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Why Are The TF-39 On C-5 Galaxy So Loud?  
User currently online747400sp From United States of America, joined Aug 2003, 3621 posts, RR: 2
Posted (8 years 1 month 2 weeks 6 days 22 hours ago) and read 14753 times:

Do not get me wrong, I like those loud TF-39 on Galaxy. But for a High by-pass turbo fan, they are pretty ear spitting. Why are these engine so loud, they got a rather high by-pass, for engine in the CF-6 family 8.0 bpr. So do any body know the answer to this question? If so please tell me, thank you.

10 replies: All unread, jump to last
 
User currently online747400sp From United States of America, joined Aug 2003, 3621 posts, RR: 2
Reply 1, posted (8 years 1 month 2 weeks 6 days 22 hours ago) and read 14759 times:

Quoting 747400sp (Thread starter):
TF-39

I meant TF39, I foregot to get ride of the dash


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17040 posts, RR: 66
Reply 2, posted (8 years 1 month 2 weeks 6 days 21 hours ago) and read 14728 times:

The Galaxy engine might well be the very first big fan mounted on a big plane. I would venture that:
- Military implementation means less focus on noise reduction.
- Early big fan technology less noise optimized.
- Not as big a bypass ratio as more modern engines.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineF14D4ever From United States of America, joined May 2005, 319 posts, RR: 4
Reply 3, posted (8 years 1 month 2 weeks 6 days 21 hours ago) and read 14723 times:

I've not seen data comparing actual measured noise levels vs. any other high bypass turbofan, so I can only take your word for the TF39 being louder than others.

Core exit velocity might be higher for the TF39; that's something the cycle designers would have reduced as turbofans evolved.

Nacelle design plays a major role in noise reduction. Look at the TF39 nacelles, paying attention to length. The fan cowl seems shorter than CF6 fan cowls. Notice also the distance between fan nozzle and core nozzle; that core nozzle sits quite far back from the fan nozzle lip, meaning the high-velocity (noisy) core efflux is not as well mixed or shrouded by the fan efflux.

Turning to the inlet, there might not be as much or as effective noise reduction treatment in the inlet as there is in modern commercial turbofans. Remember also that the C-5 was not constrained by noise factors as are commercial aircraft. Lockheed wasn't concerned about noise, and noise treatment adds weight.

Finally, fan blade shape strongly effects the forward noise lobe. Blades have come a long way since introduction of the TF39. The CF6-80C2 blades in particular are essentially four generations (-6, -50, -80A, -80C2) evolved beyond those of the TF39. CFM56 blades, newer still, and then we have those masterpieces of aero (and art), the GE90 blades.



"He is risen, as He said."
User currently offlineSinlock From United States of America, joined Dec 2000, 1647 posts, RR: 2
Reply 4, posted (8 years 1 month 2 weeks 6 days 20 hours ago) and read 14722 times:

There has always been a trade off between Power and Quietness with engines. The quietness of the TF39 was not a factor in the C-5 program due to it's military nature. Needless to say the diffrences between the TF39 and even the early CF-6 are vast. The main purpose of the TF39 at the time of it's birth was power to weight and it's low fuel burn. As flawed as the idea was a major purpose of the C-5 was to supply Euorpe during the first weeks of WWIII.


My Country can beat up your Country....
User currently offlineGrandTheftAero From United States of America, joined Nov 2003, 254 posts, RR: 5
Reply 5, posted (8 years 1 month 2 weeks 6 days 20 hours ago) and read 14702 times:

One of the features of the TF-39 is its "stage and a half" fan rotor. It was a two-rotor fan with the first stage having blades half the length of the second stage. The "half" stage was surrounded by inlet guide vanes. The highly non-uniform flow field created by this arrangement causes lots of noise.

In latter engines the half stage is placed behind the fan... what we now call a booster or LPC.


User currently offlineLitz From United States of America, joined Dec 2003, 1765 posts, RR: 0
Reply 6, posted (8 years 1 month 2 weeks 6 days 19 hours ago) and read 14676 times:
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It's been talked about over on the military forum, but the C5 is being rebuilt, one plane at a time, into the C5M :

http://209.157.64.201/focus/f-news/1653481/posts

And will be using the CF6-80C2 engine as its powerplant.

