DIJKKIJK
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Converting A Three Or Four Holer To Two.

Tue Sep 03, 2013 4:02 pm

Would it be possible to convert a DC-8, 707, A340 or MD-11 to a twin engined airplane?

Is it just a question of attaching more two powerful engines in the place of four, or are there other issues?

I guess it would make the older four/three holers more economical and would make it cheaper for some operators.

Any examples of this having been done in military aircraft?

Sorry if this has been brought up before.
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nipoel123
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RE: Converting A Three Or Four Holer To Two.

Tue Sep 03, 2013 6:34 pm

It would require a recertification of the aircraft. It will be able to use some of its parent's certification, but not a whole lot.

Secondly the wing is probably made for 2 engines on each wing (in case of a fourholer), so load balance would probably be an issue. For a triholer, a new vertical stabiliser, in case of the MD11/DC10, or a whole new tail section (727, L1011) would be necessary.

I'll leave it to the more informed to go into detail about this, but these are a few of the issues you'd encounter with such a project.

Not to mention that an iconic aircraft loses it's identity when you change it's engine set up. An MD11 would not be an MD11 if it had 2 engines.
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seabosdca
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RE: Converting A Three Or Four Holer To Two.

Tue Sep 03, 2013 6:47 pm

Quoting DIJKKIJK (Thread starter):
Would it be possible to convert a DC-8, 707, A340 or MD-11 to a twin engined airplane?

Physically? Yes.

Would it be remotely economical, when you could buy a brand-new, lighter, more aerodynamically efficient, more reliable aircraft instead?

No.

You'd have to redesign and rebuild significant parts of the aircraft, and at the end of the process you'd still have a heavy airframe with obsolete systems and less remaining useful life than a new-build aircraft.
 
roseflyer
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RE: Converting A Three Or Four Holer To Two.

Tue Sep 03, 2013 6:53 pm

Larger jets don’t have much margin for retrofit modifications built into the design any more. A few decades ago, every component was overdesigned 10-25% to allow for stretches and upgrades. That has slowly shrunk down with time to the point of almost zero on the 787 and A350 in a search for better efficiency.

Re-engining the DC-8 and 707 with CFM engines was already a big enough challenge. The structural changes to the wing with the new loads would be rather extreme. Also the DC-8, 707 and A340-200 do not have enough rudder authority for a single engine out scenario. You’d have to put a new vertical fin on those airplanes which is a very expensive modification. It already costs millions to convert a passenger airplane to a simple freighter, which doesn’t involve any flight control or propulsion changes. Once you start changing propulsion and flight controls, it will be far more money.
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bohica
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RE: Converting A Three Or Four Holer To Two.

Wed Sep 04, 2013 4:12 am

Quoting nipoel123 (Reply 1):
For a triholer, a new vertical stabiliser, in case of the MD11/DC10, or a whole new tail section (727, L1011) would be necessary.

Not only that, the L10, D10, & M11 would become very nose heavy without the tail engine. Trying to balance it would be next to impossible unless you add weight to the tail section, weight which reduces payload and increases fuel consumption.
 
DIJKKIJK
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RE: Converting A Three Or Four Holer To Two.

Wed Sep 04, 2013 7:21 am

Thanks for your replies. I was looking for instances in any aircraft, not necessarily widebodies.

It has been tested before on a Vickers Viscount. The four RR Darts were replaced by twin RR Tays.



http://en.wikipedia.org/wiki/File:Vickers_663_Tay_Viscount.jpg


I was wondering if there were other examples.
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RE: Converting A Three Or Four Holer To Two.

Wed Sep 04, 2013 7:54 am

Quoting DIJKKIJK (Reply 5):
I was wondering if there were other examples.

The A340/A330 are probably going to be as close as you can get, but they were developed simultaneously.

The reverse (2 to 4) happened with the AVRO Manchester/Lancaster.

If you count designed that evolved from others: the DHC-7 to Dash-8.
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Starlionblue
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RE: Converting A Three Or Four Holer To Two.

Wed Sep 04, 2013 8:27 am

Lockheed and Douglas studied two-holer versions of the L-1011 and DC-10 respectively. I suppose these would have been the grey area between conversion and further development. Ironically, such twins might have been very well timed, as the big twin explosion was about to begin.



[Edited 2013-09-04 01:30:04]
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citationjet
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RE: Converting A Three Or Four Holer To Two.

Wed Sep 04, 2013 1:28 pm

Quoting DIJKKIJK (Thread starter):
Any examples of this having been done in military aircraft?

Here is an example of a military aircraft going from a twin engine to a single engine. The Navy O-2A Cessna M337 went to a single engine pusher.

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zanl188
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RE: Converting A Three Or Four Holer To Two.

