PRATT & WHITNEY has used computational fluid-dynamics (CFD) to develop a remedy for fan flutter on the F119 engine for the Lockheed Martin/Boeing F-22. CFD analysis identified an airflow disturbance over the inlet guide-vanes which was causing vibration and flutter of the hollow fan blades.
The F119 is the first US military engine to have hollow fan blades, which are produced by diffusion-bonding of titanium plates, then joined to the first-stage rotor disc by linear-friction welding, to create a once-piece bladed-disc (blisk). This process saves 32kg per engine, says P&W.
Fan vibratory-stress detected in tests was traced to airflow separating from the narrow gap between the fixed forward strut and movable aft flap of the guide vane. The phenomenon has not been seen in other engines using the same guide-vane design, says Bill Richey, manager, F119 business development.
CFD aerodynamic analysis, combined with finite-element structural modelling of the fan-blade, suggest airflow separation as the cause of the problem, he says. To test the hypothesis, the gap in an inlet guide-vane in P&W's Joint Technology Demonstrator Engine was taped over and a reduction in fan-blade vibration was measured.
According to Richey, the cure developed for the F119 involves a rubber seal which is inserted in the trailing edge of the fixed strut and stays in contact with the reprofiled leading edge of the movable flap, closing the gap in the guide vane. Tests have verified that the cure has eliminated flutter and reduced vibratory stress to "well below" allowable levels, he says.
Richey says that the use of CFD to establish the link between the flowfield over the inlet guide-vane and vibration of the fan blade was made possible by computing advances which have made CFD more affordable. P&W used desktop workstations to develop the remedy, he says.
Source: Flight International
