Noise Reduction Insulation Techniques?!?!?

Discussion in 'General Experimental Aviation Questions' started by HomeBuilt101, Jan 24, 2016.

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  1. Jan 24, 2016 #1

    HomeBuilt101

    HomeBuilt101

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    Does anyone have any suggestions to reduce the noise level in the airplane?

    There are three sources of noise are engine noise, vibration from the engine mounts and propeller, and wind noise.

    The biggest culprit is engine noise so what is the best firewall insulation and or other techniques that will isolate the engine noise from entering the cabin?

    Thanks for your help!!!
     
  2. Jan 24, 2016 #2

    Aviator168

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    Wear a noise canceling headset
     
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  3. Jan 24, 2016 #3

    plncraze

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    Unfortunately the noise canceling headset is probably the easiest option but if you are really serious there are other options. There are old NACA reports which talk about noise from props, cafefoundation.org has papers on their website dealing with noise and if you are an EAA member you can go through the archives section of Sport Aviation and find info as well. Also read anything you can about the Lockheed YO-3A.
    The short answer is get the engine quiet first, then get the prop quiet and then you will hear the airframe noise. There was an article in Flying magazine years ago about Tony Bongiovi's Twin Comanche which he quieted down with much effort. Reading that will give you an idea of the effort involved in making an airplane quiet.
     
  4. Jan 24, 2016 #4

    Toobuilder

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    The cheapest, lightest, safest, and most effective way to cut the noise is the best ANR headsets you can buy - and they are portable!
     
  5. Jan 25, 2016 #5

    SVSUSteve

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    The old NACA and CAFE reports are often extremely technical and acoustics is the only thing in aircraft design I have found that makes aerodynamics seem simplistic. I was cross trained to do ultrasound while in the military and the qualification tests required a lot of study on acoustical physics but some of the old aircraft noise reduction papers make my brain hurt since they were written largely for acoustic engineers by acoustic engineers.

    While ANR headsets are great and one would be foolish to not use them, if you're building (and certainly if you're designing) it's kind of foolish to not design the cockpit to be quiet and then further reduce that with ANR. It's like designing a weak point into your wing and then retroactively adding a doubler patch to address it. It's Band-Aid engineering which might be fine for correcting a mistake coming off the line in Wichita, Duluth or somewhere in Florida but it's kind of stupid to not do better in a homebuilt.

    Balancing and isolating the engine vibration is a big one. Having a thicker windshield (which will also help keep wayward birds out) is a common recommendation. Most of the reading I have done has said that you don't get much additional benefit once the windshield gets past around 3/8".

    Eliminate or isolate anything that could rattle in your cockpit. Put damping materials on the INSIDE of the firewall otherwise even if it's stainless steel it will vibrate like the skin on a drum.

    The best source of information I have found that is explained in terms that won't make someone without a background in engineering or physics develop a nosebleed is here: Aircraft Acoustics 101, A Quick Overview - Pegasus Aeromarine, In. If you look around the site, they actually have diagrams on how to do different levels of acoustic protection.
     
  6. Jan 25, 2016 #6

    don january

    don january

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    I've seen two thing's that work. first is to run your exhaust beyond the seating location of were pilot sit's look's tacky but help's, best to go under belly if you can. And the other was found by trial and error. Do you know that liquid rubber stuff you can dip handle's in and when dry it's pliable like for pliers/ screwdriver's. Take it and brush /paint the inside of your firewall even behind carpet's if removable, let dry and it does help, Weight penalty involved but not much.. But!- yes there is alway's a But, you'll just find new noise's to bother you. Like the boy's said good head set and even ear plug's or both go along way.
     
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  7. Jan 25, 2016 #7

    Pops

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    One of the reasons I like wood airplanes.

    Dan
     
  8. Jan 25, 2016 #8

    HomeBuilt101

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    Thanks all for the information...the Aircraft Acoustics 101 article is outstanding!!!

    THANKS AGAIN!!!
     
  9. Jan 25, 2016 #9

    bmcj

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    You missed oil canning and tailcone (megaphone) amplification for sheet metal aircraft.

    For engine/exhaust noise (on a homebuilt), how about a Swiss Muffler? Swiss style muffler
     
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  10. Jan 25, 2016 #10

    gtae07

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    And it'll burn really well too.

    Go check out the VAF forums and search for posts about firewall testing by DanH. He built a propane-powered test rig that subjected material samples to the FAA-required 15 minute test. Pretty much anything and everything attached on the cockpit side of the firewall smoked or burst into open flame in short order, including paint. He recommends that there be nothing on the cabin side of the firewall, not even paint. At most, a little bit of sealant solely to keep CO fumes and other nasties out of the cockpit. On the engine side, he recommends ceramic blanket insulation overlaid with stainless foil and all the edges sealed with Firebarrier 2000, and special passthroughs filled with the same, plus a stainless shield for some distance aft of any cowl openings. I'm following his recommendations verbatim.

