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A Question About Tri-jets  
User currently offlinekatanapilot From Canada, joined Oct 2004, 170 posts, RR: 0
Posted (10 months 6 days 19 hours ago) and read 7237 times:

I have read so much about how Tri-jets were a solution to ETOPS issues before twins were allowed to go long distances over water and that planes like the MD-11 are now very inefficient because they are running three engines compared to more efficient twins...so my question is this:

Why weren't tri-jets built with two efficient (for their time) engines and an on-demand emergency engine on the tail that wouldn't run unless there were issues over water? If they had built them like this maybe they would have had a much longer service life and we'd get to see them for a few more years!

33 replies: All unread, showing first 25:
 
User currently offlineStitch From United States of America, joined Jul 2005, 30537 posts, RR: 84
Reply 1, posted (10 months 6 days 19 hours ago) and read 7236 times:
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Quoting katanapilot (Thread starter):
Why weren't tri-jets built with two efficient (for their time) engines and an on-demand emergency engine on the tail that wouldn't run unless there were issues over water?

Hauling that extra engine around would be a fair bit of dead-weight and I am not sure how easily it would be to start such an engine if it had been shutdown for a significant period of time during the flight.


User currently offlinekatanapilot From Canada, joined Oct 2004, 170 posts, RR: 0
Reply 2, posted (10 months 6 days 18 hours ago) and read 7220 times:

Quoting Stitch (Reply 1):
Hauling that extra engine around would be a fair bit of dead-weight and I am not sure how easily it would be to start such an engine if it had been shutdown for a significant period of time during the flight.

Surely hauling dead weight is more efficient than running a third engine!

As far as the start-up, couldn't an 'emergency engine' have been developed with sure-fire startup if the industry had asked for it?


User currently onlineStarlionblue From Greenland, joined Feb 2004, 16991 posts, RR: 67
Reply 3, posted (10 months 6 days 18 hours ago) and read 7212 times:

Quoting katanapilot (Reply 2):
Quoting Stitch (Reply 1):
Hauling that extra engine around would be a fair bit of dead-weight and I am not sure how easily it would be to start such an engine if it had been shutdown for a significant period of time during the flight.

Surely hauling dead weight is more efficient than running a third engine!

Actually no. It is much more efficient to run a third engine at all times than to carry a shut down third engine. The drag from a shut down engine alone is horrendous.

Quoting katanapilot (Reply 2):
As far as the start-up, couldn't an 'emergency engine' have been developed with sure-fire startup if the industry had asked for it?

Sure. It is called an APU and most airliners have one. 



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineUnited1689 From United States of America, joined May 2013, 71 posts, RR: 0
Reply 4, posted (10 months 6 days 18 hours ago) and read 7205 times:

Quoting katanapilot (Thread starter):
Why weren't tri-jets built with two efficient (for their time) engines and an on-demand emergency engine on the tail that wouldn't run unless there were issues over water?

One word: Weight

Quoting Starlionblue (Reply 3):
Actually no. It is much more efficient to run a third engine at all times than to carry a shut down third engine. The drag from a shut down engine alone is horrendous.

Bingo   The drag would kill any efficiency benefits the twin engine config would render. Not to mention the increased fuel burn from the larger engines.



The act of "driving" is only possible with a manual transmission.
User currently offlinevikkyvik From United States of America, joined Jul 2003, 9767 posts, RR: 27
Reply 5, posted (10 months 6 days 18 hours ago) and read 7185 times:
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Quoting Starlionblue (Reply 3):
Actually no. It is much more efficient to run a third engine at all times than to carry a shut down third engine. The drag from a shut down engine alone is horrendous.

Indeed. It can see strange, but you get a lot more drag from a shut down engine than from a running engine.

And, take any random tri-jet. Say you have 150,000 lbs total thrust, split among the 3 engines. If one engine is "emergency-only", you now only have 100,000 lbs total thrust, excepting an emergency. So you now need more powerful engines on the wings, which will likely be larger and heavier. So not only are you carrying around a spare third engine plus its associated drag, you're carrying two heavier primary engines.

