"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."
My understanding is that this is due to blade tip speeds - you don't want
the blade tip supersonic. Since the blade tip speed is directly
proportional to the ratio of tip to root radius, you use lots of little
blades, all far from the center of rotation. That way the outer tip in
inner portion of the blade are close to the same speed, allowing them to be
very close to mach. That way the aircraft can maintain positive power very
close to mach.
Thanks!
-David Summers
On Sun, Apr 3, 2016 at 12:27 PM, Paul Mueller <paul.mueller.iii@xxxxxxxxx>
wrote:
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.
On Sat, Apr 2, 2016 at 1:35 PM, Norman Yarvin <yarvin@xxxxxxxxxxxx> wrote:
On Sat, Apr 02, 2016 at 12:58:00PM -0400, Dave McMillan wrote:
Well, I can't speak to turbojets, specifically, but when I was
working on thrust reversers (production equipment, not the reversers
themselves) for high-bypass turbofans at AirBus, the (layman's)
explanation that I was given was that 70-80% of the thrust of a turbofan
is essentially the primary intake fan acting like a propellor, and only
20-30% was "hot gas" thrust from the combustor stage, and the outer cold
flow was what the thrust reverse interacted with. The "hot" part of the
engine primarily contributed to thrust by providing the energy to keep
the front fans turning.
That's all correct, but it's different from the practice of
attributing "thrust" to the intake of a ramjet, which was what was at
issue here.
--
Norman Yarvin http://yarchive.net/blog