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Fan Blade Tip Curl  
User currently offlineTroubleshooter From Germany, joined Feb 2005, 423 posts, RR: 5
Posted (8 years 8 months 3 weeks 2 days 5 hours ago) and read 7297 times:

Recently I came across two events of fan blade tip curl on a CFM56-3 engine. In both cases the defect was found on the leading edge corner and the blade tip was bent forward (against the airflow direction). No other damage in the intake or exhaust area to find.

How can this happen? I would suggest a FOD but there were no signs of such an occurence.


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13 replies: All unread, jump to last
 
User currently offlineVC-10 From United Kingdom, joined Oct 1999, 3702 posts, RR: 34
Reply 1, posted (8 years 8 months 3 weeks 2 days 4 hours ago) and read 7278 times:
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Using reverse thrust below min speed, around 60kts, can cause this

User currently offlineMolykote From United States of America, joined Aug 2005, 1342 posts, RR: 15
Reply 2, posted (8 years 8 months 3 weeks 2 days 4 hours ago) and read 7276 times:
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In short (lacking visual aids)....

1 - The blade is angled forward
2 - The blade is rotating at a high speed

I've seen 5 sequential blades angled forward following a bird ingestion and aborted takeoff.

Two major velocity components exist under these circumstances.
- Aircraft forward motion
- The blade leading edge cutting across the aircraft's forward direction of motion

Under the takeoff circumstances I observed, the forward velocity component was a fraction of the fan blade leading edge speed at full takeoff power. When FOD hits, the blade is likely to deform in the same orientation at which is already inclined.



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User currently offlineKaddyuk From Wallis and Futuna, joined Nov 2001, 4126 posts, RR: 25
Reply 3, posted (8 years 8 months 3 weeks 2 days ago) and read 7222 times:

When this happens, the blade is ground down to the root of the curve. Sometimes you can see fan blades with part of the tip missing...


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User currently offlineRichardPrice From , joined Dec 1969, posts, RR:
Reply 4, posted (8 years 8 months 3 weeks 2 days ago) and read 7214 times:

Quoting Kaddyuk (Reply 3):
When this happens, the blade is ground down to the root of the curve. Sometimes you can see fan blades with part of the tip missing...

Doesnt that create balancing issues?


User currently offlineFr8mech From United States of America, joined Sep 2005, 5596 posts, RR: 15
Reply 5, posted (8 years 8 months 3 weeks 1 day 23 hours ago) and read 7204 times:

Quoting RichardPrice (Reply 4):
Doesnt that create balancing issues?

It may. There are limits to the amount of blade that we can remove. At some point the blade has to come off, or the rotor balanced. Changing the blade is easier, but performing a field balance on the rotor is cheaper (in some cases).



When seconds count...the police are minutes away.
User currently offlineAogdesk From United States of America, joined Jun 2004, 935 posts, RR: 3
Reply 6, posted (8 years 8 months 3 weeks 1 day 22 hours ago) and read 7203 times:

On the PW4000, I've seen opposite fan blades curled (2 per engine) at the tips, about a half inch radius. Was odd because the blades had already been changed within a few flight legs of my spotting it, and the opposite engine had the same condition. No FOD ingestion damage either.

User currently offlineMD11Engineer From Germany, joined Oct 2003, 14127 posts, RR: 62
Reply 7, posted (8 years 8 months 3 weeks 1 day 21 hours ago) and read 7152 times:

Quoting Fr8mech (Reply 5):
Quoting RichardPrice (Reply 4):
Doesnt that create balancing issues?

It may. There are limits to the amount of blade that we can remove. At some point the blade has to come off, or the rotor balanced. Changing the blade is easier, but performing a field balance on the rotor is cheaper (in some cases).

The limits are quite big though. On the PW 2040 you can almost remove an inch from the forward corner of the fan blade tip (done it after fod damage)

Jan


User currently offlineDougloid From , joined Dec 1969, posts, RR:
Reply 8, posted (8 years 8 months 3 weeks 1 day 20 hours ago) and read 7145 times:

Bird strike.

It was common to remove the corner of a fan blade on TFE 731 engines, as far as balance was concerned you could just remove the same amount of material on the opposite blade.


User currently offlineTroubleshooter From Germany, joined Feb 2005, 423 posts, RR: 5
Reply 9, posted (8 years 8 months 3 weeks 7 hours ago) and read 6863 times:

Thanks for your inputs mates.

Quoting VC-10 (Reply 1):
Using reverse thrust below min speed, around 60kts, can cause this

Can you explain this more detailed, please?



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User currently offlineStarlionblue From Greenland, joined Feb 2004, 17108 posts, RR: 66
Reply 10, posted (8 years 8 months 2 weeks 6 days 22 hours ago) and read 6821 times:

Quoting Troubleshooter (Reply 9):
Thanks for your inputs mates.

Quoting VC-10 (Reply 1):
Using reverse thrust below min speed, around 60kts, can cause this

Can you explain this more detailed, please?

