Design and Spin Testing of Integrally Damped Hollow Core Composite Fan Blades

NASA-Glenn is developing a reduced-weight low-noise fan blade that has increased propulsive efficiency and reduced acoustic noise. The research focuses on drawing free-stream air into a hollow-core spinner and main shaft and then using a complex manifold network built into each blade to direct the air through the blade interior and out along the blade trailing edge. This exhausting air along the trailing edge energizes the airflow to increase propulsive efficiency and reduce acoustic noise. A complimentary structural research effort involves adding integral passive damping to these thin-wall open-cell blades. The damping treatment consists of a small viscoelastic damping material embedded within the lay-up of the composite fan blade skins and manifold components. Four different fan blade test specimens were fabricated, including; (1) an undamped (or baseline) blade set, (2) an “optimally” damped blade set, (3) a “maximum” damped blade set, and (4) an “ultra” damped blade set. The four blade sets were initially bench tested followed by spin testing in the NASA-Glenn Spin Facility to determine their modal properties as a function of rotational speed. The optimally damped and maximum damped blades increased the loss factors over undamped blades by 100200%, whereas the ultra damped blade set had increased loss factors by 300-400%. Embedding a viscoelastic material in these hollow core blades was found to be an effective way to increase the blade damping and decrease the vibration.