# Quiet Flight

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#### GESchwarz

##### Well-Known Member
Dauntless,

Your post has got me thinking. Because my cockpit cage is welded steel tube and carries all of the structural loads, I am free to skin it any way I want without concern for carrying primary structural loads. I'm thinking of skinning it with two layers of real thin skin, like .010" and bonding them together with an elastomeric mastic. The adhesive would be applied in a grid pattern so that there is a high percentage of airspace. The airspace would breath to the atmosphere so that there would be no peel forces applied by any trapped air at high altitude.

It seems to me that this type of sandwich may be a very efficient sound barrier, and light weight too.

I think .010" aluminum is probably available by special order. Maybe the two skins should be of disimilar thicknesses so they resonate at different frequencies, thereby better keeping in line with the principle of dissimilar materials. So hows about .016” on the outside and .010” on the inside… That would be a little more dent resistant.

#### Dauntless

##### Well-Known Member
The exhaust manifold heat shield of my Mazda rotary engine is made of two thin layers of stamp-formed sheet steel, joined along the perimeter only. This is a sandwich of steel/air/steel. When you tap it, it makes very little sound in response, aka a dead sound, no ring to it like you would expect from a single piece of steel. So I agree with you on the disimilar sandwich idea.
Yep, sounds like Mazda were thinking along the same lines. It wasn't my idea, though. I read about it more than 10 years ago when I started working on my RV. Search the web on 'sound attenuation dissimilar materials' for lots of sources of more info.

Dauntless,

Your post has got me thinking. Because my cockpit cage is welded steel tube and carries all of the structural loads, I am free to skin it any way I want without concern for carrying primary structural loads. I'm thinking of skinning it with two layers of real thin skin, like .010" and bonding them together with an elastomeric mastic. The adhesive would be applied in a grid pattern so that there is a high percentage of airspace. The airspace would breath to the atmosphere so that there would be no peel forces applied by any trapped air at high altitude.

It seems to me that this type of sandwich may be a very efficient sound barrier, and light weight too.

I think .010" aluminum is probably available by special order. Maybe the two skins should be of disimilar thicknesses so they resonate at different frequencies, thereby better keeping in line with the principle of dissimilar materials. So hows about .016” on the outside and .010” on the inside… That would be a little more dent resistant.
Sounds like that would work fine, but it also sounds potentially expensive and a bit difficult. Why not try something easier and cheaper as an experiment? Say, kevlar and fiberglass skins sandwiching half-inch or one-inch white foam? Six ounce fabric all wets out at about 10-thou in my experience, and such a sandwich will weigh next to nothing, can be bent into shape (within reason) and taped into place to see if it helps. Just peel the skin off both sides of the foam and wet out the fabric directly onto the foam. The epoxy will bond the fabric to the foam...just don't use vinylester or polyester resin on EPS foam! :gig:

#### GESchwarz

##### Well-Known Member
I'm trying to minimize the flammables in the cockpit area. EPS foam and gasoline make napalm. In a crash I have no control over what happens between my fuel on board and a whole bunch of EPS foam.

How well do rigid foams like EPS and others perform as sound barriers? I have my doubts. It's my understanding that a good filler material has a lot of give to it in the form of elasticity that allows the vibration to be converted into heat by shear action. A monolithic block of foam seems to be a bit too stiff for that to happen. I'm sure it does well at some part of the frequency range. One thing I am sure of is that you want to minimize the mechanical contact between the two skins of the sandwich, and thus you are maximizing the ability of the two skins to move relative to one another.

Only by testing can we know the truth.

A 4x12 sheet of .016" 6061T6 from Aircraft Spruce costs $18.75/yd, where as .010" Kevlar is$21.75/yd, not counting the resin which is a bit pricy. I was thinking that you couldn’t get much cheaper and easier than bonding two thin sheets of 6061 aluminum using a relatively inexpensive flooring adhesive with a toothed trowel. And when you’re done you have a perfect surface finish, no sanding required. So this sandwich would be composed of tow sheets of aluminum of different thicknesses, elastomeric adhesive, and air space. It would be interesting to do some comparative testing, but I’ve got a plane to build and only so many years to do it in before I croak.

