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Size Of 767 Engines  
User currently offlineDL767captain From , joined Dec 1969, posts, RR:
Posted (7 years 2 months 1 week 3 days ago) and read 7951 times:

I was looking at 767s the other day and wondered, why are the engines so small? The engines are large but compared to the size of the plane they seem too little. Wouldn't it make more sense to have the size be a little smaller than the 777? Wouldn't larger engines be more efficient and give more thrust to carry a heavier load with less work?

7 replies: All unread, jump to last
 
User currently offlineAmmunition From , joined Dec 1969, posts, RR:
Reply 1, posted (7 years 2 months 1 week 3 days ago) and read 7920 times:

silly engineers, why couldnt hey make them larger with more thrust...

User currently offlineTranspac787 From , joined Dec 1969, posts, RR:
Reply 2, posted (7 years 2 months 1 week 2 days 23 hours ago) and read 7843 times:

Quoting DL767captain (Thread starter):
Wouldn't larger engines be more efficient and give more thrust to carry a heavier load with less work?

As evidenced by the popularity of the 787, in the 21st century the name of the game with airlines is "fuel efficiency". Yes, larger engines can lift more, but at what cost??

Besides, the 767's are considerably smaller in both dimensions and weight compared to a 772ER (or the like). So, for being as small (relatively) as it is, a thrust range of 52k-62k per motor obviously makes sense for the type.

And, as the above poster kinda mentioned, since the 767 line has sold more than 1,000 frames, the engineers obviously did something right.


User currently offlineMestrugo From , joined Dec 1969, posts, RR:
Reply 3, posted (7 years 2 months 1 week 2 days 23 hours ago) and read 7837 times:

Er... When the 767 was developed, waaay back at the funky 70s, there were no 777 engines available, so they had to stick with what the engine suppliers had at the time. As long as they work properly, that's OK.

You may think the 767's engines look rather tiny in comparison with the plane's size, but, on the other side, one could also say that the 777's engines look way too big, too.  Smile


User currently offlineLightsaber From , joined Dec 1969, posts, RR:
Reply 4, posted (7 years 2 months 1 week 2 days 23 hours ago) and read 7772 times:
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Quoting Ammunition (Reply 1):
silly engineers, why couldnt hey make them larger with more thrust...

Oh we'd love to.

Your comparing two engines that are separated by quite a bit of technology. As technology progresses, the bypass ratios have been increasing. Thus, for the same thrust level, the diameters of the engines have been growing. For example, the current 767 engines are 94" in diameter. Replacement engines would be 102" to 115" in diameter. To put this in perspective the first generation GE-90 had a diameter of 118" later increased to 124" for the GE-90-115 but has 50% (at 118") to almost twice the thrust (115k vs. 62k). So in perspective, the next generation GE-90 should be about 144 inches in diameter. So pretty soon the GE-90's will look small.

The replacement engines? The GenX has a 111" diameter for thurst of 55k to 75k. Why the range of diameters above? There are definitive engineering compromises with fan diameter. For example, the larger the engine diameter, the higher the fuel consumption on shorter missions. A 111" diameter is optimized for 4000nm and longer runs. The 787-3 should get a smaller diameter engine. However, since the cost of developing a 2nd diameter for the same airframe is too high, its ok to go with the existing engines.

Now the Trent 1000 has an even larger diameter. 112" (ok, tiny bit larger). However, because its a triple spool, its climb performance is not as degraded during climb as a double spool. (Maybe why ANA bought it for their fleet?) Yes, optimized for a very long range mission.

As new technology comes out, the optimal diameter will continue to increase. Until then, the added efficiency of a larger fan is LESS than the added weight or increased nacelle drag.

Lightsaber


User currently offlineTdscanuck From , joined Dec 1969, posts, RR:
Reply 5, posted (7 years 2 months 1 week 2 days 22 hours ago) and read 7681 times:

Quoting DL767captain (Thread starter):
I was looking at 767s the other day and wondered, why are the engines so small?

Two big reasons:
1) They're big enough for what they need to do
2) They didn't have bigger engines available at the time

Quoting DL767captain (Thread starter):
Wouldn't larger engines be more efficient and give more thrust to carry a heavier load with less work?

Larger engines are, generally, more efficient. However, the 767 doesn't need any more thrust than it's got so it's not that you want more thrust, you want the same thrust for less fuel burn. What that means, for practical purposes, would be a slightly bigger fan with an even smaller core.

Tom.


User currently offlineStarlionblue From , joined Dec 1969, posts, RR:
Reply 6, posted (7 years 2 months 1 week 2 days 17 hours ago) and read 7621 times:

Quoting DL767captain (Thread starter):
I was looking at 767s the other day and wondered, why are the engines so small? The engines are large but compared to the size of the plane they seem too little. Wouldn't it make more sense to have the size be a little smaller than the 777? Wouldn't larger engines be more efficient and give more thrust to carry a heavier load with less work?

Apart from the efficiency issue stated above, there's the noise issue. Given the same desired thrust, a larger fan will make less noise. The principle is the usual: moving a lot of air more slowly instead of less air faster is quieter (compare with tjet vs. tfan). As larger fans become more economical (materials and all that) it makes sense to make them larger so as to meet new noise regulations.

AFAIK, the engines for the 380 are larger (and heavier) than originally planned in order to make the aircraft quieter.


User currently offlineBuzz From , joined Dec 1969, posts, RR:
Reply 7, posted (7 years 2 months 1 week 2 days 16 hours ago) and read 7610 times:

Hi DL767, Buzz here. One other factor about engine size: what happens in a crosswind landing? You can only bank so many degrees before you drag the bottom of the nacelle on the runway. There's a "de-crab" function in the autoland software, so the idea is to use that rudder thing in the back instead of doing a forward slip like the classic tailwheel airplanes that are fun to fly. Some of them get more than 1 MPH per Horsepower.

The 747-400 engines and 767-300 engines are pretty much the same where I work, programming plug to re-set the thrust limit from 52k (747-400) to 60k (767-300). I guess Boeing didn't think they needed more thrust.
Besides, to make that thrust you have to burn more fuel (it's not free).

g'day


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