Flight testing of Lockheed Martin's short take-off and vertical landing F-35B will be confined to conventional up-and-away flying until Pratt & Whitney can fully clear its F135 engine for STOVL operation. Aircraft BF-1 is planned to fly in June, but will not begin STOVL flight testing until the end of the year.
Following a turbine blade failure in the F135 planned for BF-1, P&W is proof-testing a replacement flight-test engine, but it will not be cleared to full STOVL thrust, where the vibration occurs that causes the blade to crack. "We believe the problem is a vibration in powered lift mode at very high power," says Bill Gostic, vice-president F135 programme.
P&W still hopes to clear the engine for STOVL flight testing after further tests to isolate the cause of the problem, but is working on a redesigned low-pressure turbine third stage that will be retrofitted to all flight-test F135s. "After a highly instrumented engine test in April, we will reassess the viable STOVL envelope," he says.
The redesigned LP turbine is to be tested "in the August timeframe", says Gostic, and engines retrofitted with the new design are expected to be available "by the end of the year". The redesigned third stage will have different vane spacing in the upper and lower halves, to disrupt the resonant vibration caused by the blades hitting the vane wakes.
P&W plans to deliver the proof-tested replacement engine in mid-March, allowing Lockheed to begin testing the first F-35B on the hover pit. Operation of the STOVL propulsion system will be tested, but not to full thrust. "The last few percent of power is the risk," says Doug Pearson, vice-president F-35 integrated test team.
Lockheed had planned to run the propulsion system to full power on the hover pit to characterise thrust before first flight, "but we don't need to", says Pearson. "We can wait till we have a properly qualified engine." Instead, the first 30 or so sorties will be confined to up-and-away flying. "We'll stay with conventional flight through the end of the year."
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