Aircraft

Driveshaft breaks after deliberate 'hard stall' during ground run with mock-up of lift-fan inlet planned for STOVL F-35

Pratt & Whitney is rebuilding an F135 Joint Strike Fighter engine damaged last month during ground testing of the short take-off and vertical landing propulsion system.

The F135 was damaged when a deliberate "hard stall" of the shaft-driven lift fan caused its driveshaft to break and debris was ingested by the engine, according to 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 airflow around the open inlet door while in the hover.

During testing of the engine on an outdoor stand at P&W's site in West Palm Beach, Florida, the Rolls-Royce lift fan was stalled deliberately to determine stall margin. 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." The shaft separated from the engine and fan.

"It stalled really hard," says Rob Burns, propulsion director for the Joint Strike Fighter programme office. Pieces of the hollow metal shaft and test instrumentation were ingested by the F135, breaking aero­foils through the engine, he says.

Burns adds that 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.

Gostic says that the need to rebuild the damaged engine will not affect 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.