Effects of Engine Exhaust Flow on Boattail Drag

Abstract

Wind-tunnel tests were conducted to investigate the effects of jet plume shape and entrainment on hoattail pressure drag. The results were used to determine nozzle drag levels at various engine operating conditions as well as at conditions related to airplane force models. An isolated nozzle model with a pressure-tapped exterior and changeable internal parts was tested subsonically to examine changes in drag due to alterations in internal geometry and nozzle pressure ratio. In addition, tests were run with solid plume-shaped sleeves as a means to separate plume-shape effects from jet entrainment effects. Large differences in drag were measured with changes in plume shape, and, in certain regimes, jet entrainment also had a significant effect. The results of this study include boattail pressure distributions, integrated drag coefficients, and a comparison of test data with analytically predicted drag levels. Nom enclatur e A = cross-sectional area CD = boattail pressure drag coefficient (drag/g<^4.m) Cp = boattail pressure coefficient [(P — Po)/qQ] D,d = diameter h = boundary-layer height L = length of boattail MQ = freestream Mach number NPR = nozzle pressure ratio (PTJ/PO) P = local static pressure Po = freestream static pressure PTJ = exhaust jet total pressure q = dynamic pressure R = radius /3 = boattail trailing-edge angle Subscripts boattail terminal plane jet B,b J M,m T maximum freestream total