Modern aero-engine fans (without inlet guide vanes) produce a tone at blade passing frequency by interacting with the inlet flow distortion. Because the distortion is altered by forward motion of the aircraft the tone is also different under static and inflight conditions. At low tip relative Mach numbers (less than about 0.85) the significant source of distortion is the intake wall boundary layer. At higher tip Mach numbers the principal distortion under static conditions is the atmospheric turbulence drawn into the intake, but in flight this reduces and other forms of distortion may then dominate.

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