Gasman
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A VLA Trijet?

Sat Aug 12, 2017 7:02 am

It strikes me - correct me if I'm wrong - that the A380 could have been a trijet with GE90's

Looking at one engine out figures, three Trent 980s provide 252,000 lbs of thrust; whereas two GE90's would provide 230,000 lbs. Not a whole heap of difference. Could there therefore be scope for a VLA trijet in the future?
 
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TWA772LR
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Re: A VLA Trijet?

Sat Aug 12, 2017 7:09 am

22,000 ft/lbs is a huge difference.

And the engine on the center line would be tail mounted (expensive maintenance and a hell of an engine change), or an S-duct (expensive to engineer, slightly less expensive to maintain, and still a hell of an engine change).

The next advances for commercial aircraft won't come from theust, but improvements on materials and lifting surfaces.
"It's not getting to the land of the nonrev that's the problem, it's getting back." ~~Captain Hector Barbossa
 
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Starlionblue
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Re: A VLA Trijet?

Sat Aug 12, 2017 9:38 am

As TWA772LR says, 22000 ft/lb is massive. More importantly, you can't just look at pure thrust. In an asymmetric situation, you have to look at the drag consequences. Just as an example, compare how far outboard the Tristar and DC-10 engines were. Tristar was further out, giving more bending relief. It could do this because of the larger rudder closer to centerline.

Besides all that, the massive disadvantages of tail mounting mean it is a dead end, especially with today's big fans.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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PatrickZ80
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Re: A VLA Trijet?

Sat Aug 12, 2017 9:47 am

I think the A380 was the last VLA ever to be designed and it was already too big. With engines that powerful I think they'll only shrink the aircraft a little and make it a twin.

The disadvantages of trijets have already been pointed out. A third engine causes a lot of drag so a lot of power in that third engine is wasted by the presence of the engine itself. That's not the case with a twin. The A380 was already found too big so they need to shrink it anyway. Why mount a third engine to move a part of an aircraft that'll mostly be empty anyway?
 
c933103
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Re: A VLA Trijet?

Sat Aug 12, 2017 3:26 pm

I thought GE90 was exclusive to Boeing
 
LH707330
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Re: A VLA Trijet?

Sat Aug 12, 2017 5:58 pm

When Airbus designed the 330/340, they took a serious look at the trijet layout because the engines were not strong enough to lift 257 tons on a twin. They went with 4 on the 340 because a) the engines are more easily accessible, b) better bending relief, c) no engine mass in the tail, thus no associated structural weight, reduced moment arm, and associated induced drag, and d) it looks better than an MD-11 :D

Boeing did a similar study with the 767, and concluded that a twin would do better. With modern engines, a twin or quad layout will always beat a trijet.
 
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Stitch
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Re: A VLA Trijet?

Sat Aug 12, 2017 7:03 pm

c933103 wrote:
I thought GE90 was exclusive to Boeing.


There was nothing contractual preventing GE from using the engine in other applications and evidently the core of the Engine Alliance GP7000 on the A380 is a 72% scale version of the GE90 core as used on the LR777 family (77L / 77W / 77F).
 
Gasman
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Re: A VLA Trijet?

Sat Aug 12, 2017 11:00 pm

LH707330 wrote:
When Airbus designed the 330/340, they took a serious look at the trijet layout because the engines were not strong enough to lift 257 tons on a twin. They went with 4 on the 340 because a) the engines are more easily accessible, b) better bending relief, c) no engine mass in the tail, thus no associated structural weight, reduced moment arm, and associated induced drag, and d) it looks better than an MD-11 :D

Boeing did a similar study with the 767, and concluded that a twin would do better. With modern engines, a twin or quad layout will always beat a trijet.

Interesting. I would have thought the hassles associated with the #2 engine on a trijet would be more than offset by the savings through not having to purchase, maintain and run a fourth engine. But you are saying this isn't the case?
 
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Channex757
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Re: A VLA Trijet?

