Newsgroups: sci.aeronautics.airliners Path: news From: kls@ohare.Chicago.COM (Karl Swartz) Subject: Re: 757 highest thrust to weight ratio ? X-Submission-Date: Fri, 11 Dec 1992 11:33:35 GMT References: Message-ID: Approved: kls@ohare.Chicago.COM Organization: Chicago Software Works X-Submission-Message-ID: <1992Dec11.113335.6455@ohare.Chicago.COM> Sender: kls@ohare.Chicago.COM 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 ... 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 Send sci.aeronautics.airliners submissions to airliners@chicago.com