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Pratt & Whitney Joint Strike Fighter engine damaged in ground test
By Graham Warwick

Pratt & Whitney is rebuilding an F135 Joint Strike Fighter engine that was damaged earlier this month during ground testing of the short take-off and landing propulsion system.

The F135 was damaged when a deliberate “hard stall” of the shaft-driven lift fan caused the driveshaft to break and debris was ingested by the engine, says Bill Gostic, P&W vice-president, F135 programmes.

The propulsion system was being tested with a mock-up of the lift-fan inlet planned for the STOVL version of the Lockheed Martin F-35, to check the airflow around the open inlet door while in the hover.

During the testing on an outdoor stand at P&W’s site in West Palm Beach, Florida, the Rolls-Royce lift fan was stalled deliberately to test inlet performance. This was achieved by closing the variable-area vanebox nozzle below the fan.

“We stalled the lift fan 28 times by closing the vanebox area 250% beyond normal operating conditions,” says Gostic. “On the 29th time we went to 300% for a particularly aggressive stall and fractured the shaft connecting the lift fan to the engine. It separated from the main engine and lift fan.”

“It stalled really hard,” says Rob Burns, director of propulsion for the Joint Strike Fighter programme office. Pieces of the hollow shaft and test instrumentation were ingested by the F135, breaking aerofoils through the engine, he says.

Burns says the test was “pretty far out” and, in operational use, protection algorithms in the control system would prevent the lift fan stalling. “We would never expect to see a stall as hard – or at all,” he says.

To make sure, P&W plans crosswind testing of the STOVL propulsion system with the mock-up inlet and door in place using another F135. “We need to make sure there is no restriction on wind direction when coming back on deck,” says Burns.

Gostic says the need to rebuild the damaged engine will not impact the schedule for flight clearance of the STOVL propulsion system, planned for early next year to support a first flight of the F-35B in May 2008.

http://www.flightglobal.com/articles/2007/05/31/214335/pratt-whitney-joint-strike-fighter-engine-damaged-in-ground.html

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yikes, fun stuff

Pretty cool job to be able to sit there all day and say "I wonder what will happen when I do this" and then do it Wink

From what I've read so far. No one seems to be concern and everything is moving right along........................ Salute

Elite 3K Joined: May 10, 2004 Posts: 3021 Status: Offline

Yep, someone's got to find the real limits that lead to failure sometime - you can't just rely on computer modeling when it's something this important Smile

.

Man that's the kind of job I need. Testing and breaking $#!t for a chance instead of just fixing. Laughing

Elite 1K Joined: Aug 19, 2004 Posts: 1090

Corsair1963 wrote:

From what I've read so far. No one seems to be concern and everything is moving right along........................ Salute

It sounds like the way they were simulating/causing the engine stall caused the damage. If you read closely, they were closing the vanebox nozzle area 250% beyond normal operating conditions for the first 28 tests. The 29th test went to 300% beyond normal operating conditions. Obviously something about closing the vanebox area that much led to the damage. When the F-35B flies in operational service (or even flight testing), the vanebox nozzle area won't ever be closed off by 250% or 300% of normal operating conditions. It will close off to 100% of normal operating conditions. The hard stalls they were trying to simulate by closing off the nozzle below the lift fan wouldn't actually be caused in that way in a hovering F-35B. A hard stall would be caused by a significant disruption of airflow in the vicinity of the lift fan inlet on the top of the aircraft. For example, due to high winds when bringing the F-35B back onto the deck of a carrier as the article stated.

There is obviously some critical stress limit that is reached when the engine is running at a certain power setting and the vanebox nozzle closes beyond a certain amount, inducing a stall. They probably know what this critical limit is now and can avoid going over it in future tests. It sounds like they've completed that portion of testing though. Why else would they have done 28 tests with the vanebox nozzle area closed 250% of normal and then move up to 300%? It sounds like they were going for broke after satisfying all the reasonable test conditions set forth. Little did they know that they'd end up with a "broke" engine as a result. Hahaha... Very Happy

Yes it was a fun job. When I was a wee lad I worked in the Turbo-Propulsion Lab of the Naval Post Graduate School. (insert long story of how I got there, here).

We had a interesting test set up with a turbojet engine spread out in three separate rooms. Compressor section in one room, Turbine section in another. Exhaust in a third.

We mostly did studies on blade efficiency. Our resident professor who was Australian and was part of the Concorde design team (another long story) had a great idea to put an aluminum honeycomb in front of the inlet to give a better base line of airflow into the compressor.

We we fired the engine up the honey comb lasted about two seconds and we then spent a month and a half picking aluminum shards out of the compressor section.

Yep, pretty cool job.

Enthusiast Joined: Jul 21, 2004 Posts: 27 Status: Offline

ENGINE TEST DAMAGE NOT EXPECTED TO DELAY US F-35 PROGRAM: Recent tests did some big damage to a prototype F-35 Joint Strike Fighter engine, but program officials say it won't delay the overall program. The testing pushed the limits of one engine variant, intended to power an F-35 jump jet for the U.S. Marine Corps, the U.K. and possibly others. But the Defense Department says the mid-May incident won't hamper next year's first flight of the short-takeoff, vertical landing jet, also known as the STOVL model. "No one here in the program office is working to adjust the flight test program because of this event," said Rob Burnes, propulsion director for the Defense Department's F-35 program office. The damaged engine is repairable, and the program also has other prototypes it can continue to test. The program plans additional similar tests to work out any other kinks, Burnes said. (Dow Jones)

I think it is important to note that the conditions are not ever expected to be experienced in service. This is not merely a hard stall with 300% closure of the output vanes. This was a SUDDEN stall caused by a SUDDEN shut off. I do not believe that it is the fan churning against the closed output that cause the shaft to snap. I believe that it is the shock load from the sudden spike in loading that caused it.

Let's put it this way... if in service your vane box suddenly closes while you are hovering to a landing, it'll be irrelevant whether the engine breaks its drive shaft because you WILL have crashed either way!

Not as unusual as you might think. In a past life I was involved with blade off and other destructive testing methods.

Get a copy of A891230 an open report published by the Naval Ship Research and Development Center on airframe / ship interactions. I have an old dog eared copy so I can not post it. These were some hairy trials especially when hovering in the smoke stack effect and the ship changed its course, speed or the power setting of its engines.

Remember in a stable hover at 100 feet above sea level, the deck can be pitching and rolling about 20 feet; your task is to gently land on this platform of moving metal, at night, in low visibility / rain / fog with the Bingo lights on steady.

Elite 1K Joined: May 26, 2005 Posts: 1127

dwightlooi wrote:

Let's put it this way... if in service your vane box suddenly closes while you are hovering to a landing, it'll be irrelevant whether the engine breaks its drive shaft because you WILL have crashed either way!

VERY good point! Laughing

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