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#### Dauntless

##### Well-Known Member
I'm trying to minimize the flammables in the cockpit area. EPS foam and gasoline make napalm. In a crash I have no control over what happens between my fuel on board and a whole bunch of EPS foam.
Well, I've melted plenty of EPS with gasoline, and there was no napalm-ish remnant. It literally evaporates into nothing. OTOH, there is always pvc foam of you are nervous about fire. PVC is more rigid and pretty much impervious to gasoline.

Besides, if you crash so hard that the sound dampening panel is ruptured, not to mention the fuel tank, I strongly suspect the gasoline turning the foam to napalm is the least of your worries. Just sayin'... :gig:

How well do rigid foams like EPS and others perform as sound barriers? I have my doubts. It's my understanding that a good filler material has a lot of give to it in the form of elasticity that allows the vibration to be converted into heat by shear action. A monolithic block of foam seems to be a bit too stiff for that to happen. I'm sure it does well at some part of the frequency range. One thing I am sure of is that you want to minimize the mechanical contact between the two skins of the sandwich, and thus you are maximizing the ability of the two skins to move relative to one another.

Only by testing can we know the truth.
That's why I tested it on my motorhome. Foam as the core material dampened the sound much more effectively than a hollow-core door.

A 4x12 sheet of .016" 6061T6 from Aircraft Spruce costs $18.75/yd, where as .010" Kevlar is$21.75/yd, not counting the resin which is a bit pricy. I was thinking that you couldn’t get much cheaper and easier than bonding two thin sheets of 6061 aluminum using a relatively inexpensive flooring adhesive with a toothed trowel. And when you’re done you have a perfect surface finish, no sanding required. So this sandwich would be composed of tow sheets of aluminum of different thicknesses, elastomeric adhesive, and air space. It would be interesting to do some comparative testing, but I’ve got a plane to build and only so many years to do it in before I croak.
Okay, here are the material costs for the two techniques in case anyone is interested. I have based it on the cost to make one square yard of the sound deadening sandwich.

Aluminum: $18.75 +$27 = $45.75 (.010" alum. sheet) Composite (kevlar, foam, fiberglass, epoxy):$21.75 + $5 +$8 + $18.95 =$54
(Poly-Poxy from Spruce)

I pay about half that for toughened epoxy, but I buy it in 5-gal cans, so I'll presume worst case scenario.

#### Gnarly Gnu

##### Member
How well do rigid foams like EPS and others perform as sound barriers? I have my doubts.

There are a wide variety of foams but taking a quick glance at EPS without looking at any numbers:

• Mass - it has very low mass (sound isolation poor)
• Stiffness - low stiffness (a positive for sound isolation)
• Damping - fairly poor damping (sound isolation poor)

Therefore you can conclude that EPS on its own is a poor sound barrier. Your doubt was correct. The same simple technique can be applied to any material you are considering... kevlar, fibreglass etc. No need to guess. Of course to be accurate you would need to compare measured physical properties (modulus of elasticity, mass per m2 etc) but start simple.

However Dauntless was proposing to use the EPS as a core material in a sandwich and in such a situation you are not looking for the core material itself to be a sound barrier so much as a means of decoupling the two (or more) barrier layers either side. In other words mass is less critical but you require high damping, low stiffness and ideally a material that can flex within itself so that the sound energy can be dissipated as heat (via friction) within this layer. So EPS is OK but you could do a lot better with other types of foam / rubber / caulk / mastic / glasswool etc material.

#### GESchwarz

##### Well-Known Member
Decouple is the word I should have used to describe what needs to happen between the two skins. I was thnking that a good way to achieve that with a toothed trowel for applying the elastomeric adhesive would be to apply the material in vertical rows on one skin, then let that dry. Apply the adhesive in horizontal rows on the mating skin, then join the two pieces together. This would result in a dot grid pattern bondline cross section; an intentionally poor bond line.

