Abstract:

ESDU 81024 provides a simple method of estimating the drag of axisymmetric cowls at subsonic speeds. The cowl is assumed to consist of a forebody and an afterbody having a sharp trailing-edge, with or without a cylindrical midbody. The forebodies considered are the widely-used NACA 1-series although consideration is also given to improvements attainable at low speeds by means of an increased leading-edge radius. The cowl drag is assumed to consist of three components, i.e. profile, spillage and wave drags. The method used to predict profile drag was derived from correlations of form factors calculated using measured pressure distributions. The methods used to predict the spillage and wave drag components are, however, empirical, being based on correlations of both published and unpublished experimental data. The method is applicable to most practical axisymmetric cowls at zero incidence and subsonic Mach number, with the limitations on forebody profile previously mentioned. A wide range of engine operating conditions is considered, covering the practical extremes of the low-speed, engine-failed (low intake mass flow) case to the high-speed cruise (high intake mass flow) case. A flowchart provides guidance on the use of the method, while its application is illustrated by means of three comprehensive examples. A Fortran program of the method (extended to include a check for inviscid choking conditions) is provided as ESDUpac A8124. Its input and output (which includes the options to give drag for a range of Mach numbers for fixed flow area ratio or for a range of flow area ratios at fixed Mach number) are explained and illustrated by worked examples.