Aerodynamics of propulsion system integration for modern aero-engines.

Abstract

Over the past years, the fan diameter of a turbofan engine for commercial aircraft tended to increase and based on the requirement to improve the propulsive efficiency it is expected that the engine diameter will continue to increase. In this context, the issue of the integration of a larger powerplant with the airframe is a major concern. It is anticipated that the increase of engine diameter will eventually lead to an increase of the drag due to the engine installation, which could potentially outweigh the benefits of very high by-pass ratio cycles. This research aims to quantify the aerodynamic sensitivity to the key parameters of engine installation such as engine position, size and power-setting for prospective large turbofans. A numerical study with the use of Computational Fluid Dynamics was completed to determine the effect of engine position, size and power setting on the performance of a typical 300 seater aircraft with podded engines at cruise condition. Two engines with very-high by-pass ratio and with different fan diameters and specific thrusts were simulated in isolation and in range of positions under the wing. In addition, the detailed breakdown of the interference force was investigated to determine the contribution of aerodynamic forces based on the components such as wing, nacelle and fuselage. The assessment of the flowfields was done to determine the aerodynamic origins of beneficial or penalising forces and to explain the observed trends in the aerodynamic forces. The work concluded with the evaluation of the fuel burn reduction for the larger engine configuration. Over the long-haul cruise phase, the cycle benefits for the large engine with low specific thrust deteriorated from -5.8% reduction in fuel burn to -4.8% as compared with the baseline engine due to the effect of engine weight and throttle dependent aerodynamic forces. It was also found that the large engine was less sensitive to the engine position. Over the same range of positions, the sensitivity of 1.7% of reference thrust for the baseline engine reduced to 1.3% sensitivity for the 1.23 times larger engine. Overall, to aid the preliminary design of prospective large turbofans, a model for the quantification of the engine installation effect was proposed based on the available dataset. The model aims to outline the general trend in the determination of engine installation at the early stage of the preliminary design. A basic verification of the model was completed with an agreement of 1.25 aircraft drag count in the prediction of the overall installation effect based on the available dataset.