The high thrust settings required for reverse coupled with low intake speed can result in the pressure behind the fan being higher than in front of it. If this is violent enough, the pressure can bend the blades forward.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineBuzz From United States of America, joined Nov 1999, 697 posts, RR: 21
Reply 11, posted (8 years 8 months 2 weeks 2 days ago) and read 6664 times:

Hi Troubleshooter, Buzz here. I'll have to politely disagree with some of the replies on how to get -fan- blades to tip curl. It would seem that more than a few blades would curl, or you're have a compressor stall problem... or blow some FOD forward and the fan eats it.
It could happen in the high pressure compressor due to compressor stall... but I haven't borescoped any engines for a compressor stall in the last few years.
I haven't heard of reverse thrust causing that curl. Maybe I work with the right kind of flying machines where that isn't a problem.
But I have seen that kind of tip curl from what I suspect was snow / ice ingestion, often happening to 737's coming out of DEN. Sometimes it tears up a blade (the normal FOD damage), sometimes it simply bends a couple fan blades. A blue heron bird strike on takeoff also bent some blades and went down the core... bird pate' (pardon my spelling) coming out the bleed vents. It looked like somebody tried to stuff a football between the blades. But the blades weren't torn. It was an engine change anyway.

Ever seen a piston engine propellor that had a prop tip strike something hard (like a runway) while at power? The tip digs into the asphalt and since asphalt is more dense than air the prop tip pulls forward. I'm thinking when a CFM engine eats a snowball the blade tips bend forward.
The prop tip bends aft when the engine is at idle, or stopped.

g'nite


User currently offlineDougloid From , joined Dec 1969, posts, RR:
Reply 12, posted (8 years 8 months 2 weeks 1 day 21 hours ago) and read 6635 times:

Quoting Buzz (Reply 11):
Hi Troubleshooter, Buzz here. I'll have to politely disagree with some of the replies on how to get -fan- blades to tip curl. It would seem that more than a few blades would curl, or you're have a compressor stall problem... or blow some FOD forward and the fan eats it.
It could happen in the high pressure compressor due to compressor stall... but I haven't borescoped any engines for a compressor stall in the last few years.
I haven't heard of reverse thrust causing that curl. Maybe I work with the right kind of flying machines where that isn't a problem.
But I have seen that kind of tip curl from what I suspect was snow / ice ingestion, often happening to 737's coming out of DEN. Sometimes it tears up a blade (the normal FOD damage), sometimes it simply bends a couple fan blades. A blue heron bird strike on takeoff also bent some blades and went down the core... bird pate' (pardon my spelling) coming out the bleed vents. It looked like somebody tried to stuff a football between the blades. But the blades weren't torn. It was an engine change anyway.

Ever seen a piston engine propellor that had a prop tip strike something hard (like a runway) while at power? The tip digs into the asphalt and since asphalt is more dense than air the prop tip pulls forward. I'm thinking when a CFM engine eats a snowball the blade tips bend forward.
The prop tip bends aft when the engine is at idle, or stopped.

g'nite

Hey, Buzz. Bird strike disassembly sure stinks. If it's a small bird that gets ingested what you see is an abrupt egt rise because the bird pate as you artfully describe it (thin nasty grainy yellow coating studded with feather fragments) destroys the supercritical airfoil of the internal parts and thus a lot of the efficiency. On Garrett turboprops it was about a fifty degree instantaneous rise.

With the Garretts, the compressor being a 2 stage centrifugal would in most cases do the Waring blender thing quite well. A disassembly and a session at the Diversey 909 hot sink were just the ticket-it washes off easily, except for the second stage transition duct which we'd change out. I remember one that took a seagull and actually bent one blade on the first stage compressor impeller-that one whistled when it was running. Those boys at Garrett took that compressor impeller ( a one piece titanium forging) and cut the blade back and welded new material on. Couldn't tell they'd ever repaired it.

Now...big birds are different, and thank heaven for high bypass fans that most often shuck the bird past the compressor intake. I knew a guy who worked for Flying Tigers in Los Angeles and they took a lotta seagulls there. When the shop would get a bird strike motor in for disassembly, all the smart mechanics would migrate to the rear of the engine or find something else to do.


User currently offlineMX757 From United States of America, joined Aug 2005, 628 posts, RR: 12
Reply 13, posted (8 years 8 months 2 weeks 1 day 20 hours ago) and read 6622 times:

Quoting Starlionblue (Reply 10):
The high thrust settings required for reverse coupled with low intake speed can result in the pressure behind the fan being higher than in front of it. If this is violent enough, the pressure can bend the blades forward.



Quoting Buzz (Reply 11):
I haven't heard of reverse thrust causing that curl.

I agree, CO's AMT 737 run-up and taxi guide lines allow us to use T/R's to help slow the A/C down whenever necessary. From 40 knots on down.

Quoting Buzz (Reply 11):
But I have seen that kind of tip curl from what I suspect was snow / ice ingestion, often happening to 737's coming out of DEN. Sometimes it tears up a blade (the normal FOD damage), sometimes it simply bends a couple fan blades.

I've seen the same thing on our a/c coming out of EWR.



Is it broke...? Yeah I'll fix it.
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