Advanced grid stiffened structure

CO2 and NOx emissions are a major cause of concern if we must strive to develop a Clean aircraft as such being envisioned here, at CleanEra. Reduction in CO2 and NOx emissions is a direct consequence of reduction in the amount of fuel utilized by the aircraft in operation. A major part of the weight carried by the aircraft is it own weight which is contributed largely by its structure; and there is still scope to reduce the weight of the aircraft by changing the material used to build it, to light-weight composites. This, in addition to certain changes in the structure, brings us closer to the goal.

A promising configuration for the structure of the aircraft is a rib-skin configuration evolved from early isogrid stiffening concepts called Advanced Grid Stiffened (AGS) Structure. The Aluminum isogrid structure was patented by McDonnell-Douglas. Further, the Air Force Philips laboratory targeted the design and manufacture of composite AGS Structure for a shroud close to 1997. This type of structure i.e., isogrid along with other configurations shown in the picture below showed higher reliability & reduced cost due to adaptability to automated manufacturing techniques. In addition to this, compared to conventional structure, it also has higher strength, better resistance to moisture and damage tolerance which makes it suitable for operation environments as studied by Huybrechts and Meink. Reaping the benefits of such a structure is only possible once the difficulties associated with it are overcome. The design of such structures has been based on assumptions of sequence of failures – whether it is the skin, the stiffener or both at the same time.

Types of grid Stiffened Panels

Composite shell sturctures of AGS nature are being researched by scientists now to determine their reliability and fidelity. Determining the optimum configuration for a given application requires the use of very detailed and computationally expensive models. This magnitude of work is lengthy and time constraint leads to difficulties in the practical applications of these structures. Also, it is an exremely difficult task to model the changing orientation and location of stiffeners which can be simplified by modelling the stiffener and panel as two separate entities. They shall have the freedom to be analysed independently as well as a coupling will be derived between them inspite of the non-matching meshes. Hence, there is a need to develop automated design tools which shall be used to design and optimize these complex composite shell structures which can be used as parts of the fuselage or wings for the CleanEra aircraft which is currently being directed towards the Blended Wing Body design. Such tools should be very efficient to use without compromising the accuracy that is necessary for such designs. This program aims at developing an automated design tool to fit that need. This tool seeks to include grid patterns and grid density as design parameters which would be then optimized for particular design cases. For a given loading situation and overall geometry, the design tool will be able to determine the grid geometry and spacing along with the failure mode sequence (e.g. skin buckling followed by skin failure, followed by grid failure) that will minimize the weight and still generate a producible and cost-competitive component.

The tool will initially model the grid and panel/skin as independent entities. This will be followed by sensitivity studies.

The AGS Structure is primarily promising for the Blended Wing Body due to its inherent characteristic of a rectangular fuselage shape. In a circular pressurized fuselage the pressure causes only a membrane stress but in a rectangular fuselage, this causes the pressure to create bending deformation on the flat upper cabin ceiling. A conventional circular cross-section fuselage has its structure in the form of stiffeners running length-wise as shown in the picture below. But this extra bending stress would have to be taken up by the skin and not the stiffeners. To relieve the skin of this excess deformation, stiffeners in different orientations would be most suitable.

From a more practical point of view, this automated program shall provide us with an optimized structure/panel for several types of loading conditions which shall be applied to the fuselage/wing of the aircraft and will be effectively lighter than its conventional skin-stiffener counterpart. Also, being thinner than the conventional structure, it will provide increased space for storage and comfort in the fuselage along with increased space for fuel in the wing. It will also find use in spacecrafts due to its better characteristics of damage tolerance and high strength for operating environments.

Some examples of AGS Structures