[AR] Re: P&W GTF
- From: Kevin Ho <rebelwithoutajob@xxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Sat, 4 Apr 2020 15:55:56 -0500
On Apr 4, 2020, at 2:24 PM, Henry Spencer <hspencer@xxxxxxxxxxxxx> wrote:
On Sat, 4 Apr 2020, Henry Vanderbilt wrote:
There is some similarity, to be sure. Note, though, that the fan shroud is
not just ornamental :-), it modifies and controls the airflow so that the
"propeller" can operate efficiently at higher speeds. The Achilles Heel of
conventional propellers is that when the blade tips go supersonic, propeller
efficiency goes down the drain and noise rises tremendously. (You can flirt
with Mach 1 a little bit, and the props on some turboprop aircraft -- like
the P-3 Orion -- do, but there are limits.)
That’s actually the great thing about geared fans - tip speeds are below M1. In
your usual two spool turbofan, the N1 turbine drives the fan. As you increase
the bypass ratio, you increase the diameter of the fan relative to that of the
N1 turbine.
A rotational speed that allows a slow enough tip speed for the fan will be far
too slow for the turbine, and a rotational speed that is fast enough to allow
the turbine to work efficiently is far too fast for the fan.
This imposes an effective upper limit on the ratio between fan diameter and
turbine diameter, and thus bypass ratio. Putting a reduction gearbox on the fan
allows both the fan and turbine to both run at an optimal speed, which in turn
allows an enormous fan to turbine diameter ratio, and consequent high bypass.
It’s a solved problem for bizjet-sized engines, but P&W was the first to scale
it up to narrowbody size.
The end result is a huge fan the size of that on a CF-6 off a widebody 767,
with a tiny core section that looks like it came off a regional jet.
Now that all the early teething issues have been resolved, it’s an amazing
engine.
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