Design and acoustic shielding prediction of hybrid wing-body aircraft

Abstract

Recent research and developmental efforts in aircraft design have focused on the growing concerns about the environment impact of aviation and the rising costs of fuel. Under NASA's N+2 subsonics fixed-wing project, hybrid-wing-body (HWB) aircraft are investigated with the goal to meet the N+2 noise, fuel burn, and emissions requirements. As part of the N+2 program, this thesis is focused on the design and assessment of an HWB aircraft and the development of a prediction method for turbomachinery noise shielding. Based on MIT's previous experience in the Silent Aircraft Initiative, the SAX-40 aircraft concept was further developed into the N+2 HWB aircraft. The design effort resulted in two aircraft configurations: the N2A aircraft with conventional podded engines, and the N2B aircraft with a distributed propulsion system embedded in the airframe. The initial performance assessment shows that the N2A and the N2B aircraft can both meet the N+2 fuel burn goal and that the N2A aircraft is 5.7 EPNdB short of the noise goal. Also, the assessment revealed that current noise prediction methods cannot model the advanced propulsion system of the N2B aircraft, requiring the development of noise assessment tools for advanced engine-airframe configurations. NASA's Aircraft Noise Prediction Program employs the barrier shielding method to predict the airframe shielding of engine noise. However, it is an empirical formulation for straight edges and thus it is not appropriate for the planform shape of an HWB aircraft.

(cont.) At the same time, high fidelity methods such as boundary element methods and ray tracing methods are too computationally expensive if used in the early aircraft design and assessment stage. A compromise is the previously formulated diffraction integral concept based on the Maggi-Rubinowicz representation of Kirchhoff's diffraction theory. The diffraction integral method was implemented and applied to the N2A and the N2B aircraft. A noise reduction of over 20 dB in OASPL due to airframe shielding was predicted, demonstrating the shielding benefit of the HWB configuration. This shielding method is shown to be applicable to any aircraft configuration and planform geometry. The contributions of this thesis are the design of an HWB aircraft to be used as a platform for the development and evaluation of advanced analysis methods. In addition, a fast and improved-fidelity method for noise shielding prediction was developed, applicable to conventional and advanced airframe configurations such as, for example, the N2A and the N2B HWB aircraft.