Carbon composite blades really cut the mustard

University of California, San Diego (UCSD) structural engineers are working with NASA and the US Airforce to design a new generation of turbine fan blades.

University of California, San Diego (UCSD) structural engineers are working with NASA and the US Airforce to design the next generation of turbine fan blades that should significantly improve safety and reliability, reduce noise, and lower maintenance and fuel costs for commercial and military planes.

Initial feasibility tests reportedly indicate that these carbon composite ‘NASA UEET/QAT Efficient Low Noise’ fan blades are far superior to the metallic, titanium blades which are currently used.

John Kosmatka, a structural engineering professor at the UCSD Jacobs School of Engineering, is leading the project and noted that titanium fan blades have always been problematic.

‘The leading cause of engine failure is damaged fan blades. Failure may occur from the ingestion of external objects, such as a bird, or it may be related to material defects,’ said Kosmatka. ‘If it’s a metallic blade and it breaks, it can tear through the nacelle (a large chamber that contains wind flow to generate more power) as well as the fuselage, and damage fuel lines and control systems.’

In contrast, if a composite blade breaks, it disintegrates and does not pose a threat to the structure of the plane.

However, breakage is said to be less likely because composite materials are tougher and lighter than metallic blades and exhibit better fatigue characteristics. Regardless, a multi-engine plane can shut down an engine and continue to fly if a blade is lost and no other damage has occurred.

‘A composite blade disintegrates into many small pieces because it’s really just brittle graphite fibers held together in a polymer resin,’ said Kosmatka. A titanium blade, however, will fail at the blade root, causing large, four to six-foot blade(s) to fly through the air.

As designed, the composite blades are essentially hollow with an internal rib structure. These ribs-like vents are said to direct, mix and control airflow more effectively, which reduces the amount of energy needed to turn the blades and cuts back on noise.

Kosmatka said most engine noise comes from wind turbulence that collides with the nacelle. ‘By directing air out the back of the fan blades, the noise can be reduced by a factor of two. And by drawing more air into the blades, engine efficiency is improved by 20 percent.’

Kosmatka has also embedded elastic dampening materials in the blades, which minimises vibrations to improve resiliency. Durability is further increased by the fact that the blades are lighter and experience lower centrifugal forces.

Small-scale wind tunnel tests show that they last 10 to 15 times longer than any existing blade. The new blades should lend themselves to more efficient production techniques.

‘If you use titanium, you need to buy a big block of it and machine it down to size, wasting a lot of material,’ noted Kosmatka. ‘This is very time consuming and you have to worry about thermal warping.’

The composite material allows for mass production as it is fabricated into a mold, making the process more precise and ensuring that the blades are identical.

These new blades will be tested in large-scale wind tunnels next year at NASA-Glenn in Cleveland, Ohio, and if proved successful, could be installed in airplanes in 2004.