A turbomachinery engine comprises a ducted fan, a high-pressure compressor, a low-pressure turbine, a gearbox, and a ducted engine correlation parameter (DECP). The DECP is within a range of 0.10-0.25 and is defined as 10*D/GR/NLPT/NHPC, where D is a fan blade tip diameter at a leading edge of the ducted fan measured in feet, GR is a gear ratio of the gearbox, NLPT is a stage count of the low-pressure turbine, and NHPC is a stage count of the high-pressure compressor.