CFM56 fan blades position

[MonarchOrBust]

The blades sit within a couple of hair widths away from the engine casing. Is it correct that they move inwards (thus creating more separation) upon start? I'm perplexed by this. Hopefully there's a simple answer! Cheers

[Fursty Ferret]

Nope, the tolerances are just that good. The acoustic lining can be rubbed slightly by the blades though in extreme conditions.

[Sig229]

There is some play in the blades, as in they slide into slots in the hub but are not secured by bolts or anything. But I'm not aware of a design feature where they would slide inward to increase clearance. I would think the opposite would be true, in that centrifugal force would cause them to move outward during rotation. You would also have to account for the force imparted by the airflow and thrust production, but that would mostly be fore/aft. I too have seen some rubbing on the cowl interior which is normal.

[Capt Quentin McHale]

Fursty Ferret is indeed correct. As an aside, most rubbing is at the bottom of the fan blade rub strip (roughly between the 5-7 o'clock position) due to gravity, and an indication of blade tip rubbing is a "blueing" in colour of the blade tip. All ok PROVIDED it's within Maintenance Manual tolerances.

Rgds McHale.

[tdracer]

Actually, most fan rubs are related to the gyroscopic forces at rotation - in fact that's a major structural design consideration for the fan. Obviously, gyroscopic forces on the fan are not limited to rotation - they occur during normal maneuvers and turbulence, but they are usually largest at rotation.
Obviously, things like abusing the T/R cutback speed, engine surges, and birdstrikes can also cause fan rubs - sometimes quite large. The fan rub strips are generally painted with a thin coat of a contrasting color so that new rubs are readily apparent. As McHale notes, so long as the magnitudes are within MM tolerances, it's no big deal.

[733driver]

[QUOTE=tdracer;11256772]
Actually, most fan rubs are related to the gyroscopic forces at rotation - in fact that's a major structural design consideration for the fan. Obviously, gyroscopic forces on the fan are not limited to rotation - they occur during normal maneuvers and turbulence, but they are usually largest at rotation.
/QUOTE]

Very interesting. Makes a lot of sense. Thanks for that.

[FlightDetent]

Term "abradable seal" springs to mind.