Ericmetallica From , joined Dec 1969, posts, RR:
Reply 3, posted (14 years 7 months 3 days 19 hours ago) and read 3686 times:
Ok i guess i should have named my topic diffrent. Basicly the engine on top looks like all the air that is drawn in is used through the compressors and so on. while to one on the bottow has an opening to let some air go right through. What are difrences in thrust etc????
Superslushy From , joined Dec 1969, posts, RR:
Reply 4, posted (14 years 7 months 3 days 18 hours ago) and read 3682 times:
I understand your question and if I may I'd like to add a little of my knowledge. I know that the first engines of the 747 (like the one's in the second photo) were quite revolutionary in design because of the cool-air bypass design. What this means is that not all of the air sucked into the engine goes through the core, but it is compressed and sent around the inside and is fed out those gaps you are referring to. That way the cool air that comes out those gaps meets up with the thrust from the engine and when these two combine, somehow it creates more power. As far as how the first photo works, I dunno?
I'd appreciate it is someone would correct my story, I really only know the basics. I always appreciate it
Fanoftristars From United States of America, joined Jul 2000, 1639 posts, RR: 4
Reply 6, posted (14 years 7 months 3 days 14 hours ago) and read 3664 times:
Just think of the outermost metal on the bottom picture extending farther towards the back of the engine, and then you'll understand the RR design. The RR engine has the same nacelle design when fitted to the 757. It's just a different design. Now, what I would like to know are the advantages/disadvantages to the two different designs. The Trent engines do not incorporate this as far as I know.
Mikeybien From , joined Dec 1969, posts, RR:
Reply 8, posted (14 years 7 months 2 days 23 hours ago) and read 3632 times:
I think i know the answer to this but im not sure
The RR engine has a long fan duct which has lower drag than the PW model, but the extra long fan duct also adds weight.
For carriers that do mostly long haul flights, the RR might be a better choice because of a lower fuel burn. On the other hand, the PW might be a better choice for a high density short distance carrier because the aircraft has more usable payload.
Of course there are a bunch of other variables that go into engine selection for an airline like specific fuel consumption.
The only figures i have give the RR RB.211 a SFC of 0.57 and the PW 4152 a SFC of 0.58. For those of you new to engines, SFC is the pounds of fuel burned divided by the pounds of thrust developed. The lower the SFC the better.
The same source also claims the RR makes 60K trust versus 52K for the PW. However this is hardly comparing apples to apples so it doesnt mean long fan ducts are the way to go.
RayChuang From United States of America, joined Jun 2000, 8195 posts, RR: 4
Reply 9, posted (14 years 7 months 2 days 16 hours ago) and read 3609 times:
I do know that Pratt & Whitney JT9D and PW4000 installations on 747's tend to have relative short fan ducts--look at them on UA's 747-400 fleet. What's interesting is that on the 767, the JT9D/PW4000 installations have longer fan ducts.
I believe that the General Electric CF6-50 and Rolls-Royce RB.211-524 installations on the 747 always had longer fan ducts. Look at the CF6-50/80 installations on 747's and you notice the longer fan ducts pretty easily.