747400sp
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Why Are The TF-39 On C-5 Galaxy So Loud?

Wed Aug 02, 2006 1:02 am

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.
 
747400sp
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RE: Why Are The TF-39 On C-5 Galaxy So Loud?

Wed Aug 02, 2006 1:05 am

Quoting 747400sp (Thread starter):
TF-39

I meant TF39, I foregot to get ride of the dash
 
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Starlionblue
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RE: Why Are The TF-39 On C-5 Galaxy So Loud?

Wed Aug 02, 2006 2:03 am

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." - John Ringo
 
F14D4ever
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RE: Why Are The TF-39 On C-5 Galaxy So Loud?

Wed Aug 02, 2006 2:16 am

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.
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Sinlock
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RE: Why Are The TF-39 On C-5 Galaxy So Loud?

Wed Aug 02, 2006 2:18 am

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.
 
grandtheftaero
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RE: Why Are The TF-39 On C-5 Galaxy So Loud?

Wed Aug 02, 2006 3:09 am

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.
 
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litz
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RE: Why Are The TF-39 On C-5 Galaxy So Loud?

Wed Aug 02, 2006 3:49 am

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
 
prebennorholm
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RE: Why Are The TF-39 On C-5 Galaxy So Loud?

Wed Aug 02, 2006 6:42 am

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
 
texfly101
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RE: Why Are The TF-39 On C-5 Galaxy So Loud?

Wed Aug 02, 2006 7:34 am

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.
 
ImperialEagle
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RE: Why Are The TF-39 On C-5 Galaxy So Loud?

Wed Aug 02, 2006 8:34 am

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.
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N231YE
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RE: Why Are The TF-39 On C-5 Galaxy So Loud?

Thu Aug 03, 2006 8:35 am

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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