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Old March 14th 04, 08:25 PM
Peter Stickney
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Cub Driver wrote in message . ..
However, it is an honored tradition to hand out misleading ceiling
information. The B-36 had a service ceiling I believe in the high
30Ks, but was known to have flown at 50K and above.


Well, the roughly 40Kft service ceiling of the B-36s was true,
pretty much, (As in it's climbing at 100'/minute at that height)
but it's still climbing, and the ceiling changes as weight decreases
as fuel is burned off. (And a climb at less than 100'/minute is
still a climb, after all.) At B-36 cruise speeds and endurance, they
had a lot of time to drift up to some amazing heights. The Engine
Operation Charts in the -1 go up to 53,000', with instructions for
the Flight Engineers on how to set things up for maximum performance
above that height. Of course, with that small an amount of reserve
power, any maneuvering will cost some height. (But it'll cost any
intercepting fighters even more. They had even less of a maneuver
margin, and a much higher stall speed, and, in order to make a gun
or rocket pass, would have to pull more G than the B-36. Eiither the
-36 turns inside them, or the interceptor (If you're talking an F-86
era jet, like a Mig-15 or 17, or a Yak-25) stalls out and has to fall
a couple of miles before it can recover.

I'm amazed at the notion of a 50K-plus service ceiling being thrown
around here. This modern fighters must be more like rockets than jet
planes. In the 1950s-1960s the only planes that could get up that high
were semi-gliders like the B-36 and the U-2.


Well, could get there, and stay there for a while. The early
supersonic fighters had supersonic combat ceilings in the low
50Kft range, for the most part. But some though, like Walt
Bjorneby's Hotrodded F-104As, with the J79-19 engine used in
the F-4E, could, when supersonic, sustain more than 65,000'.
A B-58 on its Design Mission profile, with a 500 NM supersonic
run-in to the target, was expected to be at somewhere around
63,000'. (And it had power to spare - The B-58's flight limits
were set by airframe and compressor imlet tempertures, not thrust
available)

What is the practical altitude limit at which a jet can no longer
ingest enough air to keep it operational?


Ah... That's one of those "That Depends" questions. Basically, the
answer is "The height at which the engine can no lonvger burn enough
fuel to supply more energy than the turbine take out to drive the
compressor". But that's much too simple. The big factor is the ability
of the compressor to take in enough air and compress it sufficiently
without mucking up the flow though the engine. This is a fairly
complex relationship of Mass Flow, Pressure Ratio, and Temperature,
and it's described by the Compressor Map for the engine. This can
be tricky. Westinghouse, the U.S. pioneer of Axial Flow turbojets.
(They had the J30 flying before anybody on the Allied side had ever
seen a German jet engine) had pretty much sewn up the Navy's jet
engine business woth its very successful and very reliable first
generation engines, the J30 (FH-1 Phantom, & some prototypes), the
J32 (A 9" diameter midget turbojet for guided missiles), and the J34
(Which powered the F2H Banshee, the F3D Sky Night, and was stuck,
in pods, onto a zillion P2V/P-2 Neptunes and civilianized C-82s and
C-119s as boosters). At that point, they could do no wrong.
When it came to the followon J40, however, things didn't turn out
so well. The J40 was to be a big engine, producing about 10,000# of
thrust for the next generation of Navy Jets. (A3D, F4D, F3H, F10F,
and even the original F-102.) The problem was, the engine's
compressor wasn't too good at handling off-design airflows.
(Jet engines , especially back then, were designed to operate
most efficiently at a single set of conditions.) It was so bad
that the engine would compressor stall and flame out while climbing
through 30,000', in straight, unaccelerated flight. And they
coudn't straighten it out. The somewhat smaller J46, which wasn't
o be a J34 replacement, was pretty much the same. The Navy nearly
lost all of their fighter follow-ons to the original straight-wing
jets, the Manufacturers had to scamble to find alternate engines,
(Douglas lucked out, more or less. Ed Heinemann never trusted the
J40, so he designed sufficient stretch into his airplanes that they
could take Pratt & Whitney's J57. McDonnell had to settle for the
Allison J71, which wasn't good, but not as bad as the J40. Grumman
just abandoned the F10F), and Westinghouse ended up out of the Jet
Engine Businesss.

--
Pete Stickney
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