Thyengine From Turkey, joined Dec 2005, 4 posts, RR: 0 Posted (9 years 2 months 2 weeks 6 days 12 hours ago) and read 6832 times:
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 From United States of America, joined Sep 2005, 5837 posts, RR: 15
Reply 1, posted (9 years 2 months 2 weeks 6 days 1 hour ago) and read 6707 times:
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.
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.
Manzoori From UK - England, joined Sep 2002, 1516 posts, RR: 33
Reply 5, posted (9 years 2 months 2 weeks 3 days 11 hours ago) and read 6462 times:
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.
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