Pratt & Whitney is rebuilding an F135 Joint Strike Fighter engine that was damaged earlier this month during ground testing of the short take-off and landing propulsion system.
The F135 was damaged when a deliberate “hard stall” of the shaft-driven lift fan caused the driveshaft to break and debris was ingested by the engine, says Bill Gostic, P&W vice-president, F135 programmes.
The propulsion system was being tested with a mock-up of the lift-fan inlet planned for the STOVL version of the Lockheed Martin F-35, to check the airflow around the open inlet door while in the hover.
During the testing on an outdoor stand at P&W’s site in West Palm Beach, Florida, the Rolls-Royce lift fan was stalled deliberately to test inlet performance. This was achieved by closing the variable-area vanebox nozzle below the fan.
“We stalled the lift fan 28 times by closing the vanebox area 250% beyond normal operating conditions,” says Gostic. “On the 29th time we went to 300% for a particularly aggressive stall and fractured the shaft connecting the lift fan to the engine. It separated from the main engine and lift fan.”
“It stalled really hard,” says Rob Burns, director of propulsion for the Joint Strike Fighter programme office. Pieces of the hollow shaft and test instrumentation were ingested by the F135, breaking aerofoils through the engine, he says.
Burns says the test was “pretty far out” and, in operational use, protection algorithms in the control system would prevent the lift fan stalling. “We would never expect to see a stall as hard – or at all,” he says.
To make sure, P&W plans crosswind testing of the STOVL propulsion system with the mock-up inlet and door in place using another F135. “We need to make sure there is no restriction on wind direction when coming back on deck,” says Burns.
Gostic says the need to rebuild the damaged engine will not impact the schedule for flight clearance of the STOVL propulsion system, planned for early next year to support a first flight of the F-35B in May 2008.