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twinjet vs trijet

Old 03-01-2010, 07:34 PM
  #21  
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Originally Posted by rickair7777 View Post
Yes.



The main factor actually was the requirement that you have three or more engines for extended over-water ops. When ETOPS relaxed that requirement to two engines, the trijet was done.




Two engines are more efficient...

- Less rotating-machinery drag (saves energy).

- Fewer support systems (saves weight).

- Less flow energy-loss (no long s-duct).

- Larger fans are actually more efficient anyway. The core is going to be a little bit heavier, but it can optimized for max fuel efficiency at cruise speeds vice T/O.

If the weight and fuel savings were not enough, the reduced Mx cost (by 33%) seals the deal.
ETOPS- "Engines Turn Or People Swim". I'll take as many engines as you'll give me
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Old 03-01-2010, 08:05 PM
  #22  
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Originally Posted by tuna hp View Post
Airlines are still buying plenty of 4 engine planes. How do the A330 and A340 compare? Again, as I said, I can't see how 3 engines would make sense with the way that larger airliners have evolved to be built. It would have to be something smaller that couldn't fit the engines on the wings.
The A330 is a twin.

ETOPS (two engine over-water) flights are limited to something like three hours to a divert field (used to be two hours, but I think that changed for some operations).

Pacific long-haul operations require more than two engines for regulatory reasons, so the larger airplanes which are designed for that have four.

I think there are currently technical limitations on designing an engine large enough to only need two for a 747 or A380. Also the fan diameter, and hence landing gear length, would be very large which would create issues for ground handling (nobody could reach the fuselage).




Originally Posted by tuna hp View Post
Correct me if I'm wrong, but I believe that the reason larger aircraft mid-mounted wings is for ground clearance for the engines. Airplanes that have fuselage engines can have a 1 piece wing with the fuselage resting on top. Its supposed to be more structurally efficient.
Like I said before, I think the weight savings would be minimal. Carrying the spars all the way through might be a little more efficient, but engineers can do wonders with complex structures and arched structural objects (like a barrel-shaped wing-box) are inherently strong.
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Old 03-01-2010, 08:50 PM
  #23  
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Everybody is correct!

Boeing designed the 727 as a compromise to meet the requirements of United, American, and Eastern for a domestic aircraft that could operate out of shorter runways on medium routes to smaller airports.

United wanted a new aircraft for high density altitude airports; American wanted a twin engined aircraft for efficiency; and Eastern wanted a third engine for its Caribbean ETOPS requirements; and all three wanted shorter field capability. The first 727's even had nosewheel brakes!

Boeing designed the 727 with three rear mounted JT8D engines that increased the ETOPS of the time and allowed for a full uninterupted wingspan of the most advanced lift enhancing devices on a commercial aircraft at that time (Triple-slotted trailing edge flaps, Krueger flaps on the inner leading edge and slats on the outer). The third engine improved engine-out capabilities on the shorter runways and thus allowed greater takeoff weights.

It was also designed to operate independently of most ground support equipment and on gravel strips (Built-in rear airstair, APU, and reverse taxi capability).

Boeing 727 Family

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Old 03-01-2010, 09:21 PM
  #24  
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As with many technical discussions this one quickly went beyond the scope of the interwebs. Tuna, go to aerospace design school for a full reply to your questions, or at least buy the books listed below. You might try Eng-Tips Forums for aerospace engineers because this is mostly a pilot forum. The big picture on how to choose engines, how many to choose, where to put them is complicated preliminary design topic which includes many factors such as:

Number of engines to be used

1. power required and currently available engines on the market
2. relationship-critical factors such as climb requirements, field length, and engine failures
3. other safety considerations (ETOPS etc.)
4. cost- exponential cost increase with more engines
5. fuel available (biofuel, JP-8 etc.)
6. customer specs

Mounting factors

1. effect from engine loss on stability and control, thrust line, etc.
2. weight and balance effects
3. weight
4. engine efficiency effects due to location
5. vibration and noise
6. maintenance access
7. aerodynamic effects on aircraft from engine exhaust and mounting hardware
8. engine clearance
9. attachment points
10. thrust reversing requirements
11. engine driven accessory requirements
12. fire requirements
13. FOD exposure

A good basic design series on this is Airplane Design, Jan Roskam, Parts I-VIII. DAR Corporation ($350). Hope this helps.
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Old 03-02-2010, 04:14 AM
  #25  
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Originally Posted by rickair7777 View Post
The A330 is a twin.

