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Thread: Augmenter Tube

  1. #1
    Registered User mstull's Avatar
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    Augmenter Tube

    I'm breaking in the new engine on my new plane now. (Yes, I finished the plane. I'll post about it once it flies... probably this week.) The augmenter tube rings loudly, particularly at certain RPMs, like a wind chime. What is the simplest, lightest way to quell the ringing metal? The tube is 4.5' long, 2.5" diameter, .035" wall, 6061-T6.

    For those who aren't familiar with the term, an "augmenter tube is a tube or pipe through which the exhaust gases from an aircraft reciprocating engine are directed." Mine is open on both ends, not sealed to the muffler's tail pipe. In fact it doesn't even touch the muffler at all. The tail pipe just blows into the open end. I'm using it to route the oily exhaust down by the right main gear tire.

    My previous plane used a short augmenter tube to keep the oily exhaust off the reduction drive belt. The tube doesn't get all that hot, so it could be wrapped with rubber or something.

    I was thinking of wrapping it tightly in a few places with neoprene rubber sheeting, held on with cable ties or hose clamps. I don't want to add more weight or bulk than necessary, particularly since it is in my view and would add drag. Does anyone have any experience with this, or suggestions for possible solutions? It would be easy to try different things.

    Another idea I had would be to clamp a length of 1/2" by 1/16" thick aluminum angle to it with some hose clamps. The angle could be located so it doesn't add any drag or obstruction to view.

    It may ring less as oil and carbon build up inside over time. But it's very loud at certain RPMs now.
    Mark E. Stull
    mstull@wtxs.net

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    Re: Augmenter Tube

    Mark,
     
     
    A photo would help a lot in visualizing this. You say the tube is 4.5ft long, so I guess that it is supported in at least one or more locations. It's obvious that it is resonating at certain frequencies. I really don't know how a augmenter tube it used. If it is connected directly to the engine, then it could be vibration that is causing it to resonate. If not directly connected to the engine, then it would be the exhaust beat that is causing the issue. You did not say, but I assume this is on a two cycle engine.
     
     
    My first thought would be to change the clamping locations or add some more to change the resonate frequency to such a high frequency that it would not matter.
     
     
    Richard

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    Moderator Dana's Avatar
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    Re: Augmenter Tube

    Why use a metal tube at all? How about aeroduct, flexible wire spiral and silicone coated fabric ducting, or something like that?

    -Dana

    Dullard: someone who can open an encyclopedia or dictionary and only read what they'd planned to.

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    Registered User mstull's Avatar
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    Re: Augmenter Tube

    Thanks guys,

    I didn't attach a picture because I'm waiting to unveil the plane once it flies (in case the plane might need some major change), and I haven't taken a picture of it after I added the augmenter tube yet. The augmenter tube doesn't connect to or touch the engine or muffler at all. It's resonance is excited by the exhaust wind pulses. It is only supported at its ends for good reason... there isn't any airframe to connect it to anywhere else along its length. I could add a couple pounds and make additional support. But I try to at least start out with everything as light as possible on my U/L designs.

    Yes, I thought about using flexible duct and a lot of other ideas. There's a couple problems I thought of with duct: Its slightly rippled interior might create a slight resistance to flow and require going to a larger diameter that would block my view more. Even with my smooth 2.5" diameter tube, a little of the exhaust spills out the front of the tube sometimes in run-ups. Also, the ripples would collect the condensed oil and might get heavy. And again there's no airframe part there to support the duct. Even so, I don't totally rule out using duct, maybe for part of the augmenter tube.

    Here's a picture of the short augmenter tube on my last plane to give you a better mental picture of what it is and does.
    Attached Thumbnails Attached Thumbnails Augmenter Tube-sep25a.jpg  
    Mark E. Stull
    mstull@wtxs.net

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    Re: Augmenter Tube

    cant wait to see this...

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    Re: Augmenter Tube

    The ringing is probably a resonance at certain RPM's, but that resonance could be a sonic resonance or a resonance of the pipe itself. The ringing may be the pipe vibrating and adding some mass (dampening material) at the locations along the tube would tend to detune the pipe from that resonance, like putting your finger on a guitar string changes the note the string wants to make. You said it doesn't get that hot, so can you touch the tube when it is ringing and see where along the tube the maximum amplitude is? It shouldn't take much material to change the resonance frequency of the pipe.

    The other option I see would be to split the pipe up into smaller segments to tune the resonance frequency of each segment beyond what the engine would produce. Rolling a bulge in the pipe at intervals along the pipe or changing its shape in some fashion to change its resonance point could do the trick.

