C-5M engine sound difference?

[sandiego89]

I have yet to hear a C-5M in person, and youtube does not fully answer it for me- can anyone comment on the change in noise with the new engines? I understand the decible level has gone down, but is the very characteristic whine gone too?

Big electric motor sound on the A's and B's (and C), truly a neat sound. :ok:

I used to see them all the time, and rode in a few from Dover.

[KenV]

Yes. The new GE CF6 engines require entirely new nacelles. These nacelles use a civil style pylon mounted design with extensive acoustic insulation. The old nacelles had almost no acoustic insulationi which resulted in the VERY loud turbo machinery noise radiating down and to the sides. Although C-5M does not meet even Stage 3 noise requirements, the new engine/nacelle installation is much quieter than the old one, eliminating most of the turbo machinery noise. The noise that remains is mostly aero acoustic noise which sounds very different. C-5M sounds a lot like an older 747 which also has four CF6 engines.

BTW, the old nacelles were engine mounted rather than pylon mounted. Most of the maintenance charged against the engine on the C-5A/B was actually nacelle/thrust reverser maintenance. The old TF-39s after all of the upgrades was actually quite reliable. USAF could have gotten 90+% of the reliability/maintainability improvement of the C-5M by just putting on new nacelles rather then new engines and nacelles. Typically, USAF screwed the pooch on the C-5 engine swap.

[tdracer]

IIRC, the TF-39s had fan inlet guide vanes - which do a very good job of imitating how sirens work and create a similar, extremely loud, noise. The CF6 doesn't have fan inlet guide vanes - that all by itself makes it considerably quieter.

KenV, the CF6-80C2 has significantly higher thrust and much better fuel burn relative to the TF-39. So it's not just about maintenance costs (although if the performance improvements justify the costs is a different question).

Several times over the years, we've looked at doing a re-engine of the B-52. About 15 years ago I was personally involved in a study of using a single 757 engine on each pylon (either the RB211-535 or PW2000). The improvements in aircraft performance and range were massive - as were the maintenance savings. But we never got the go-ahead. I read something in the WSJ not too long ago that explained the B-52 re-engine made massive sense, but never got the go-ahead for two reasons. First the government budget rules make it difficult to spend money today to save money tomorrow. Second, when the USAF did their cost analysis, they made a fundamental error - they didn't account for the fact that aerial refueling fuel costs a lot more than stuff loaded on the ground - by a factor of ~15 :eek:. Had we gotten the go-ahead 15 years ago, the re-engine would have more than paid for itself by now, and the USAF would have a far more capable B-52 :ugh:

[KenV]

IIRC, the TF-39s had fan inlet guide vanes - which do a very good job of imitating how sirens work and create a similar, extremely loud, noise. The CF6 doesn't have fan inlet guide vanes - that all by itself makes it considerably quieter

All axial flow turbine engines have inlet guide vanes. The TF-39 was unique in that it had a higher than usual bypass ratio which required the fan stage to have snubbers integral to the fan blades. This resulted in the inlet guide vanes being in front of the fan rotor rather than behind. This combination resulted in unique turbo machinery acoustics.


KenV, the CF6-80C2 has significantly higher thrust and much better fuel burn relative to the TF-39. So it's not just about maintenance costs (although if the performance improvements justify the costs is a different question).


Indeed. However, the CF6s on the C-5 are derated to produce only a few percent more thrust than the TF-39s, just enough to overcome the additional weight. The C-5's wing/pylon interface can't handle the additional thrust and beefing up the wing would add so much to the cost of the project that savings go out the window. And while fuel burn is better, it is not "much" better. The CF6 is a development of the TF-39 and does not employ any big jumps in technology. In fact the CF6 has a significantly lower bypass ratio than the TF-39. The CF6 gets its fuel advantage from its (slightly) higher core pressure ratio. This re-engine is very different than the KC-135 re-engine which went from a pure turbojet J-57 to a high bypass fanjet CFM56. The CFM56 is also far far more reliable than the old J-57/JT3.

As for the B-52 engine replacement fiasco, there were multiple re-engine proposals. The first used the PW2000, same as what USAF was flying on C-17. That went nowhere because the entire B-52 fleet was supposed to be replaced by the Bomber X (B-1 and then B-2). But that never happened. The next one involved RB211 but that failed on two grounds: 1) the proposal was to LEASE the RB211s rather than purchase them. SAC was really hinky about having leased engines on a nuclear strike platform. And 2) USAF "accidentally" failed to take into account the costs of tanker fuel. (no, this was not an "oversight", although USAF describes it that way now.) Both the PW2000 and RB211 are now out of production so those options have closed. And with the B757 out of production there are no commercial engines now in service in the thrust range of those engines.

GE has proposed installing eight CF-34s. That's not gaining much traction. PW has proposed using four of their PW1135 geared turbo fans. But that engine is still in development so it's not gotten much traction either. One of the problems now is that the B-52 fleet is much smaller than it used to be and nearer to its service end date. So development and installation costs need to be amortized over far fewer airframes and flight hours.