Sat Aug 12, 2017 11:25 pm

The only path for a VLA Trijet in the future would be via some new thinking, and it won't be easy. The various lifting body and theatre body/BWB designs with three ultra high bypass between twin tails. Even this has started to be overtaken by thoughts of hybrid designs where a mechanical coupling between the fans and the engine core is replaced by electric drive.

I doubt we'll ever see anything beyond a twin and tube design until one of the manufacturers feels ready to make the jump.
 
WPvsMW
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Re: A VLA Trijet?

Sun Aug 13, 2017 2:04 am

Bring back the P-39 Airacobra (https://en.wikipedia.org/wiki/Bell_P-39_Airacobra for the younger set)... think "big APU" with a long driveshaft to a rearmost open rotor. :rotfl: :rotfl:
 
aeropix
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Re: A VLA Trijet?

Mon Aug 14, 2017 8:44 am

Starlionblue wrote:
As TWA772LR says, 22000 ft/lb is massive.


Yes siree bob! That's a huge amount of torque! But I'm not sure how torque is related to thrust, unless you're going to drive the wheels on the undercarriage directly, or hook it up to a whopping big propellor!
 
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Starlionblue
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Re: A VLA Trijet?

Mon Aug 14, 2017 9:21 am

aeropix wrote:
Starlionblue wrote:
As TWA772LR says, 22000 ft/lb is massive.


Yes siree bob! That's a huge amount of torque! But I'm not sure how torque is related to thrust, unless you're going to drive the wheels on the undercarriage directly, or hook it up to a whopping big propellor!


Now a plane like that would make the A.nut crowd happy. :D
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Gasman
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Re: A VLA Trijet?

Tue Aug 15, 2017 7:56 am

TWA772LR wrote:
22,000 ft/lbs is a huge difference.

And the engine on the center line would be tail mounted (expensive maintenance and a hell of an engine change), or an S-duct (expensive to engineer, slightly less expensive to maintain, and still a hell of an engine change).

The next advances for commercial aircraft won't come from theust, but improvements on materials and lifting surfaces.

Did I enter into a parallel universe? How can we possibly refer to a 22 000lbs difference as being "massive" without reference to the denominator?

Four Trent 980s offer approx 336,000 lbs thrust. Three GE90s; approx 345,000 lbs. So all is fine when all engines are operating. With one out, you have 230,000 lbs with GE90s and 252,000 with Trents. That's less than a 9% reduction in one engine out power - and these figures are all approximate depending on exact engine rating. So it's hardly valid to assert that a VLA trijet wouldn't be feasible with existing engine technology.

Whether or not the market needs VLA anyway is a whole other argument.
 
GalaxyFlyer
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Re: A VLA Trijet?

Tue Aug 15, 2017 3:20 pm

You need to understand moreover about FAR 25, the effects of an engine loss on "specific excess thrust". Four engine planes need less total thrust, usually, than trijets to meet performance.
 
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keesje
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Re: A VLA Trijet?

Tue Aug 15, 2017 4:12 pm

A trijet is beneficial in 1 engine out situations. I remember a concept that had three engines, the centerline being a small one. Big enough to add TO power to 1 GE90 in critical situations, small enough to not create an enormous tail. In would be used during TO / engine out situations only. https://youtu.be/Ln7UIN5NlCc

Image
"Never mistake motion for action." Ernest Hemingway
 
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TWA772LR
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Re: A VLA Trijet?

Tue Aug 15, 2017 7:09 pm

Gasman wrote:
TWA772LR wrote:
22,000 ft/lbs is a huge difference.

And the engine on the center line would be tail mounted (expensive maintenance and a hell of an engine change), or an S-duct (expensive to engineer, slightly less expensive to maintain, and still a hell of an engine change).

The next advances for commercial aircraft won't come from theust, but improvements on materials and lifting surfaces.

Did I enter into a parallel universe? How can we possibly refer to a 22 000lbs difference as being "massive" without reference to the denominator?

Four Trent 980s offer approx 336,000 lbs thrust. Three GE90s; approx 345,000 lbs. So all is fine when all engines are operating. With one out, you have 230,000 lbs with GE90s and 252,000 with Trents. That's less than a 9% reduction in one engine out power - and these figures are all approximate depending on exact engine rating. So it's hardly valid to assert that a VLA trijet wouldn't be feasible with existing engine technology.