I believe that this would maximize the amount of airspace in the bond line, thereby maximizing the amount of decouple.

Glasswool is an excellent decoupler, but cannot be used to join two skins.

#### Rienk

##### Well-Known Member
Decouple is the word I should have used to describe what needs to happen between the two skins. I was thnking that a good way to achieve that with a toothed trowel for applying the elastomeric adhesive would be to apply the material in vertical rows on one skin, then let that dry. Apply the adhesive in horizontal rows on the mating skin, then join the two pieces together. This would result in a dot grid pattern bondline cross section; an intentionally poor bond line.

I believe that this would maximize the amount of airspace in the bond line, thereby maximizing the amount of decouple.

Glasswool is an excellent decoupler, but cannot be used to join two skins.

Sounds like a heavy solution, as most of the adhesive would be doing nothing except adding weight. Why not just dabbs of adhesive in a grid? I guess you would have to decide on how much weight you want, or how much time you want to spend.
We've done things similar to this (but nor for acoustic reasons) by drilling a bunch of holes in a sheet of 1/8" plastic, and then troweling the adhesive over it, leaving said 'dabs' of adhesive (and then clean off the mask for additional use - we used water based to make this easy).

#### Gnarly Gnu

##### Member
Glasswool is an excellent decoupler, but cannot be used to join two skins.

Ah but do you need to join them?

If you are mechanically fixing the panels around the perimeter then it doesn't matter. For example a sandwich with a thin glasswool core would make a superb firewall, typically screw or rivet fixed around the outside edge (less points is better). But if you plan to adhesive fix the panels only then the bonding is clearly important.

#### GESchwarz

##### Well-Known Member
I don't understand what should take so much time. It's a pretty quick operation to prep, trowel, and join a couple sheets of aluminum. When you're talking about sound barriers, a little weight is a given. The amount is easily controlled by the type of trowel and technique of application. As a former General Building Contractor, I can do this in my sleep. That mask sounds like a big chocolate mess to me.

The adhesive is doing a number of things: 1. bonding the two skins, 2. creating enclosed airspaces, 3. adding mass, 4. is an elastomer that converts sound energy into heat through shear motion, and 5. it's a disimilar material that forces sound energy to change velocity at as many as four separate interfaces. All five of these functions are key elements of a good sound barrier.

I have a bunch of rockwool that can be peeled off of the bundle in any thickness I want. It's surprisingly heavy stuff...designed to line kilns. I've been looking for a good place to use it. The firewall and any heat shield in the engine compartment would be obvious applications. Joining the two skins around the perimeter is the obvious solution with this material.

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##### Super Moderator
Staff member
I'm suggesting it's perhaps not wise to ignore every text book ever written on acoustics and hope that just this once increasing the panel stiffness will actually reduce sound transmission coz it somehow worked for auto in an alternate universe.... :tired:
I'm not as much ignoring physics, but we're talking about different things, which GES summarized here.

Since he was asking about the best shape for a canopy (in terms of noise) resonance isn't a factor, but the major contributor to cabin noise, at least according to the guys, earning their living in making aircraft quieter. Increasing the canopy/window stiffness a lot really helps in lowering that resonance and thus cabin noise.

I don't doubt any of your conclusions about dampening

Interesting discussion by the way

#### Bart

##### Well-Known Member
Rock wool or mineral wool may be somewhat heavier than even fiberglass, but less of it may suffice, being superior for dampening. Also, its heat range is vastly better, having already been burned.

Automotive products like Dynomat work well, but are heavy.

But, what if only used sparingly, might products like Dynomat have a role here? In other words, could narrow strips of it, placed where vibration is most active, get most of the benefit but without having to cover an entire surface?