ETOPS (two engine over-water) flights are limited to something like three hours to a divert field (used to be two hours, but I think that changed for some operations).

Pacific long-haul operations require more than two engines for regulatory reasons, so the larger airplanes which are designed for that have four.

I think there are currently technical limitations on designing an engine large enough to only need two for a 747 or A380. Also the fan diameter, and hence landing gear length, would be very large which would create issues for ground handling (nobody could reach the fuselage).






Like I said before, I think the weight savings would be minimal. Carrying the spars all the way through might be a little more efficient, but engineers can do wonders with complex structures and arched structural objects (like a barrel-shaped wing-box) are inherently strong.

Etops at 180 minutes with a flight by flight exception in the pacific for 207 minutes
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Old 03-02-2010, 04:51 AM
  #26  
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Yep, a UAL 7777 went about five hrs on one engine when they blew one at their etp on the way out of SYD. Diverted to HNL. Very impressive.
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Old 03-02-2010, 04:59 AM
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Originally Posted by acl65pilot View Post
Yep, a UAL 7777 went about five hrs on one engine when they blew one at their etp on the way out of SYD. Diverted to HNL. Very impressive.
That is a long 5 hours!
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Old 03-02-2010, 05:17 AM
  #28  
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You are telling me. I think it happened in November or early Dec. I will look for the article. They did it though.
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Old 03-02-2010, 05:31 AM
  #29  
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Originally Posted by tuna hp View Post
In terms of performance characteristics, what are the tradeoffs between a 2-engine and 3-engine design?

Lets assume that we are comparing a twinjet with fuselage mounted engines and a trijet with an s-duct layout and all engines on the fuselage. I think its unfair to compare to a trijet that would have 2 wing-mounted engines. It seems structurally inefficient on its face.

I read all over the internet that 2 engines is always more efficient than 3. But is that necessarily true?

It seems that a main factor is the requirement that a plane be able to complete its takeoff run with one engine out. On a twinjet you're losing 50% of your power while on a trijet you are losing 33% of your power.

So take two hypothetical airplanes and hold pretty much everything equal except the number of engines at the back. If the plane requires 0.5X thrust to complete takeoff, then the twinjet needs total of 1X thrust while the trijet needs 0.75X thrust to takeoff with the same runway requirement.

At cruise, which plane is more efficient? The one that is uses the three smaller less efficient engines or the one whose 2 larger engines that are incredibly overpowered for cruise? By how much?
There is no such thing as an overpowered airplane. It's like having too much gas.. the only time there is too much is when you are on fire.
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Old 03-02-2010, 05:42 AM
  #30  
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Originally Posted by oneflynfool View Post
Three engines does not equal three pilots on relatively modern aircraft.
MD-11, Falcon 7X, etc.
You are absolutely correct. The discussion of "two engines vs. three" is very valid; however, one must narrow the parameters somewhat. If one takes the broad scope of "aircraft" the discussion runs the gamut from corporate aircraft up to wide body transport category aircraft. The Falcon 7X, for example, with three engines/two pilots is economical as a corporate jet and very functional, yet the three engine/two pilot wide body is not economical because of crew costing primarily. The MD-11, for example, did not meet performance requirements and promised fuel burn figures and was never purchased or used in significant quantity by the majors. Crew costs were an advantage; however, the overall economics were not and it went by the wayside as did the DC-10 and L-1011, which economics drove to the desert.

The international commercial market has always been served by four engine aircraft for a variety of reasons. At the size and weight of a B-747-400, A-340, 380, one has to design and build a four engine aircraft for the design loads and performance requirements. Also, as mentioned, a four engine aircraft deletes the ETOPS requirements, which are significant ! Incidentally, we are up to 207 minute ETOPS in the Pacific. In the north Atlantic theater, 180 minute ETOPS is all that's required.

In working with a client who wanted to buy two B-777's for an international corporate operation, once one examined all of the parameters, the B-747-400 won out hands down for the reasons stated above.

This is a good, valid discussion. Let's just narrow down the discussion parameters and enlighten ourselves.

G'Day Mates
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