Whether or not the market needs VLA anyway is a whole other argument.

22,000 lbs is still a huge difference even in an engine out scenario. If you're going to post topics like this, include ALL relevant data, not just cheery picking the ones you want; because that's how I read your post the first time that 22K was the difference between 3 GE90s and 4 Trents.

Btw, snarky replies like yours is what is bringing down a.net.

*rant off*
"It's not getting to the land of the nonrev that's the problem, it's getting back." ~~Captain Hector Barbossa
 
GalaxyFlyer
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Re: A VLA Trijet?

Tue Aug 15, 2017 8:49 pm

22,000 pounds may not seem like much, but it might be 30% of the excess thrust available for OEI climb
 
Gasman
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Re: A VLA Trijet?

Tue Aug 15, 2017 11:00 pm

GalaxyFlyer wrote:
22,000 pounds may not seem like much, but it might be 30% of the excess thrust available for OEI climb


As calculated above, it's less than a 9% difference in OEI power from Trent 900s (depending on exactly which Trent 900s you're talking about).

Anyway - I think we've shown that a trijet of something in the ballpark MTOW of an A380 is possible with existing powerplants. But of course, if what someone said above is true - that three engines is no more economical than four - there really would be no point.
 
GalaxyFlyer
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Re: A VLA Trijet?

Wed Aug 16, 2017 3:08 am

Look at this way, if the four-engine plane loses one engine; it has lost 25% of its total thrust. If a tri-motor loses one engine, it loses 33% of its thrust and a twin loses 50%. The question isn't about 9%; it's about how much thrust is available above what it takes to maintain level flight in the take-off configuration. If it takes 200,000 pounds of thrust just to keep the OEI A380 level, take-off configured, the quad has 52,000 of thrust to meet the 3% gradient requirement, while the tri-motor version in your nightmare has only 30,000 pounds. It's here that 22,000 pounds is huge.

If the quad A380 requires 252,000 pounds of thrust to meet the OEI requirement, the tri-motor version would require about the same or three of 126,000 pounds thrust each, minimal. And perhaps more, as the wings would weigh a good bit more due to less wing bending relief and the added structural weight in the tail. Wings probably 10,000 to 15,000 more structure and tail the same. There's a reason no one has designed a tri-motor in 40 years. Totally impractical.

GF
 
Gasman
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Re: A VLA Trijet?

Wed Aug 16, 2017 5:11 am

But presumably the designers of 70's trijets knew all this? Why, for instance, did the L-1011 not have four downrated RB-211s all on the wings?
 
kitplane01
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Re: A VLA Trijet?

Wed Aug 16, 2017 6:06 am

PatrickZ80 wrote:

A third engine causes a lot of drag so a lot of power in that third engine is wasted by the presence of the engine itself.


Is your claim that a tail mounted engine causes more drag than a wing/pylon mounted engine? If so, what would that be?

I would be surprised if a tail mounted engine caused more drag than TWO wing/pylon mounted engines, which might be the alternative.
 
Gasman
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Re: A VLA Trijet?

Wed Aug 16, 2017 7:03 am

kitplane01 wrote:
PatrickZ80 wrote:

A third engine causes a lot of drag so a lot of power in that third engine is wasted by the presence of the engine itself.


Is your claim that a tail mounted engine causes more drag than a wing/pylon mounted engine? If so, what would that be?

I would be surprised if a tail mounted engine caused more drag than TWO wing/pylon mounted engines, which might be the alternative.

Exactly. People here seem to be comparing 3 engines vs. 2, as opposed to 3 vs. 4 which was the point of the thread.
 
GalaxyFlyer
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Re: A VLA Trijet?

Wed Aug 16, 2017 2:02 pm

I don't know, ask Lockheed. My opinion is that given the rated engines and the size of the two planes (DC-10 / L1011), three was enough. Don't discount fashion in design either. The L1011 wasn't very successful, 284 frames built. Three and four engine airliners are done and dusted for very specific economic reasons.

