Afterbody Drag Measurement at Transonic Speeds on a Series of Twin and Single Jet Afterbodies Terminating ct the Jet-Exit

SUMMARY A systematic sex-188 of twin-jet afterbodaes all terminating at the jet-exit has been tested at Mach numbers from M = 0.7 to 1.3 on a strut-mounted rig employing high pressure air for the Jet streams. The afterbody drag was derived from balance measurements of thrust-minus-drag and wind-off calibrations of thrust, and interpreted with the help of pressure plotting data. Two single-nozzle afterbodies were tested for comparison. Parameters investigated in the programme include jet size, base area with the changes obtained by infilling the valley between the nozzles and by extending as a fairing, afterbody boattail angle, nozzle shroud area ratio coupled with changes in shroud length, and shroud angle. At subsonic speeds and likely operational jet pressure ratios, the afterbody drag coefficients based on fuselage cross-sectional area lie in the range 0.03-0.06 of which typically, 0.026 can be ascribed to skin friction. Under these conditions, much of the variation between configurations can be expressed as a linear increase in afterbody drag with effective base area suitable defined to represent the area over which the external stream IS necessarily separated. In particular, this concept appears to be a means of reconciling the drag of single-and twin-nozzle installations of the present type with the nozzle exits close to the base. The rate of increase is given by 0.12 x effective base area/fuselage cross-sectional area. At M = 1.3, or at higher jet pressure ratios at any Mach number, this simple type of correlation is less successful. The increase in afterbody drag with shroud area ratio is greater at the higher jet pressure ratios. An increase III shroud angle from 15" to 2C " with a boattail angle of 15" increases the afterbody drag coefficient by 0.004 subsonlcally or 0.007 at M = 1.3; a reduction in boattall angle from 15" to IO' IS beneficial at M = 1.3 but gives a small penalty subsonically, due to a poorer pressure recovery at the base. The tests have produced some " design rules " ; the report stresses the factors that should always be borne In mind when applying these rules in practice.