Originally Posted by
waito
Time has passed.
1. Noise Level is expected on par with LEAP now
2. Only one rotating fan, the second is variable stator
3. the above enables a simpler and single gearbox, fan speed can be reduced and noise will meet near future limitations.
4. efficiency step up by 20% compared to best in class engines today for single aisle market
I personally see nose level in 3. still as a dead end if no more improvement can be made thereafter.
You still have the vortices from the fan hitting that stator (basically the same noise generation mechanism as a siren - and we all know how quiet those are
) with no way to attenuate the sound.
In short, I'll believe it when I hear it. Nobody thought the unducted fan on the 7J7 was going to be as nearly noisy as it was until they stuck one on aircraft and flew it around at a few hundred knots airspeed
.
There is also the issue of losing a fan blade without a containment system.
BTW, the engine for the 7J7 didn't have a gearbox - the fans were driven by free turbines.
Everybody knows the efficiencies that you can get from going to what's effectively a higher bypass with an unducted fan (without having to carry all that weight and drag of the duct). The trick is making it efficient and quiet in the 0.7-0.8 Mach speed range.