Re: 757 highest thrust to weight ratio ?

From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works
Date:         11 Dec 92 03:35:21 PST
References:   1 2 3
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In response to Derek H Cedillo's reply to Gregory R. Travis' reply to
my post ...

Greg sez ...

  However, I am confused by Karl's statement that Lufthansa chose the A340
  over the A330 because of concerns that the twin-engined A330 would place more
  stress on its two engines whereas the A340 would enjoy higher engine
  reliability as its four engines loafed along.

Just to make it clear, this was a rather fuzzy memory.  I believe I
read the details, which I may have distorted badly, in AW&ST, but
cannot place it better than that.  At the time, it kinda made sense
to me, but I'm not sure it does now.

If anyone can fill in the missing details I would be most appreciative.

Greg continues ...

  Since a twin engine jet is nominally overpowered compared to a four engine
  jet, it should be able to operate, on aggregate, at a lower thrust setting
  during takeoff or be operated at high thrust for a shorter overall climb.

  Since a four-engine jet has all four operating closer to the margin, in
  normal operation, the engines should suffer from higher demands all around.

and Derek replies ...

  I think the thing here, is that you are imagining two different engines
  entirely.  This isnt exactly the case.

Eh?  Don't you have that backwards?  Since an A330 is little more than
an A340 with two big engines instead of four little ones, it's very
much the case that the engines are entirely different.  And this may
well be the case -- at least by some metrics, a 67,500 lb. thrust
engine endures more stress than a 31,200 lb. thrust engine.  Of course
it depends on the core from which one started, and a lot of other
factors.

Or perhaps Lufthansa feels the CFM56 is inherently more reliably than
any of the engine options for the A330 (CF6-80 first, then PW4000, and
RR Trent and GE 90 options later).  Not that there's anything *bad*
about any of the larger engines, but the CFM56 has the best record of
any of the larger jet engines if I'm not mistaken.

Derek continues ...

  I dont have thrust data handy,
  so I cant compare the A330 engine performance with the A340, but as a quick
  example, I'd like to point out that the A340, A320 and A321 all have
  the same GE engine spec (CFM56-5) while the A340 is a four engine plane
  and the A320 and 21 are two engine planes.

It's actually a CFM International (GE and Snecma are equal partners, I
believe) engine, and the -5 simply means its for an Airbus as far as I
can tell.  In its various incarnations a CFM56 ranges from 20,000 lbs.
thrust up to 34,000 lbs.  The A320 uses a -5A2 (25,000 lbs.) or -5A3
(26,500 lbs.) version, while the initial A340 version uses a -5C2
(31,200 lbs.).

  Althought the 340 is definately bigger than the 320/1, I dont think the
  engine loading would be twice as much.  (can someone come up with stats 
  to prove or disprove please)

I'm not sure this will answer the question or not, but I dug out the
power/weight specs I gathered earlier, added numbers for the A330/A340,
and added a column for power/weight ratio with one engine out.  Here's
what I came up with:

model		pass	range	MGTOW	engines		thrust	p/wt	1out
-----		----	-----	-----	-------		------	----	----
A320-200	140-179	    ?	162	2 CFM56-5A3	26500	0.3272	0.1636
A321-100	180-220	    ?	181.2	2 CFM56-5B2	31000	0.3422	0.1711
A330		280-440	    ?	467.5	2 CF6-80E1A2	67500	0.2888	0.1444
A340-200	220-440	    ?	558.8	4 CFM56-5C2	31200	0.2233	0.1675
A340-300	280-440	    ?	558.9	4 CFM56-5C2	31200	0.2233	0.1675

The one-engine-out numbers are remarkably similar for the A320 and A340.

  Another look is the 767-200ER/300/300ER, MD-11 and 747-200/300/400 which 
  can choose the GE CF6-80C2.
  All are relatively large planes, and they gradually increase
  in size, but is it huge enough to say the 747 is Twice as heavy/aero dyn
  loaded, etc, to require twice the thrust as the 767?

Well, sucking a few more figures out of my files:

model		pass	range	MGTOW	engines		thrust	p/wt	1out
-----		----	-----	-----	-------		------	----	----
747-400		412-509	 8380	870	4 PW4056	56000	0.2575	0.1931
767-300(ER)	204-290	 6650	400	2 PW4060	60000	0.3000	0.1500

I picked these two particular airframe/engine combinations as the best
comparison points but have more data if anybody wants it.  In any case,
yes, the 747 *is* twice as heavy, more than that in fact, but because
it uses a slightly lesser rated engine has a bit less than twice the
thrust.

  Again, I would appreciate any airframe data to support the 
  weight/thrust/loading claim, or shoot me out of the sky as seen fit.

  Can someone help?

There's some data, but I'm not understanding Lufthansa's position much
better.  Maybe it's just the late hour, or the flu I've been fighting
off.

How 'bout one of you folks from Boeing?  (Better yet, Airbus, if any
of you are out there!)

-- 
Karl Swartz	|INet	kls@ditka.chicago.com		
1-415/854-3409	|UUCP	uunet!decwrl!ditka!kls
		|Snail	2144 Sand Hill Rd., Menlo Park CA 94025, USA
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