Airliner question

Let'sgoflying!

Let'sgoflying!

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Dave Taylor
On several of my recent airline flights I noticed a sound in the cabin that is certainly reminiscent of the harmonics of engine speed asynchronicity.
Like a prop plane that has the sync switch off.
I think this was on a 757, can look it up.
On the little CJ I fly, we have a switch that claims to sync the fan, or the turbine - but no one can tell any difference if it is in either position, or off.

Do larger turbofans have any kind of engine sync capability?
What else could sound like two large engines but with a frequency that waxes and wanes for hours, with a periodicity of 5-6 seconds?
 
I don’t remember seeing one in 727-200s, 737-200s, 757/767-200/300s, A330-200/300s, L1011-100/200/500s or S-3Bs.

There’s one in MD-88s, but not sure about the MD-90s (very different motors).

I put the S-3 in there since I believe it has a motor from the same family as the CJ you fly. I do remember on long cross countries fiddling with the throttles so they wouldn’t do that async thingy.

And I definitely remember having no clue if the thing actually worked in the -88, we just flipped the switch as part of the very piloty 7 step after takeoff checklist.

My 1967 Chris Craft Commander has a thing to sync up the motors… Chevy 327s.
 
Let'sgoflying! said:
On several of my recent airline flights I noticed a sound in the cabin that is certainly reminiscent of the harmonics of engine speed asynchronicity.
Like a prop plane that has the sync switch off.
I think this was on a 757, can look it up.
On the little CJ I fly, we have a switch that claims to sync the fan, or the turbine - but no one can tell any difference if it is in either position, or off.

Do larger turbofans have any kind of engine sync capability?
What else could sound like two large engines but with a frequency that waxes and wanes for hours, with a periodicity of 5-6 seconds?
Click to expand...

No such system on A310/300, B777 or B727 (except for the FE). Perhaps an Air Cycle Unit?

Were there voices, too?
 
Keep in mind that there are two (or more) spools on each engine, so syncing one doesn’t necessarily mean the other will be synced.
 
MauleSkinner said:
Keep in mind that there are two (or more) spools on each engine, so syncing one doesn’t necessarily mean the other will be synced.
Click to expand...
I’m having trouble picturing that. So here’s what I think I know about jets. The front thing compresses the air and makes it hot enough to ignite fuel. Then you squirt some fuel in there, it burns and turns the thing in the back, which is connected to the thing in the front to keep it turning and compressing. So is there some kind of Clutch, like a Tourque Converter between them?
 
luvflyin said:
I’m having trouble picturing that. So here’s what I think I know about jets. The front thing compresses the air and makes it hot enough to ignite fuel. Then you squirt some fuel in there, it burns and turns the thing in the back, which is connected to the thing in the front to keep it turning and compressing. So is there some kind of Clutch, like a Tourque Converter between them?
Click to expand...
There’s a shaft between the thing in the back and the thing in the front, but there are two sets of things that have concentric shafts, and airflow created by the one thing drives the other thing.
 
MauleSkinner said:
There’s a shaft between the thing in the back and the thing in the front, but there are two sets of things that have concentric shafts, and airflow created by the one thing drives the other thing.
Click to expand...
Ok. So like a Torque Converter, right?
 
luvflyin said:
I’m having trouble picturing that. So here’s what I think I know about jets. The front thing compresses the air and makes it hot enough to ignite fuel. Then you squirt some fuel in there, it burns and turns the thing in the back, which is connected to the thing in the front to keep it turning and compressing. So is there some kind of Clutch, like a Tourque Converter between them?
Click to expand...
You have the basic idea. No imagine that shaft connecting the front and back thingies has a straw that the shaft is stuck trough. That straw also has some front thingies compressing the air and things in the back to make it turn.

the straw is one spool. The shaft is a different spool. They don’t turn the same speed
 
MauleSkinner said:
Probably similar enough to say yes.
Click to expand...
Ok. Another thing I’m wondering about. Is there any reason to sync the fans other than annoying noise? I’m kinda thinking about the Lockheed Electra thing. Obviously an extreme analogy, but is there a reason other than noise?
 
luvflyin said:
Ok. Another thing I’m wondering about. Is there any reason to sync the fans other than annoying noise? I’m kinda thinking about the Lockheed Electra thing. Obviously an extreme analogy, but is there a reason other than noise?
Click to expand...
That’s about it for all of the airplanes I’ve flown.
 
luvflyin said:
Ok. Another thing I’m wondering about. Is there any reason to sync the fans other than annoying noise? I’m kinda thinking about the Lockheed Electra thing. Obviously an extreme analogy, but is there a reason other than noise?
Click to expand...

i read a paper on harmonics this morning about that.
In some cases, yes.

for the other question about physical connection between compressor & turbine… “Free Turbine”
I have seen where a person can hold the prop stationary while the turbine is started…for a while.
 
luvflyin said:
Ok. So like a Torque Converter, right?
Click to expand...

