Yesterday I was able to witness part of the final ground tests for the Pratt & Whitney F135 before the development phase finally completes later this year.
The accelerated mission test (AMT) forms part of the initial service release (ISR) qualification for the F135 version powering the F-35B short take-off and vertical landing (Stovl) variant. ISR is required before regular squadron pilots, rather than test pilots, can begin training on the F-35B at Eglin AFB in 2011.
The evaluation, which includes crosswind tests of the lift fan inlet on an adjacent stand in Florida as well as tests at AEDC, also marks the finale of a long-running test effort dating back to the start of what was then called the JSF119 concept demonstration phase way back in 1998.
As its name suggests, AMT is designed to fast track engine running time through a series of composite missions that replicate the sort of scenarios the F-35B propulsion system (not simply the engine) will see in service. For Stovl AMT, a set of five composite missions has been created in conjunction with Lockheed Martin to represent 17 scenarios ranging from air-to-air and air-to-ground maneuvering to combat air patrol and vertical landings.
Mission Type Three that I witnessed took the engine and its complex lift system through 55 minutes of simulated air-to-air operation, including two periods of powered lift to represent a short take-off and vertical landing.
C12 stand in Pratt's sprawling 7,000 acre West Palm Beach test site in Florida, I heard the engine spool up from ground idle to begin the AMT test. Within a few minutes I was surprised to hear the distinct whine of the lift fan gear system as the clutch engaged, despite the dampening provided by the serious ear protection given to me by Pratt.
The new test stand is more stable for Stovl tests than the original vertical gantry (P&W)
Standing only a few feet away from the spider-like
The whine, relatively high-pitched compared to the background engine sound, soon dissipated as the clutch plate locking mechanism engaged and power was applied for the rolling take-off and climb. Over a roughly two minute period some four to six minutes into the test, a lot happened at once. Most noticeable was the Rolls-Royce developed 3-bearing swivel duct at the back of the engine as it moved down through 60 deg to vector thrust down for lift off.
Close-up of the 3-bearing swivel duct in action yesterday (P&W), and how it looks before integration (below) at Rolls-Royce. (RR)
At the same time, though not as obvious from my vantage point, was the small (+/- 1 deg) left-right yawing movements of the swivel duct as the 'take-off' occurred. Also invisible were the 0 to 60 deg variations in lift fan exit vane angle during the event, or the fact the roll posts were pumping out stabilization jets at the rate of up to 3,200 ft/pounds force.
At around six minutes the engine, FX648, was in 'up and away' mode and ready for the first of six phases of running in full afterburner. When this kicked in the ground shook and my chest was pounded as the engine pushed out around 43,000-lb thrust. Behind the stand, hundreds of feet from the exhaust, the open water of the swampy section of Everglade rose up in fury. Although the exhaust plume was only visible as heat distortion and a trace of particulates, photos of the same test at night showed a spectacular series of shock diamonds cascading into the darkness.
In all the test included six periods of full after-burner, 10 at ground idle and 15 at part-power setting. The 'flight' ended with a vertical landing which saw the swivel duct pivot down to 90 deg, the roll posts push out up to 2,500 ft/pounds force and the lift fan exit vane angle down to 90 deg.
A lift fan module is loaded into the original X-35 (Lockheed Martin/RR)
While the engine wound-down I was able to inspect the crosswind test on engine FX640 at the adjacent site C14. The tests are aimed at verifying model predictions for the behavior and performance of the lift fan with the inlet door open at its two main operating angles 35 and 65 deg, and include baseline evaluation of the engine and fan running with zero crosswind, as well as runs with the door open at the two angles and a cross wind of 20 kts.
The ‘wind’ is provided by a PT2/T34 turboprop - originally an engine used on the long-retired C-133 transport - and ducted across the engine via a long tunnel-like structure.
The group wanders underneath FX640 - the cross wind test unit. (PW)
Unlike the AMT profile, which takes the system throughout its operating envelope, the crosswind tests deliberately probe performance at an 'off-nominal' schedule. The focus is mainly on the ability of the fan's inlet to cope with the cross flow which is set at a 90 deg "worse case" position relative to aircraft position. The inlet is configured with 10 support struts over a set of 33 inlet guide vanes, each of which can be moved between -10 deg and +72 deg.
So far the test crew report that the only surprises are good ones which is an encouraging sign as the effort moves into the final phases of its 270 test point target.
... and finally, just to remind us how this all started. A short clip of an interview with the F-35B Stovl concept inventor, Lockheed Martin's Paul Bevilaqua.
Click here to read Guy's story on P&W's F135 increased-thrust announcement.