Dash 80
Topic Author
Posts: 294
Joined: Wed Nov 07, 2001 4:26 am

Older Turbofans With Mid-Span Shrouds

Thu Jan 20, 2005 11:37 pm

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,

Steve
...where the rubber hits the runway...
 
747NUT
Posts: 77
Joined: Sun Sep 05, 2004 12:24 am

RE: Older Turbofans With Mid-Span Shrouds

Fri Jan 21, 2005 12:36 am

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 !
 
B747FE
Posts: 217
Joined: Mon Jun 21, 2004 11:09 pm

RE: Older Turbofans With Mid-Span Shrouds

Fri Jan 21, 2005 12:58 am

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.

Regards,
B747FE.
"Flying is more than a sport and more than a job; flying is pure passion and desire, which fill a lifetime"
 
SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

RE: Older Turbofans With Mid-Span Shrouds

Fri Jan 21, 2005 1:51 am

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.
 
broke
Posts: 1299
Joined: Wed Apr 24, 2002 8:04 pm

RE: Older Turbofans With Mid-Span Shrouds

Fri Jan 21, 2005 3:12 am

The primary purpose of the midspan shrouds is to prevent the fan blade from twisting due to the aerodynamic loads on the airfoils.
 
Dash 80
Topic Author
Posts: 294
Joined: Wed Nov 07, 2001 4:26 am

RE: Older Turbofans With Mid-Span Shrouds

Fri Jan 21, 2005 3:56 am

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

Is that how it gets the nickname "clapper?"

Thanks for the info.

Steve

[Edited 2005-01-20 19:58:12]
...where the rubber hits the runway...
 
manzoori
Posts: 1459
Joined: Sun Sep 22, 2002 7:08 am

RE: Older Turbofans With Mid-Span Shrouds

Fri Jan 21, 2005 6:50 am

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.

Cheers!

Rez
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