The shaft-driven lift fan propulsion system that will enable the Lockheed Martin F-35B Lightning II stealth fighter to perform short takeoffs and vertical landings (STOVL) recently operated for the first time in the aircraft during ground testing at the newly constructed hover pit at Lockheed Martin’s STOVL Operations Test Facility in Fort Worth, TX.
BAE Systems pilot Graham Tomlinson performed two conversions from conventional (wing-borne) to STOVL (jet-borne) mode with the aircraft anchored to the specially instrumented hover pit. The test was part of the final series of ground tests prior to the F-35B’s first flight in the following weeks.
“The F-35B’s STOVL propulsion system operated exactly as expected, providing the power output that our models forecast and transitioning very smoothly from conventional to STOVL mode and back,” said Bobby Williams, Lockheed Martin Vice President and F-35 Deputy Program Manager. “We expect the same kind of seamless transition when the F-35B begins STOVL-mode flights in early 2009.”
At full power, the F-35B’s propulsion system generates more than 40,000 lb of lifting force, about 170% more than the current-generation STOVL fighters. Lockheed Martin is developing the F-35 with its principal industrial partners, Northrop Grumman and BAE Systems.
The propulsion system has logged more than 1900 h of operation on test stands thus far. In 2001, the X-35B, a proof-of-concept STOVL aircraft using a prototype of the same propulsion system, completed 14 short takeoffs, 17 vertical takeoffs, and 27 vertical landings. That year it became the first aircraft in history to perform a short takeoff, accelerate to supersonic speed in level flight, and descend for a vertical landing in a single mission.
The STOVL propulsion system comprises a Pratt & Whitney F135 turbofan engine, a driveshaft leading from the engine face to a gearbox and clutch connecting to a counter-rotating Rolls-Royce lift fan located directly behind the cockpit, a three-bearing swivel duct at the rear that vectors the engine thrust downward and provides yaw control, and a roll nozzle under each wing for lateral stability.
During conversion from conventional to STOVL flight, all doors associated with the STOVL propulsion system begin to open, including the lift fan inlet and exhaust doors, roll-nozzle doors, auxiliary inlet doors atop the fuselage, and the aft fuselage three-bearing swivel duct doors. Once all doors are opened, the clutch engages and the lift fan begins turning. When the lift fan reaches full speed, the clutch locks, providing a direct physical connection between the engine and lift fan. The STOVL propulsion system is then used to provide flight control.
To evaluate overall flying qualities and airworthiness, the F-35B will operate in conventional mode during its initial series of flights. In preparation for the F-35B’s first flight, pilot Tomlinson flew the F-35B for the first time in late May, assessing the aircraft’s handling at various power settings. In early 2009, the F-35B will undergo initial STOVL flight operations before moving to the Naval Air Station in Patuxent River, MD, for further testing.
In addition to being the first operation of the shaft-driven lift fan propulsion system, the tests marked the first use of the hover pit and its supporting facilities.
“Our aim is to retire technical risk well before we deliver F-35s to the fleet,” Williams said. “Our investment in facilities like the hover pit is helping us do just that while building confidence in the test program.”