[AR] Re: P&W GTF
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sat, 4 Apr 2020 15:24:59 -0400 (EDT)
On Sat, 4 Apr 2020, Henry Vanderbilt wrote:
Is it just me, or is this the commercial airliner turbofan evolving back
toward the old reduction-gear driven turboprop? Seems like a
halfway-point, this does.
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.)
Only they'll never remove the fan shroud, because then it wouldn't be a
"jet" and people might notice that, even aside from Concorde, airliners
are cruising slower than the planes our parents flew in.
The engine industry has flirted with removing the shroud, which involves
using weird-looking blade shapes to try to retain efficiency and control
noise output in its absence. (And yes, customer acceptance is an issue,
and one symptom of this is that the designs tend to be called "unducted
fans" rather than "turboprops again". :-) But people will put up with a
lot for cheaper fares...) The idea comes up regularly, but so far,
nobody's made a go of it. Mind you, ten years ago the same was true of
geared fans.
Henry
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