Basically, you'd be hemorrhaging money at the end of the day.  



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlineTristarSteve From Sweden, joined Nov 2005, 3976 posts, RR: 34
Reply 6, posted (10 months 6 days 18 hours ago) and read 7174 times:

Before the B777-300ER was invented, there was a proposal from Boeing for a B777 with a much larger take off engine in the APU position. This would be the APU, but provide thrust for take off only. I remember an article in Flight Global about it.

And then lets not forget the HS Trident 3B. This was a tri-jet with three Spey engines, but also had a tke off only Boost engine fitted in the tail above the Nbr 2 engine. This engine was switched on during taxy out, then ran up to take off when the main engine throttles were advanced. It was made mostly of fibreglass, had a total loss oil system. made really loud noise, but worked most of the time!


User currently offlinebohica From United States of America, joined Feb 2004, 2668 posts, RR: 0
Reply 7, posted (10 months 6 days 15 hours ago) and read 7081 times:

Quoting United1689 (Reply 4):
One word: Weight

  

I don't have the exact numbers but the weight of an MD-11 engine is well over 10,000 pounds. Carrying that engine around as a spare means 10,000 pounds of lost payload.

If the average weight of each passenger is 200 pounds, that means a loss of 50 passengers, a huge loss of revenue.

Then we get into the issue of drag, extra fuel, etc, etc, etc. It ain't gonna fly.


User currently offlinekatanapilot From Canada, joined Oct 2004, 170 posts, RR: 0
Reply 8, posted (10 months 6 days 14 hours ago) and read 7044 times:

Awesome answers, thanks everyone!!

User currently offlineViscount724 From Switzerland, joined Oct 2006, 24796 posts, RR: 22
Reply 9, posted (10 months 5 days 11 hours ago) and read 6739 times:

Quoting TristarSteve (Reply 6):
And then lets not forget the HS Trident 3B. This was a tri-jet with three Spey engines, but also had a tke off only Boost engine fitted in the tail above the Nbr 2 engine. This engine was switched on during taxy out, then ran up to take off when the main engine throttles were advanced.

Rolls-Royce RB162-86, the Trident 3B's booster engine.


View Large View Medium
Click here for bigger photo!

Photo © Paul Markman

http://www.bredow-web.de/Triebwerke_und_Flugzeugmotore/Rolls-Royce_RB_162/Rolls-Royce_RB_162.jpg


User currently offlineYQBexYHZBGM From Canada, joined May 2009, 204 posts, RR: 1
Reply 10, posted (10 months 5 days 10 hours ago) and read 6709 times:

Interesting, I knew nothing about the takeoff boost engine on the Trident 3 -- I guess it wasn't as close to the 727 in weight and performance as I thought. What other trivia or interesting tidbits regarding differences between the two types are you aware of?

Al


User currently offlineKuja From Bermuda, joined Aug 2013, 80 posts, RR: 0
Reply 11, posted (10 months 5 days 9 hours ago) and read 6692 times:

Quoting YQBexYHZBGM (Reply 10):

The Trident was similar in original conceptual role to the 727, I think. However, due to the BEA's demands, HS fitted it with comparatively small Rolls-Royce Spey engines which left it quite underpowered (indeed it was known by its crews as the 'ground gripper') and necessitating the fourth booster engine for the Trident 3.
(Of course, BEA then turned around and said that the Trident was underpowered and that they wanted the 727, but it seems the government told them they had made their bed and now they had to lie in it.)

Like the 727, the Trident was designed for high speed, but as far as I can tell, did not share the 727's short field performance. The Trident did have fascinating landing gear, though. Offset nose gear that retracted sideways and quad wheel but single axle main gear that rotated 90-degrees to retract.
See this video for a brief demo.


