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Accueil >> Forum >> A 747 with 2 engines Forum Aviation civile
A 747 with 2 engines
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Message de speedbird9468 - Envoyé le 12 Mar 23:53 |
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I often wonder if a 747 could be developed with 2 engines. My reason is that we see the 777, A350, and in general loads of twin engined ultra long routes being flown. I'm sure the engine makers could come up with something.
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Message de EI-DUB - Envoyé le 12 Mar 1:17 |
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I could be mistaken, but I am fairly sure I heard that a 747 and a 777-300 are basically the same fuselage, obviously without the hump, so you basically have a 747 there with two engines, so the R+D costs probably would prohibit such a development when there is a very close alternative already available.
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Message de DEMIS - Envoyé le 12 Mar 6:07 |
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I believe from engineering point of view , that such a modification is impossible , since this , probably would mean a redesign from almost scratch paper and thus for a plane that has been around almost 40 years , would mean an unjustifiable feasibility study.
Besides with new models coming out soon (i.e.787-XXX) , I don't think there is such a market share , that a modified 747 could fit in.
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Message de FLX - Envoyé le 12 Mar 12:36 |
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This is an interesting question and actually, I hv been trying to figure out exactly why so many major airlines are replacing their 744s with 773s/77Ws. Some key observations:
1. In terms of max cabin width, 747 is wider than 777 throughout most of the maindeck length(e.g. 10abreast vs 9abreast in Y-class) so fuselage design can't be the same. However, 747 nose+tail shapes are much more tapered/curved-in(In other words, more streamlined externally) than 777 and therefore less space.
2. In terms of available cabin floor space/max seats, a 744(Biggest 747 variant today) is still a bit bigger than a 773/77W. EI-DUB is right about the 747 hump(Especially for the older 741/742 which has a tiny upperdeck). Without the upperdeck mini-cabin, a 773/77W has nearly the same cabin space as a 744.
3. The way space is used today on 773/77W is a lot more efficient than in the past on any 747s:
a) Stair connecting both decks on any 747 takes up significant floor space that could hv be used for revenue seats.
b) Unlike any current 747, Boeing has designed the 77W's spacious crown area above the main deck as crew rest areas(A must for extra/ultra long range flights) connected by tiny ladders(Ok for crew) to the maindeck. This is a popular option for many 77W operators as it free-up valuable space on the maindeck.
4. In terms of belly cargo space, any 747 is significantly smaller than 773/77W and only a bit bigger than all 772 variants. It should be noted that air cargo is a much bigger rev$ generator in pax flights today than 40yrs ago when the 1st 747 was under development.
5. Max total thurst of a 744 is about 240,000lbs fm 4 turbofans. Max thurst of a 77W is about 230,000lbs fm only 2 GE90s. Yet in terms of fuel burn, 77W is about 60% of 744 for a typical long range mission.
6. 744 EIS in 89(Even the latest 744ER came out not long after that) while the GE90-115B(The most powerful turbofan in the world) on the 77W debuted in 2004. Such powerful engine was simply not available 15yrs ago.
7. The upcoming 748F/I(2009-10) is a significantly bigger/heavier jet than the current 744 or 77W that requires even higher total max thrust level(Over 280,000lbs). If use only 2 turbofans, it means each must produce at least 140,000lbs - nearly 22% increase over the GE90-115B and well over the design limit of the GE90 family architecture(Already the biggest in the world).
8. GE90 is by far the most costly engine family project GE has ever developed. They started with the GE90-85(Max thrust about 85,000lbs) back in the late 80s for the earlier/smaller 772s and took them over a decade to arrive @ the GE90-115B for the latest/heaviest 77W.
We really hv to appreciate how big/expensive a tech leap is required for a 22% increase in thrust. It can never be acheived by small tweaks of the existing GE90-115B(In many ways, the ultimate GE90). GE has to come up with a completely new family design which can take over a decade to develop and won't meet the development schedule of the next 748F/I. And we can forget about RR and PW. Both are totally tied-up with smaller thrust engine projects(e.g. RR's Trent XWB turbofan for the future A350-1000 is planned not to require as much thrust) and neither has announced projects that will yield thrust level equivalent or higher than GE90-115B.
I think by the time the engine makers come up with something, Boeing will hv abandoned any further 747 development long time ago(When the 748I EIS in 2010, the whole 747 program will be over 45yrs old!).
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Message de FLX - Envoyé le 12 Mar 16:00 |
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Are U guys aware that early 747s sometimes fly with 5 engines?
Early 747s are designed with an extra engine attachment point under the wing and between the fuselage and the 1st portside engine closest to the fuselage. This is for ferrying a spare engine to busy overseas airports located far away fm main op bases. In those days, oversized air freighters and well-stocked spare part centers/bases were uncommon around the world. I hv seen old photos of a NW 741 taking-off with 5 engines on the way fm the U.S. to HND(The old Int'l gateway to Tokyo) and I couldn't believe my eyes!
