N79969 From , joined Dec 1969, posts, RR: Reply 7, posted (11 years 7 months 5 days 23 hours ago) and read 1932 times:
Does anyone know the purpose of the nozzles at the end of CF6 engines on MD-11? They don't appear on PW powered MD-11 nor do they appear on CF6-powered 747 or 767. They reappear on the A300/310/330 that are powered by CF6.
Also they appear on all engines available on the 777. I have wondered what determines when these exhaust nozzles are used or whether they inside the cowling or actually stick out.
Dynkrisolo From United States of America, joined Feb 2001, 1845 posts, RR: 8 Reply 11, posted (11 years 7 months 5 days 19 hours ago) and read 1871 times:
I think Hkniceguy has indirectly answered the question. I believe it's of different vintage. Varig's plane was delivered in 1993 while EVA's was delivered in 1997. The older ones must have similar tailcones that can be found on other -80C2 on the B747, B767, A300, and A310. The newer ones must have the -80E-type tailcones. Remember, the A330 came after the MD-11. My guess is GE found some improvement with the -80E-type tailcone . Also, remember, both GE and P&W had problems meeting fuel consumption guarantees on the MD-11. So both of them did a lot of performance improvement packages on the MD-11 engines. So, this might well be one of features introduced in those improvement packages.
N79969 From , joined Dec 1969, posts, RR: Reply 12, posted (11 years 7 months 5 days 18 hours ago) and read 1843 times:
Dynkrisolo, I think you're correct in pointing out HKniceguy's reply. The length and appearance of the tailcone is tied to vintage. But I also indirectly answered my own question regarding the last digit/letters of the CF6 model number. (See above)
My question remains, what is that tail cone there for exactly? What function does it perform that it is required on some engine/aircraft combinations and not on others?
Dynkrisolo From United States of America, joined Feb 2001, 1845 posts, RR: 8 Reply 15, posted (11 years 7 months 5 days 18 hours ago) and read 1826 times:
Not quite. On the CF6, the B designation is for Boeing applications, and the D designation is for Douglas/MDC applications.
All turbofan engines have tailcones or mixer devices. They "regulate" the flow coming out from the engine core. The flow will expand to the ambient in a controled manner. If you don't have one, the engine core flow might "violently" mix with the ambient flow. Then it means a potential loss in efficiency and increase in noise. That's why Hkniceguy observed different noise patterns with different tailcones on the CF6.
Engine companies often provide optional retorfit kits for a charge. It's up to the airlines whether they want the kits or not. Some airlines are willing to pay an upfront charge and expecting to recover the cost over the life of the equipment. Some airlines are willing to live with a higher operating cost and save the upfront charge. So, I am not surprised that KLM has retrofited their CF6 engines on the MD-11.
N79969 From , joined Dec 1969, posts, RR: Reply 16, posted (11 years 7 months 5 days 18 hours ago) and read 1815 times:
Thanks, Dynkrisolo. That was the answer I for which I was looking. Given what you said, the PW has its mixer device on the inside of the tailpipe. Do you know the advantage/disadvantage of that design feature?
Dynkrisolo From United States of America, joined Feb 2001, 1845 posts, RR: 8 Reply 17, posted (11 years 7 months 5 days 18 hours ago) and read 1811 times:
Look carefully at the Delta picture in this thread. The PW engine does have a tailcone, too. It just does not protrude behind the exhaust. A mixer device is usually used when you mix the engine core and fan bypass flow. Some examples are the newer RB211s on the B767 and B747-400, Trent 700 on the A330, V2500, and the CFM56-5C on the A340.
CX773 From Hong Kong, joined Sep 2001, 365 posts, RR: 7 Reply 19, posted (11 years 7 months 4 days 21 hours ago) and read 1743 times:
I hope this can answer your question.
for example, A330 (Trent 700) is a mixed engine (hot and cold nozzles are merged inside the integrated exhaust nozzle) which has convergent jet nozzles and is used for examining the effect of the component performance ( efficiency for the compressors and turbines, pressure loss for the combustor) on the engine performance. once the fan pressure ratio is selected, the bypass ratio is determined for a given turbine inlet tempreture and overall pressure ratio.
With the mixed engines, the choice of fan pressure ratio fixes the pressure downstream of the LP turbine and hence the power output from the turbine. For a given choice of turbine entry temp. and overall pressure ratio, the fan pressure ratio therefore fixes the bypass ratio. ( this is in contrast with the unmixed engine eg."PW 4000" for which the bypass ratio and fan pressure ratio can be selected independently, at least over some range.)
The benefit conferred by the working line being further to the right for the mixed engine augments the small increase in thrust arising directly from the mixing of core and bypass flow. it means the pressure ratio falls more rapidly as turbine inlet tempreture decreases, but the mass flow thru the fan decreases proportionally less. this steeper working line of the fan is a primary reason for the use of the mixer ( sorry can't show graph here). This maximum fan pressure ratio occurs during climb, but of far greater concern to the overall fuel consumption are conditions when the engine is throttled back somewhat, such as cruise, because the engine operates for much longer at this condition. A further benefit is that there is a smaller drop in rotational speed required to reduce thethrust with the steeper working line; this higher rotational speed, relative to that for the unmixed engine, means that at the reduced thrust condition for cruise the LP turbine will operate at lower value of change in stagnation enthalpt/blade speed(square) which is likely to give a further increase in efficiency.