thyengine
Posts: 4
Joined: Wed Dec 14, 2005 12:14 am

Fan Blade Tip Clearance

Thu Dec 15, 2005 11:19 pm

You know, in ESM's (Engine Shop Manual) are Fan blade tip clearance limits(min,max). This tip clearance gives different performance resullts (EGT,EPR..etc.) between test cell and on-wing.
Could anybody make a comment what the result would be both in test cell and on-wing If we either adjust the clearance to min or max?
 
fr8mech
Posts: 6585
Joined: Mon Sep 26, 2005 9:00 am

RE: Fan Blade Tip Clearance

Fri Dec 16, 2005 10:01 am

Greater tip clearance results in reduced engine efficiency due to the air escaping between the tip and shroud.

A clearance closer to the minimum will increase efficiency but depending on how close the tolerance is, may run the chance of fan lade rub. This may be a simple rub on the abraidable lining or cause an unacceptable vibration condition.
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liedetectors
Posts: 323
Joined: Thu Jul 21, 2005 10:44 am

RE: Fan Blade Tip Clearance

Fri Dec 16, 2005 1:06 pm

Quoting Fr8Mech (Reply 1):
, may run the chance of fan lade rub.

This correct. One way they get around this is for the tips to actually slice into a foam like material to better seal around the tips. I was skeptical at first but when i saw this in person i was made a believer. Another techique is to determine how much the blade will expand due to thermal expansion and design the casing to expand as well to avoid the casing deterioration.

I do believe the engine makers use a combination of both, thus when the blade expand they slice into the foam to make a good interference to prevent air leakage.
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2H4
Posts: 7960
Joined: Tue Oct 19, 2004 11:11 pm

RE: Fan Blade Tip Clearance

Fri Dec 16, 2005 1:14 pm




Quoting Thyengine (Thread starter):
This tip clearance gives different performance resullts (EGT,EPR..etc.) between test cell and on-wing.



What variables account for the difference between performance results on-wing and off-wing?




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thyengine
Posts: 4
Joined: Wed Dec 14, 2005 12:14 am

RE: Fan Blade Tip Clearance

Fri Dec 16, 2005 10:11 pm

Quoting 2H4 (Reply 3):
What variables account for the difference between performance results on-wing and off-wing?

What variables? We can just take on EGTM for the whole performance variables...
 
manzoori
Posts: 1459
Joined: Sun Sep 22, 2002 7:08 am

RE: Fan Blade Tip Clearance

Mon Dec 19, 2005 12:33 am

Quoting 2H4 (Reply 3):
What variables account for the difference between performance results on-wing and off-wing?

Carcass bending due to G loading primarily.

On a testbed the engine is very static whereas on wing at the end of the take-off run when the pilot pulls back on the stick, the rotation of the aircraft an hence the pylon introduces all sorts of G and gyroscopic loads into the engine carcass which will result in some degree of deformation. This can lead to increased tip rubs.

hth

Rez
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2H4
Posts: 7960
Joined: Tue Oct 19, 2004 11:11 pm

RE: Fan Blade Tip Clearance

Mon Dec 19, 2005 2:13 am




Quoting Manzoori (Reply 5):
Carcass bending due to G loading primarily.



Ahhhhh....thanks, Manzoori! It seems so obvious now...




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