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Older Turbofans With Mid-Span Shrouds  
User currently offlineDash 80 From United States of America, joined Nov 2001, 309 posts, RR: 1
Posted (11 years 4 months 1 week 6 days 17 hours ago) and read 9382 times:

Maybe you engine buffs could help me. I read an interesting article on early high-bypass turbofans and their need for mid-span shrouds to dampen fan blade vibrations due to the heavier titanium used.

I was just wondering, in this picture of a P&W JT9D, is the mid-span shroud the part that appears to be the two rings in fan blades? If not, what exactly are they there for, and where is the mid-span shroud?

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Photo © Jack Hannen

Thanks in advance,


...where the rubber hits the runway...
6 replies: All unread, jump to last
User currently offline747NUT From Australia, joined Sep 2004, 78 posts, RR: 0
Reply 1, posted (11 years 4 months 1 week 6 days 16 hours ago) and read 9347 times:

Yes the shrouds are those two rings around the fan, and as you have mentioned before it is to do with vibration damping.
Also the shrouds are lubricated with a graphite grease where they touch together, this is usually done on a weekly basis.

If it's not broken, don't fix it !
User currently offlineB747FE From Hong Kong, joined Jun 2004, 230 posts, RR: 4
Reply 2, posted (11 years 4 months 1 week 6 days 16 hours ago) and read 9331 times:

Yes, the midspan shrouds are the rings in the fan disc.
It protrude from both sides of the blade airfoils interlocking with shrouds on adjacent blades to form a continuous ring which provides torsional rigidity and dampens vibration.
In the P&W JT9D-7R4, the fan is constructed with a single midspan shroud.
This shrouds are normally lubricated during "A"checks.


"Flying is more than a sport and more than a job; flying is pure passion and desire, which fill a lifetime"
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 66
Reply 3, posted (11 years 4 months 1 week 6 days 15 hours ago) and read 9308 times:

There is a small clearance between the surfaces on adjacent blades. When you turn the engine over by hand at low speed, when a blade passes over top dead center, it falls forward and hits the blade in front of it. At slightly higher speed this becomes the rattle or clatter you hear when an engine is windmilling on the ramp. Passengers are sometimes alarmed by the rattling sound. I've heard them say:
"That engine sounds like it's falling apart!"

At running speed, the individual blades "fly" more or less evenly spaced, the disc is balanced.

Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineBroke From United States of America, joined Apr 2002, 1325 posts, RR: 3
Reply 4, posted (11 years 4 months 1 week 6 days 13 hours ago) and read 9266 times:

The primary purpose of the midspan shrouds is to prevent the fan blade from twisting due to the aerodynamic loads on the airfoils.

User currently offlineDash 80 From United States of America, joined Nov 2001, 309 posts, RR: 1
Reply 5, posted (11 years 4 months 1 week 6 days 13 hours ago) and read 9253 times:

So that's what makes that clacking sound. Interesting.

Is that how it gets the nickname "clapper?"

Thanks for the info.


[Edited 2005-01-20 19:58:12]

...where the rubber hits the runway...
User currently offlineManzoori From UK - England, joined Sep 2002, 1516 posts, RR: 26
Reply 6, posted (11 years 4 months 1 week 6 days 10 hours ago) and read 9226 times:

Broke has got it just right.

Modern engines with ever larger wide chord fan blades (such as the RR Trent series) don't need the clappers due to improvements in internal strengthening.


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