Happy-flier
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How Far Could Such A DC-8 Fly...

Sat Mar 03, 2012 5:43 pm

As I was sitting here, thinking about the new PW geared turbofan and its variants, I wondered:

"What if they took a DC-8-73, swapped the CFMs for four PW1521Gs, added some winglets to it, and did some flight tests... How far would it fly on a full fuel load?"

The thrust class of the PW1521G is stated as 21,000–23,300 lbf. This would correspond nearly evenly with what the current CFMs put out.

Any wild guesses as to how far a Stretch Eight could go with these newest engines?

What would we call it: DC-8-83 ... or DC-8-2013?  
May the wind be always at your back . . . except during takeoff & landing.
 
prebennorholm
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RE: How Far Could Such A DC-8 Fly...

Sun Mar 04, 2012 1:53 am

It's of course pure speculation since it sure ain't gonna happen. But:

1. It may go nowhere at all since the wings may not have been designed to accommodate the heavier weight of the PWs.

2. At least the outer engines will for ground clearance have to be raised somewhat higher on the wing. That will give a drag penalty which reduces the range.

3. The heavier weight of the engines will reduce combined fuel and payload weight. Due to altered weight distribution it may affect max fuel load downwards. Maybe not.

Apart from that, present day CFM56-5Bs are a lot more efficient than the 25 years old CFM56-2 on DC-8-73. And we are told that the new PW is some 16% better than the -5B. So ferry range will surely be increased substantially. (As well as 20+ years old -5As on old 320s are no match for present day -5Bs - the -2 and the -5A are in fact a lot more similar than -5A and -5B).

But with a meaningful payload, due to increased OEW, max fuel load will be reduced substantially. Whether that will mean increased, unchanged or decreased range will demand a qualified analysis. But sure it will be more fuel efficient.

At the end of the day, re-engining an airliner is a lot more than just bolting on another blower.

Therefore the 320neo, which is going to take to the sky with the new PWs, will be modified substantially. Wing structure reinforced, and a whole lot of other changes to allow an increased MTOW as compensation for the increased OEW. And it is highly unlikely that any non-neo 320 will ever in the future be upgraded to 320neo. If viable, that would make a lot more sense than doing the same to the DC-8-73.
Always keep your number of landings equal to your number of take-offs
 
Happy-flier
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RE: How Far Could Such A DC-8 Fly...

Sun Mar 04, 2012 4:59 pm

Thanks for the great reply - much appreciated.

I agree that the DC-8 is an ancient breed whose innovative potential is at an end... though I admit I would love to see a new narrow-bodied twin designed for the kind of intercontinental flight that the 787 does, making use of composite and geared turbofan technologies. Time will tell.
May the wind be always at your back . . . except during takeoff & landing.
 
rmm
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RE: How Far Could Such A DC-8 Fly...

Tue Mar 06, 2012 5:23 am

I've wondered what the DC8 & 707 would be like with the CFM56-3. Much the same thrust class as the -2 but lighter, smaller diameter frontal area (reduced drag) and more ground clearance. Then throw in the advanced upgrade kit for better SFC.
 
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Francoflier
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RE: How Far Could Such A DC-8 Fly...

Tue Mar 06, 2012 5:57 am

I am guesing a 20-25% fuel burn improvement, which would very roughly translate in that much extra range...

Now, the real question would be: How far would an A343 go with PW G engines. But we don't want to open that can of worm again, do we...

  
I'll do my own airline. With Blackjack. And hookers. In fact, forget the airline.
 
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mayor
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RE: How Far Could Such A DC-8 Fly...

Wed Mar 07, 2012 6:12 am

Quoting francoflier (Reply 4):
I am guesing a 20-25% fuel burn improvement, which would very roughly translate in that much extra range...

When the DL DC-8-61s were converted to -71, we understood that the company was very happy with the better fuel consumption, somewhere in the vicinity of 27% better than the -61.
"A committee is a group of the unprepared, appointed by the unwilling, to do the unnecessary"----Fred Allen
 
A342
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RE: How Far Could Such A DC-8 Fly...

Tue Mar 13, 2012 12:07 pm

If you hang GTFs onto the wing of a KC-135A instead of the J57 turbojets...  
Exceptions confirm the rule.
 
flipdewaf
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RE: How Far Could Such A DC-8 Fly...

Tue Mar 13, 2012 12:31 pm

Quoting francoflier (Reply 4):
I am guesing a 20-25% fuel burn improvement, which would very roughly translate in that much extra range...

Now, the real question would be: How far would an A343 go with PW G engines. But we don't want to open that can of worm again, do we...

But it will have 4 engines so by default will not be as good as a T7, this is what I have learnerised from A.net  

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prebennorholm
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RE: How Far Could Such A DC-8 Fly...

Sun Mar 18, 2012 2:13 am

Quoting francoflier (Reply 4):
Now, the real question would be: How far would an A343 go with PW G engines. But we don't want to open that can of worm again, do we...

No problem, since here we are actually talking about something which might be technically realistic. (Technically realistic, but hardly economically realistic).

The CFM56-5C4 on the A343 is some 4,000 lbs (or some 75%) heavier than the CFM56-2 on the DC-8-7x. That is much more in line with a relevant PW G version, so contrary to the DC-8 the A343 would hardly need much rebuilding of the wing structure.

However, the efficiency gain on the A343 would be a lot smaller since its CFM56-5C is already a lot more efficient than the CFM56-2. It's an entirely different beast, much larger, much more powerful and efficient - and developed 15 years later.

Differences are a lot more than scaling. They don't even share the same number of compressor and turbine stages.

(I sometimes have the feeling that some a.netters imagine that CFM56 is mostly one engine design. There are similarities, like on Ford engines whether they are V8, V6 or 4 inline. But take for instance CFM56-5B (A320) and CFM56-5C (A340): That little suffix - B or C - means that the former is 5,300 lbs of metal while the latter tips the scale at 8,800 lbs).
Always keep your number of landings equal to your number of take-offs

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