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Message de speedbird9468 - Envoyé le 13 Mar 0:50 |
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Message de FLX - Envoyé le 13 Mar 7:40 |
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Yes, imagine 1hr for boarding/disembarking, extra long queue @ security+immigration+custom, below deck baggage space needed for 3decks of pax, wait upto hrs @ the baggage claim, no room to sit or stand @ the departure gates even in the most modern terminal designs today, etc., all are truly romantic images we can associate with flying on a giant jet into ultra-busy superhubs(The only places where such jet make economic sense for an airline)......
When I was younger, I adored flying on jumbo jets - basically the bigger the better. Remember all those revolutionary cabin feature possibilities(e.g. Gym, lounge, internet cafe, shopping arcade, private mini cabin, etc.) Airbus shown us in the 388 cabin mock-up? In service, only a little of those survived in the ultra-premium F-class on SQ's 388 while the remaining pax continue to fly in a cabin environment, frankly, little different than in a 346,77W or 744. I suspect all other 388 operators(Except may be EK which even industry experts don't exactly know how they breakeven, let alone being profitable) will follow a similar set-up as SQ is already THE benchmark in premium inflight products.
As long as oil prices stay high and airlines continue to focus on short-term profit to keep their stock prices fm falling in the nex financial quarter, ultra-large jet is pretty meaningless to pax like me-the avg Joe.....
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Message de captain bill - Envoyé le 13 Mar 17:50 |
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re the first question about 747 with two engines. It would cost a fortune to do as the wing would need to be re-designed which would mean a different spar which would mean a redesign of the section of the fuselage that carries the spar. Then there is all the hydraulics and electronics and uncle Tom Cobbly and all so it would be more cost effective to design and build a new airliner.
Yes I remember speaking with Pan-Am in the late 1960s and they reckoned they could even fit a swimming pool.
Captains log star date 2345 the Star Ship Enterprise is grounded so we can change the chlorine in the swimming pool. beam me up Scottie.
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Message de FLX - Envoyé le 14 Mar 12:53 |
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Yes, it would be more cost effective to design and build a new airliner which Boeing did and calls it 77W. In many ways, 77W IS the modern 747 with 2 engines minus the upperdeck. Both do essentially the same type of job.
I'm old enough to remember that the basic 747 concept was originally developed for the USAF ultra-large transporter proposal. The upperdeck cockpit+a nose cargo door was Boeing's solution to meet the quick loading/unloading requirements. After Lockheed C5 won the contract, Boeing converted the concept into a civilian airliner with minimum design changes(e.g. Retaining upperdeck cockpit) to save R&D costs. It was PanAm, not Boeing, which suggested to use the tiny space behind the cockpit as a pax lounge and later for a few more seats(Boeing initially planned to use it for crew rest).
If the 747 program didn't hv a military background, I truly believe Boeing would hv designed it in a structurally simpler single deck layout, again, to save cost+weight and suffer only a tiny lost in cabin space. It would hv looked something like a 77W but definitely with 4 engines. In those days when 747 already struggled for the lack of power+reliability fm 4 engines, any aerospace engineer who dared to propose the crazy idea of 2 engines for such a large jet would probably get fired! Today, thurst produced by a single GE90-115B is equal to the total fm nearly 2.5 engines on early 747 or about 6.8 engines on early 707. The gains in reliability are even more staggering.
Isn't it amazing that after 35yrs, 2 engines are not only possible for a jet of similar capabilities, it's fast becoming critical to hv no more than 2 engines(I feel a bit sorry for the A346)?
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Message de speedbird9468 - Envoyé le 14 Mar 15:05 |
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Message de captain bill - Envoyé le 15 Mar 3:52 |
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I remember during my Rolls Royce days doing tests on the RR Conway that powered the old B.O.A.C. / British Airways Boeing 707-436 and the Rolls Royce RB-211 that powered the 747 and the conclusion was that all 4 RB-211s used the same amount of fuel per hour as did 2 Conway's so the old 707 was using twice as much fuel over the same route as the 747.
Engine technology today is fantastic and with the massive reduction in noise, fuel burn and low maintenance with only two engines the sky is the limit.
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Message de FLX - Envoyé le 17 Mar 4:54 |
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captain bill:
My info could be a bit outdated but I think 2 airlines are still running scheduled 707 pax flights. 1 in Argentina(May be Aerolineas or something like that) and the other in Iran(May be Iran Air or something like that).
On one hand, we've VS, CO and others embarking on cutting-edge bio-fuel demo projects to save our planet(Ok, these could be just PR stunts..) and their own bottomlines(Now, that's core Corp objectives..). On the other hand, we've these 707 flights probably 4-6Xs more effective in spilling emissions than a single 777/A333 flight and these 707 operators which obviously got $ to burn QUICKLY or receive free gasoline fm their gov'ts..... Such a diverse avaition world we hv today!
Sometimes I wonder what if the world didn't hv deregulation+national airline privatization or major flag carriers continued to receive subsidies fm gov'ts or oil prices remain low? Would we still see advances in propulsion tech as amazing and as quickly as we've got today? Would Boeing and Airbus be interested in developing 2-engine jumbo jets at all?
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