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SpaceshipDC10
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CF6-50 start: what is that sound?

Sun May 03, 2020 1:05 pm

It's about at mid-lendth of the video, when the engine is spining ever faster. What produces that growl? I've often wondered when t was more common to hear it.


https://youtu.be/cQH48XCQJDs
 
GalaxyFlyer
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Re: CF6-50 start: what is that sound?

Sun May 03, 2020 1:54 pm

I think it’s transition thru sub-idle vibration range. The engine goes from being balanced on the geometric center (the shafts) to balanced on the center of mass.
 
Okie
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Re: CF6-50 start: what is that sound?

Sun May 03, 2020 2:25 pm

The noise "growl" you hear at about the 50 sec mark is the harmonics of the flame front when the fuel is introduced. You can see the condensation from the air starter up until the turbine becomes self sufficient to accelerate rotational speed up to idle and the starter is dropped out.
The GE 90 on 777's are grumbly as well.

Generally caused by excess O2 which is required to accelerate the turbine.

We have serious issues with harmonics on large steam boilers used to produce electricity whether on gas or oil. Rockets that use RP-1 (kerosene) for fuel have harmonic issues as well.

Okie

.
 
SpaceshipDC10
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Re: CF6-50 start: what is that sound?

Sun May 03, 2020 7:35 pm

Okie wrote:
The noise "growl" you hear at about the 50 sec mark is the harmonics of the flame front when the fuel is introduced. You can see the condensation from the air starter up until the turbine becomes self sufficient to accelerate rotational speed up to idle and the starter is dropped out.


Ok, that very clear, thanks. Btw, is it the rotating fan that is producing the so characteristic sound of the CF6-50?
 
Okie
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Re: CF6-50 start: what is that sound?

Mon May 04, 2020 1:11 am

SpaceshipDC10 wrote:
Ok, that very clear, thanks. Btw, is it the rotating fan that is producing the so characteristic sound of the CF6-50?


The LM2500 turbine which is basically a CF6 is used often in power generation.

The flame front in the burners and entropy waves and vorticity waves through the turbine blades.

Here is one paper. Google is your friend.

http://www.aerospacelab-journal.org/sit ... L07-05.pdf

Okie
 
surrodox2001
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Re: CF6-50 start: what is that sound?

Mon May 04, 2020 2:58 am

Okie wrote:
The noise "growl" you hear at about the 50 sec mark is the harmonics of the flame front when the fuel is introduced. You can see the condensation from the air starter up until the turbine becomes self sufficient to accelerate rotational speed up to idle and the starter is dropped out.
The GE 90 on 777's are grumbly as well.
.


I think the RB211 also has the hum when starting but more visible (exhaust smoke), but I don't sure.
 
surrodox2001
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Re: CF6-50 start: what is that sound?

Mon May 04, 2020 3:01 am

SpaceshipDC10 wrote:
Ok, that very clear, thanks. Btw, is it the rotating fan that is producing the so characteristic sound of the CF6-50?


Are you thinking about the initial start sequence with the whine? It's all present from CF6-6s all the way up to -80E1s (+LM2500) from the videos I heard.
 
JayinKitsap
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Re: CF6-50 start: what is that sound?

Mon May 04, 2020 8:01 am

Okie wrote:
SpaceshipDC10 wrote:
Ok, that very clear, thanks. Btw, is it the rotating fan that is producing the so characteristic sound of the CF6-50?


The LM2500 turbine which is basically a CF6 is used often in power generation.

The flame front in the burners and entropy waves and vorticity waves through the turbine blades.

Here is one paper. Google is your friend.

http://www.aerospacelab-journal.org/sit ... L07-05.pdf

Okie


waves, vibration, cavitation and the like is what makes compressible fluid flow so damn complicated. Scaling a turbine to a different thrust entails a lot of black arts that comes from experience, why so few jet engine manufacturers - GE, RR, P&W, Safran, and Honeywell basically have it cornered. They are the best and it can still take years and years in service to get them running right.
 
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747classic
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Re: CF6-50 start: what is that sound?

Mon May 04, 2020 8:19 am

AFAIK the CF6-6 and 50 series were actually prown for vibration, because of the length of the engine, giving a tendency for the shafts to bend.
Especially after short turn around stops the heat of the engine went upwards while the lower side remained relative cooler. During very fast turn arounds, we motored the engine(s) somewhat longer, before "fuel on " was selected.
When starting the engine sometimes (not always, depending the time on wing and turn around time) also a spike in high speed rotor vibration was noted in the cockpit during engine start.
The consequence of this excessive bending was significant blade rubbing of the blade tips , resulting in a higher rate of performance degradation and higher removal and overhaul rates.
When the degradation became worse, the engine showed also an increased tendency of displaying high speed rotor vibration at TOD, after retarding the P/L and operating in (flight)idle.