    Some links:
    Firewall sealant and Fire Safety - VAF Forums
    Firewall Insulation - VAF Forums
     
    Last edited: Jan 25, 2016
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  11. Jan 25, 2016 #11

    TFF

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    Most noise is from wind. One of the reasons for the flip up canopy vs the sliding one for a RV6-7 is they are quieter; other wise they are worthless. If you are hearing resonance from the engine, the engine mounts might be done and need replacing. That resonance is why there are about a 100 versions of Lord mounts for engines that use the same physical shape. If after you do the firewall and you think it is noisy still, a 12" square put in the center of large unsupported structure may be enough to change the resonance. You don't want to go crazy with sound material as you can easily add 50-100 pounds to the airplane quick. This is after you have you nice ANR headsets.
     
  12. Jan 25, 2016 #12

    Midniteoyl

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    Rock Wool...
     
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  13. Jan 25, 2016 #13

    Turd Ferguson

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    Foam earplugs. The cheapest, lightest, simplest and most effective solution.
     
  14. Jan 25, 2016 #14

    Pops

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    Flying a Cessna 337 Skymaster with the engines out of sync feels like your head is going to explode. About as bad as it gets.


    Dan
     
  15. Jan 25, 2016 #15

    Turd Ferguson

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    I've flown some conventional twins that would do the same. One big turboprop twin freighter I flew sounded like the plane was falling apart if you cycled the props out of sync in flight, lol. We would do this on final approach over a coworkers house and people called to report a "plane had crashed" so we had to quit. Sometimes.
     
  16. Jan 25, 2016 #16

    SVSUSteve

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    There's another compelling reason that is not more common and it boils down to two words: carbon monoxide.
     
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  17. Jan 25, 2016 #17

    SVSUSteve

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    The major source of that isn't leakage through the firewall. It's from the silly little muff heaters people still use as a throwback. Given how high of BTU even a small electric heater can put out, there's no reason to include the holdover "Carbon Monoxide Expressway" in a modern aircraft unless you have something standing in your way like the need to adhere to a type certificate which, no.....no we don't.

    There are ways around it...but then almost anything will burn with direct propane flame impingement for a couple of minutes. Even the turnout gear we wore on the fire department isn't rated to sustain that.

    If one is terribly concerned about flammability but still wants to reduce noise simply stiffening the firewall either by making it thicker (which would increase fire and crash resistance but have a significant weight penalty) or by using stringers on the backside of the firewall to stiffen it. Either would change the vibration frequency and decrease sound transmission.

    You'll also notice that at least one engineer pointed out that his original approach is overly complicated and going to be heavy as hell. A couple of other people also pointed out, most likely correctly given a dearth of hard data since catastrophic uncontrollable in-flight engine compartment fires like he is talking about are relatively rare, that by the time the firewall heated up that much the cowling probably would be burned off and the flame would be destroying the bubble canopy. It's probably overkill to a degree like arguing that one should have a bombproof baggage compartment on their RV just in case Daesh decides to bomb your plane. He makes some good points about being careful with material selection and placement but following his recommendations blindly and without critical evaluation of the risks and benefits is probably a less than ideal solution because he's talking about a doomsday scenario that isn't common.

    That goes for the advice anyone gives you including myself, TFF, BMCJ, Pops, etc. One of the biggest problems with VAF is- because it's a type club/cult- that people latch on to someone else's idea and run with it simply because it sounds good at first blush. At the same time, one of the reasons why I like and respect Van so much is that he will flat out call people he has sold kits to on their more ludicrous ideas for modification.

    Honestly, if you're that concerned about a massive fire in the engine compartment the coating on the **** firewall should be a last line of defense. Cutting off the fuel supply and deploying a fire extinguisher (available from any racing supply store) would probably be the more prudent choice if executed along with a very quick emergency landing or ballistic chute deployment. Taking a "well, I'm not going to actively fight this and put my faith in the firewall design of some random dude with a flamethrower" approach (which, that said I happen to respect the hell of Dan- I've learned a lot from reading his posts on other subjects over there- but see my previous comment) is neigh on suicidal.

    Once you close the fuel shutoff (which should be step #1), the only liquid source of fuel up front should be the oil which isn't going to burn even in a 150+ mph relative wind at the temp that blowtorch of his is producing. If I recall the temp of most motor oil when burning is somewhere around or under 500 degrees.