Are you aero engineer?

GF
 
kitplane01
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Re: A VLA Trijet?

Wed Aug 16, 2017 6:58 pm

GalaxyFlyer wrote:
I don't know, ask Lockheed. My opinion is that given the rated engines and the size of the two planes (DC-10 / L1011), three was enough. Don't discount fashion in design either. The L1011 wasn't very successful, 284 frames built. Three and four engine airliners are done and dusted for very specific economic reasons.

Are you aero engineer?

GF


No, I'm not.

I just wondered why a tail mounted engine might cause significantly more drag than a wing/pylon mounted engine.
 
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keesje
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Re: A VLA Trijet?

Thu Aug 17, 2017 1:00 am

2 Engines are preferred these days.

If a design needs >150k lbs in 1 engine out emergency scenario's, that doesn't bring you anywhere however.

Then it is 3,4 or more.
"Never mistake motion for action." Ernest Hemingway
 
GalaxyFlyer
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Re: A VLA Trijet?

Thu Aug 17, 2017 1:07 am

With all tri-motors involving a tail-mounted engine, the drag penalty is due to the weight of additional structure in the tail. There is also the issues of separation of control paths which is more stringent now than when the earlier tris were designed. Weight means less efficiency. No one, other than business jets for some specific reasons builds a tail mounted design regardless of the number of engines

GF
 
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flyingturtle
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Re: A VLA Trijet?

Thu Aug 17, 2017 7:31 am

What I had imagined once was the middle engine mounted on the underside of the belly, for easy removal of that engine. A #2 engine that is part of the vertical stab is just too cumbersome to maintain.

The other more serious idea that has been floating around is a #2 engine that also doubles as an APU, with a closable inlet and a closable exhaust.

David
Keeping calm is terrorism against those who want to live in fear.
 
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Matt6461
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Re: A VLA Trijet?

Thu Aug 17, 2017 7:52 am

GalaxyFlyer wrote:
Look at this way, if the four-engine plane loses one engine; it has lost 25% of its total thrust. If a tri-motor loses one engine, it loses 33% of its thrust and a twin loses 50%. The question isn't about 9%; it's about how much thrust is available above what it takes to maintain level flight in the take-off configuration. If it takes 200,000 pounds of thrust just to keep the OEI A380 level, take-off configured, the quad has 52,000 of thrust to meet the 3% gradient requirement, while the tri-motor version in your nightmare has only 30,000 pounds. It's here that 22,000 pounds is huge.

If the quad A380 requires 252,000 pounds of thrust to meet the OEI requirement, the tri-motor version would require about the same or three of 126,000 pounds thrust each, minimal. And perhaps more, as the wings would weigh a good bit more due to less wing bending relief and the added structural weight in the tail. Wings probably 10,000 to 15,000 more structure and tail the same. There's a reason no one has designed a tri-motor in 40 years. Totally impractical.

GF


It's not quite that simple.
  • Having more TO thrust implies achieving higher speed for a given runway.
  • Higher speed implies higher L/D at takeoff. Higher L/D means less drag.
  • The climb gradient is equal to (T/W) - 1/(L/D)

Given that a higher-powered trijet would be going faster V2 than a quad, and given that its minimum gradient at V2 with OEI is lower than a quad's, we'd have to run some numbers to see which effect would dominate.

I don't dispute, btw, that more engines generally means lower total takeoff thrust. It's just not as simple as you portray. Plus both trijets and quads could be climb-limited instead of takeoff-limited, meaning that your engines won't get any smaller even if the trijet needs less TO thrust (although derate for TO would have some benefits).

TO thrust pretty much always determines engine size for twins - though this might change given future very-high-AR wings. If that does happen, we'll likely never see another quad/trijet for mainstream applications.
 
GalaxyFlyer
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Re: A VLA Trijet?

Fri Aug 18, 2017 3:46 am

I was simplifying things. Yes, the specifics vary and sometimes overlap.

GF
 
LH707330
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Re: A VLA Trijet?

