Cost effective acoustic liners for jet engines

Thermally Assisted Piercing of Acoustic Liners (TAPACO)

Background

Aircraft engines contribute significantly to noise pollution (Jones, 2014). An increase in air travel has increased the demand for quieter aircraft. Intake acoustic liners encase large turbofan aircraft engines and attenuate their noise. Kempton (2011) identified the lightweight of acoustic liners as one of the major challenges. The best-performing broadband absorbing liner configuration is a sandwich structure with a perforated facing sheet (see Figure 1) exposed to the air stream and a specially designed cell structure. The perforation pattern and the design of the sandwich cell are of chief importance to the acoustic behaviour of the liner. Composites can offer significant weight reductions due to their anisotropic properties and low-density constituents.

State-of-the-art liners are mainly made of thermoset composites and the perforation is done by orbital drilling or robotic laser heads. However, both methods lead to cutting of the load-bearing fibres which results in a reduction of the composite efficiency. This forces the designer to add more material to the design which reduces the potential fuel savings. Perforating without cutting fibres could be a key approach to maximising noise reduction without compromising on weight savings. In addition, both drilling and laser drilling can create localised heat areas that damage the composite material. Furthermore, Thermoset matrix composites are used extensively in various industries including aerospace. The application of thermoplastic matrix materials, however, has not currently reached the same levels of development as those attained with thermosets. Thermoplastic materials have clear advantages in comparison to the thermoset matrix composites such as the capability to be recycled and welded. The weldability of thermoplastic can eliminate the need for bonding and riveting and the cost and weight issues associated with each of these. Table 1 presents a comparison between thermoset and thermoplastic resin properties.

Objective

The project will address the challenge of producing cost effectively acoustic liners and their key component the perforated sheet made of thermoplastic composites.

The TAPACO solution focuses on the use of the Thermally Assisted Piercing (TAP) technique in order to perforate the acoustic liners on the jet engine. TAPACO, a concept developed by Brunel Composites Centre, will enable the perforation of a one-piece acoustic liner without the need for drilling. Thermally Assisted Piercing (TAP), is a material displacement technique that has been used previously to create holes in composite materials without damaging the load-bearing fibres. In this technique, the thermoplastic composite material is heated which allows the movement of the fibres in the molten matrix. The TAP rig can then be pressed onto the composite material in order to displace the load-bearing fibres and create a hole pattern according to acoustic design specifications. The process will be automated and continuous piercing of the facing sheet liner is possible.

Benefits

The proposed technology will eliminate the main disadvantage of current state-of-the-art perforation methods which is the cutting of the load-bearing fibres while performing the drilling operation. This technology will allow the user to utilise the capability of thermoplastics to melt and reshape upon heating to their processing temperature which in case of engineering thermoplastics is well above the expected service temperature. In addition, thermoplastic composites are tougher and more damage tolerant than thermosets and are easier to repair and can be shaped using simpler equipment. For the manufacturing of acoustic liners more than 50% of the cost is attributed to shaping and drilling and with material cost being similar between high-performance epoxies and suitable engineering thermoplastics we expect a cost reduction of 15 to 20% for TAPACO technology. The methodology can be tailored to account for different geometries is scalable and can be automated. Other important advantages are the use of non-complex equipment, the short processing time, the simple operation principle, and the elimination of hazardous by-products such as dust or fumes.

Brunel Composites Centre is involved technically by offering expertise in the TAP technology and leading the finite element modelling, future planning, dissemination, and exploitation work packages of TAPACO.

Project Partners

• Denroy
• Spirit AeroSystems
• Involute Engineering
• Brunel Composites Centre