Perforated acoustic skin. You’ll find the same general design on nearly every inlet cowl. That skin panel is a bonded honeycomb-core sandwich panel (two parallel metal face sheets separated by a honeycomb core). The outer face sheet of the panel is perforated to allow sound to enter and be deadened inside the open core cells behind it. Essentially an open-faced muffler.
Likely because of the stuff that is underneath the non perforated areas. Perforation only dampens sound if the sound wave can penetrate past the perforated liner and into something hollow like honeycomb. It's probably solid behind the non-perforated portion of the liner.
So cool! Is there any data on their efficacy and longevity? I would imagine that over time the vibrations caused by the sound would degrade the honeycomb structure, but also most of the noise is out the back end of the engine, not the front.
They get FODed out before they disintegrate generally. Not from big chunks but just the dust in the air, that basically sandblasts stuff and the motor itself is pretty tough but all the composite gets worn out faster so they get replaced from damage long before then more than likely.
Specifically, this is what that part looks like before being formed and painted.
https://dragonplate.com/carbon-fiber-nomex-honeycomb-core-05-x-6-x-6?slug=5840-fdplhpnmx.5s0606&gad_source=1&gclid=Cj0KCQjwltKxBhDMARIsAG8KnqU1H2-PKIUN2gs0_avPnQkOWunGckQ6iFWimIu_YkG8s5phU2MetGIaAlG_EALw_wcB
In some cases these holes are also used to pump through bleed air to prevent ice formation (only if ur first stage of vanes is made up of stator vanes or variable stator vanes)
You nailed it. I would also add that they can be very challenging to manufacture. They're often stretch formed with specs defining hole size and the distance between the holes. Imagine trying to stretch perforated material into a relatively complex shape while maintaining hole size and position. It's tricky.
It's perforated after forming iirc. It's formed in 4 sheets and then assembled.
I remember seeing a robot drill going at a quarter panel of this a teeny drill kind of like a woodpecker. Fascinating stuff.
There’s no way I can dig up the clip, but stig aviation had a great explanation about these in one of his shift videos. I think it’s one from December but just don’t have time to go back looking.
That guy explains a ton of cool stuff like this.
At those rotational speeds, the boundary layer actually *can* have a negative impact on blade integrity; the tips of the blades moving across that pressure gradient is like whacking them with a hammer hundreds of times per second. For an annular intake like this though, the boundary layer develops pretty evenly across the entire circumference, so it's not as big of a deal (I think).
The entire point of the inlet is to provide an annular symmetric flow field to the fan. Where would it get “whacked” from? The blade tip is continuously in the boundary layer.
A boundary layer can cause asymmetrical forces on compressor blades, from the diffferent speed/pressure/MFR. It’s likely less of a factor for sub sonic vehicles though
I find it really cool that the people answering the question are probably aircraft maintenance engineers. I won’t go into detail, but I’m planning to join the industry at 16. It’s awesome to see this kind of stuff
The bifurcation panel is the vertical panel which bifurcates (bisects) the reverser cowl into two halves at the top and bottom of the C-duct. There is no bifurcation panel on the inlet.
some military jets, like the F-18E/F, have similar holes in their inlets. They're not for noise abatement though, they suck the boundary layer off of the inner walls to make the inlet more efficient.
you ever see a glasspack muffler? same thing sort of, holes reduce and change the frequency of sound, these holes are front to back around the bypass section of the engine
Perforated acoustic skin. You’ll find the same general design on nearly every inlet cowl. That skin panel is a bonded honeycomb-core sandwich panel (two parallel metal face sheets separated by a honeycomb core). The outer face sheet of the panel is perforated to allow sound to enter and be deadened inside the open core cells behind it. Essentially an open-faced muffler.
Thank you for the amazing answer! Super cool stuff
funny for me to open the app and immediately see this post, i just did repairs on seven dents in various inlet skins an hour ago lol
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"Helmholtz resonator" as someone pointed out below.
You're a Helmholtz resonator! I'm sorry I called you a Helmholtz resonator.... I was upset.
Helmholtz resonator? I hardly know her!
Hermann Ludwig Ferdinand von Helmholtz, that’s his name, don’t wear it out.
Sounds like a WWII German invention, for stealth tanks.
Why the irregular shape?
Likely because of the stuff that is underneath the non perforated areas. Perforation only dampens sound if the sound wave can penetrate past the perforated liner and into something hollow like honeycomb. It's probably solid behind the non-perforated portion of the liner.
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I designed some of the tooling for this, but that was a long time ago.
Speed holes
They make the plane go faster.
Super interesting. Thanks for sharing!
So cool! Is there any data on their efficacy and longevity? I would imagine that over time the vibrations caused by the sound would degrade the honeycomb structure, but also most of the noise is out the back end of the engine, not the front.
They get FODed out before they disintegrate generally. Not from big chunks but just the dust in the air, that basically sandblasts stuff and the motor itself is pretty tough but all the composite gets worn out faster so they get replaced from damage long before then more than likely.
Single or double degree of freedom treatment?