It's quite strange hearing that sucker buzz around Marietta, it just doesn't sound right!

 Smile

- litz


User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6451 posts, RR: 54
Reply 7, posted (8 years 1 month 2 weeks 6 days 16 hours ago) and read 14636 times:

Low noise is a lot more than just high bypass ratio.

The most efficient noise reduction methods are:
- clever mixing of hot core gas and fan air.
- good shielding of fan up front.

Those noise reduction methods cost weight increases and nacelle drag. I think that it's a fair bet that Pentagon didn't want one single oz extra emtty weight or even the slightest drag increase for noise reduction to reduce payload and range.

It is also a fair bet that the CF6-80 on the C-5M will be more noisy than when the same engine is installed on a 747-400.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineTexfly101 From United States of America, joined Jan 2006, 351 posts, RR: 0
Reply 8, posted (8 years 1 month 2 weeks 6 days 15 hours ago) and read 14616 times:

Quoting F14D4ever (Reply 3):
Finally, fan blade shape strongly effects the forward noise lobe. Blades have come a long way since introduction of the TF39. The CF6-80C2 blades in particular are essentially four generations (-6, -50, -80A, -80C2) evolved beyond those of the TF39. CFM56 blades, newer still, and then we have those masterpieces of aero (and art), the GE90 blades.

Great info, thanks for the post

Quoting GrandTheftAero (Reply 5):
One of the features of the TF-39 is its "stage and a half" fan rotor. It was a two-rotor fan with the first stage having blades half the length of the second stage. The "half" stage was surrounded by inlet guide vanes. The highly non-uniform flow field created by this arrangement causes lots of noise.

ditto in this post too...thanks

I always thought that the C-5 noise was almost more the high pitched whine than the decibels. Having lived right next to both the approaches of McCoy AFB and Patrick AFB, I felt that the C-5 was more intrusive than the B-52. Its distinctive whine would always make me sit up and listen while I would sometimes not notice a 52 in the pattern, even the D's during Nam.


User currently offlineImperialEagle From United States of America, joined Jan 2006, 2601 posts, RR: 22
Reply 9, posted (8 years 1 month 2 weeks 6 days 14 hours ago) and read 14595 times:
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C-5's were pretty common in the skies above the Atlanta area from the first stages of testing right on up through the 70's. The yearly "open house" just north of town, at the base up in Marietta, always included a C-5 or two-----one static with both ends open for walk-through, and one for touch and go's. Very kewl.

The sound those engines make is very distinctive. And plenty of smoke for good visual effect.
Recently, while visiting with a friend in the Air Force, we were able to observe a few t/o's and the memories came pouring back.



"If everything seems under control, you're just not going fast enough!"
User currently offlineN231YE From , joined Dec 1969, posts, RR:
Reply 10, posted (8 years 1 month 2 weeks 5 days 14 hours ago) and read 14497 times:

Quoting F14D4ever (Reply 3):
Nacelle design plays a major role in noise reduction. Look at the TF39 nacelles, paying attention to length. The fan cowl seems shorter than CF6 fan cowls. Notice also the distance between fan nozzle and core nozzle; that core nozzle sits quite far back from the fan nozzle lip, meaning the high-velocity (noisy) core efflux is not as well mixed or shrouded by the fan efflux.

Turning to the inlet, there might not be as much or as effective noise reduction treatment in the inlet as there is in modern commercial turbofans. Remember also that the C-5 was not constrained by noise factors as are commercial aircraft. Lockheed wasn't concerned about noise, and noise treatment adds weight.

Correct. Notice the auxiliary intake doors on the TF-39 engine are always open. This, in effect, allows noise to secape. The first Boeing 747s had doors such as these in their nacelles; after the noise regulations where passed in the 1970's, Boeing (in conjunction with P&W?) redesigned the nacelle for the JT9D to eliminate these auxiliary doors for the later 747s, thus to eliminate noise. The downside however, is that this nacelle added much more weight to the airframe.

In addition, its probably true that the military does not care about noise. If it did, there would probably be no after-burning (or reheat), and the all of the KC-135s would have CFM-56 power (I forget the military designation).


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