Subject: 757 highest thrust to weight ratio ?

From: Bob Coggeshall <[email-masked]>
Date: Sat, 28 Nov 1992 21:48:00 +0800

I've heard that the 757 has a very high thrust to weight ratio. Just
how high is it ? Is it the highest of any commercial jetliner ?

I've also heard that there is a boeing-internal video of it 
during tests doing an [almost?] straight vertical climb. 

What are the facts here ?

Thanks.

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From: [email-masked] (Terrell D. Drinkard)
Date: 01 Dec 92 23:15:24 PST

>how high is it ? Is it the highest of any commercial jetliner ?

My back-of-the-envelope calculations would suggest about a .38 thrust to
weight ratio.  That is assuming the RR RB211-535E4B gives around 42,000 lb
thrust (SLST) at a MTOW of 220,000 lb.  I'm not aware of anyone who
actually bought that configuration, though.  Oh, and on their acceptance
flight tests, (I got to fly one on a 737-400 once) these babies are *empty*
and climb like homesick angels.  I have seen VSIs pegged (6,000fpm+).

>during tests doing an [almost?] straight vertical climb. 

I've never seen it, but then I don't do any flight test stuff.

>What are the facts here ?

Excellent question! :-)

-- 
Terry
[email-masked]
"Anyone who thinks they can hold the company responsible for what I say has
more lawyers than sense."

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From: [email-masked] (Karl Swartz)
Date: 02 Dec 92 03:49:51 PST

>actually bought that configuration, though.

The various RB.211-535 variants used on the 757 range from 37,400 lb
thrust up to 43,100; the PW2037 is rated at 38,250.  Most airlines
choose a 230,000 or 240,000 lb MGTOW, however.  The only one I could
find on a quick scan with only 220,000 was Northwest, which with the
PW2037 works out to 0.3477.

Curiosity drove me to dig out the March 16, 1992 AW&ST with the big
specifications section and look up the power/weight ratios for some
common transports.  Where a choice was available I tried to pick the
heaviest version with the highest thrust verion of a common engine
type, and tossed in a couple of other interesting cases.  Over 24
samples the power/weight ratios ranged from 0.2428 to 0.3477.  The
higher figure is the 757 in Northwest's configuration; a better
comparison point is the 240,000 MGTOW 757 which comes in at 0.3187,
trailing only the A320/A321, MD-90, and 737-500 (and of course the
lighter 757).

Not surprisingly, the top 11 were all twins.  At the other end of the
scale, the heaviest 737-200 Advanced, at 0.2488, was only ahead of the
727-200 Advanced.  Judging by these numbers it would seem the 737-200
is a rather underpowered aircraft.

Subjectively, pilots I've talked to love the 757 for its abundant
power and from the cabin once I was impressed with how quickly a 757
got us out of a wind-shear situation at O'Hare.

Another factor that may give an illusion of power is that most 757s
are used on medium-range routes, yet they've got a 4550 mile range.
Typical flights probably carry a light fuel load and thus may be well
short of MGTOW.

As for highest of any commercial airliner, even the lightest 757 is
still well short of the Concorde's 0.3725 power/weight ratio.  :-)

-- 
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From: [email-masked] (Fred Christiansen)
Date: 09 Dec 92 18:45:30 PST

In article <[email-masked]> [email-masked] (Karl Swartz) writes:
>Not surprisingly, the top 11 were all twins.

Why "not surprisingly"?  As a layman with no background in this stuff,
I would have tho't that manufacturers would keep the thrust-to-weight
in some ballpark range (for economic reasons).  And that the advent
of larger twins is because more powerful engines have become available,
eliminating the need for a 3rd (or in the case of, say, MD-10 class
aircraft, the need for a 4th).
-- 
Fred Christiansen, Motorola, 2900 S Diablo Way, Tempe, AZ 85282  "Canajun, eh?"
[email-masked]  ||  uunet!phx.mcd.mot.com!fredch  ||  +1 602-438-3464
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From: [email-masked] (Karl Swartz)
Date: 10 Dec 92 00:52:14 PST

>aircraft, the need for a 4th).