    There's also the vaguely 9/11 conspiracy theory-esque comment someone made about the temperature difference between the temp of burning avgas versus the melting point of steel ("Avgas can't melt steel firewalls"). The important thing to remember is that if you're blowtorching the firewall at those temps like he is supposing, one has to wonder- given the behavior of metal under heat- whether the engine would just separate from the aircraft's weakened fire wall under the air and gravitational loads within the test period the FAA mandates or even the time it takes to execute a maximal performance gliding descent from the low teens (because, remember you shut your engine off right?). Then again, it brings up the point of the reality of someone having a strong fuel fed fire for that long assuming they weren't ****ing stupid (which is the nicest way to describe it) and didn't shut off the fuel.

    The question I have about his methodology is pretty simple. You'll notice that many of his tests (Koolmat) have flames licking around the corners of a sheet of metal not much bigger than the torch. Notice that most of the ignition for stuff on the backside of the firewall seems to start at the edges versus the center of the flame where radiated heat should- in theory- be greatest. It would lead me to wonder whether that ignition was due to direct flame exposure- completely unrealistic if we're supposed to be looking at something on the cockpit side of a firewall versus an engine compartment fire- rather than transmitted heat. A more realistic test if you're looking at how the material behaves to transmitted heat versus direct flame impingement would be to have the metal sheet larger than the test material. Neither his tests nor the FAA burn certs should be taken as an excuse to simply "go with it". You still have to stop and think about the difference between what could happen and how it would actually go down versus what is likely to happen and how that will proceed.

    I'm probably the most risk averse person on this forum but the trick is remaining true to the reality of in-flight fires. The far greater risk is from an electrical fire inside the passenger compartment subtly creeping up until it renders the cockpit full of smoke and the aircraft non-flyable than from the dramatic Hollywood style engine exploding and flames licking down the sides of the fuselage uncontrollably. It's still a threat and something that should not be taken lightly but there are a lot better ways to manage that threat than the approach he suggested.

    Which is exactly why the common idea that you have to fill the voids with material to decrease noise which is so prevalent in the homebuilders who try to off the cuff acoustically insulate the aircraft is so laughable. A little bit of reading would teach them that it's overkill.
     
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  18. Jan 25, 2016 #18

    Turd Ferguson

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    No reason NOT to use an exhaust heat exchanger for cabin heat. The gold standard of simplicity, been in use for many, many years and has a proven reliable track record. Of course, it may require periodic testing in addition to inspection and repair. Technology has made it possible to perform operational CO checks via the use of high performance, low cost CO detecting equipment. One of the best values in aviation. Yea, I'll stick with my silly little "throwback" muff heater, thank you.

    Firewall leakage can indeed be a major source of CO in the cockpit. This is common knowledge to A&P's in the field, which is why it's not a problem. It's addressed before it becomes such.
     
  19. Jan 25, 2016 #19

    Marc Zeitlin

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    Kind of getting off track from the main topic here, but I will disagree with the "no reason..." logic. Using electricity to create heat is extremely inefficient. Let's look at the #'s.

    A 1500W room space heater is equivalent to 5100 BTU/hr. But to produce 1500W in a 14V airplane requires 107A - way more than any normal alternator can put out.

    Let's assume you've got a 60A alternator, with a normal load of 20 - 30A. That'll leave you with about 15A - 20A left over for heat without overloading the alternator (you generally want to keep them at 80% capacity or below). 20A will give you about 1000 BTU/hr. of heat. If your airplane is sealed up REALLY well from air leaks, maybe that'll be somewhat useful. But it's not a lot of heat - it's the equivalent of a 300W incandescent light bulb.

    Now let's look at using exhaust pipe heat. Assuming that our engines are about 33% efficient, if we're running an O-360 at 75% power (135 HP, approximately), then 270 HP is going overboard somewhere as heat, through the cylinders, oil and exhaust. 270 HP is equivalent to 687K BTU/hr. Obviously, we don't capture a whole lot of that, but even if we're only 1% efficient at capturing the waste heat, we get 6,870 BTU/hr. of heat - over 6 times as much as the electric heater (and we don't need to stress our alternator or buy a larger one).

    While I agree that CO is a potential issue and needs to be monitored/mitigated, there is no doubt that using waste heat from the engine is far superior as far as providing heat to the cabin is concerned. Especially in larger aircraft/cabins.
     
  20. Jan 25, 2016 #20

    SVSUSteve

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    So is air cooling an engine but we still do it.

    Eh...maybe from an academic standpoint but it's one of those design issues that comes down to what one sees as more important. I'd rather "waste" engine heat if means eliminating a significant safety risk for minimal electrical demand compared to the other ones on the aircraft.
     

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