As one might expect, the study indicates that there are tradeoffs for each design. They even plug Ultra High Bypass engines into the formula, as well as GTF, variable vane nozzles and a number of other experimental variables to test the extremes of turbo fan concepts.
Perhaps this is old news and has been discussed already. In case it is new news, here is the complete study;
If that's a bit much, here are the conclusions;
UHB engines are feasible fora 737/A320-class vehicle. In general, the larger diameter
associated with UHB engines can be accommodated on this class of vehicle with relatively
simple measures such as increased landing gear length or changes to wing dihedral (changes
to wing dihedral were not explored in this study).
Optimum engine fan pressure ratio depends on the metric of interest, as well as the ground
rules, basic engine architectures, and assumptions used in the analysis. With the ground rules,
architectures, and assumptions used in this study: empty and ramp weight (often surrogate
indicators of cost) are minimized with high fan pressure ratio; block fuel consumption is
minimized with a fan pressure ratio of ^1.6; block NOx emissions are minimized with high
fan pressure ratio; and LTO NOx and certification noise are minimized with fan pressure ratio
as low as possible. These fan pressure ratio trends do not change with variation in engine
overall pressure ratio or design Mach number.
The primary benefit of the geared fan approach is to enable viable propulsion systems at lower
fan pressure ratios than possible with a direct drive fan. The geared fan approach is preferred
for fan pressure ratios (top-of-climb) below 1.5 (roughly BPR >13). At a fan pressure ratio of
1.5, a low work LPC, direct drive engine can provide outcomes similar to a geared engine.
Above a fan pressure ratio of 1.5, a low work, direct drive engine provides a better overall
aircraft system (for the metrics tracked in this study) than the geared engines do.
• If the design goal is to minimize ramp weight, block fuel, or block NOx, with the design
ground rules and technology assumptions of this study geared fan engine technology is not
necessary (since the minimums for these metrics occur at fan pressure ratios greater than 1.5.).
• If the design goal is to minimize airport area environmental impacts (i.e., aircraft noise and
LTO NOx), a geared system would be the preferred approach because it enables a practical
low fan pressure ratio engine design.
• Among the cases analyzed, the best balanced designs, performing well across all the metrics
of interest (ramp weight, fuel consumption, emissions, and noise), are fan pressure ratio 1.5
designs; either the high work LPC with a geared fan, or the low work LPC with either a geared
or direct drive fan.
• Relative to 1998 EIS technology, the advanced 2015 EIS configurations have the potential for
significant benefits: up to 29%reduction in fuel consumption and 25 EPNdB cumulative noise
reduction. These benefits do not occur with the same engine design, however. The minimum
fuel consumption designs have ^12 EPNdB (cumulative) higher noise than the minimum noise
designs and the minimum noise designs have up to 6% higher fuel consumption than the
minimum fuel consumption designs.
[Edited 2010-12-14 02:49:04]