The catch here is that once committed to takeoff, the aircraft must be
able to fly long enough to return for a safe landing with one engine
out.  With four engines, this in effect means you've got to carry 33%
more power than the bare minimum to keep you in the air; with two you
need a full 100%.  Note that this does not *necessarily* imply a much
higher thrust/weight ratio -- depending on how the certification regs
are written, one could figure maybe twenty minutes to dump fuel and
return and certify the engine for a substantial overload thrust quite
a bit above the normal "maximum" thrust.  But since essentially the
same engines would also be used on three- and four-engine aircraft
this probably wouldn't make much difference.

Looking at this from the view of normal operations, the engines on a
twin will normally not be worked as hard which should lead to higher
reliability and lower maintenance costs.  And, as mentioned in regard
to the 757, better climb performance when needed.

The Airbus A330 and A340 offer an interesting comparison, since they
are identical save the engines (two on the A330, four on the A340),
their wing attachments, and those parts of the controls directly
affected by the engine differences.  (Fuselage lengths differ too
though this is true even amongst A340 models.)  The A340, with four
engines, offers freedom from ETOPS restrictions for long, overwater
routes.  The A330, with two larger engines, offers the maintenance
economies of fewer engines for shorter or overland routes.

It's interesting, then, that Lufthansa chose the A340 even though
the A330 would have served most of the routes they had in mind.  As
I recall, they felt that four less-stressed engines would be cheaper
to operate and maintain than two larger engines.  Given the overall
popularity of twins this doesn't seem to be a widespread view; it may
simply reflect greater confidence in the more mature engines of the
A340 compared to the A330.

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From: [email-masked] (Gregory Wright)
Date: 10 Dec 92 02:09:23 PST

>aircraft, the need for a 4th).

  The reason for the this may, as you say, may not be so obvious so 
alow me to put in my two cents. The FAA, along with the CAA and others,
require that we demonstrate a minimum climb out gradient with one engine
inoperative. In the case of a two engine aircraft this requires that
the airplane be able to climb out with a 50% reduction in gross thrust.
For a four engine platform this one engine out condition represents only
a 25% reduction in thrust. You can see then, that for a two engine plane
it is required to "over" engine the plane...

Hope this helps to clear things up,
greg
HIgh lift development

-- 
 ________Greg Wright____________   "Sure my YMP is fast, but if it doesn't
|  [email-masked]    |   run OS2 I don't need it....."
|     [email-masked]       |                             
|____uunet!bcstec!gregory_______|               NOT A BOEING SPOKESPERSON.

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From: [email-masked] (Gregory R. Travis)
Date: 10 Dec 92 16:07:07 PST

Karl makes some excellent points regarding the reasons twins tend to be
overpowered compared to 3 and 4 engined jets.

I would just like to add that FAA certification requires not only that an
airplane be able to finish takeoff, return, and land with AN engine out,
but that it also meet certain climb performance criteria during a
single-engine climb.

Karl is absolutely right that, on a 747 that loses one, the remaining three
need to make up, individually, much less of the lost thrust.  Whereas on
a 757 the remaining engine must make up ALL of the lost thrust.

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.

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.

greg
--
Gregory Reed Travis				D  P  S  I
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From: [email-masked] (Derek H Cedillo)
Date: 11 Dec 92 00:38:19 PST

In article <[email-masked]> [email-masked] (Gregory R. Travis) writes:
[good outline of engine loading]

>normal operation, the engines should suffer from higher demands all around.

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

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?
Note that they have 2,3,4 engines respectively
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?
Thanks,

Derek

-------------------------------
"He lived a life of going-to-do, 
   and died with nothing done"	       
		-J. Albery	     

In other words---JUST DO IT!

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From: [email-masked] (Karl Swartz)
Date: 11 Dec 92 03:35:21 PST

In response to Derek H Cedillo's reply to Gregory R. Travis' reply to
my post ...

Greg sez ...

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 ...

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,

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.).

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.

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.

  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!)