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Abstract

A twin-engine, low-wing transport model, with a supercritical wing of aspect ratio 10.8 designed for a cruise Mach number of 0.77 and a lift coefficient of 0.55, was tested in the Langley 16-Foot Transonic Tunnel. The purpose of this test was to compare the wing-nacelle interference effects of flowthrough nacelles simulating superfan engines (very high bypass ratio (BPR ss 18) turbofan engines) with the wing-nacelle interference effects of currenttechnology turbofan engines (BPR ss 6). Forces and moments on the complete model were measured with a strain-gage balance, and extensive external static-pressure measurements (383 orifice locations) were made on the wing, nacelles, and pylons of the model. Data were taken at Mach numbers from 0.50 to 0.80 and at model angles of attack from ?4ffi to 8ffi. Test results indicate that flow-through nacelles with a very high bypass ratio can be installed on a lowwing transport model with a lower installation drag penalty than for a conventional turbofan nacelle at a design cruise Mach number of 0.77 and lift coefficient of 0.55.

Introduction

Aircraft manufacturers have focused much of their research and development efforts on improving the performance of commercial transport aircraft by increasing the aerodynamic efficiency, by utilizing turbofan engines with improved (lower) specific fuel consumption (SFC), and by improving the installed performance of the turbofan engine nacelles. The airframe-associated improvements stem from advances in structural materials, machining methods, and computer-aided design techniques that have allowed the use of more efficient, high-aspect-ratio (ratio of wing span squared to wing area) wings. The propulsion-related improvements in turbofan engine efficiency are primarily a result of an increase in the ratio of fan flow to engine core flow (i.e., bypass ratio (BPR)); thus, marked decreases in SFC are provided. Consequently, the current design trends for commercial aircraft are toward higher turbofan engine bypass ratios (increased nacelle diameter relative to thrust) and higher wing aspect ratio (reduced wing chord relative to wing span and area, ref. 1). As a result, nacelle sizes have grown much larger with respect to the wing chord and could result in large nacelle-wing mutual interference effects for low-wing transports with conventional underwing nacelle-pylon layouts. Interference caused by this type of installation may degrade the performance of the new supercritical wing airfoils (which are much more sensitive to small flow disturbances) by causing premature shock formation

and flow separation on the wing, which leads to severe drag penalties. These penalties may be large enough to negate the decrease in SFC realized from increased BPR.

Since the early 1980's, Langley Research Center has been investigating the problems and solutions related to the installation of twin turbofan nacelles on transport-type aircraft with supercritical airfoil wings. Previous investigations with a 1/24- scale high-wing transport model (refs. 2 to 10) were conducted in the Langley 16-Foot Transonic Tunnel at Mach numbers from 0.40 to 0.85 and at angles of attack from ?4ffi to 6ffi. The wing of this model had a quarter-chord sweep of 30ffi, a wing aspect ratio of 7.52, and a design cruise Mach number of 0.80. However, the flow-through nacelles tested on this model represented turbofan engines with lower bypass ratios (BPR = 4 to 6), and the wing aspect ratio was low relative to current-technology designs. Additionally, the high-wing design of this model is not typical of current and future commercial transport designs, which have predominantly low-wing locations. Therefore, a new model based on the response of the airframe industry to the fuel crisis of the late 1970's|designed to obtain better fuel economy by cruising at a slightly lower Mach number (Mdes = :77)|was fabricated. The model design incorporated a high-aspect-ratio wing (10.795) with a quarter-chord sweep of 21.0ffi. On this model, flowthrough nacelles for engines with very high bypass ratios (BPR = 10 to 20) were tested to determine the best location and orientation on the supercritical wing for minimum drag of the wing-body-nacelle combination. This model had sufficient pressure instrumentation to provide details of the flow around the nacelles, pylons, and wing; therefore, performance differences between various nacelle installations indicated by aerodynamic force and moment data could be explained.

The present investigation was conducted to examine the aerodynamic characteristics of this new windtunnel model and to compare the installed interference effects of current-technology turbofan-engine (BPR ss 6) nacelles with the installed interference effects of turbofan engine nacelles with very high bypass ratios (BPR ss 18).

Symbols and Abbreviations

ATF advanced turbofan-engine nacelle (BPR ss 6)

BL model buttline (lateral dimension from centerline of model, positive in
spanwise direction), in.