| CPC F02K 3/06 (2013.01) [F02C 7/045 (2013.01); F02C 7/24 (2013.01); F04D 29/526 (2013.01); F04D 29/541 (2013.01); F04D 29/542 (2013.01); F04D 29/664 (2013.01); F04D 29/667 (2013.01); F01D 9/042 (2013.01); F01D 25/24 (2013.01); F05D 2220/36 (2013.01); F05D 2260/14 (2013.01); F05D 2260/96 (2013.01)] | 20 Claims |

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1. A gas turbine engine defining an axial direction and a radial direction, the gas turbine engine comprising:
a turbomachine;
a fan rotatable by the turbomachine, the fan comprising a fan blade, the fan blade defining an outer tip along the radial direction, a trailing edge at the outer tip, and a length, LFB, at the outer tip along the axial direction; and
an outer nacelle surrounding the fan and surrounding at least in part the turbomachine, the outer nacelle comprising a stage of pre-swirl inlet guide vanes located upstream of the fan, the stage of pre-swirl inlet guide vanes having a pre-swirl inlet guide vane defining a 15% span location and a leading edge at the 15% span location, the pre-swirl inlet guide vane further defining a length, LIGV, along the axial direction at the 15% span location, the outer nacelle further comprising an inner surface along the radial direction and an acoustic treatment coupled to or integrated with the inner surface, the acoustic treatment defining a length, LAT, along the axial direction; wherein the gas turbine engine defines a length, LS, from the leading edge of the pre-swirl inlet guide vane at the 15% span location to the trailing edge of the fan blade at the outer tip of the fan blade, and wherein the length, LAT, of the acoustic treatment is as follows: (LIGV2/LS)×UCF1<LAT<(LFB3/LIGV)×UCF2, wherein UCF1 is a first unit correction factor equal to 1−1 inch and UCF2 is a second unit correction factor equal to 1−2 inch.
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