency. The factor curve gives a picture of the drop off in propulsive efficiency with altitude. In this document, curves for the Liberty "12" and the Hispano-Suiza "300" engines are given.