    If it is a sonic resonance (pressure waves resonating in the pipe) then shortening or lengthening the pipe is what you probably need to do. The pipe seems really short to have a standing wave type resonance in it, but who knows? The calculator here may be useful (if crude) Autolounge.net | Calculators | Exhaust Resonance

  7. #7
    Registered User Norman's Avatar
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    Re: Augmenter Tube

    I remember some talk on the pulsejet site about cutting a fish-mouth on the outlet end to lower resonance induced noise in a pipe with a pulsating flow. In fact I believe this is a feature of the Gluhareff Pressure Jet engine
    Norm
    "For some years I have been afflicted with the belief that flight is possible to man" ~Wilbur Wright 5/13/1900

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    Registered User Autodidact's Avatar
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    Re: Augmenter Tube

    The guy at the place where I bought my anvil showed me how a magnet the size of a silver dollar attached to the anvil would stop it from ringing. A square of adhesive weight tape applied at the right location might do it. It might not have to be very big. Don't they have something like that to balance tires?

    Upon finding the right location, you might attach something more permanently to keep it out of the prop if it lets go.
    "Milk cures wing dope poisoning."

    Flying and Glider Manual

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    Registered User mstull's Avatar
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    Re: Augmenter Tube

    Thanks guys. After thorough testing today, it turns out it's not the metal ringing, it's sonic. The air/exhaust inside the 4.5' long tube resonates with a loud ring around 3,000 RPM. I can stop the ring with my hand over the end of the tube. I think I can just avoid that RPM, since it is well below cruise and well above idle.

    I finished the run-ups, tuning, and adjustments today, and did my first taxi test. No problems. I'm planning a first flight for tomorrow (Tuesday).
    Mark E. Stull
    mstull@wtxs.net

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    Re: Augmenter Tube

    OK Mark - I'm glad you sorted that out without having to make modifications. Now if you could just explain what you think the benefit of the augmenter tube is. I did a search on Google and found very little about it and no explanation of how it works and what kind of boost it gives. I did find something about using the tube on small model airplane jet engines to increase the power, but nothing on how it might be used on 2 or 4 cycle engines to add boost to the aircraft. It was noted briefly that Cessna use it on there 310 planes.
     
     
    Richard

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    Registered User Joe Fisher's Avatar
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    Re: Augmenter Tube

    On the early Cessna 310 airplanes the augmenters were used to reduce engine cooling drag. They were dropped on later model airplanes in favor on engine nacelle baggage lockers. So apparently the storage space was more valuable than the drag reduction.

  12. #12
    Registered User mstull's Avatar
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    Re: Augmenter Tube

    Good Question, Richard,

    My augmenter tube is not intended to augment at all. It just directs the oily exhaust to a place where it won't get all over me or my plane.

    The tail pipe that sticks out of many mufflers is the point of greatest flow restriction in the exhaust system. So if you lengthen the tail pipe to direct the exhaust somewhere, flow will be more restricted, engine tuning will be changed, you could lose a lot of horsepower, and the muffler will get quieter. Using a large, open-on-both-ends augmenter tube retains the engine/exhaust tuning, yet allows you to direct the oily exhaust.

    I think augmenter tubes were used on some WW2 military planes to actually augment efficiency, and that's how they got that name. Designers needed to squeeze every possible bit of speed and endurance into fighter planes.
    Mark E. Stull
    mstull@wtxs.net

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    Registered User pwood66889's Avatar
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    Re: Augmenter Tube

    I am familiar with "augmenter tubes," Mark, from my A&P education. They are on the Cessna 310, and collect the exhaust pipes. The exiting exhaust helps pull heated air out of the plentum chamber on the engine making for better cooling. You are not interested in that; just vectoring the exhaust away from the plane.
    I know you don't want more weight or drag or restriction to vision. But you may have to change tubes to get rid of what appears to be a harmonic of that RPM. Do you have several lengths of that tube to experiment with? Or a "throw-a-way" that you could cut down to see if a better frequency results?
    Percy in SE Bama

  14. #14
    Registered User mstull's Avatar
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    Re: Augmenter Tube

    Thanks Percy,

    I can cup my hands to extend the length of the tube, or put something up into the end of the tube and change its pitch slightly, like you'd expect on a pipe organ. But that doesn't solve anything, other than show that it's a sonic oscillation. As it is (or seems so far) the RPM when it happens is actually pretty convenient for avoiding.

    Part of the problem is... the augmenter tube I used is the only thing that works out with the available support points and exhaust components I had on hand. It came out simple, light, inexpensive, and fairly low drag. It was the lesser of evils by far. I'll let y'all know if it turns out to be enough of a problem that I have to come up with something else... that would probably include customizing the tail pipe on the muffler to point a different direction and make it larger diameter so I could use some extreme temp flex duct instead of an augmenter tube. That would probably be even more ideal, since I could route the duct where it doesn't block my view as much.
    Mark E. Stull
    mstull@wtxs.net

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