Wed Sep 04, 2013 2:31 pm

Lockheeds JetStar went from 2 to 4...


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neutrino
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RE: Converting A Three Or Four Holer To Two.

Wed Sep 04, 2013 2:45 pm

Quoting CitationJet (Reply 8):
Quoting DIJKKIJK (Thread starter):
Any examples of this having been done in military aircraft?

Here is an example of a military aircraft going from a twin engine to a single engine. The Navy O-2A Cessna M337 went to a single engine pusher.

How about the Tigershark from the Tiger II? Does that count?
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Viscount724
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RE: Converting A Three Or Four Holer To Two.

Thu Sep 05, 2013 1:48 am

The Dassault Falcon 2000 twin is closely based on the Falcon 900 trijet.

Falcon 900


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Falcon 2000


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jetstar
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RE: Converting A Three Or Four Holer To Two.

Thu Sep 05, 2013 2:33 am

Quoting zanl188 (Reply 9):
Lockheeds JetStar went from 2 to 4...


IIn the early 1990's a aftermarket shop tried to convert a 4 engine JetStar to 2 engines, they successfully test flew a conversion with GE Capital supplying the financing because the conversion used the GE CF-34 engines. But GE Capital had second thought about the re-enjoining and also did not think the conversation will sell enough to justify the investment so GE Capital quietly pulled out of the program, and without any other financing available the program folded up and the test airplane was scraped.

It was called the FanStar, I have a picture of it somewhere and hopefully I can find it.

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RE: Converting A Three Or Four Holer To Two.

Thu Sep 05, 2013 5:03 am

Quoting zanl188 (Reply 9):
Lockheeds JetStar went from 2 to 4...

Wasn't the twin config a kind of oddball Rolls-Royce engine? Also, I'd heard the design change from two to four was to compete with the McDonnell 220 for an Air Force contract...which the JetStar won  
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jetstar
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RE: Converting A Three Or Four Holer To Two.

Thu Sep 05, 2013 9:06 pm

There are 2 stories out there if the JetStar was designed to be a 2 engine or 4 engine airplane

From what I was told by a JetStar salesmen back in the early 1970’s the JetStar was designed originally as a 2 engine airplane using the British Siddley Orpheus engine of about 4500 pounds of thrust each, but from what I heard it could not meet the US Air Force requirements for second segment climb so Lockheed decided to go with 4 P&W JT-12-6 engines which had 3000 pounds of thrust each, it’s competition for the contract was the McDonnell 119, later called the 220 which was a 4 engine airplane.

The P&W JT-12–6 JetStar was underpowered to begin with so I can imagine how bad it was with the Orpheus engines, only after P&W came out with the –8 engines of 3300 pounds of thrust at least gave it some more power, but at the same time Lockheed upped the max gross weight from 40,921 pounds for the –6 to 42,000 pounds for the –8 which took away some of the advantage of the higher thrust engines, but it was still better than before..

The other story I heard from an instructor when I attended JetStar maintenance school, then Lockheed did all the maintenance training at the factory in Marrietta GA, was the JetStar or the McDonnell 119 would be used to fly the President into small airports or for personal use, the Air Force had back then a requirement that the President always flies on a 4 engine airplane so if Lockheed wanted to remain in the competition then the JetStar would have to have 4 engines.

The P&W JT-12 engine had delays in certifying the design, I believe it was developed from a small sub sonic guided missile engine and because of the delay Lockheed at least wanted to fly the airframe to start flight testing so Lockheed used the British Orpheus engines, the only other small engines available

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Jetlagged
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RE: Converting A Three Or Four Holer To Two.

Fri Sep 06, 2013 12:09 pm

Quoting bohica (Reply 4):
Not only that, the L10, D10, & M11 would become very nose heavy without the tail engine. Trying to balance it would be next to impossible unless you add weight to the tail section, weight which reduces payload and increases fuel consumption.

Not that difficult. No need for ballast, you need to stretch the fuselage aft of the wing and possibly reduce the length of the fuselage ahead of the wing. That puts the wing at the new more forward CG.

Quoting Starlionblue (Reply 7):
Lockheed and Douglas studied two-holer versions of the L-1011 and DC-10 respectively. I suppose these would have been the grey area between conversion and further development. Ironically, such twins might have been very well timed, as the big twin explosion was about to begin.

Airbus had already beaten them to it with the A300B (unencumbered by US airline requirements).
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Starlionblue
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RE: Converting A Three Or Four Holer To Two.

Fri Sep 06, 2013 1:27 pm

Quoting Jetlagged (Reply 15):
Quoting Starlionblue (Reply 7):
Lockheed and Douglas studied two-holer versions of the L-1011 and DC-10 respectively. I suppose these would have been the grey area between conversion and further development. Ironically, such twins might have been very well timed, as the big twin explosion was about to begin.

