Abstract:

ESDU 00017 presents a semi-empirical graphical method for estimating boat-tail drag at zero angle of attack. A correlation of experimental data showed that the base pressure ratio (that is the ratio of the base pressure on the boat-tailed body to that on the cylinder without boat-tailing) was independent of boat-tail length and angle and of jet pressure ratio. An equation for that ratio is given in terms of the diameter ratio (base to maximum). Its use to obtain the base pressure on the boat-tailed body requires information on the base pressure on the cylinder for which ESDU 99010 may be used. For boat-tail pressure drag, the method of characteristics was used to generate plots of the theoretical drag coefficient against freestream Mach number from 1.5 to 4 and afterbody length/maximum diameter ratio up to 2.5, each plot being for one value of boat-tail angle. The graphs are for boat-tail angles of 2, 4, 6, 8 and 10 degrees. For those cases for which the base pressure is less than the pressure at the end of the boat-tail, an empirical correction factor is applied to the theoretical values. Such cases are determined from a further set of graphs, also derived using the method of characteristics, that present the pressure coefficient at the end of the boat-tail as a function of the same variables over the same ranges as those used for the pressure drag graphs. For cases when the base pressure exceeds the pressure at the end of the boat-tail, the boat-tail pressure drag is reduced and a further empirical correlation leads to an equation for that decrement as a function of freestream Mach number, the base pressure coefficient and the pressure coefficient at the end of the boat-tail. The ranges of the geometry and of jet and freestream conditions applying in the experimental data analysed for ESDU 99010 and ESDU 00017 are tabulated, and plots of the predicted results against the experimental data are provided that show that base drag coefficient is estimated to within 0.005, the boat-tail pressure-drag coefficient to within 0.003 and the total afterbody pressure-drag coefficient to within 0.007. Worked examples illustrate the use of the method.