Numerical analysis of the multi-section nature of the regional blended wing body aircraft at low speed

Abstract

The blended wing body aircraft, or BWB for short, is an innovative concept that gathers all of the elements of an aircraft inside a unique wing envelope. This configuration promises a lower environmental footprint coupled with sizable fuel savings. Numerous scientific papers already either use or are expanding on this configuration; particularly for passenger transport and UAV applications. Concerning the passenger transport, the existing aerodynamic works on the BWB focus mainly on large-capacity, long-range designs under cruise conditions. However, current market forecasts show an expected increase in demand for regional, medium capacity aircrafts for 100-150 passengers. Moreover, this aircraft`s unique geometry incorporates multiple airfoil, chord, sweep and twist distributions along the wingspan. For the BWB, these distributions are not necessarily constant or mathematically smooth; giving raise to different aerodynamic behaviour depending on which section of the aircraft is analysed. This multisection nature of the BWB hints at the hypothesis that the BWB designer may have the ability to consciously align the aerodynamic behaviour of the centerbody with that of the outer wing to obtain predictable low speed behaviour, in particular, to generate a stronger natural lift at low speeds. The objective of this thesis is to develop BWB-specific design tools, based on the hypothesis of the existence of a unique interaction between the centerbody and the outer wing, with respect to natural lift at low speeds for this aircraft configuration. This original work would allow designers to incorporate said predictive coupling in the conceptual design phases of future BWBs. The proposed methodology is based on the numerical analysis of a regional BWB at low speed. First, a geometrical model of the baseline regional BWB was generated by applying classic aircraft design methods coupled with BWB-specific constraints. This was followed by the comparison of the low speed behaviour between a conventional aircraft and a BWB. The aerodynamic behaviour of each aircraft was simulated via RANS simulations applied over a series of angles of attack spread around the stall angle. Based on the behavioural differences, a coupling between the centerbody and the outer wing is proposed. Specifically, a study of the impact of twist distribution on maximum lift is carried out. Once again, RANS simulations at low speed are applied, this time, over three BWB geometries with different twist distributions. The results validate the hypothesis of a strong coupling between centerbody and outer wing at low speeds. Furthermore, these original results are used to calibrate a low-fidelity model of the maximum lift coefficient of a BWB at low speeds. This thesis brings forth some known results from classic wing theory like the importance of stalling close to the wing root and the stall delay effect introduced by the twist. On the original side, a baseline regional BWB geometry is proposed for use in future works. This geometry has already been used in other theses and scientific works. Furthermore, a mathematical low fidelity model of the maximum lift coefficient of a BWB has been developed. Moreover, analysis of the behavioural differences between a conventional aircraft and a BWB show that the centerbody of a BWB generates a stall-resistance phenomenon which is non-existent in conventional aircrafts. Analysis of the twist distribution on the BWB shows that it is possible to couple the aerodynamic characteristics of the outer wing with those of the centerbody to amplify natural lift generation. In particular, it was found that, on top of delaying stall, increased twist can also lead to increased maximum lift in the case of a BWB aircraft. The numerical results are used to calibrate the proposed maximum lift coefficient model. This model is a contribution to the scientific community interested in the development of complex wings and all-wing aircrafts like the BWB. In conclusion, this thesis shows the unique characteristic of the BWB to allow the designer to couple the outer wing and the centerbody behaviours to increase maximum lift and optimize its stall behaviour at low speed. Future work should focus on the use of more accurate simulation methods as well as the exploration of a larger BWB model database so as to enhance the robustness of the maximum lift prediction model. Nevertheless, the general results of this thesis are satisfactory and unveil previously undocumented advantages of the BWB with respect to conventional aircrafts at low speed.

Date	4 Dec 2020
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	François Morency (Supervisor) & Julien Weiss (Co-supervisor)