[GreenKnight121]

I have read a statement from a guy who was part of those early studies, and he says that the real issue with all of the 4-engined B-52 proposals was that the thrust line of the engine was always significantly further from the wing chord-line than that of the 8 engines were - which would exaggerate the stresses on both the pylons and the wing.

In fact, he said that their modeling showed that the wings would twist (leading edge going up and trailing edge dropping) far more than it currently does, which would both shorten the wing life and create handling problems. This held even if there was no overall increase in available thrust - the problem was worse if total power was to be increased.

So basically, all stories about fuel costs aside, there were real technical problems that precluded 4-engined solutions - and still do. Hence the focus on a 1-for-1 replacement proposal (CF34). And even then the total power is only expected to increase a little - due to the wing-stress/twist issue.

[chopper2004]

Visually, this might help as a couple of my photos taken in July and Dec last year.. Travis based C-5M and a West Virginia Air Guard C-5A (shortly to be retired)

http://i57.photobucket.com/albums/g209/longranger/Sunday_Travis_C5_zps274d3a8e.jpg

http://i57.photobucket.com/albums/g209/longranger/Sunday_Travis_C55_zpse5794007.jpg

and the A model

http://i57.photobucket.com/albums/g209/longranger/WVANG_C5_1_zps446365b7.jpg

Cheers

[LowObservable]

TDRacer - WSJ my :mad:!

Opinion: Bumbling Caused B-52 Reengining Delay | Defense content from Aviation Week (http://aviationweek.com/defense/opinion-bumbling-caused-b-52-reengining-delay)

What do you think of GK's wing-twist point, by the way? He's correct in observing that GE is pitching 8 x CF34s (as used on the big Embraer jets.) I have also heard that four engines were an asymmetric-thrust issue.

[KenV]

With two engines per pylon, the engines were effectively on the SIDE of the pylon rather than hung below it. Thus the thrust line of both engines was close to the bottom of the wing. The four engine proposals hung the engine BELOW the pylon, and these engines are much larger in diameter. So the thrust line is much farther below the wing. Increased thrust AND a longer moment arm results in significantly higher twisting loads on the wing. Each four engine proposal included new pylons and structural beef up to the wing to handle the additional loads. SAC really did not like the stiffer wings because they made the ride at low altitude rougher on the crew and mission systems and SAC was doing a lot of low altitude penetration flying. The wing beef up also increased the price of those proposals. But analysis showed the fuel and maintenance savings would offset the overall design, development, and installation costs. But that was for a large fleet operating for many decades. The fleet has since shrunk considerably and the number of service years has shrunk as well. This makes a four engine solution problematic financially. Hence the new eight engine proposal which reduces or eliminates the need to stiffen the wing. But it's still not clear if the savings will off set the costs.

As for thrust asymmetry, the eight engine layout has all sorts of engine out issues. The proposed four engine layouts improved most of the engine out issues. I don't know how the new eight engine proposal behaves.

[Turbine D]

Hi KenV,
I've read with interest your postings and others regarding the TF39 engine for the C-5s. Hopefully, I can add a little information to what has already been posted plus some history. First a photo of the engine:
http://i1166.photobucket.com/albums/q609/DaveK72/page1image1016_zpseb6c7910.png (http://s1166.photobucket.com/user/DaveK72/media/page1image1016_zpseb6c7910.png.html)
Unlike today's high bypass turbo fan engines, the manufacturing technology limited the size of fan blades that could be produced in the mid-1960s, so what you see, looking in the front, is what was known as a "1.5 fan", part of it rotated and part of it didn't, e.g., most of what was the bypass air not going through the engine core received no compressive work. Acoustical noise from jet engines mostly emanates from the front of the engine and not the rear except for after-burning engines. So in the instance of the TF39 engine, there was no sound absorbing materials in the nacelles, hence the unique "electric" sound on TO and climb out.

The engine design and development started at GE in late 1963 using a building block GE1 core (compressor + high pressure turbine) and adding the fan feature and low pressure turbine. The so called GE1/6 was roughly one-half the size of the planned engine. When it was tested, it confirmed an 8 to 1 bypass ratio, 15,830 pounds of thrust and a sfc of .336 remarkably low for its day. A lot of new technology went into the engine including variable compressor vanes, a 25 to 1 compressor pressure ratio and turbine airfoil cooling techniques to permit 2,500℉ turbine temperatures. When the full size TF-39 engine was built it all was nearly perfectly matched, the fan, the compressor, combustor, turbine and exhaust nozzle resulting in a 40,000 pound thrust engine. Later refinements increased the thrust to 43,000 pounds.

Interestingly, the first TF39 engines were flight tested on a B-52 testbed aircraft taking the place of two engines on the starboard inner pod.

What was learned on the TF39 engine paved the way for the CF6 and subsequent GE90 engine families of today.

TD

[KenV]

Thanks for the photo and the additional info. They provided a lot of insight.

