Dash 80 From United States of America, joined Nov 2001, 309 posts, RR: 2 Posted (8 years 5 months 23 hours ago) and read 4159 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?
747NUT From Australia, joined Sep 2004, 78 posts, RR: 0 Reply 1, posted (8 years 5 months 22 hours ago) and read 4123 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.
B747FE From Hong Kong, joined Jun 2004, 230 posts, RR: 3 Reply 2, posted (8 years 5 months 22 hours ago) and read 4107 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.
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SlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 71 Reply 3, posted (8 years 5 months 21 hours ago) and read 4084 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.
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