Jags+30jugs wrote:
For a controversial tid bit, the balancing of the burtterflys is
strictly for idle and coming off idle operation. From there on the
differences are buried. IE: wide open= what is butterfly balance at
idle??
You wouldn’t believe how much trouble I went to trying to explain
this rather simple concept to a bunch of managers at Pratt & Whitney
Aircraft. Without success, I might add.
The items in question were the afterburner spray rings for the F-100
engine in the F-15 and F-16. The spray rings are concentric tubes
with holes that spray great heaping gobs of fuel into the tail end of
a jet engine when you want to go reeeeally fast and don’t care how
much fuel you burn in the process.
For this engine, P&WA had developed an entirely new idea in spray
rings. Rather than being round tubes, they were oval. They had
holes around the ID, and there were “pintles” that were attached to
the OD surface, ran through the center of the tube (the short way
across the oval), and jammed into these holes to plug them. When
fuel pressure was applied, it would distort the tube, making it less
oval and more round, which would pull the pintles out of the holes
and allow the fuel to pass through and spray. Really, a stroke of
genius.
The implementation was awful, unfortunately. The pintles were
supposed to remain closed up to 30 psi, and then the flow varied with
pressure up to about 250 psi. They calibrated the flow at 100 psi,
being logically about midspan in the flow range. This makes about as
much sense as synchronizing your butterflies at 1/3 throttle rather
than at idle.
The critical operating point for a fighter jet is “upper left hand
corner” – meaning, as you view the operating realm on a chart of
speed vs. altitude, very high and very slow. It’s where having an
afterburner that doesn’t flame out on you means life or death in a
dogfight. High and slow means the air is very thin, so even at full
afterburner these spray rings were operating at just barely cracked
open.
These rings were ugly things. Simple idea, but once you attached
mounting bosses and plumbing connections things get messy. Each of
these bosses stiffened the oval cross-section in that location, so it
wouldn’t de-ovalize as much adjacent to a boss when pressure was
applied. To compensate, they made the holes close to the bosses a
bit larger. And then they calibrated each pintle – using a method
that screwed up the ovalization of the tube, only allowing the local
area to change shape rather than the whole tube – to provide the
same flow at 100 psi. When operated at 30.5 psi, which is where they
operated at upper left hand corner, half of the pintles were still
stuck shut, and the flow from the others was all over the map. You
could even have one entire side of the engine getting all the fuel
while the other side got none at all. The fuelling spray pattern was
a mess, so the performance was poor and unpredictable. Some engines
would work better than others just by luck.
I tried and tried and tried to explain to these nimnuls that they
should be calibrating these spray rings at crack-open pressure. Who
cares if one pintle flows a little more than another at sea level
take off (where air is really thick, those spray rings are operating
near max pressure), you’ll never see the difference and there’s zero
chance of a flameout even if there was a difference. But getting a
uniform spray pattern at upper left hand corner would be worth its
weight in gold. And no changes to the hardware at all, just a
calibration procedure change.
If you wanna know how that worked out, just read any Dilbert cartoon
where Dilbert tries to explain something to his pointy-haired boss.
Later on, these dipwads came up with their own solution to the spray
ring issues: They redesigned the oval spray rings to operate at
higher pressures, popping open at 100 psi and operating all the way
to 600 psi. Goody, goody, the calibration procedure at 100 psi will
work great! No such luck, they changed the calibration pressure to
250 psi so it’d be mid-range again. And, to their amazement, they
never seemed to work significantly better at upper left hand corner.
Sometime later, they did away with the oval spray rings altogether,
since experience had proven that they didn’t perform well at upper
left hand corner. And with that, yet another good idea got flushed
down the crapper and will probably never be heard from again.
– Kirbert
// please trim quoted text to context only