My understanding was that the unique sound of the TF-39 was not caused by the fixed portion of the fan, but the "snubbers" on the rotating portion. They are visible in the photo as the inner most "ring" outboard of the center cone. The lack of acoustic treatment in the nacelles just made that noise very loud. But you may be right, the unique sound may be due to the fixed fan section. I do know this: I work at Kelly Field where C-5s are based and C-5s take off here nearly daily. They are exceptionally loud from the sides and from below, and less so from the front. Don't know if that is due to the rotating or the fixed portion of the fan.

I also understand that the fixed fan portion was optimized for "ram drag" recovery of the intake system when the aircraft is at cruise. It adds little to thrust at low speed during takeoff and initial climb. In the photos just above (post #6) one can see the openings in the nacelle that admit air to the fixed portion of the fan.

If memory serves, there were two B-52s modified for engine testing. One for the TF-39 and one for the JT9D. The B-52 wing at the inboard engine location is fairly thick and stiff and could handle the torsional loads. The wing outboard is significantly thinner and will twist under the torsion loads of a single big engine. There was a straight forward solution, but SAC didn't like it. Now that ACC owns the aircraft, maybe they'll be more amenable to stiffening the wing. It would seem that using the B-52 as a manned penetrating bomber are over.

[tdracer]

LowObservable, yea you got me - that's the article. I read a lot of stuff from WSJ and Av Week, I got confused :O
KenV The fan 'snubbers' as you call them are not unique to the TF39 - they were on all the early big fan engines - JT9D, CF6-6, CF6-50, and the CF6-80A (I'm thinking they are on the CF6-80C2 as well but I wouldn't swear to it). We refer to them as the 'mid span shroud' (also called 'clackers' due to the noise they make as the fan slowing spins in the wind). It wasn't until the wide cord fans started showing up (PW4000/112", GE90, etc.) that the fan blades could be made stiff enough that the shroud wasn't needed.
All fan jet engines have fan exit guide vanes, however the TF39 was unique among big fan engines in having what were effectively fan inlet guide vanes (the old low bypass engines such as the JT8D all used inlet guide vanes). As I noted previously, having fan inlet guide vanes does a good job of mimicking how a siren works, with a similar sound.

Those doors on the inlet are known as 'blow-in doors' (early 747s had them as well). At low speed/high airflow conditions (i.e. the start of takeoff roll) the inlet lip could separate which did nasty things to the fan, so those doors were fitted (the problem was worse on the 747/JT9D since it was designed for very high cruise speed and the inlet lip was very sharp and narrow). There were spring loaded closed, the suction during low speed/high airflow would cause them to open and add additional inlet airflow. The blow-in doors worked fine, but the were also a noise problem - vortices for those doors generated huge noise when they fit the fan. By the time the 747-200 came along, we'd learned how to design big inlets that aren't likely to separate at low airspeed/high airflow (baring big crosswind)so no current production engines need them.

[Turbine D]

tdracer & KenV,

I didn't mean to imply that the unique TF39 noise came only from the stationary fan portion. What I suspect is that some of noise may have also come from the rotating fan blade tips as there was no sound absorbing materials inside the nacelle. We called snubbers "part span shrouds". They are on the entire CF6 family of engines including the -80C. It is an important design feature for airfoil stability as is the aerodynamic shape of them. Most are also custom designed for bird ingestion resistance among other things such as unlatch prevention, etc.

When the CF6-6 engine entered the commercial market on the DC-10, one of the first problems the airlines complained about was the fan blades "clanking" when the wind was turning the fan while passengers were embarking or disembarking using the staircase. The noise frightened the passengers. The clanking was from the part span shrouds hitting against one another, so we had to come up with an anti-clank devise to stop the noise which we did.

[KenV]

tdracer and TurbineD, thanks for all the great info regarding the blow in doors, snubbers/clankers, etc etc. Your insights are appreciated.

[sandiego89]

Thanks guys, some really great insight here- good stuff

- I will miss the "electric" noise, you have no doubt when one spools up.

The clanking was from the part span shrouds hitting against one another, so we had to come up with an anti-clank devise to stop the noise which we did.

I recall some Harrier techs would push a broom handle down the mouth of the Pegasus main compressor to keep it from spinning in the breeze. Guess that solution might cause the pax some alarm. :}

[stilton]

KenV


Aren't you understating the thrust and capability increase from the C5A to the -M model ?


It's not 'a few percent' more it seems to be more than twenty, even derated a jump from 43000 to 50000 pounds of thrust times four is a significant increase in power (like adding more than half another engine)


Plus the effect on climb rate, altitude capability and range are considerable.
Am I missing something ?

[GreenKnight121]

16.28% increase.

[Autobahnstormer]

Sandiego,
they same high-tech fix was applied to the C-5A. When the engines were idle they would spin in the wind causing bearing wear and noise from the christmas tree joints. The solution was a piece of 2X4" with an RBF tag stuck in the intake. I saw it with my own eyes while being given a guided tour, allegedly this problem did not exist with the C-5B.
ABS