Blend Wing Body

The structure design of bended wing body aircraft for commercial airline transport By Yue Sung (Lance) Fung 10/17/2012 Introduction Today many new design aircraft concept use blended wing body theory, one of the biggest challenge of this aircraft is to design a strong and pressurized structure for safe commercial Airline transport. According to V. Mukhopadhyay the structure of Blended-Wing -Body (BWB) flight vehicle has been a one of the major challenging problem for many year. By comparing the fuselage of a conventional aircraft which is a cylinder shape, the stress level of a flatter shoebox shape type BWB fuselage has a higher magnitude ,because the internal pressure causes blending stress of the whole fuselage instead of the stress act on the skin membrane. Due to the primary design of the conventional fuselage structure are focus on membrane stress, a new design and material are needed ,in order to increase the bending moment of inertia without increase the weight requirement while the aircraft is pressurized . Although a whole flowing Blended Wing Body fuselage provide structure weaker and no as pressurize as conventional aircaft, but new designs of the blended wing body aircraft structure can provide satisfying stress, deflection and buckling safety factors, pressurized body during the critical flight and ground loads. There are three articles discussed in this literature review. According to R. H. Liebeck, the BWB structure is separated into two major components: the centerbody and the outer wings. The Y braced box type fuselage design structure concepts was based on a thick stringer outer surface structure, where the stringers are about 5–6 in. deep in order. Then using internal ribs have Y braces where they meet the skin, to reduce the bending moment on the skin created by the internal pressure across the center body and the outer wing. As a result ,the complete center body pressure vessel is  omposed of the upper and lower surface panels, the rounded leading edge the rear main spar, the outer ribs and the internal ribs payload and does not carry wing bending loads therefore the load can equally balanced however the cabin inside of the aircraft is hard to keep pressurized for commercial flight. At the same time , V. Mukhopadhyay has been stated that using the alternative multi bubble structure configuration instead create a whole body structure on a BWB aircraft. The multiple bubbies body can easily apply to the BWB aircraft internal structure, where the center distance between each segment was kept as same as the radius, therefore the outer side and the inner cabin wall junction are facing 120 degree each other, as a result a equal angel geometry given an advantage structure when the membrane stress equilibrium force acting between the cabin wall and the outer shell by balanced by inter cabin wall tensioning, which can keep the inter cabin more pressurized. Comparison to V. Mukhopadhyay , L. U. Hansen, stated that the loading of the structure leads to high bending loads in every part of the cabin can be reduced by structural elements carrying the vertical force components in result the reduce the blending force in the cabin which keep the fuselage pressurized . The solution is to use a strong structural components panel in the membrane which connecting both upper and lower side of the fuselage ,and there are four fuselage surface composite panel configurations were analyzed and optimized for minimum weight, under required internal pressure and estimated compressive loads with both stress and buckling constraints. Although the skin/stringer outer surface structure can apply to the whole structure of the BWB with really strong component against the blend stress , the fuselage surface composite panel also will be a good consideration for strengthen the structure of the outer body of BWB aircraft since it can enlarge the skin stress of the aircraft the keep it more pressurize. Yet the multiple bubbies cylinder configuration can be most effective when it applied to the center body of BWB because of the circulate configuration will reduce the membrane stress and easier to keep the fuselage pressurized than skin/stringer configuration. Discussion R. H. Liebeck method can applied to can applied to the blended wing body aircraft while the Y braces box structure is really sturdy over the upper and lower surface panels, the rounded leading edge the rear main spar, the outer ribs and the internal ribs payload. The design does included carry wing bending loads therefore the load can equally balanced . However the design cabin pressure load is experienced on every fight and fatigue becomes the design conditions. Since the structure is no in a circle it cause the cabin pressure loads are taken in bending, the margin required for aluminum could be forbidden , therefore and advance carbine composite. Other studies said that if the structure are not built by composite material the structure of the BWB will be heavier than the conventional aircraft which in result having less advantage of building BWB aircraft as predicted . Although the Y braces with skin and stringer structure from R. H. Liebeck article will required higher weight without advance composite, L.U. Hansen, stated that the BWB body aircraft outer shell structure can be connect with different type of the . The strong structural components panel can be bonded and connect each other , the different size of the panel and represented the different airfoil surface of the BWB aircraft. When compared to the stringer and the membrane , components panel on every can provide better connecting both upper and lower side of the fuselage also it blend moment can be reduced by structural elements carrying the vertical force. The challege of the design is to hold the panels connection during higher G loading and abnormal maneuver ,and the each panel have to the make it as less tolerance as possible to reduce the induce drag and the turbulence drag from the gap from each panel. To improve the first two solution, the alternative solution state by V. Mukhopadhyay is to use the multiple bubbies fuselage ,during the experiment the multiples cylinder body can remodel the advantage of the pressurized cylinder body of the conventional aircraft. The experimental result shown the design loaded of the cylinder fuselage his the same pressure stress and loading when using the two bubbies and three bubbies structure. When the number bubbies structure increase the pressure loading decreased while the blend moment increase. Although compare to the first two method the blending moment may if the bubbies number increase to three of four , however the design show that the pressurized load of the multiple bubbies construction can provide more pressure load than the skin / string structure and the component panel structure. From the test result of V. Mukhopadhyay article Von-Mises stress of the top surface of the aircraft combined the top and bottom internal cabin pressure. These stresses were well within allowable limits and about 25% lower than the four-bubble design with about 10% increase in unit weight/floor area. Therefore multi-bubble fuselage appear to be significantly better compared to the component panel design and Y brace skin / string structure. As future evidence, NASA build a unmanned BWB aircraft to optimize the structure design due reality flight situation, their structure design was using the multiple bubbles Structure and component panel adding on top and the bottom to prevent the blend moment. The multi-bubble type fuselage which has better stress distribution, for same material and dimension, can be the most effective , further design will be more focus on the combination of the component panel with multiple bubbies fuselage with can reduce the blending force and buckle force while keeping the pressure loading which required for commercial flight.