The succesor of the CF6-6/50 series, the CF6-80A/C series was for that reason designed shorter.(approx 0,4 meter)
Operating a twin over the ocean, you're always one engine failure from a total emergency.
 
SpaceshipDC10
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Re: CF6-50 start: what is that sound?

Mon May 04, 2020 10:27 am

surrodox2001 wrote:
SpaceshipDC10 wrote:
Ok, that very clear, thanks. Btw, is it the rotating fan that is producing the so characteristic sound of the CF6-50?


Are you thinking about the initial start sequence with the whine? It's all present from CF6-6s all the way up to -80E1s (+LM2500) from the videos I heard.


No, it's the high-pitched sound when the engine is at idle, like at the end of the video, sound that becomes even louder at higher rpm.
 
surrodox2001
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Re: CF6-50 start: what is that sound?

Mon May 04, 2020 10:46 am

SpaceshipDC10 wrote:
surrodox2001 wrote:
SpaceshipDC10 wrote:
Ok, that very clear, thanks. Btw, is it the rotating fan that is producing the so characteristic sound of the CF6-50?


Are you thinking about the initial start sequence with the whine? It's all present from CF6-6s all the way up to -80E1s (+LM2500) from the videos I heard.


No, it's the high-pitched sound when the engine is at idle, like at the end of the video, sound that becomes even louder at higher rpm.


It's probably related to engine design, I heard the whine on CF6-50s on 747/DC10-30 and less on CF6-6 on DC10-10s.
 
Max Q
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Re: CF6-50 start: what is that sound?

Mon May 04, 2020 5:01 pm

747classic wrote:
AFAIK the CF6-6 and 50 series were actually prown for vibration, because of the length of the engine, giving a tendency for the shafts to bend.
Especially after short turn around stops the heat of the engine went upwards while the lower side remained relative cooler. During very fast turn arounds, we motored the engine(s) somewhat longer, before "fuel on " was selected.
When starting the engine sometimes (not always, depending the time on wing and turn around time) also a spike in high speed rotor vibration was noted in the cockpit during engine start.
The consequence of this excessive bending was significant blade rubbing of the blade tips , resulting in a higher rate of performance degradation and higher removal and overhaul rates.
When the degradation became worse, the engine showed also an increased tendency of displaying high speed rotor vibration at TOD, after retarding the P/L and operating in (flight)idle.

The succesor of the CF6-6/50 series, the CF6-80A/C series was for that reason designed shorter.(approx 0,4 meter)



Interesting, I’ve not operated the CF6-50 but have quite a few hours between a pair of -80’s on the 767. A superb engine, it was noticeably more prone to vibration in a certain thrust range than the RR power plant on our 757’s
The best contribution to safety is a competent Pilot.


GGg
 
strfyr51
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Re: CF6-50 start: what is that sound?

Tue May 05, 2020 12:52 am

surrodox2001 wrote:
Okie wrote:
The noise "growl" you hear at about the 50 sec mark is the harmonics of the flame front when the fuel is introduced. You can see the condensation from the air starter up until the turbine becomes self sufficient to accelerate rotational speed up to idle and the starter is dropped out.
The GE 90 on 777's are grumbly as well.
.


I think the RB211 also has the hum when starting but more visible (exhaust smoke), but I don't sure.


There were Fuel nozzle mods that eliminated that smoke, on start. When United first got the L1011-500's from Pan Am the start scared the crap out of most of the Mechanics because it sounded like the engine was going to blow up and with all that smoke? It looked as if it was blowing up! After a couple of engines were ruined with false engine fire signals from Ground guys? We installed new start nozzles in all of them and that eliminated the problem. The start was as smooth as silk from then on.
That airplane was actually a Caddie of airplanes in my book.
 
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767333ER
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Re: CF6-50 start: what is that sound?

Sat May 16, 2020 2:13 am

Every CF6 seems to do some version of this but anything -80A and newer doesn’t do that whine that these older ones do. This rumble I’ve noticed on some 767s for a split second hits just the right frequency to vibrate what seems to be ever panel in the cabin! Definitely a menacing sound.
Been on: 732 733 734 73G 738 752 763 A319 A320 A321 CRJ CR7 CRA/CR9 E145 E175 E190 F28 MD-82 MD-83 C172R C172S P2006T PA-28-180

2 ears for spatial hearing, 2 eyes for depth perception, 2 ears for balance... How did Boeing think 1 sensor was good enough?!

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