User currently offlineprebennorholm From Denmark, joined Mar 2000, 6385 posts, RR: 54
Reply 12, posted (10 months 5 days 8 hours ago) and read 6655 times:

When British Dan Air many years ago operated a substantial fleet of Comets, then they found out that when they had to shut down an engine, then they actually burned slightly less fuel when continuing high altitude cruise on three engines.

That was of course an entirely different ball game compared to modern high bypass ratio fan engines since those RR Avon engines had a very small frontal area causing a lot less drag. Also turbojet engines generally lose less power at altitude than fan engines do. The remaining three engines were actually slightly more fuel efficient when operating at the somewhat higher power setting needed.

It also helped that the Comet had a very slow cruise speed.

Dan Air proposed to make it SOP to shut down one engine during cruise to save a little fuel. The authorities, however, didn't approve it, likely because in case of one more (unscheduled) shut down they would lose too much redundancies on electric, hydraulic and/or pheumatic systems.

But the military Comet, the Nimrod, skimmed the North Atlantic at low altitude and low speed for Russian submarines for decades during the Cold War, always with two of its four RR Spey engined shut down. They only used all four engines for climb and high altitude cruise. It extended the available loiter time considerably.

But for modern airliners cruising at Mach 0.8 or faster with large fan engines it doesn't work at all. The windmilling fan would have tip speed well into supersonic causing a horrendous supersonic and transsonic drag. Losing an engine means descending to lower altitude and slowing down in order to fight the drag penalty, and even then the fuel flown goes steeply north.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently onlineStarlionblue From Greenland, joined Feb 2004, 16991 posts, RR: 67
Reply 13, posted (10 months 5 days 7 hours ago) and read 6648 times:

Quoting YQBexYHZBGM (Reply 10):
What other trivia or interesting tidbits regarding differences between the two types are you aware of?

The offset and sideways retracting nose gear mentioned by Kuja comes to mind. This meant more space for the forward cargo hold.


View Large View Medium
Click here for bigger photo!

Photo © Alex Pan



Apparently the main gear rotated through 90 degrees during the retraction cycle.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineYQBexYHZBGM From Canada, joined May 2009, 204 posts, RR: 1
Reply 14, posted (10 months 5 days 5 hours ago) and read 6585 times:

Quoting Kuja (Reply 11):
The Trident was similar in original conceptual role to the 727, I think. However, due to the BEA's demands, HS fitted it with comparatively small Rolls-Royce Spey engines which left it quite underpowered

Also interesting. I know the Spey was small, since it was used on the BAC 1-11 and Fokker F28, which are both quite small when compared with the Trident Three. But, I would have thought that three Spey engines with larger intakes would have been enough for the Trident. Guess not!

I had no idea about the offset landing gear either. I can't even imagine a crosswind landing -- crabbing without wing scrapes must have been a very touchy operation with a setup like that.

Al


User currently onlineStarlionblue From Greenland, joined Feb 2004, 16991 posts, RR: 67
Reply 15, posted (10 months 5 days 5 hours ago) and read 6582 times:

Quoting YQBexYHZBGM (Reply 14):
I had no idea about the offset landing gear either. I can't even imagine a crosswind landing -- crabbing without wing scrapes must have been a very touchy operation with a setup like that.

I don't see how it would have made a big difference. By the time the nose gear touches down you should be going straight. Besides, it is only barely off centerline.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineTristarSteve From Sweden, joined Nov 2005, 3976 posts, RR: 34
Reply 16, posted (10 months 4 days 23 hours ago) and read 6461 times:

Quoting YQBexYHZBGM (Reply 14):
I had no idea about the offset landing gear either. I can't even imagine a crosswind landing --

The Trident was designed for autoland. The designers were so sure of themselves that they were worried about the nose wheels running over the centreline lights on the runway, This drove them to fit the nosegear offset to avoid the drumming as it hit the lights.

