RTP Company: The Global Leader in Specialty Compounding

End-Use Case Study —

Helicopter Fan Blade

• Long fiber polypropylene
• Reduced cycle times save customer money
• Lightweight structural compound replaces thermosets

There is a growing use of thermoplastics as alternative materials to costly metal and prepreg composites for the aircraft/aerospace industry. Reinforced thermoplastics have the properties to meet specifications for many interior and exterior applications.

Injection Molded Helicopter Fan Blade

An injection molded fan blade has been developed for use on McDonnell Douglas helicopters equipped with NOTAR fan systems. NOTAR uses a constant circulation pressure generated by the fan to counter main rotor torque. The system eliminates the need for a tail rotor.

In an effort to reduce cost, a hand lay-up composite blade, which consisted of a foam core and a fiberglass / epoxy skin, was replaced by an injection molded blade made from an RTP Company compound of long glass fiber, polypropylene and a blowing agent. After completion of extensive material and structural qualification testing, the injection molded blade has been certified by the FAA for a one lifetime (7500 hours) service life. There haven't been any failures for the blade since it was put into service in January 1992.

90% Cost Savings

A cost savings of 90% has been achieved with the injection molded blade. The short cycle time (4 minutes vs. 10 hours) and low scrap rate (3% vs. 20%) are credited for this tremendous savings.

The Custom Compound

Chemically-coupled polypropylene provides a good balance of cost with processing ease, excellent chemical resistance and acceptable mechanical properties. The long glass-fiber reinforcement provides greater surface and linear distance of interface with the resin for better support strength and mechanical properties than a short fiber. A blowing agent was incorporated for weight reduction and for consistent weight and mass distribution because of the high operating RPM.

Processing

RTP Company's Technical Service group fine-tuned the processing parameters to keep skin thickness and weight consistent, and added a positive shut off valve on the injection molding equipment to assist in consistent shot / weight control. Initial prototyping and qualification production of blades were performed in Winona. The custom molder was able to control the processing to produce blade assembly weight within +/-1.0 gram.

Testing

The thermoplastic compound was tested in accordance with MIL-HDBK-17B requirements for melting temperature, tension, shear and bearing strength. Static, fatigue, and flight tests were used on the blades to substantiate the expected service life for compliance to an FAA certification. The blades were statically tested in a pullout strength test designed to determine the failure load required to separate the molded-in spar from the blade. They were also fatigue tested in an environment which produces the lowest bearing and shear strengths to achieve a product that would stand up to the 7,500 lifetime service hours required. Ground test rigs of assembled fans were subjected to high (5,000) rpms and pitch angles to generate the maximum centrifugal forces. And finally, flight tests involving 5,000 flight hours were run.

Test results showed that both the injection molded blade and the compound qualify for use in the NOTAR fan system. The unique combination of the chemically-coupled polypropylene with the long glass-fiber reinforcement and the blowing agent has proven capable of providing the required stiffness, weight and load strength desired for the system.

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