PRELIIIINARY DESIGN OF TURBOFAN ENGINE PROPULSION SYSTEM ON MODERN FUEL EFFICIENT AIRCRAFT

PRELIIIINARY DESIGN OF TURBOFAN ENGINE PROPULSION SYSTEM ON MODERN FUEL EFFICIENT AIRCRAFT H. O. Sowers. General Electric Co. Evendale, OH; and W. Tabakoff, University of Cincinnati, OH DESIGN CQNSIDERATIONS Installation of high bypass ratio turbofan engine nacelles on modern fuel efficient jet aircraft oust satisfy a variety of physical installation requirements wh i1 e ach ieving near optimum aerodynamic performance. Consideration of installation criteria in the preliminary design phase of nacelle development requires simplified and rapid techniques for evaluation of the nacelle external lines and their relationship to the installation on wing. A discussion of installation criteria and experimental and analytical techniques now available to assess the aerodynamic lines is presented and recommendations made for utilizing the techniques in the preliminary design phase. objective i; not always met and interference drag levels of 3% or more are often encountered. INTRODUCTION: The wing designer accounts for the fact that a strut and nacelle will be located below the wing in three basic areas as described in Reference 1. The channel created by the wing, nacelle cowling and pylon, Figure 1, will induce higher velocities and thus modify the lower surface oressure distribution of the forward portion Of the wing; the nacelle will create a change in leading edge incidence; and the nacelle will require sufficient ground clearance thus influencing wing dihedral andlor landing gear len th. Basicall however, the wing is designed wit;tout the nacelre and strut assuming they will be located properly to ,avoid installation penalties. The objective is to have the interference dra increment, be zero or positive at the desiqn 11% coefficient. However. this Preliminary design studies of high bypass ratio turbofan engine nacelles and their installation on modern jet aircraft considers both mechanical and aerodynamic criteria. considerations will usually result in a compromise of performance, weight and cost that impacts the nacellelpylonlwing combination and result in a basic installation Configuration appropriate for further more extensive detailed studies. Throughout this design phase, the question is asked "llhat is the performance penalty if the nacelle is moved from Position A to Position B?" This question is usually asked prior to the existence of any wind tunnel test data for the confiaurations beino studied. and this leads to These the egtablishment o i a test program with a matrix of variables, including position, that will allow a configuration to be selected. However, to establish this test matrix, the same question must be answered in order to provide a small matrix of geometric variations that can be built and tested within a reasonable budget. There has been an extensive amount of test data generated by the aircraft and engine manufacturers that establish the installed performance of specific installations and in some cases leads to empirical relationships which are then used for estimates in the preliminary design studies and to determine the impact of moving from Position A to B. matrix relates vertical and axial spacing, normalized by chord, to an aircraft drag penalty. These conditions do not account for nacelle geometry or wing shape and many times are found to predict the wrong trend when new configurations are tested. The most.common type of The nacelle design establishes the cowl aerodynamic lines that will achieve the objective performance on an isolated basis. Generally, the inlet design criteria will establish a maximum nacelle diameter which is contoured within boattail angle limits to establish the fan nozzle outer diameter. The required fan nozzle area will then establish the inner cowl diameter at the fan nozzle exit plane. Mechanical installation requirements will determine a point on the core engine where the aft mount is to be located and thus the minimum diameter of the cowl at that axial station. Between these two oints, the aerodynamic and mechanical designer wiTl establish a core cowl and core nozzle configuration that satisfies weight and performance objectives. This selection may . consider internal core nozzle plugs, short and long separate flow core cowls and mixed flow long duct nacelles. All of the variations can result in significantly different cowl contours which will react differently to the channel flow of Figure 1, depending u on the nacelle to wing selected by the wing designer. spacing and the wing 7 ower surface contour The following discussion presents an approach to the reliminary desi n problem that accounts for bot{ wing and nacelye aerodynamic design considerations. Figure 1. Xaceile/i$inc Channel Flow Description.