No, like a drive shaft. No slip.

And two shafts. One for the compressed are that has fuel injected and burns, the other for a lot of air going around the outside for much of the thrust.

This is for a turbo FAN engine. A turbo JET only has one shaft.
 
Let'sgoflying! said:
i read a paper on harmonics this morning about that.
In some cases, yes.

for the other question about physical connection between compressor & turbine… “Free Turbine”
I have seen where a person can hold the prop stationary while the turbine is started…for a while.
Click to expand...

For turbo prop engines. All turbofan engines are free turbine. You can see them spin in the wind when shut down.
 
Some if not most jets have two sets of thingys.

The torque converter analogy applies to two SETS of thingys, not between the front thingy and back thingy. So basically between one back thingy and the other back thingy.

On the -88 you had your choice of which set of thingys to sync. Like it mattered….
 
Pinecone said:
No, like a drive shaft. No slip.

And two shafts. One for the compressed are that has fuel injected and burns, the other for a lot of air going around the outside for much of the thrust.

This is for a turbo FAN engine. A turbo JET only has one shaft.
Click to expand...
Turbojets can and have had multiple shafts. The difference between a turbofan and a turbojet is that in a turbofan there is bypass air that does not go through the core compressor, combustor, and turbine. Bypass air only goes through the fan, gains some energy by doing so, and then is accelerated through the fan nozzle to produce thrust. Modern turbofans produce some 85-90% of their thrust from the fan flow only, the core flow contributing a small amount but primarily there to power the fan. Simply put, to produce the same amount of thrust it’s much more efficient to accelerate a lot of air a little than to accelerate a little air a lot (in technical terms this is the propulsive efficiency)
 
The 757 does not automatically sync the fan speeds. Once the after takeoff items are done I always sync them up by adjusting one of the thrust levers otherwise you can hear what your describing and I find it very annoying.
The 767 the N1s are synced and as long as the thrust levers aren’t obviously split they stay equal.
 
I've heard that riding in the back of airliners. I figure sometimes it might be some harmonic between an engine and some other thing...a cabin air fan, a pump, etc...
 
I’m a 757 pilot. We do not have sync for the engines. Some airplanes it’s less noticeable than others, but sometimes upfront It’s noticeable even if we are just barely out of sync. I’ve noticed it’s a bit more noticeable on the PW 757’s v the RR 757’s. But it’s noticeable in both, typically we will just match up the engines, but manually as there’s no sync
 
AKiss20 said:
Turbojets can and have had multiple shafts. The difference between a turbofan and a turbojet is that in a turbofan there is bypass air that does not go through the core compressor, combustor, and turbine. Bypass air only goes through the fan, gains some energy by doing so, and then is accelerated through the fan nozzle to produce thrust. Modern turbofans produce some 85-90% of their thrust from the fan flow only, the core flow contributing a small amount but primarily there to power the fan. Simply put, to produce the same amount of thrust it’s much more efficient to accelerate a lot of air a little than to accelerate a little air a lot (in technical terms this is the propulsive efficiency)
Click to expand...

Which turbojet engines have multiple shafts
 
X3 Skier said:
Pratt and Whitney J57 and J75 and their civilian derivatives JT3 and JT4 are two ubiquitous engines of the 60’s and later.
Click to expand...
Yup.

In the quest for higher pressure ratios in the 60s, GE continued down the single shaft architecture with variable geometry route with as many as 17 stages on the J79. PW invested instead in the two shaft architecture with its J57 and J75 (JT3C being the civilian designation of the J75) engines to solve the matching problem. Later of course both companies would use both technologies as we entered the turbofan era of the 70s to today, with the addition of interstage and inter compressor bleeds as well. Compressor matching is a tough problem.
 
Pinecone said:
Which turbo fan engine is not free turbine??
Click to expand...
"Free turbine" isn't a term that is applicable to a turbofan or tubojet engine at all really, it is only meaningful in the context of turboshafts.

In a turboshaft the point is to extract power via a shaft to run some kind of process (a gearbox & propeller for an aviation engine, a gearbox & propeller for marine engines, an electrical generator for land-based gas turbine power plants). That shaft is powered by a power turbine which requires a gas generator/core to produce the high energy gasses that drive the turbine. The gas generator fundamentally consists of a core compressor(s), a combustor, and a core turbine(s) (which powers the core compressor). The remaining energy in the gasses after the core turbine(s) is what is used to drive the power turbine. "Free" in this context refers to whether the power turbine is mechanically on the same shaft as the core compressor(s) and turbine(s). Free turbine designs have the power turbine on an entirely separate shaft than the core compressor(s) and turbine(s), as opposed to being on the same shaft. In a free turbine design the power turbine can run at a different rotational speed than the core shaft which is a large design advantage (and why you see that basically all turboshaft engines today are free turbine designs). Land-based gas turbines for power generation, however, are often not free-turbine designs like the LM6000 which is an aero-derivative of the aviation CF6 engine. In that gas turbine, the power turbine that drives the electrical generator is on the low-speed shaft directly and the entire LP shaft spins at 3600RPM (60Hz).

