Pratt & Whitney has completed durability and performance testing of the short take-off and vertical landing version of its F135 engine, as it works to keep development of the Lockheed Martin F-35B Joint Strike Fighter on track.

The manufacturer has completed 635h of sea-level durability testing on a ground-test engine that accumulated 1,032 accelerated mission cycles and included 636 dynamic clutch engagements for the shaft-driven lift fan.

Another test engine has completed 134h of altitude testing to demonstrate performance, operability, augmentor ignition, failure detection and accommodation, and control software, says Bill Gostic, F135 programme manager.

F135 on STOVL test stand
F135 on STOVL test stand                                             © Pratt & Whitney

P&W expects "up-and-away" flight clearance for the STOVL F135 next month, enabling the first F-35B to begin conventional take-off and landing flight testing in June.

Clearance for STOVL operation is pending further testing to verify the cause of previous turbine blade failures.

The manufacturer has delivered the engine and lift fan for the first F-35B, aircraft BF-1, to Lockheed. This engine has been cleared for up-and-away flying, but is limited to low-power ground testing in STOVL mode to avoid exciting turbine blade vibration.

"They can run at a low power condition and demonstrate the effectors move," says Gostic, referring to the lift system's various doors and nozzles.

P&W is still hopeful of clearing this engine for a limited STOVL flight envelope after further ground testing. This would allow aircraft BF-1 to begin powered-lift flight testing around September.

But if the engine has to be removed and replaced with an F135 with redesigned third-stage low-pressure turbine, this would delay STOVL flight testing until around December, says Gostic.

Durability testing was conducted at sea-level conditions in a new ram test facility at P&W's West Palm Beach, Florida site. Testing included 300 simulated STOVL flights, and the engine accumulated 240h in powered-lift mode.

The lift-fan clutch, redesigned for development F-35Bs, is being torn down for post-test inspection, but "looked good" during the tests, says Gostic. "We did not see any unusual wear."

Altitude testing was conducted at the Arnold Engineering Development Center in Tennessee, and focused on steady-state and transient performance and operability characteristics.

The General Electric/Rolls-Royce team developing the F136 alternative engine for the F-35 also recently completed altitude testing of a pre-development engine at the site.

Source: Flight International