Airbus reinvents the Trijet?

Airbus Industrie has filed a patent for a new Trijet airliner design that is quite different from the ones we are used to seeing.

The new patent features the third engine mounted on a short pylon above the rear fuselage but with a twin tail arrangement to help keep the noise down, think An-225 tail with a DC-10 rear engine in between.

This is not the first time this arrangement has been proposed, the Avro 740 on the 1950's had a top mounted rear engine but this (possibly better) arrangement featured a butterfly, or V, tail instead of twin endplate fins. One benefit that Airbus is exploring is that it allows for the use of smaller engines of greater efficiency (enough to justify a third unit on the plane?) thought the improved safety margin is obvious.

Any thoughts?

Flight report

I guess it depends on which engines are being used, but all other things equal, I'd expect that three smaller engines would consume more fuel than two larger ones, add more weight, more cost and more down time. I'm not sure I like the idea. But I'm not an investor or purchasing agent for an airline either.

Thats exactly what I was hinting at, though Airbus claims weight savings in other areas (not sure how this comes about, or why they can't be applied to twins?).

Heres a picture of what the Airbus patent is for (bear in mind its a layout patent, not an actual design)



But here is how Avro wanted to it. On balance I think if you imagine it with the other two engines carried under the wings as is the current trend, this would be better. It would certainly be lighter as you only have two control surfaces and they have a lot less stress on them without the forces created by the endplate fins.

As long as they make the total V-tail surface as large as the combined total surface of the conventional tail, it should be no problem. And yes, it has significant advantages in weight savings and strength at the cost of control complexity. But everything now is FBW anyway. I think I like the Avro design better overall for an airliner. All three engines are behind the passenger compartment. The only problem is at high AOA -- are the wings interrupting airflow to the engines?
I just don't see a significant advantage to the tri-motor. If you really think safety is an issue, why not go four engines and use Upper Surface Blowing? You'd still save on the undercarriage and be able to use a much smaller wing area to cut drag and/or a smaller span to ease ground handling and reduce turbulence. With four engines you also remove the main draw back of USB which is EOI scenarios.

they are suggesting that physically smaller engines can be used - given the current trends of engines bigger than the fudelage benefit can be seen.

Unless they are going to get three turbofans with a much higher bypass ratio than the two larger ones, my bet is they end up consuming more fuel. I find it almost equally unlikely that the smaller engines together weigh less than the two bigger wing mounted engines. Any structural savings (including shorter landing gear) are going to be offset by the higher aggregate weight of the three engines, imo.
Additionally, you now have three engines to borescope or rebuild during overhaul or inspection instead of two. Cynically, I have a 50% higher chance of engine failure.
I don't see the benefit of three smaller engines. I'm not saying there can't be one, only that I don't see an obvious one.

By using 3 smaller engines (with smaller nacelle diameters) the landing gear can be shorter => lighter.



I think its a bad idea, and won't fly.

-> maintenance (as mentioned by del)
-> at take-off, the 3rd engine is in crap airflow (less than ideal power)
-> routing of fuel
-> cabin noise
-> greater strain on the wing spars in flight

Thats what I meant about why it can't be applied to twins. The BAe 146 and Bombardier CRJ (two very different designs) also have short landing gears, why would this be such a benefit (to the Airbus in particular)?

In fact if you built the Avro 740 (or even the BAC Three Eleven if you want to specifically cover widebodies) you could have a landing gear even shorter and lighter, wasn't this one of the many recognised benefits of rear mounted jets as far back as the 1950's? How come they went out of fashion?

I agree that it will never be flown, do you remember the other Airbus designs I posted quite a while ago (I tried to find the thread but can't)? Canard, Joined wing and BWB type designs have all be patented, just in case.

Its an interesting proposal, but:

1) Fuel economy. Even with weight savings the fuel economy issue would be the same that killed off the DC-10/MD-11

2) As Kilcoo316 noted fuel savings would be offset by the increased MRO for 3 instead of two engines in addition to the increased complexity of an additional fuel system and the like.

I get the impression most people here are assuming Airbus is considering a three engine arrangement rather than a two engine arrangement, and pointing out all the negatives relating to such a move.

What about moving from a four engine to a three engine? You have to go to a trijet, tail mounted arrangement in such a case. Also a trijet arrangement for large amounts of thrust is better than two, for the basic reason that you don't have to have a 50% safety factor, which means you aren't carrying around a bunch of extra weight.

