Drag Measurements of an Axisymmetric Nacelle Mounted on a Flat Plate at Supersonic Speeds

AbstractAn experimental investigation was conducted to determine the effect of diverterwedge half-angle and nacelle lip height on the drag characteristics of an assemblyconsisting of a nacelle fore cowl from a typical high-speed civil transport (HSCT) anda diverter mounted on aflat plate. Data were obtained for diverter wedge half-anglesof 4.0 °, 6.0 °, and 8.0 ° and ratios of the nacelle lip height above a flat plate to theboundary-layer thickness (hn/5) of approximately 0.87 to 2.45. Limited drag datawere also obtained on a complete nacelle/diverter configuration that included foreand aft cowls. Although the nacelle/diverter drag data were not corrected for basepressures or internal flow drag, the data are useful for comparing the relative drag ofthe configurations tested. The tests were conducted in the Langley Unitary Plan WindTunnel at Mach numbers of 1.50, 1.80, 2.10, and 2.40 and Reynolds numbers rangingfrom 2.00 x 106 to 5.00 x 106 per foot. The results of this investigation showed thatthe nacelle/diverter drag essentially increased linearly with increasing hn/_ exceptnear 1.0 where the data showed a nonlinear behavior. This nonlinear behavior wasprobably caused by the interaction of the shock waves from the nacelle/diverter con-figuration with the flat-plate boundary layer. At the lowest hn/_ tested, the diverterwedge half-angle had virtually no effect on the nacelle/diverter drag. However, ashn/_ increased, the nacelle/diverter drag increased as diverter wedge half-angleincreased.IntroductionThe renewed interest in high-speed civil transport(HSCT) configurations with extended supersonic rangehas spurred investigations into aircraft drag reduction atsupersonic cruise conditions. Mutual aerodynamic inter-ference between the engine nacelles and airframe canhave a significant impact on efficient propulsion-airframe integration. By paying close attention to theflow field interactions of the nacelle and airframe, thedesigner can exploit the favorable interference effects tominimize the total aircraft drag (refs. 1-4).Linear analysis methods have been shown to roughlypredict the drag levels and basic interference effectsassociated with nacelle-airframe interaction (refs. 4-7).Also, linear design methods (refs. 3 and 8) have beenfairly effective in improving the overall integrated dragcharacteristics. However, a more detailed and accurateunderstanding of nacelle-airframe integration character-istics is needed to support the development and applica-tion of advanced computational fluid dynamics (CFD)analysis and design methods.Numerous experimental studies have been con-ducted to identify the basic interaction of the nacelle andairframe and to evaluate various analysis and designmethodologies. Typically, the nacelle drag increment isobtained by subtracting the clean aircraft drag from thedrag of the aircraft with nacelles (refs. 2 and 9-11). Theprimary advantage of this technique is that it is a gener-ally accepted method to obtain the installed nacelle drag.The primary disadvantage of this technique is that sepa-rating the various drag components that contribute to thetotal installed nacelle drag is impossible. These includenacelle-on-aircraft interference drag, aircraft-on-nacelleinterference drag, nacelle-on-nacelle interference drag,and isolated nacelle drag. Another disadvantage of thistechnique is that the data accuracy suffers because thestrain-gauge balance must be selected to measure thedrag of the entire model instead of just the nacelles.Another technique that has been used to measurenacelle drag increments was developed at the AmesResearch Center (ref. 5). In this technique, the aircraftmodel is mounted to one strain-gauge balance and sup-port mechanism, whereas the nacelles are mounted on anindependent flow-through strain-gauge balance andmodel support mechanism. This technique allows thenacelles to be positioned anywhere underneath the air-craft wing. The primary advantage of this technique isthat the various drag components previously discussedcan be determined from the separate aircraft and nacelledrag measurements. In addition, the accuracy of thenacelle drag measurements is improved because thenacelle strain-gauge balances are sized to measure onlythe nacelle drag. However, this technique is limited inthat the nacelle diverters are not modeled.Recent experimental store-carriage drag studies atthe Langley Research Center have been useful in deter-mining the drag characteristics of isolated stores as wellas the mutual interference between stores that weremounted on a flat plate (ref. 12). In this technique, the