Read DI Online - DefenceIntegration.org

Cover Page

Editors Comment

Profile: The Performance Review Institute

Defence IQ Air Weapons Integration Conference, March 2006

MIRA flies high using Composite Wing Ribs.

Back Cover

Back

Nodal Optimization of Truss Structures (NOTS)

A DTI funded project was recently completed that investigated a novel, wholly composite, lightweight aerospace truss structure. The project consisted of eight partners from research and industry.

Advanced composite truss structures are a radical enabling technology for large composite structures. Currently perceived by the aerospace industry as a highly beneficial yet far-from-market concept, the application of composite truss structures has been limited in the aerospace sector because the underpinning joining technology has not been sufficiently developed. Consequently, the potential weight savings and manufacturing costs have failed to meet expectations. The performance of traditional metallic truss structures has been proven, but to maximize future weight savings a wholly-composite structure is required.

MIRA were responsible for the simulation work and guided the design. The main challenge was to optimise the structure based on the simplified operational loading conditions. The initial approach was to understand the load paths in the existing structure and to assess if reinforcement of the structure in the direction of load paths was an economical method of optimisation.

MIRA commonly uses a software package known as “Optistruct” for the structural optimisation of design concepts. Optistruct is a topology optimisation tool often used in the automotive industry for the development of castings. The software algorithm treats elements as potential voids and modifies their density in response to applied loads and design parameters in order to achieve an efficient structure. It works on the principal of maximising the minimum total potential energy, which leads to a maximum global stiffness (the Prager Theory). This technique helped in optimising the design concepts before detailed design was carried out.Airbus UK defined three load-cases for the wing rib, shown in the table below. Each load-case represents a different combination of loads

Airbus UK first identified that the internal structures of aircraft wings had the potential to be made using carbon truss structures. Their initial studies suggested that a carbon composite truss structure could provide both higher stiffness and greater tolerance to damage whilst delivering a lower weight wing.Previously these advantages were lost when the carbon composite struts werecombined with metallic

Load Case	Title	Represents	Represents
1	Shear flow	Aerodynamic, fuel, structural and inertial loads	Complete wing
2	Maximum Compression Load	Brazier loadsand fuelsuctionload	Complete wing
3	Pull of load	Aerodynamic pressure on the wing and fuelpressure load	Complete wing

nodes and inefficient bolted fixings.

Few attempts had been made to manufacture an entirely carbon composite truss structure, so a 3-year long DTI supported project began in December 2002. The aim of the project was to develop a planar carbon composite wing rib truss using novel manufacturing techniques that can accommodate complex geometries, for use in real civil aerospace applications. Critical to the successful development and implementation of such structures are the expert application of software tools that assist in the structural design and performance analysis, coupled with cost-effective manufacture and repair. Successful realisation of such structures would find ready implementation in aerospace applications as well as land transport. With so much as stake, the project was split into 9 work packages which ran over a 3 years period.

Airbus UK determined that rib 17 of the A330 was to be the target design. One of the most important design requirements was to produce a truss structure weighing less than the current aluminium rib (15kg).

that the wing was required to meet.Each load-case has an Ultimate and a Design level, the Design level being one-third of the Ultimate.

In order to define the local loads for the wing rib in load-cases 1 and 2, the displacements created at the junction of the rib castellations and the skin were extracted from the global wing model. The wing rib was then re-run in isolation from the wing to determine the forces required to produce these displacements. This was required to define the loads for MIRA’s Optistruct analysis. In this instance the Ultimate load-case was used.

Design Space
The model required that the area from which material could be removed be identified. This is commonly referred to as the ‘design space’. MIRA, in consultation with Airbus, defined this as the 2D profile shown in the illustration. The system holes are areas required for fuel lines and electrical applications. The cleats, required to prevent stringer buckling on the upper surface are joined to the castellations and therefore require a specific area over which they can be fastened.

Defence Integration

Back

Site Designed at: Versatile Solutions