Quiet Flight

[Dauntless]

Interesting article from Switzerland about reducing airplane noise. Nothing really new, but interesting discussion and a couple of great photos. Be sure to follow the link at the bottom to the author's personal installation.

Aircraft Silencers

 

[GESchwarz]

In making an aluminum assemby which required extra stiffness, I added a bead of Gorilla glue to the rivited joint.

What I discovered is that an assembly that has glue in its rivet joints yields a quieter part. When you strike it, it does not ring like an all metal part. Rather it makes a dead sound. The vibration is killed of at the glue joint. I wonder how effective this might be in reducing noise if an entire cockpit is assembled by adding this or similar adhesive to all rivet joints.

Adding adhesive to joints to eliminate noise is a practice used in architectural construction. The joints between the subfloor and the floor joists receive a bead of glue prior to assembly to prevent the floor from squeeking when walked on.

Caution:

Although Gorilla glue can be used to add strength and stiffness to a riveted assembly, it should never be used as a substitute for true structural adhesives like epoxy or methacrylate. I am not aware of any testing done on Gorilla glue to withstand exposure to solvents, temperature, stress, etc. To my mind its greatest capability is its foaming, gap-filling characteristic. This gives added strength because there are there are no large voids if applied with a reasonable degree of care.

 

[berridos]

To make the tube for a muffler or a swiss muffler there are ceramic resins that are used with basalt cloth. That would withstand 1500º C and would maybe be lighter than steel. Should deserve a test.
I looked at that resin for my molds. Have to get back to the guy to know if that muffler appliation is feasible.
Apart from the temperature resistance and weight ceramic tube intuitively should resonate less than steel.

 

[GESchwarz]

I don't know if steel tube resonates as much as it transmits sound. I would think that working with ceramics introduces a host of other problems that you don't have with steel, such as its brittleness a proneness to cracking. Although pound for pound ceramic may be lighter than steel, steel tube can probably be much thinner, and therefore lighter. Talk to your guy and see what he says.

 

[berridos]

just reviewed the docs and it states it is very vibration resistant and appropiate for thin walled tubes.
minimum thickness 1mm
weight 1,6g/cm3
80mpa
can be used with all sorts of fibers.

enclosed the brochure but it is in german. has a picture.
http://www.keraguss.de/Infoblaetter/keraguss_p_infoblatt.pdf

 

[Tom Nalevanko]

I used Gorilla glue on outside-based (Southern California) wooden patio furniture. It does not hold up well over time. NEVER would I use this on an aircraft. When it crumbles, you have a mess and a gap between the pieces you applied it to.

But I do believe in 'glue and screw' as a good technique. The steel sections of my Stallion's cage were both glued and screwed to the fiberglass fuselage. Adhesive was Hysol 9430. The theory (as with house floors and stairways) is that the glue takes the light and vibratory loads and minimizes the steel's fatigue loading. But when push comes to shove, the steel takes the load.

Blue skies,

Tom

 

[GESchwarz]

Thanks for the data on Gorilla glue, Tom.

While on the subject of applying glue, and sealant in particular, to rivited or screwed joints, as in fuel tanks, the key is to apply to both surfaces before mating, then, and this is the hard part, join them in one careful movement and apply the fasteners in such a way that the parts cannot separate. If the parts do separate after the adhesive has squeezed out of the mating area, there will be voids in the bond line and thus a leak path.

 

[berridos]

Hi GES

To be able to calculate the equivalent weight, what steel tube thickness would you use for the inner perforated tube and the outer tube in a schwarzpipe? Just to see if we are talking about a meaningful difference.

 

[GESchwarz]

Maybe .020" on the inside and .050" on the outside depending on need for strength at attach points. Material is 321 cres. Any lesser steel would wear away at rotary temperatures. Inconel or titanium would also work.

The outer can on mine gets welded directly to the exhaust port tubes at the engine, so it doubles as the manifold for a significant weight saving. I cut away the collector portion of the stock manifold and I'm putting a 5" diameter tube (can) in its place. This is where the high velocity gas pulses spin around the circumference, and the smaller diameter perforated collector tube runs down the center, in the eye of the huricane, so to speak. The exhaust pulses tend to cling to the outside of the curved flow path due to their higher density and velocity. The centerline collector tube collects the slower, less dense gas at the center of the can. Slower, less dense gas is quieter than the high velocity (supersonic) pressure pulses.