Mon Aug 21, 2017 9:06 pm

Gasman wrote:
LH707330 wrote:
When Airbus designed the 330/340, they took a serious look at the trijet layout because the engines were not strong enough to lift 257 tons on a twin. They went with 4 on the 340 because a) the engines are more easily accessible, b) better bending relief, c) no engine mass in the tail, thus no associated structural weight, reduced moment arm, and associated induced drag, and d) it looks better than an MD-11 :D

Boeing did a similar study with the 767, and concluded that a twin would do better. With modern engines, a twin or quad layout will always beat a trijet.

Interesting. I would have thought the hassles associated with the #2 engine on a trijet would be more than offset by the savings through not having to purchase, maintain and run a fourth engine. But you are saying this isn't the case?

Nope. The engine count is a cost driver, but to a much lesser extent than fuel burn and overall aerodynamic optimization. That's why we have not seen any new trijet designs since ETOPS went into effect, with all OEMs preferring either a twin or a quad.
 
Nean1
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Re: A VLA Trijet?

Thu Aug 24, 2017 12:05 am

keesje wrote:
A trijet is beneficial in 1 engine out situations. I remember a concept that had three engines, the centerline being a small one. Big enough to add TO power to 1 GE90 in critical situations, small enough to not create an enormous tail. In would be used during TO / engine out situations only. https://youtu.be/Ln7UIN5NlCc

Image


I like the concept of 2.5 engines, 2 big ones on the wings and a smaller on the tail. Maybe even with the retractable intake duct. The central engine would be derived from narrowbody aircraft, produced at high cadence.
 
prebennorholm
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Re: A VLA Trijet?

Sat Aug 26, 2017 12:38 am

Gasman wrote:
But presumably the designers of 70's trijets knew all this? Why, for instance, did the L-1011 not have four downrated RB-211s all on the wings?

If ETOPS had been "invented" a couple of decades earlier, then the Junkers Ju52 would likely have been the last three engined airliner. Post ETOPS no new trijet design has been considered anywhere in the world.

You may look at ALL trijets as "twins which are modified to be allowed to fly further than 60 minutes away from an airport".

The disadvantages are many and substantial. Maybe the major disadvantage is structure weight. The fuselage has to be a lot stronger and heavier to absorb a bumpy landing with the weight of one engine in the tail. The heavier fuselage plus the weight of the tail engine both add to the bending moment on the wing, so also the wing has to be stronger and heavier. All that extra structure weight can be subtracted pound for pound from payload and fuel load.

Some of the disadvantages apply to quads as well, even more so than trijets. But quads happen to have the absolutely most favorable weight distribution for wing bending. That is the reason why an A343 can take on so much more load than an A333 (even if that gap - due to A333 improvements - has been narrowed after the A343 went out of production).
Always keep your number of landings equal to your number of take-offs
 
GalaxyFlyer
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Re: A VLA Trijet?

Sat Aug 26, 2017 2:27 am

A factor in the 727 design was the take-off weather mins was 1sm for a twin and 1/2 sm for three- engines. Plus the wing was optimized for field performance at KLGA prior to the piers extended the runway available.
 
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Starlionblue
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Re: A VLA Trijet?

Sat Aug 26, 2017 2:45 am

GalaxyFlyer wrote:
A factor in the 727 design was the take-off weather mins was 1sm for a twin and 1/2 sm for three- engines. Plus the wing was optimized for field performance at KLGA prior to the piers extended the runway available.


Tristar and DC-10 were also somewhat driven by the requirement to operate at LGA.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
PC12Fan
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Re: A VLA Trijet?

Sat Sep 02, 2017 1:51 pm

A few years back, an A.netter (can't remember who) made up some schematics of what looked like an L10 on steroids. He even made up a bunch of them with several liveries which many enjoyed. Would be neat to see those again.

Realistically, you'll never see it happen. The VLA (748/A380) is a niche market and a dying one at that. The 777-9 will be the biggest aircraft to come out for a very long time.
Just when I think you've said the stupidest thing ever, you keep talkin'!

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