Patent of the month: Optimised unit load device

Materials World magazine
1 Aug 2017

Dr Jennifer Unsworth of intellectual property firm Withers & Rogers describes a novel composite structure for aircraft loading pallets.

The drive to reduce the weight of components in order to improve fuel efficiency and lower operating costs for end products has led to the development of new materials and structures for a range of applications in numerous industries, including aviation.

These developments include the optimisation of structures to ensure that, in addition to providing weight savings, the components in which the structures are used are reliable when in service.

Fergusson’s Advanced Composite Technology has developed and secured a UK patent for an optimised structure for use on unit load devices (ULDs).

ULDs are pallets or containers that are used to load luggage and freight onto aircraft. They allow cargo to be packaged into a single unit, reducing the total number of items that need to be loaded into the hold. In service, ULDs are moved via roller tracks and ball mats, meaning that the base panels of the devices travel many kilometres under load during their service life. 

Traditionally, ULDs have been made from high-strength aluminium alloys, which are suitable from a cost and manufacturability perspective. However, the limitations of the stiffness-(and strength-)to-weight ratio and their susceptibility to plastic deformation has led to the development of new composite structures for use in ULDs.  

Composite materials, in particular composite sandwich structures, are known for their high stiffness-(and strength-)to-weight ratios, so attempts have been made to incorporate them into ULDs.

While composite sandwich structures address the issue of increasing the stiffness to weight ratio of the base panel of a ULD, their low core density and correspondingly low compressive strength meant that the base panels made from composite sandwich structures were at risk of suffering from local indentation failure during use. The formation of indentations on the base panel of a ULD would make it more difficult to unload from the aircraft, so composite structures have not been used in ULDs before.  

The structure developed by Fergusson’s Advanced Composite Technology meets the same design load and operational requirements as the previously developed structures, while also offering increased resistance to plastic deformation and damage formation.

The base panel includes a central core that is sandwiched between a fibre-reinforced upper surface layer and a fibre-reinforced lower surface layer. The central core includes high-strength microspheres, surrounded in a matrix material to form a composite foam. The upper and lower surface layers include a matrix material that binds them to the central core, resulting in a lightweight and rigid structure.  

In some examples of the invention, barrier layers may be provided between the core and the upper and lower surface layers. A wear layer may also be bonded to the lower surface layer.   

The size of the microspheres and the density of the core can be optimised to provide a panel that has a stiffness-to-weight ratio that is slightly higher than conventional low-density foam cores, but with a much higher compressive strength in the direction perpendicular to the panel surface.

A static indentation test compared the forces applied to a panel of the newly invented structure and a conventional high-strength foam core, each of the panels tested having the same core thickness, surface layer material and surface layer thickness. The loading condition of a base panel in service was replicated using a spherical steel ball, the type used in aircraft cargo flooring.  

The maximum indentation of the panel made from M200 (the conventional foam core) was six times larger than that of the structure developed by Fergusson’s Advanced Composite Technology. In practice, this means that the new structure is at less risk from indentations that will make unloading aircraft cargo difficult and is more likely to be adopted by other manufacturers of ULDs.

Filed in the UK on 10 June 2015, the patent application for the new structure was granted just over two years later on 14 June 2017. The fast processing of this patent application arose from the manufacturer’s ability to take advantage of the UK Intellectual Property Office’s ‘Green Channel’. Patent applications are eligible for processing through this route when the invention provides environmental benefits. In this case, the incorporation of the innovative structure into ULDs reduces the weight of the device, meaning that less fuel would be required to transport the container, resulting in lower levels of emissions.