Large twins are all very well and good, but they can't replace three or four engines for VLAs.

A few things I like:

Loss of one engine means a reduction of 1/3 power available instead of 1/2 if it was a twin.

The twin tail arrangement allows for a less costly hangar stucture due to it wouldn't have to be high enough to clear a typical single vertical stabilizer
The lower gear due to the smaller engines would help as well.

Maintenance costs?
Perhaps higher, but not as high as you would think because the parts are physically smaller and there's a small savings there.
Still costs about the same for labor whether machining components or making repairs/doing maintenance.

Seems the horizontal stabilizer for a twin vertical stab would have to be heavier than a single vertical stab configuration since it's carrying all the load at the fuselage attach point.

V-tails are interesting, but didn't the Bonanza single engine private airplane suffer from more stall-spin accidents than did the more conventional vertical/horizontal stab aircraft?
These accidents due to aircraft design characteristics and not to pilot screwups which are the leading cause.
Seems they were tougher to land in crosswinds than a conventional tail airplane


I wonder too if noise insulation for the rear mounted engine adds a lot of weight over a similar engine sized twin?

Fuel economy may be better than the huge engine twins?

Does anyone have cruise speed fuel flow figures for both the big engines and the medium sized ones used on a triple?

[edit on 20-4-2008 by Desert Dawg]

[edit on 20-4-2008 by Desert Dawg]

Boeing looked at that arrangment in the 60's I beleive with a short 3 engine version of the 747.

Well I think that we're looking at this all wrong.

We're comparing the Three-Engine layout to a Two-Engine layout.

As Richard said, what if you got an aircraft of comparable size with Four engines and them compared the two?

I mean an A340-like design could possibly benefit greater from three engines. On weight and fuel and maintenance. Maybe not as much on range and speed.

Maybe I'm reading the article wrong? Is the whole point of the patent to be compared to a twin-engine?

Shattered OUT...

Perhaps, but im not sure it really makes a difference. (All assume a 3 class layout)

The A340 has a range of PAX from the intial model 261 to 380 for the -600 variant.

The twin engine 777 is from 270 -200 to 350 for the -300ER model
The A350 has a notational capacity of 305 to 365.

So to 3 engine a A340 will never give you the efficenency of the 777 or the A350.

Now if you want to talk about a A380 with 3 engines then you may be onto something.

Hmmmm, interesting idea. Airbus were lookin at a shortened, twin jet A380 model to take on the 777 before they settled on the A350-1000. Who knows?

The CRJ has the engines mounted at the back of the fuselage, the whisper jet has a high wing. This means nothing under the wing to hit the ground - hence lower landing gear.




Unfortunately, there isn't really a free lunch with one particular mounting position - your always compromising something. Both of these designs have compromises, for instance mounting the engines on the fuselage means more loading at the wing root, and having a high wing means separate load paths for landing gear and wing.

How about take off with 3 cruse with 1 or 2. The automakers are looking at so many different angles perhaps the airline manufacturers are digging a little deeper too.


mikell

Personally I quite like the look of this, airbus has always followed fairly conservative approaches to aircraft, even with their 'groundbreaking' A380 (similar to Lockheed and MD designs). Beoing at least persued to an extent the somewhat more original path with the 7E7
Also, would this aircraft not require different hangers for maintanace?

Jensy

It's possible, but then you've added alot of dead weight during cruise phase in addition to the increased purchase and maintenance costs. And the third engine would have less than ideal airflow at high aoa -- exactly when it is planned to be used.
Perhaps two smaller engines under wings and one very large BPR -fan on the centreline slot? Use the large BPR for cruise? Still, you've essentially introduced two engines that add drag, cost and complexity for the (probably smaller) benefit of extra power at take off and flutter dampening.
I'm still not sold. I can't picture a position where I'd prefer three engines to two. If a civil design is too large for two, then it's probably easier, cheaper and lighter to strap four engines under the wing than mount one in the tail and two on the wing. Also makes removing and replacing the engines easier and faster.

They could be thinking about the P&W 8000 or some similar engine with a geared LP fan. The advantage of a geared fan is that one can use a small core at continuous thrust and shift gear in the LP fan to adjust thrust.

It's like the jet equivalent of constant speed props.

The core of such an engine can always be run at maximum efficiency, but the downside is they can't make them individually as powerful as current large fans.