In a turbojet or turbofan there is no process power extraction via a shaft, all turbines are used to power core compressors (and a fan in the case of a turbofan). Even in 3 spool turbofans, there is always a core compressor on each shaft (the fan and low-pressure compressor on the low speed shaft, the intermediate-pressure compressor on the intermediate speed shaft, and the high-pressure compressor on the high- speed shaft). Even if you built a turbofan where the fan was on an entirely different shaft that was powered by a "fan turbine", I would hesitate to call that a "free turbine" design as the fan itself does feed air to the core and is considered part of the core compression process. But at this point it is somewhat semantics because there is no separate process shaft. Free turbine vs not is all about the power turbine that runs the process shaft. Note that I have occasionally heard the opposite of a free turbine design referred to as a "through turbine" design but am not sure if that is an actual term of art. I have not worked with turboshafts much, all of my work being focused on turbofans.
 
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Minor quibble. All aircraft turbojet/turbofan engines I worked on extracted power from the low spool to drive alternator/generator and other auxiliaries like engine fuel pumps and hydro mechanical fuel controls mounted on a gearbox or shaft, albeit minuscule compared to that required to drove the high spool or fan.

Cheers
 
Fan - the part you see in the front
Compressor- located behind the fan and behind the inner most part of the fan
Turbine- located in the rear and can only be seen by looking in the tail pipe (not recommended during engine operation)

there are multiple turbine wheels in sets. One set spins the fan, the other spins the compressor
 
X3 Skier said:
Minor quibble. All aircraft turbojet/turbofan engines I worked on extracted power from the low spool to drive alternator/generator and other auxiliaries like engine fuel pumps and hydro mechanical fuel controls mounted on a gearbox or shaft, albeit minuscule compared to that required to drove the high spool or fan.

Cheers
Click to expand...
Yeah I glossed over that because customer power extraction, while basically omnipresent in turbofans, isn't really an architecture thing. Didn't want to muddy the waters.
 
texasclouds said:
Maybe the spark plugs were getting worn? Or a failed thermistercombodulation valve. Either way, sounds like you were on a budget airline.
Click to expand...

jet engines do indeed have spark (plugs) igniters

no familiar with at that other thing though
 
Timbeck2 said:
Turbine- located in the rear and can only be seen by looking in the tail pipe (not recommended during engine operation)
Click to expand...
Reminds me of one of my first jobs as a young engineer of the F-111 Program. The TF30 engine had a bad habit of having a minor fire in the engine after shutdown. My job was to go out to Nellis AFB and check it out.

This consisted of waiting on the ramp until a F-111 parked and then looking up the tailpipes to see the flash of the fire (if any). Apparently young engineers were considered somewhat expendable. However, no damage to engine, airframe or me during this process.

The solution was to adjust the fuel pressurization valve which was dumping fuel into the combustor when the residual heat affected the the setting. I do blame this experience from the premature hair loss later in life:D

Cheers
 
luvflyin said:
I’m having trouble picturing that. So here’s what I think I know about jets. The front thing compresses the air and makes it hot enough to ignite fuel. Then you squirt some fuel in there, it burns and turns the thing in the back, which is connected to the thing in the front to keep it turning and compressing. So is there some kind of Clutch, like a Tourque Converter between them?
Click to expand...

Jets don't use compression ignition, at least not any that I'm familiar with. The compressor and turbine are directly connected, like a direct drive propeller.
 
X3 Skier said:
Reminds me of one of my first jobs as a young engineer of the F-111 Program. The TF30 engine had a bad habit of having a minor fire in the engine after shutdown. My job was to go out to Nellis AFB and check it out.

This consisted of waiting on the ramp until a F-111 parked and then looking up the tailpipes to see the flash of the fire (if any). Apparently young engineers were considered somewhat expendable. However, no damage to engine, airframe or me during this process.

The solution was to adjust the fuel pressurization valve which was dumping fuel into the combustor when the residual heat affected the the setting. I do blame this experience from the premature hair loss later in life:D

Cheers
Click to expand...

Ah yes, the P and D valve. I worked on the J-69, J-85, J-79, F-110, and P&W 200. I also worked when you didn’t have to catch the fuel from said P&D on shut down.
 
747-400/747-8 does not have any sort of engine synchronization system that I'm aware of, although the auto throttle system, which is engaged for 99.99% of every flight (usually only turned off during the last few hundred feet before landing) does match the N1 (fan) RPM of all 4 engines which usually results in no noticeable harmonic vibration. Although it has been over 10 years since I flew it, I seem to remember that the EMB-145 did have a synchronization system through the engine FADECS even though the airplane didn't have autothrottle. If the thrust levers were matched to a relatively close angle by the pilot the system would tweak engine thrust to minimize any vibration.
 
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