From what I have learned (and I am merely an operator of turbomachinery and in
no way a designer) this has to do with tip loss.
Comparing large vs small turbofans:
The gap between the tip of the fan and the inside of the cowl is a region of
pressure loss. Much of the required fan blade to cowl clearance is based on
machining and balancing factors that do not scale up with fan size, and tip gap
is more similar than you might think between turbofans of vastly different
size.
The result is that thrust scales up with the square of diameter while tip loss
scales only slightly more than linear. The TSFC difference between a GE90 and
FJ44 is enormous!
Regarding ducted fans in general:
At the low mach end, it's easier to deal with the tip / blade loss issue by
having an efficient long span propeller rather than have a short span ducted
fan with a cowl acting as wingtip device.
At high mach that long fan airfoil will break Mach someplace along its span,
causing inefficiency. Thus it's better to use the short span fan blade with the
cowl as a duct. Even so, most modern fans are breaking Mach at takeoff thrust,
hence the buzz saw noise.
Which leads to geared fans, but now I am really OT and will stop.
-------- Original message --------
From: Paul Mueller <paul.mueller.iii@xxxxxxxxx>
Date: 04/03/2016 11:27 AM (GMT-07:00)
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: to the stars, soon
Sorry to dwell on a somewhat OT subject, but thank you for all the
explanations! Interesting!
So in a sense, a turbofan (moving away from turbojets now) is basically a
ducted fan, driven by a smaller turbine engine. And in the case of big
high-bypass turbofans, they are essentially big ducted fans. Makes me wonder
why smaller "traditional" ducted fans (like on airships or the E-fan aircraft)
have much fewer blades, not a big blade disk like a turbofan.
Yes, this is OT and I apologize.