Airbus had already beaten them to it with the A300B (unencumbered by US airline requirements).

Quite. However if Lockheed and/or Douglas had launched a big twin in the mid to late 70s they would have been well ahead of Boeing.
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vzlet
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RE: Converting A Three Or Four Holer To Two.

Fri Sep 06, 2013 6:43 pm

Zero to two to four to 2+2:

http://upload.wikimedia.org/wikipedia/commons/thumb/8/8d/Chase_XG-20_glider_USAF.jpg/800px-Chase_XG-20_glider_USAF.jpg

http://upload.wikimedia.org/wikipedia/commons/5/55/Chase_XC-123A.jpg

http://www.nationalmuseum.af.mil/shared/media/photodb/photos/050322-F-1234P-006.jpg
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mandala499
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RE: Converting A Three Or Four Holer To Two.

Fri Sep 06, 2013 10:57 pm

Quoting DIJKKIJK (Thread starter):
Would it be possible to convert a DC-8, 707, A340 or MD-11 to a twin engined airplane?

Airbus has a very nice A340 to twin engine conversion program...
Forget all the common systems, center tank plumbing and outboard engine plumbing, extra landing gear etc... (A340-200/300 was basically an A330 with 4 smaller engines and and extra landing gear...)...
Just go send it to Airbus and ask for a twin, they'll convert your 340-200/300 to a 330-200/300... Just don't check the aircraft's serial number...   
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bohica
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RE: Converting A Three Or Four Holer To Two.

Sat Sep 07, 2013 5:54 am

Quoting Jetlagged (Reply 15):
Not that difficult. No need for ballast, you need to stretch the fuselage aft of the wing and possibly reduce the length of the fuselage ahead of the wing. That puts the wing at the new more forward CG.

That idea may work for a new design for a twin based on a tri jet. For an existing DC-10, if you removed the #2 engine, there is no way in hell you can stretch the aft fuselage and shorten the forward fuselage to be able to balance it. To do that would cost more than a brand new airplane.
 
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Jetlagged
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RE: Converting A Three Or Four Holer To Two.

Sat Sep 07, 2013 2:14 pm

Quoting bohica (Reply 19):
That idea may work for a new design for a twin based on a tri jet. For an existing DC-10, if you removed the #2 engine, there is no way in hell you can stretch the aft fuselage and shorten the forward fuselage to be able to balance it. To do that would cost more than a brand new airplane.

As the studies for a twin DC-10 referenced above show there certainly was "a way in hell" to do it. Airliner fuselages are stretched and shortened frequently as they are developed. There would be no problem in doing this to ensure the wing was positioned at the new CG. It certainly would not cost more than a brand new aeroplane to develop. Did the 727-200 cost as much as a new airframe to develop for example, or one of the numerous 737 or DC-8 variants? Douglas were masters of this art.
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bohica
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RE: Converting A Three Or Four Holer To Two.

Sat Sep 07, 2013 4:28 pm

Quoting Jetlagged (Reply 20):
As the studies for a twin DC-10 referenced above show there certainly was "a way in hell" to do it. Airliner fuselages are stretched and shortened frequently as they are developed. There would be no problem in doing this to ensure the wing was positioned at the new CG. It certainly would not cost more than a brand new aeroplane to develop. Did the 727-200 cost as much as a new airframe to develop for example, or one of the numerous 737 or DC-8 variants? Douglas were masters of this art.

I was talking about taking an existing physical airplane sitting on the tarmac and modifying it from three engines to two, not a redesign based on the original aircraft.
 
mandala499
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RE: Converting A Three Or Four Holer To Two.

Sat Sep 07, 2013 6:01 pm

Quoting bohica (Reply 21):
I was talking about taking an existing physical airplane sitting on the tarmac and modifying it from three engines to two, not a redesign based on the original aircraft.

The only widebody jet you can do this most on today would be... the A340-200/300 and convert it into a mutant A330.
If you don't want a jet, you can always chop the tail off a Trislander and convert it into an Islander by replacing the tailfin !   
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musang
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RE: Converting A Three Or Four Holer To Two.

Tue Sep 10, 2013 10:31 am

BAe had plans to build the RJ series (advanced 146s) with two engines and call it the -NRA, New Regional Aircraft. Models and images exist in the Avro Heritage Museum at Woodford Airfield, Manchester. One of the reasons the 146 originally had 4 was the lack of a suitable engine for a twin design at the time.

http://www.google.co.uk/imgres?imgur...gfuloHYDQ&ved=0CEQQ9QEwAw&dur=1488

To visualise this, see above link or look at an Antonov 148.

Regards - musang

[Edited 2013-09-10 04:05:38]

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