davescj
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1 Engine, 2 Planes?

Mon Jun 25, 2007 7:31 pm

Lately we've been hearing about thee engines for the A350, and the fact that GE has said they will not invest in making a new engine. This raises my question: why not use an existing engine? For example, if the engine for the A340 would create suffient power using 2 engines on the A350, couldn't use simply use the same engine for both AC? Thanks!
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Starlionblue
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RE: 1 Engine, 2 Planes?

Mon Jun 25, 2007 9:10 pm

The engines on even the 340-500/600 are ten year old technology. Putting these on a plane due to enter service in 2011 (?), and thus throwing away 15 years of advances, doesn't make a lot of sense. This is especially true if you consider how cutting edge the competition is.
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HAWK21M
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RE: 1 Engine, 2 Planes?

Mon Jun 25, 2007 9:30 pm

Quoting Starlionblue (Reply 1):
how cutting edge the competition is.

Why Isn't Airbus considering a Bleedless Engine.
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Jetlagged
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RE: 1 Engine, 2 Planes?

Mon Jun 25, 2007 9:50 pm

Quoting HAWK21M (Reply 2):
Why Isn't Airbus considering a Bleedless Engine.

Because that has no advantage without a total re-design of aircraft systems.
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Pihero
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RE: 1 Engine, 2 Planes?

Mon Jun 25, 2007 10:13 pm

It goes a little bit further than that, Jetlagged.
Airbus still thinks that the engine power pick-up budget remains just about the same : You'd need bigger engine drven generators, you'd need an electrical driven air cycle machine, water separators, heaters....
What yoiu'd save in piping would have to be balanced with electrical complexity...
On the other hand, there is always some "bleeding" of the engine compressor, if only for surge prevention, so why not use it ?
The last argument is about redundancy.
The jury on the "all electrical" airplane is still out and it could well become the future, and Boeing is scouting that trail...
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darkblue
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RE: 1 Engine, 2 Planes?

Mon Jun 25, 2007 11:20 pm

Quoting Davescj (Thread starter):
Lately we've been hearing about thee engines for the A350, and the fact that GE has said they will not invest in making a new engine. This raises my question: why not use an existing engine?

Good question! GE offered Airbus a GEnx derivative for the A350XWB and Airbus said it wasn't good enough. It wasn't that long ago that airframers recognized that the engine is the single most complex and expensive component of the aircraft. Financially it made sense to design an aircraft around an existing engine. This is why GE's CF6 engine has become the most widely used engine found on:

A300
A330
747
767
DC-10
MD-11
Lockheed C-5
Japan C-X

More recently there has been an even stronger demand for more efficient aircraft. Boeing and Airbus have put the lion's share of this responsibility on the engine. So I think we're seeing a new trend now. Everytime a new aircraft is proposed, the airframers want a new engine, not just an improved version of what is already exists.
 
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Jetlagged
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RE: 1 Engine, 2 Planes?

Tue Jun 26, 2007 2:20 am

Quoting Pihero (Reply 4):
It goes a little bit further than that, Jetlagged.

Whether or not Airbus think all electric is a good idea, simply using a bleedless engine achieves nothing unless the systems are completely redesigned to take advantage. That was the only point I sought to make.

Overall power consumption will be about the same, but pneumatic power is harder to tailor exactly, so excess bleed has to be provided in some circumstances.

Quoting Pihero (Reply 4):
You'd need bigger engine drven generators, you'd need an electrical driven air cycle machine, water separators, heaters....

Boeing are installing two generators per engine, i.s.o the usual one (frequency wild so no constant speed drive). The 787 has two packs, supplied by electrically driven air pumps. Obviously the air will need to be heated or chilled, but not necessarily by an ACM (I have no info on this). The air from the pumps will be much lower pressure and temperature than engine bleed air.

Quoting Pihero (Reply 4):
What yoiu'd save in piping would have to be balanced with electrical complexity...

Not sure about that, those ducts are heavy and the new electronically controlled electrical distribution systems should be lighter.

Quoting Pihero (Reply 4):
On the other hand, there is always some "bleeding" of the engine compressor, if only for surge prevention, so why not use it ?

Agreed, but surge bleed is usually only needed at low power. The "bleedless" engine must be designed so that the excess air used for service bleed is not necessary for surge stability.

Quoting Pihero (Reply 4):
The last argument is about redundancy.

Six generators, including the two on the APU, that's a lot of redundancy.

Quoting Pihero (Reply 4):
The jury on the "all electrical" airplane is still out and it could well become the future, and Boeing is scouting that trail...

Agree 100%. The theoretical gains have yet to be proven in practice.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
N231YE
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RE: 1 Engine, 2 Planes?

Tue Jun 26, 2007 7:36 am

I recall reading that the 747-400 and the 767-300 were designed engine commonality in mind: one could literally take the engine off a 747-400 and mount it on a 767-300, or vice versa. Could someone enlighten me on this?
 
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Jetlagged
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RE: 1 Engine, 2 Planes?

Tue Jun 26, 2007 11:13 pm

More details here: RE: RR Engine On 767 (by Tristarsteve Apr 10 2006 in Tech Ops)#36
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
prebennorholm
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RE: 1 Engine, 2 Planes?

Wed Jun 27, 2007 6:46 am

Quoting Davescj (Thread starter):
...why not use an existing engine? For example, if the engine for the A340 would create suffient power using 2 engines on the A350, couldn't use simply use the same engine for both AC? Thanks!

No engine on any A340 would be anywhere nearly powerful enough for the A350.

But with the RR engine the A350 engine will have a lot in common with the RR Trent 900 used on the A380. But at least on the larger A350 versions even more power is needed.

Then comes ETOPS issues. On a twin a lot of things must be differently configured to maintain redundancy on various functions. Just imagine how valuable a single electric generator would be in case of an engine shut down. No, it simply doesn't work. On some planes the APU is part of the complete redundancy, which means that the APU must be started every, say, 30 minutes just show that it is functional, and you possibly divert if it isn't.

Redundancy on a twin is a very tricky thing, while on a quad you basically always have three functional systems in case of an engine shut down.

While the Trent on the A350 will be heavily based on the Trent 900, then it is very likely that the Trent 900 on the A380 will be upgraded long time before the A350 service entry. And those performance upgrades, which are many years in the design, testing and certification procedure, are most likely already anticipated in the A350 performance predictions.

Large turbofan engine development has been totally evolutionary during the last 40 years. Minor and not so minor improvements have been made from time to time, and some scaling have taken place as well. They can all be traced back to three grandfathers, the P&W JT9D, the RR RB-211 and the military GE TF-39. Only the TF-39 was substantially changed into the civil CF6 because the former was considered the inferior engine, especially concerning noise. Those engines are the roots of present day P&W 4xxx, RR Trent 5-, 6-, 7-, 8- and 900, as well as GE90 and GEnx versions.

One of the major changes done over the last almost 40 years was between the Trent 800 and the -900. The 900 was the first one to take advantage of contrarotating N2 and N3 shafts.
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