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Research on Composite Fan Blades May Improve Airplane Safety, Performance

26 October 2001

University of California, San Diego (UCSD) structural engineers are working with NASA and the United States 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 indicate that these carbon composite ""NASA UEET/QAT Efficient Low Noise"" fan blades are far superior to the metallic, titanium blades which are used currently.

Turbine fan blades play a critical role in overall functionality of an airplane. They are attached to the turbine engine located in the nacelle, a large chamber that contains wind flow to generate more power. These six-foot long external blades create high wind velocity, and 80 percent of the thrust that moves that plane.

John Kosmatka, a structural engineering professor at the UCSD Jacobs School of Engineering, is leading the project and notes that titanium fan blades have always had problems. ""The leading cause of engine failure is damaged fan blades,"" says Kosmatka. ""Failure may occur from the ingestion of external objects, such as a bird, or it may be related to material defects. If it's a metallic blade and it breaks, it can tear through the nacelle as well as the fuselage, and damage fuel lines and control systems."" When this happens, the safety of the aircraft and its passengers is threatened and the likelihood of a plane crash increases.

In contrast, if a composite blade breaks, it simply crumbles to bits and does not pose a threat to the structure of the plane. However, breakage is 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. According to Kosmatka, ""A composite blade disintegrates into many small pieces because it's really just brittle graphite fibers held together in a polymer resin."" 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 direct, mix and control airflow more effectively, which reduces the amount of energy needed to turn the blades and cuts back on noise. Kosmatka says most engine noise actually 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 minimizes vibrations to improve resiliency. Durability is further increased by the fact that the blades are lighter and thus experience lower centrifugal forces. Small-scale wind tunnel tests show that they last 10 to 15 times longer than any existing blade. ""The number one maintenance task is the constant process of taking engines apart to check the blades,"" says Kosmatka.

Finally, these new blades should lend themselves to more efficient production techniques. As Kosmatka notes, ""If you use titanium, you need to buy a big block of it and machine it down to size, wasting a lot of material. This is very time consuming and you have to worry about thermal warping."" The composite material allows for mass production. 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.






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