842. A Merit Factor Suitable for Evaluation of a Variety of Aircraft Types and Missions

Paper

Abstract

A Merit Factor is developed which is based on the concept that the best aircraft can do the biggest job (pounds of payload x n.m.) at the lowest weight, and in the least time. It differs from the commonly used productivity factor in that a baseline mission is not required to determine the relative merit of different designs. The Merit Factor can be calculated using aircraft data frequently appearing in the literature. The manner in which cruise speed affects the Merit Factor is described for jet and shaft driven aircraft. The equation derivation can also be used to match aircraft characteristics to mission load, range, and speed requirements. An approximation is presented which permits use of the Merit Factor for helicopters and other aircraft exhibiting a drag increase with speed which is more severe that that predicted by the conventional drag polar. For subsonic shaft driven airplanes additional equations are presented which permit calculation of the Merit Factor solely in terms of: Empty weight/Gross weight, Special fixed weight items required for the mission, Engine fuel specifics, Thrust power/shaft power, Zero lift drag, Aspect ratio and span efficiency, Cruise altitude, Wing loading, Fuel reserve requirements. Thus, the quantitative sensitivity of the Merit Factor to each of these terms can be evaluated during early design concept evaluations.