Quoting YQBexYHZBGM (Reply 14):
But, I would have thought that three Spey engines with larger intakes would have been enough for the Trident. Guess not!

The boost engine was not used on every departure, just the long distance, high weight flights. This was its problem. The aircraft might spend a week on short range shuttle flights, and then go off to Athens. The boost engine was not started until after the aircraft was taxying, (It was soooo noisy), and then it didn't start and the aircraft had to return to the gate.


User currently offlineCALTECH From Poland, joined May 2007, 2175 posts, RR: 25
Reply 17, posted (10 months 4 days 19 hours ago) and read 6384 times:

Quoting katanapilot (Thread starter):
have read so much about how Tri-jets were a solution to ETOPS issues before twins were allowed to go long distances over water and that planes like the MD-11 are now very inefficient because they are running three engines compared to more efficient twins...so my question is this:

Why weren't tri-jets built with two efficient (for their time) engines and an on-demand emergency engine on the tail that wouldn't run unless there were issues over water? If they had built them like this maybe they would have had a much longer service life and we'd get to see them for a few more years!

The tri jets did have the most efficient engines installed. Compare the DC-10/L1011 to the A-300 B4 and see what a third engine could get you.

For your on-demand emergency engine in a tri-jet, the closest thing would be the FedEx 727s after re-engining. The number 2 engine was supposedly left at idle during flight while number 1 and 3 engines did most of the work. On the other hand, UPS re-engined their 727s with Rolls-Royce Tays in all 3 positions.



UNITED We Stand
User currently offline113312 From United States of America, joined Apr 2005, 569 posts, RR: 1
Reply 18, posted (10 months 3 days 17 hours ago) and read 6080 times:

You are assuming that there were more efficient engines available at the time. There were not. The CF-6, original RB211 and the JT-9D were the most efficient engines available at the time the DC-10 and L-1011 were developed.

User currently onlineMax Q From United States of America, joined May 2001, 4359 posts, RR: 19
Reply 19, posted (10 months 3 days 8 hours ago) and read 5921 times:

Quoting CALTECH (Reply 17):

For your on-demand emergency engine in a tri-jet, the closest thing would be the FedEx 727s after re-engining. The number 2 engine was supposedly left at idle during flight while number 1 and 3 engines did most of the work.

That is not correct, all three engines were used normally.



The best contribution to safety is a competent Pilot.
User currently offlineYQBexYHZBGM From Canada, joined May 2009, 204 posts, RR: 1
Reply 20, posted (10 months 3 days 6 hours ago) and read 5899 times:

Quoting CALTECH (Reply 17):
On the other hand, UPS re-engined their 727s with Rolls-Royce Tays in all 3 positions.

I've read so many accounts of the 727s being withdrawn from most fleets due to the difficulty of replacing the JT8D with more efficient and less noisy engines, particularly the S-ducted #2 engine. How did UPS manage it, then? Why didn't others follow suit -- just not economical?

Al


User currently onlineMax Q From United States of America, joined May 2001, 4359 posts, RR: 19
Reply 21, posted (10 months 3 days 2 hours ago) and read 5859 times:

Quoting YQBexYHZBGM (Reply 20):

I've read so many accounts of the 727s being withdrawn from most fleets due to the difficulty of replacing the JT8D with more efficient and less noisy engines, particularly the S-ducted #2 engine. How did UPS manage it, then? Why didn't others follow suit -- just not economical?

Well, UPS only re-engined their lower powered -100's with the Rolls Royce Tay engine.
Even so that required an enlarged inlet lip on the #2 engine, a bit like the MD11.


Other operators re-engined their -200's though, like Fed Ex who installed the Valsan conversion, fitting JT8D 217's or 219's on the #1 and #3 position and retaining the same engine but removing the reverser on the centre #2 powerplant.



The best contribution to safety is a competent Pilot.
User currently offlineCALTECH From Poland, joined May 2007, 2175 posts, RR: 25
Reply 22, posted (10 months 1 day 8 hours ago) and read 5526 times:

Quoting Max Q (Reply 19):
That is not correct, all three engines were used normally.