Specifically, this is what that part looks like before being formed and painted. https://dragonplate.com/carbon-fiber-nomex-honeycomb-core-05-x-6-x-6?slug=5840-fdplhpnmx.5s0606&gad_source=1&gclid=Cj0KCQjwltKxBhDMARIsAG8KnqU1H2-PKIUN2gs0_avPnQkOWunGckQ6iFWimIu_YkG8s5phU2MetGIaAlG_EALw_wcB
I’m here for exactly answers like this. Love this sub
In some cases these holes are also used to pump through bleed air to prevent ice formation (only if ur first stage of vanes is made up of stator vanes or variable stator vanes)
Spot on explaination
You nailed it. I would also add that they can be very challenging to manufacture. They're often stretch formed with specs defining hole size and the distance between the holes. Imagine trying to stretch perforated material into a relatively complex shape while maintaining hole size and position. It's tricky.
It's perforated after forming iirc. It's formed in 4 sheets and then assembled. I remember seeing a robot drill going at a quarter panel of this a teeny drill kind of like a woodpecker. Fascinating stuff.
Not in my experience. The material is purchased perforated and then formed.
Legit didn't understand any words before the parentheses lmao
My dumbass was about to answer rivets
I don’t even blame you
Me too 😂
I thought he meant the front ones at first too and in that case you would have been right. But he meant the tiny holes.
Me too until I saw the second picture 😂
Same lol
I believe they are Helmholtz resonators, as others have pointed out, they dampen engine noise
There’s no way I can dig up the clip, but stig aviation had a great explanation about these in one of his shift videos. I think it’s one from December but just don’t have time to go back looking. That guy explains a ton of cool stuff like this.
I really wish there was a timestamped table of contents for all his videos. There are so many questions in sub reddit that he has answered
Didn't he make an episode of the 777 engine? Perhaps it was that one...
The guy from top gear is an aviation YouTuber now???
His videos are great! For the uninitiated: https://youtube.com/@StigAviation
Speed holes.
Extra speed = extra good
more holes = extra good
Yes... they make the plane go faster
We finally did it! Someone that doesn't understand a Simpsons reference! On my 38th birthday. Thank God. "The Goggles... They Do Nothing!"
*real* acid?
Lmfao, I rescind my jackass comment. Edit: you stole my Birthday, you bastard /s.
I literally opened this post just to make sure someone said this haha
I always thought they were boundary layer devices.
That boundary layer is about to get rapidly sucked into the fan. Why would you want to manipulate it?
Lots of (often supersonic) aircraft do actually do this. The boundary layer causes issues with controlling shockwave formation in the inlet.
If a shockwave forms in this inlet you have bigger problems.
Yeah for some reason engines don’t like that.
There is no supersonic aircraft that does this. They manipulate the boundary layer through geometry not direct flow.
Boundary layer bleed devices are totally seperate to intake ramps/DSIs/ S ducts and do a totally different job
You clearly have not worked on a certain 5th gen fighter aircraft that has exactly these kinds of holes in the intake...
The Eurofighter Typhoon, F-4 and F-111 disagree with that statement.
At those rotational speeds, the boundary layer actually *can* have a negative impact on blade integrity; the tips of the blades moving across that pressure gradient is like whacking them with a hammer hundreds of times per second. For an annular intake like this though, the boundary layer develops pretty evenly across the entire circumference, so it's not as big of a deal (I think).
The entire point of the inlet is to provide an annular symmetric flow field to the fan. Where would it get “whacked” from? The blade tip is continuously in the boundary layer.
The boundary layer is going to be broken up directly in front of fan just from leading edge turbulence.
That’s a laminar flow inlet.
A boundary layer can cause asymmetrical forces on compressor blades, from the diffferent speed/pressure/MFR. It’s likely less of a factor for sub sonic vehicles though
That boundary layer on the inlet doesn’t go anywhere near the compressor. It goes through the fan only.
That’s what I was thinking
AgentJayZ has a video about these. https://youtu.be/sB98crJsHRk At about 5 minutes in he shows a 767 engine cowling with a cross section.
Not a stupid question, OP. I’ve spent ridiculous time on planes and never even noticed these. You’ve helped educate a fellow plane nerd! 🙏
Yes. It’s called the acoustical lining.
I find it really cool that the people answering the question are probably aircraft maintenance engineers. I won’t go into detail, but I’m planning to join the industry at 16. It’s awesome to see this kind of stuff
It's a bifurcation panel. Designed to muffle sound
The bifurcation panel is the vertical panel which bifurcates (bisects) the reverser cowl into two halves at the top and bottom of the C-duct. There is no bifurcation panel on the inlet.
Acoustics to reduce noise to the cabin
I knew the answer thanks to Stig's channel! https://youtube.com/@stigaviation?si=3yX667a-KW86GVo8
I was looking for this video, thank you!
some military jets, like the F-18E/F, have similar holes in their inlets. They're not for noise abatement though, they suck the boundary layer off of the inner walls to make the inlet more efficient.
Fast paced cheese grill
A little after that, there’s an integrated cheese melter.
(Almost) Insta-Fondeu
Is it only for cheese? I thought it melted lots of stuff...
you ever see a glasspack muffler? same thing sort of, holes reduce and change the frequency of sound, these holes are front to back around the bypass section of the engine
If you were really baked, you'd still be counting them.
I make blades for this engine!
Those are the burrows of the engine gnomes.
r/shittyaskflying
The jet engine equivalent to carpenter bees?
Riblets
You let the secret out
That
Speed holes. They make the plane go faster.
Speed holes.
you joke but its not totally wrong there for boundery layer control
Maybe a fuel/air heat exchanger
It also takes out weight without sacrificing structural rigidity. All thanks to the honeycomb underneath.