ESDU 00026:2000

ESDU 00026 presents a semi-empirical graphical method for estimating the pressure drag coefficient of boat-tails with length/maximum diameter ratio up to 2.5 at zero angle of attack for Mach numbers from 1.5 to 4. The graphs, each for a value of boat-tail angle, present theoretical results obtained using the method of characteristics as a function of freestream Mach number and boat-tail length/maximum diameter ratio; for conical boat-tails the graphs are for boat-tail angles of 2, 4, 6, 8 and 10 degrees and for circular-arc boat-tails for 5, 10, 15, 20 and 25 degrees. The graphs for circular-arc boat-tails also apply to parabolic-arc boat-tails and for both types the boat-tail angle is defined as the local angle at the boat-tail/base junction. The theoretical results are modified to correlate the available experimental data; for conical boat-tails a single factor developed for ESDU 00017 is used while for the curved boat-tails an equation in terms of the boat-tail angle is given. Equations for the boat-tail angle in terms of the boat-tail dimensions are provided. The method applies when there is a jet efflux, provided the base pressure is below that at the end of the boat-tail; conditions for that to occur may be predicted using ESDU 00017 for the case of conical boat-tails. The method assumes there is no interference from any forebody, which requires that there are at least three diameters of cylindrical body ahead of the boat-tail. It is expected to predict the drag coefficient to within 0.003 for conical boat-tails and to within 0.005 for the smooth boat-tails. Worked examples illustrate the use of the method. For boat-tails with length/maximum diameter ratio greater than 2.5 theoretical results are available in ESDU Aero B.S.02.03.02, and other ESDU documents give prediction methods for subsonic and transonic speeds.