Strange engine sound during start up and shut down

[Capt Speirs 52]

I have been around quite a few different types of jet and turbine engines and don't recall ever hearing this sound before (xPMDG737NGX_JET62e.wav and xPMDG737NGX_JET61e.wav) in the PMDG 737-800 sound folder. This sound is very prominent during the last phase of shut down and initial phase of start up. The PMDG 777-200 and 777-300 do it also, I suspect it is quite authentic, since PMDG went to great lengths to put this sound in. I find the sound makes the engine sound as if it has FOD in it. For now I have replaced the sound files with silence until I can figure what it is. Anyone know what this sound represents?

[kevinh 409]

Sounds like the fan blades rattling in their mountings as the shaft rotates slowly. This is normal, the fan blades are mounted in slots in the disk and are slightly loose until the fan gets up to speed and centrifugal force holds them in place.

 

I've not listened to a start externally so never heard this in the simulation. Extraordinary PMDG have captured this.

[scandinavian13 5,192]

I'd have to check it out when I get home, but if it's what Kevin is saying (and I don't doubt it), it's pretty normal for engines to make this noise when not at normal running speed.

 

If you've ever spent any time out on the ramp on a windy day, you'll know that this is definitely the true:

 

If you let the video run into the next video, it should be a guy explaining this with an RB211.

[PMDG777 231]

 

 

This is normal, the fan blades are mounted in slots in the disk and are slightly loose until the fan gets up to speed and centrifugal force holds them in place.

 

Centrifugal force acts outwards on the blades. What do you mean by "loose"? Surely the blades can't be loose in the radial direction?

[kevinh 409]

Centrifugal force acts outwards on the blades. What do you mean by "loose"? Surely the blades can't be loose in the radial direction?

The blade root has a flange on the end, so centrifugal force can never pull the blade out of the slot. You can just see it in the picture on this page.

 

http://www.docstoc.com/docs/71759264/CFMI-CFM56-3-STAGE-1-FAN-BLADE

[KriVa 307]

Centrifugal force acts outwards on the blades. What do you mean by "loose"? Surely the blades can't be loose in the radial direction?

 

They actually are... If only slightly. The dovetail on the fanblade is ever so slightly smaller than the mounting it's fitted in. That's what produces the very characteristic rattling sound.

[Jack C 8]

It's actually great that stupid the rattling. As soon as the blades heat up they expand and fit snugly and no more rattling

[KriVa 307]

The fact that they don't rattle once the engine spools up is not due to them heating up, but due to the centrifugal forces exhibited on them.

They actually need to be rather loose, even when rotating. This comes from the fact that they pass through the resonant frequency when spooling up. If they were fixed to the engine, the engine would vibrate severely.

A second reason why they are loose in the fitting, is because the rotor can balance itself out this way. (We're talking about tiny amounts of mass here).

However, due them them being (somewhat) loose, they can vibrate in their fittings, causing the engine no harm.

If the fan blades heat up enough to fill the gaps between blade and and disk, you've got bigger problems than them rattling. (Talking about fan blades here, not compressor or turbine blades, those do actually heat up and expand quite a bit, relatively speaking.)

 

Oh, and also: Full names are required on the PMDG subforums, as per the forum rules.

[kevinh 409]

Turbine and compressor blade roots do expand with heat as well as move under centrifugal force. They too are loose when the engine first starts, but you probably wouldn't hear them rattling from outside the engine. Fan blades wouldn't get very hot so thermal expansion isn't so much of a factor.

 

Apart from the resonance issues Thomas mentioned, blades need to be easily removable for maintenance purposes. Bolting them to the disk would be impractical as well as prone to the blades being thrown off at high RPM. Mounting them in slots in the compressor and turbine disks makes for a very secure installation yet with straightforward assembly and disassembly.

[Capt Speirs 52]

So basically what I am getting is that the sound in the sim of the "Blade Clappers" seems a bit exaggerated. I will cut the volume down to (in my opinion) a more believable sound level. After all it is about immersion not realism, what? Thanks all for your inputs, I feel better about that ever so annoying sound of the "Clappers".

[PMDG777 231]

Some really great info on this thread learnt a lot just reading it! It all makes perfect sense!

[scandinavian13 5,192]

So basically what I am getting is that the sound in the sim of the "Blade Clappers" seems a bit exaggerated. I will cut the volume down to (in my opinion) a more believable sound level. After all it is about immersion not realism, what? Thanks all for your inputs, I feel better about that ever so annoying sound of the "Clappers".

 

Honestly, you're better off leaving it alone. Immersion is at its best when you're getting the true, full experience, and not the experience you think you should be getting. If that were the case, most people would be getting a "realistic" experience of the flight deck from the perspective of seat 10A...

[Tabs 2,790]

Centrifugal force acts outwards on the blades. What do you mean by "loose"? Surely the blades can't be loose in the radial direction?

 

Just an aside here since I vividly remember my mechanics professor telling the class we'd all get zeroes if we wrote "centrifugal force" on an exam:

 

There's no such thing as centrifugal force. The acceleration in circular motion is actually inward toward the center of rotation. The apparently "force" outward is actually just a consequence of Newton's First Law (inertia), not a force that's acting on the body.

 

http://www.physicsclassroom.com/class/circles/Lesson-1/The-Forbidden-F-Word

[Capt Speirs 52]

Honestly, you're better off leaving it alone. Immersion is at its best when you're getting the true, full experience, and not the experience you think you should be getting. If that were the case, most people would be getting a "realistic" experience of the flight deck from the perspective of seat 10A...

I just got off the phone with a friend of mine that flies a 767 and told me that the "Clackers" as he calls them are not very noticeable on the walk around until you get right up to the engines (and the wind is blowing them around) because of all the ambient noise on the tarmac. Inside the cockpit you don't really hear them. He also said that they don't make noise unless the engines have been off for hours or overnight. When they are hot they don't make noise. So with that said, I'm on a mission to quite them down. He is scheduling a visit here next week at that point I will let him hear the PMDG 737 and see what he thinks.

[PMDG777 231]

Just an aside here since I vividly remember my mechanics professor telling the class we'd all get zeroes if we wrote "centrifugal force" on an exam:

 

There's no such thing as centrifugal force. The acceleration in circular motion is actually inward toward the center of rotation. The apparently "force" outward is actually just a consequence of Newton's First Law (inertia), not a force that's acting on the body.

 

http://www.physicsclassroom.com/class/circles/Lesson-1/The-Forbidden-F-Word

 

Yes it's the centripetal force which acts towards the center of the circle. I guess centrifugal force is the fan blades wanting to move tangentially to the circle hence being "thrown outwards".