How critical is perforation direction to acoustic damping? 3

[KirbyWan]

Howdy all,

So for nacelle structure that have perforated skins for acoustic damping, how critical is the direction of the perforation to the acoustic damping effect?

Usually the perforation is a 60° pattern so there is a direction where the holes are closely spaced and a direction 90° to this where the holes are staggered on every other row. Currently I'm dealing with fan case liner panels and I took a survey of the articles we have in house and about 75% of them have the perforation direction so the line of closely spaced holes is in the airflow direction and I believe this is the OEM configuration. The other 25% have the perforation pattern so the airflow is in the direction of staggered holes. I'm not sure if the OEM design allows both or if over years of service and repair, face sheets were replaced without holding to this convention. We do, I've made our bond shop take perf sheet off and redo it in the correct direction, but I'm wondering if I'm being overly conservative. Any people with acoustic damping expertise who can weigh in on this?

Thanks,

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[WKTaylor]

KW... THIS is rocket science stuff... Reading the intros to a few reports on sonic structure bleed-off made my head swim.

First question... are ALL your nacelle structures for EXACTLY the same engine configuration... or are there theme/variations for the engine models? IF all engine models are exactly the same... then I would expect to see exactly identical perforated sheet material//orientation engine-to-engine. IF not the same engine models thane the trend should be exactly the same for 'like models'. IF the engines are identical models, and the hole pattern, is different... the Your comment regarding repair/overhaul spares is likely true.., vendor got something wrong.

For the jets I've worked-on, each engine model had specified perforated skin/septum design combinations. The drawings should be very clear regarding the exact perforated skin specification which includes hole sizes and layout pattern. One of the greatest factor is hole-size/density per square-inch. Inevitably this leads to a unique spec material for size/pattern-layout/hole-edge-quality/sheet-thickness etc... along with the specifics of the honeycomb-septum design behind/below the perforated skin. This 'system' has to account for primer/topcoat paint... which creeps over the hole edges... and also for random hole blockages due to potting or 'in-reticulated' adhesive or honeycomb, etc.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[KirbyWan]

Wil,

We're working on CF6-50's at the moment. There is an old style liner segment that has .063" diameter holes on 0.155" centers with double diamond core, an older style and the newer style of liner segment uses .073" diameter holes on 0.107" centers, more than twice as dense, with a honeycomb core. The 75% figure I called out was for the exact same part number, and considering the age, these have been through multiple shops. Though I have seen the same rough variation on CF6-80 liner segments and the liner segments that get installed in translating sleeves and fixed structures of thrust reversers. We don't have drawing outside what we have RE'd based on parts received and maybe a purchased OEM panel.

My understanding was that acoustic damping was more like black magic where there were some general rules of thumb and then you need to go test to see if they work in each specific situation. Considering the same perforation pattern, only turned 90° they would have the same hole size and density per area. My thought was that if the air flow disturbance of one hole impinged on the next hole at roughly the dimension of the holes it could cause an effect. If the disturbance was much larger or smaller any differences would be averaged out and not make any difference. But this would be more of a drag issue and not an acoustic issue. I'm guessing with those acoustic panels that have wire mesh bonded there would be no change in acoustic or air resistance regardless of the perforation orientation.

Thanks for your response.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[WKTaylor]

KW...

The net effect of the tiny/smooth perforations on air-flow is 'none/negligible'... they are simply there to allow acoustic energy to bleed into the septum.

There are 'other issues' with the perforated skin that create long term mechanical/structural problems, such as moisture [rain/wash-water, snow, etc] that can enter the septum and accumulate/freeze at altitude... but this rarely affects acoustic properties... until massive blockage occurs.

Unsure about why You are seeing variations within model-type... except as a 'spares' issue. The slightly larger holes for a higher thrust variant seems rational. Hopefully these variations [if unintended by spares] were allowed by engineering disposition by the Nacelle OEM.



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[rb1957]

I'd talk to the OEM, if they'll talk to you is a different question !?

another day in paradise, or is paradise one day closer ?

 

[KirbyWan]

Wil,

The parts are listed in the IPC with the new style "replaces" the old style that uses the double diamond core and the GE IPC says that means they are fully interchangeable. Maybe the double diamond core had better acoustic performance with less open area.

RB1957,

I've never had good luck getting information out of GE.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[GregLocock]

Are these helmholtz style absorbers? That is the mass of the slug of air in the hole acts as a piston on the elasticity of the air in the chamber behind?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[KirbyWan]

GregLocock,

I know not of what you speak. At least not yet. I'll look this up and learn about it. Thank you.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[WKTaylor]

Most engine OEMs do not design the nacelle components for their engines... but simply install the airframer-supplied nacelle parts as needed to test-run new/overhauled engines.

In the case of the CFM56 folks... when I had similar questions... the service folks redirected me to the airframe manufacturer that did the original re-engine mod and designed all the nacelle parts/pieces [corporate mod-drawings]... they were the experts!

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[LiftDivergence]

Kirby,

I don't think you are being too conservative by having them redo the perforated face sheet to have the same direction as OEM.

For the aircraft models/minor models I've worked on, as I'm sure you're aware, there are allowable damage limitations and repair options specifically for the perforated face sheets of the acoustic panels. As Wil pointed out - this is in the SRM from the airframe OEM rather than the engine manufacturer, usually ATA 54.

Looking at this (again for A/C I have data for) they are very specific about total area covered by repairs or affected by allowable damage removal since of course they have to comply with noise limits in the FARs. They don't want to block too many of the 1. Face sheet holes or 2. core cells.

I have seen one statement "the acoustic panel area loss limits are related the the maximum estimated increase in noise of 0.1 effective perceived noise, decibels (EPNdB)".

I strongly suspect that this is mostly a net perforation area thing, probably the exact orientation of the holes is irrelevant. However, not being OEM we can't say for sure how those limits were developed. They may have tested the limits of their noise compliance with certain assumed blocked perforation area, and this would have been for the OEM hole orientation.

I really have no inherent barometer for "0.1 EPNdB" so this could be a pretty tight "margin". To maximize future repair-ability with confidence I would always want the hole orientation to match the as-delivered configuration.

Just my two cents!

Keep em' Flying
//Fight Corrosion!

 

[KirbyWan]

LiftDivergence,

Thanks for your response. I bet I can do a search on EPNdB and get some useful scholarly articles that will teach me something about the black magic that is acoustics.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[WKTaylor]

KW, LD... From what I can tell, hole density/inch^2 and %-open/inch^2 is actually far-more important than the hole-pattern.

Hole-pattern/layout is how the holes of various Dia's are integrated into position.

ONE CAUTION NOTE.
Hole pattern/Dia' MAY have a (+) or (-) [potential] effect for sheet strength/durability.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]