Fan Tip Speed and Shock Waves

[Brian Abraham]

Having a few idle moments and calculated that the tip speed for the fan on a GE90-115B at 100% N1 is Mach 1.18 at sea level and 1.35 at 35,000 feet. (Just been reading "Not Much of an Engineer" by Sir Stanley Hooker). To me that means shock waves a plenty and hence inefficiency (if I read across from propeller theory correctly). Put me out of my misery, whats going on?

[Mad (Flt) Scientist]

Are you allowing for compression in the inlet? A bit of heating there may be pushing up the speed of sound. I'd be pretty sure the tips aren't sonic, whatever the explanation ...

[Mark 1]

Turbo fans do indeed run with super-sonic tip speeds. The RB211s typically had Mach 1.4 at the design point.

Standard propeller theory is not very useful for two reasons; the blades are cascades rather than isolated airfoils (i.e. the distance between blades is only a few chords, so flow is essentially parallel between blades), and the fan is in a duct which inhibits radial flow around the tips. This leads to an aerodynamic design considerably different from a propeller.

The shock waves generated at the tips propagate non-linearly and generate the rasping 'buzz-saw' noise familiar on some types - most noticeable when the tips have only just gone sonic.