Abstract:

ESDU 01012 provides an empirical graphical method for boat-tail pressure drag at zero angle of attack and for jet pressure ratios of 1.5 to 4 derived from a correlation of wind-tunnel data. The boat-tails may be of circular- or parabolic-arc or other curved profile and the method also applies to cases in which the curved portion of the profile runs into a final conical portion tangentially. The body upstream of the boat-tail must run into the boat-tail without discontinuity. The nozzles were convergent and the minimum annular base area was defined as base diameter minus jet diameter not exceeding 0.01 of body maximum diameter. The best geometric correlating parameter was found to be the dimensionless slope of the cross-sectional area distribution, weighted by projected area, and five graphs give the boat-tail drag for Mach numbers of 0.6, 0.8, 0.9, 0.92 and 0.94 as a function of that parameter and jet pressure ratio.Equations are provided for that correlating parameter for the circular-arc boat-tail and for a circular-arc profile running tangentially into a conical portion. The ranges of the flow conditions and dimensions of the models used in the correlation are tabulated (the jet flow was cold) and sketches show the drag coefficient based on the body maximum area was generally correlated to within 0.01. One case generated values outside that range and the reasons for that are considered. A brief discussion shows that the use of a cold jet flow leads to a pessimistic value of boat-tail drag. Two fully-worked examples illustrate the use of the method, one relating to a curved profile for which only the profile coordinates are known.