 

[berridos]

That could mean a weight saving of 7-8lbs on bingelis design with your tube specifications.
Are 7-8lbs worth it?

 

[GESchwarz]

The "bingelis design" is a lot like the cherry bomb glass pack muffler that teenagers use to make their hot rods louder than the stock muffler, and still be legal. The only advantage of this muffler is that it's light weight. Does it reduce noise? I'll bet it's quieter than no muffler at all.

 

[Starman]

I cut away the collector portion of the stock manifold and I'm putting a 5" diameter tube (can) in its place. This is where the high velocity gas pulses spin around the circumference, and the smaller diameter perforated collector tube runs down the center, in the eye of the huricane, so to speak.

The centrifugal force could cause unacceptable back pressure as the gasses speed up. Someone visited this forum a while ago who had that problem.

 

[GESchwarz]

The centrifugal force could cause unacceptable back pressure as the gasses speed up.

Why?

The fact of the matter is that this geometry is highly effective in reducing noise. Furthermore it acts as a high velocity venturi (low pressure) at the exhaust ports which facilitates the scavenging of exhaust gasses from the combustion chamber.

Another thing that my testing revealed is that a long straight pipe created the greatest back pressure of all my attachments.

 

[Starman]

That's good, but at high gas velocities the centrifugal force causes the muffler outlet to be at a much much lower pressure than the muffler inlet. The pressure at the outlet would go bellow atmospheric and stop outletting until the pressure at the muffler inlet was extremely high, but the engine can't supply that exhaust pressure so it creates a huge back pressure in the exhaust which can kill the engine. It's simple physics. Of course you may not agree with the physics of it so let's look at reality: there was an actual person on this forum who tried that actual design, and it actually killed the engine every time it reved up, and that's an actual fact =)

Of course the devil's in the details but I think with that design if you go either way on it you either have a loudspeaker or a cork.

 

[GESchwarz]

That's good, but at high gas velocities the centrifugal force causes the muffler outlet to be at a much much lower pressure than the muffler inlet. The pressure at the outlet would go bellow atmospheric and stop outletting until the pressure at the muffler inlet was extremely high, but the engine can't supply that exhaust pressure so it creates a huge back pressure in the exhaust which can kill the engine. It's simple physics. Of course you may not agree with the physics of it so let's look at reality: there was an actual person on this forum who tried that actual design, and it actually killed the engine every time it reved up, and that's an actual fact =)

Of course the devil's in the details but I think with that design if you go either way on it you either have a loudspeaker or a cork.

Starman,

I'm the "actual person" on this forum that published results of muffler testing. Read it again.

You are claiming all kinds of huge pressures and velocities and stoppages. What do you base all of this on?

 

[Starman]

I wondered if maybe you were =) Did you test it on an small industrial engine?

 

[berridos]

Hi GES
How long have you build your phase splitter?
Couldn't you pose a pick?
What difference would it make to build the outer tube in a conic form?

 

[GESchwarz]

Briggs & Stratton. It's also been used on some rotary powered planes with excellent results. The pics are there. What would conic do for you that a cylinder doesn't?

 

[Starman]

Briggs and Stratton sound familiar. What I recal reading is that someone put a centrigfugal muffler on a B & S (I think) and it ran fine at low RPM, but when it tried to rev up it quit/lacked power/ran erratically/wouldn't rev. I cant recall exactly which, but in any case it was no longer be a functioning lawn mower engine due to the muffler. Was that you? Did you correct the problem? Sorry, I haven't been reading all the posts here lately.

 

[GESchwarz]

That is not what I reported. I would suggest that you re read it.

All of the comparisons were done at a fixed idle setting. At that setting, when I put the long pipe to the exhaust port, the engine died. It did not die with the centrifigual muffler; it ran well and it was very quiet at all speeds, with no reduction in performance.

What you read and what you think you read are vastly different. You got to work on that or it's going to bite you. We are our brothers' keeper.