Kettle drum analogy: Kettle drum (about the same size as a small aircraft firewall) resonates at given frequency, due to its size and surface tension. Now, we could slather Dynamat or similar material over the whole drum head, but that would be pretty heavy. Instead, why not put a small patch of Dynamat right at the center of the drum head, dampening most of its vibration? More bang for the buck, and much lighter.

Thoughts?

#### GESchwarz

##### Well-Known Member
I have just enough experience with vibration testing to know that you won't really know until you try it out. You begin with a theory based on some experience. I think your idea is worth trying.

#### GESchwarz

##### Well-Known Member
Does anyone have any experience with perforated metal as an interior surface treatment. I know that is used in command and communications rooms on U.S. Navy ships. I imagine that it is the outer skin with some sort of porous acoustic material in between.

I'm thinking that if it is available in real thin wall aluminum it may be worth a test.

#### WonderousMountain

##### Well-Known Member
Pop put together a recording room for a pianist once. Everything was at an angle. This geometric idiosyncracy would keep the waves from marrying or arriving at the same time at any one point.

So my suggestion is: Random angles of quieting stuff:gig:

If this doesn't work you could always try turning the engine off and thermaling:whistle:

Okay the voice of my dead mother is chiding me I have to stop

#### rtfm

##### Well-Known Member
It will be fascinating indeed to see this aircraft designed to invert the laws of physics!

In other advice offered water is not wet, the sun is very cold and gravity repels objects from the earth. onder:

Hi,
Easy on old chap. This is the sort of sharp comment which characterises some other forums - not this one. One of the best things about our forum is the fact that folks are polite. You'll be needing advice one day - possibly from the very folks you're being rather snide with.

Duncan

#### rtfm

##### Well-Known Member
Speaking of quiet flight - what progress on the "Schwarzpipe"? Have you done any more work on it?

Duncan

#### Bart

##### Well-Known Member
I have just enough experience with vibration testing to know that you won't really know until you try it out. You begin with a theory based on some experience. I think your idea is worth trying.

Well, to expand on my theory about minimal dampening with maximal quieting, consider another musical analogy: To quiet a guitar or piano string, you only need to touch a small part of it, not the whole thing. I suppose the trick is to know which part of the item is vibrating most, and target that.

A fuselage is sorta combination of wind and percussion instruments. Some places vibrate and boom like a big bass drum, while others whistle. We could slather stuff over the whole works, but that would be too expensive and heavy. Maybe a dab of stuff here and there would work as well, at a fraction the cost.

And, add me to the list of folks wanting to hear more about Swarzpipe.

#### GESchwarz

##### Well-Known Member
Regarding what's been dubbed "the Schwarzpipe", I feel the results that I have reported in this forum are pretty conclusive. If I remember right I was going to gather and report some hard data on sound level. That effort stalled out when the free software I downloaded failed to run even after receiving advice from the vendor of said product. I have since downloaded another product, then I discovered that my teenage daughter has misplaced the microphone for the laptop.

Read my report and if you need clarification don't hesitate to ask. I'am sure that if you build it pretty much as I did that you will get similar results. I couldn't be happier with the results I got. It truly exceeded my expectations.

Anybody considering building a similar design please post your drawings on this post for comment and critique before cutting any metal. I'd hate to see somebody spend time on something based on some kind of misinterpretation.

#### berridos

##### Well-Known Member
Hi Ges

Could you estimate how hot your schwarzpipe gets?
I was thinking of something similar embeded into a tunnel in the fuselage however I dont want to melt the fuselage.
I like the tunnel idea because it would act as a stiffening stringer to the composite fuselage while improving drag.

#### GESchwarz

##### Well-Known Member
I have only heard that piston engines run at about 1500 F and rotaries are up around 1800 F.

That's why nobody makes aluminum exhaust manifolds.

In order to imbed an exhaust pipe you're going to need heat shields, insulation, and forced air ventilation. The forced air ventilation is often augmented by blowing the exhaust stream through a venturi tube which will suck air out of the surrounding cavity. There are many examples of this, start by checking out the Beechcraft T-34 Mentor.