That is why I wrote 'supposedly.' Might be mixing it up with the Valsan Super 27 project that rermoved the #2 T/R, which wasn't needed anymore during landing and lessened the noise footprint. Thought it was written that the number two on the Valsan 727 engine mods was able to be left at a lower thrust than #1 and #3 engines because of the extra thrust, which increased thrust and performance.

http://www.freepatentsonline.com/4836469.pdf

'An airplane equipped with the system requires a runway at least about 17% shorter for takeoff, if the center engine is operated at full power."

Quoting YQBexYHZBGM (Reply 20):
I've read so many accounts of the 727s being withdrawn from most fleets due to the difficulty of replacing the JT8D with more efficient and less noisy engines, particularly the S-ducted #2 engine. How did UPS manage it, then? Why didn't others follow suit -- just not economical?

Just not economical. Re-engine some old frames or buy newer ones ? Two engined 737/757s were coming into their own.

UPS re-engined all of their 727-100s, 50 of them ?, with the R&R Tays.

FedEx did a hush kit first with all 3 of the original JT8Ds for most of their 727s.

http://www.fedex.com/us/hushkit/helpdecide/index.html

It would be the last 15 727-200F Advanced that received the Valsan mod, not sure exactly how many did. Continental Airlines was supposed to do the Valsan retrofit also.



UNITED We Stand
User currently onlineMax Q From United States of America, joined May 2001, 4359 posts, RR: 19
Reply 23, posted (10 months 1 day 7 hours ago) and read 5493 times:

Quoting CALTECH (Reply 22):
Continental Airlines was supposed to do the Valsan retrofit also.

That was never done.



The best contribution to safety is a competent Pilot.
User currently offlineCALTECH From Poland, joined May 2007, 2175 posts, RR: 25
Reply 24, posted (10 months 17 hours ago) and read 5328 times:

Quoting Max Q (Reply 23):
That was never done.

That is incorrect. It sure was. Continental was looking at the re-engining program.

http://www.flightglobal.com/FlightPDFArchive/1988/1988%20-%202089.PDF

"13 August 1988

The second re-engined 727 is for Continental Airlines and is scheduled to be complete with new Pratt & Whitney JT8D-200s in October, entering service on November 1, Johnson says."

Just like people not believing a Continental Airlines DC-10 flew with winglets.



UNITED We Stand
25 Max Q : Fascinating, you get your information from an Aviation magazine who derived theirs from a program that, obviously never came to fruition. I know, I w
26 KenanC : Question: In trijets such as the L1011, the engine mounted on the tail doesnt have anywhere for the thrust to output? Where does it go? In others like
27 Post contains links and images Kuja : The type of arrangement you refer to is known as an S-duct, and as far as I know is used on all trijets apart from the DC-10 and MD-11. The DC-10 and
28 LH707330 : Ok, the net/net of Caltech and Max Q's discussion is that CO 727s never had the Valsan retrofit, though there were plans that never materialized, and
29 Post contains links and images Kuja : I will compare the two designs, although firstly, a disclaimer - I am not an aerodynamicist, so my understanding of this is limited to what I have he
30 ferpe : Well made, now Douglas were no pushovers and no dummies. Their reasons for their somewhat unusual design must have been well documented and argued at
31 Post contains images Flighty : As a Concorde and Soyuz captain of many years ('supposedly' ) my colleague on B744 showed the 747 burns more fuel on 3 engines versus 4. Recall the B
32 Post contains images United1689 : Interesting, and hardly surprising. The dead weight, added drag, and increased thrust (and likewise fuel burn) from the remaining three engines would
33 Post contains images CALTECH : Looking at that, I can believe that, supposedly. Long ago, had to put a # 3 engine inlet cover on a 727-200, for a ferry flight back to Denver from K
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