F135 Engine Cleared For F-35B STOVL Testing

Feb 3, 2009
By Graham Warwick

Pratt & Whitney’s F135 engine has been cleared for powered-lift flight-testing in the F-35B Joint Strike Fighter (JSF).

Approval follows a review of design changes made to eliminate vibration that caused turbine blade failures during ground tests of two engines.

The first redesigned engine was delivered to Lockheed Martin last week for installation in aircraft BF-1, the first short takeoff and vertical landing (STOVL) F-35B. Testing is expected to begin on the hover pit at Fort Worth, Texas, early this month, leading to a first vertical landing at Naval Air Station Patuxent River, Md., in June or July.

The third-stage turbine-blade problem has delayed the start of powered-lift testing by four to six months, but has not extended the overall JSF flight-test program as other work was moved around to accommodate the delay, says Bill Gostic, vice president for the F135 program.

The first turbine blade failed on Aug. 30, 2007 during powered-lift qualification testing of ground-test engine FX634. The second blade failed on Feb. 4, 2008 during proof testing of flight-test engine FTE06. The proof test was designed to determine whether the engine was susceptible to blade failure.

The failures were traced to high-cycle fatigue cracking due to vibration caused when the blades struck the wakes from third-stage stator vanes upstream. The fix was to switch to asymmetric vane spacing, to disrupt the excitation causing the vibratory stress, and to redesign the blades to remove the stress concentration.

Pratt conducted a series of ground tests to validate that the asymmetric vanes eliminated the vibration and did not interfere with turbine cooling, and has completed a new proof test on the redesigned FTE06 flight-test engine. This is the engine delivered to Lockheed Martin for installation in aircraft BF-1.

The engine manufacturer plans an additional ground testing in May/June with strain gauges on the new blades to confirm the redesign has eliminated the stress concentration. “We’re confident we have sufficient margin,” Gostic says.

F135 production is scheduled to begin in July with the first four engines for the conventional takeoff and landing (CTOL) F-35A. This will be followed by a second production batch of eight CTOL and eight STOVL engines.

Photo: Lockheed Martin

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