LIGHT AIRCRAFT INSULATION: THERMAL AND NOISE CONTROL IN YOUR AIRCRAFT Stearman owners need not atten

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LIGHT AIRCRAFT INSULATIONTHERMAL AND NOISE
CONTROL IN YOUR AIRCRAFT(Stearman owners need
not attend!)
Presenter Dan Newland
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Why Insulate?

• For comfort. ANY insulation WILL make the
  aircraft thermally and acoustically more pleasant
  perhaps with very little weight increase. Proper
  selection of the materials will make the most
  from your weight and volume restrictions. If
  upholstered over, it can also add a plush look to
  the interior.

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NOISE

• What makes aircraft noise?
• Engine, prop aerodynamics and rattles
• What is the primary unit of measurement of noise?
• The decibel (named after Alexander Graham Bell).
  It is a logarithmic scale. THIS IS IMPORTANT! A
  drop from 100dB to 90 dB is NOT a drop of 10
  but sounds like ½ the noise! But a sound can also
  be measured in terms of pressure.

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How does noise get in the cabin?

• True noise first enters through gaps in the
  aircraft. THE FIRST, LIGHTEST AND CHEAPEST
  METHOD TO CONTROL NOISE IS TO CLOSE OFF GAPS AND
  GASKET EVERY AIR LEAK IN THE CABIN!

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How does noise get in the cabin?

• Why do I say true noise? Because most noise in
  the cabin starts as vibration and as a secondary
  effect, turns into noise.
• Stay tuned, this is a big concept and is covered
  later!

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What is the frequency (pitch) unit?

• Hertz (Hz, also known as cycles per second. 50 Hz
  50 CPS (about the lowest level humans can
  hear). 20,000 Hz to 20,000CPS about the highest
  frequency people can hear noise. Sound is simply
  waves traveling through a fluid like air that
  cause alternating high and low pressure (called
  compression and rarefaction).

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What is sound?

• We perceive sound when our eardrum is hit with
  these alternations in pressure that causes the
  membrane to vibrate and visa versa since they are
  just simple transfers of energy according to the
  first Law of Thermodynamics).

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4 Why is frequency important?

• Humans perceive volume differently at
  different frequencies. Human hearing is
  Non-linear meaning that even though some sounds
  may be louder than others according to a meter,
  we dont necessarily hear it that way.

4a
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4 Why is frequency important?

• Sound travels differently through different
  materials depending on the frequency

4b
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4 Why is frequency important?

• Higher frequencies are most easily stopped
  (easily apparent above 1000 Hz)

4c
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4 Why is frequency important?

• Most of the noise in an aircraft is lower
  frequency
• Example 6 cylinder engine at 2400 RPM (40 Hz)
  is firing 3 cylinders per revolution120 Hz power
  strokes (40 x 3). So you can expect a lot of
  narrow band noise at 40 Hz and even more at 120
  Hz. You will also see noise spikes at resonant
  frequencies at 240, 360, and 480 Hz etc. Note an
  octave is a doubling of the primary frequency
  like at 120 Hz, 240, 480, 960 Hz etc.

4d
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4 Why is frequency important?

• NOW CONSIDER, 3 bladed prop at 2400 RPM 120
  Hz. Expect that the engine/prop frequencies add
  up from a 6 cylinder engine to have a lot of
  noise at 120 Hz. You might consider a different
  prop that doesnt have a coincidental frequency.

4e
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5. ABSORPTION OF NOISE ADDING 1 OF FOAM
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6. BLOCKING NOISE WITH A ½ lb. sq. ft. BARRIER
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7. EFFECT OF THICKNESS ON ABSORPTION
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7A. Absorption

• Absorption is proportional to the thickness.
  Maximum absorption is achieved with the thickness
  of the absorber material equal to ¼ the
  wavelength.

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8. EFFECT OF MASS ON BARRIER, doubling weight of
barrier.
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9. What did we learn?

• The thicker the absorber, the lower the
  frequencies absorbed.
• The more massive the barrier, the lower
  frequencies blocked

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10. What is this absorber and barrier stuff?

• If you think of noise like water and you are in a
  boat, you have a good analogy, (more on this
  later). An absorber like a sponge soaks up sound
  and converts it to heat, (another form of
  energy). A barrier is used to prevent noise from
  entering an area. This needs mass (weight), and
  like an absorber, what is not reflected is
  converted to heat. Since noise is a source of
  energy and all energy must obey the 1st law of
  thermodynamics, most noise control involves
  converting noise to another form of energy, heat.

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10. What is this absorber and barrier stuff?

• There are only TWO direct methods of passively
  treating noise. This may seem surprising but to
  treat NOISE (not vibration), you either use an
  absorber or a barrier.

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10. What is this absorber and barrier stuff?

• A barrier ONLY works to stop noise AS IT COMES
  THROUGH FROM A NOISE SOURCE! (like keeping water
  out of the boat)
• An absorber soaks up SOME noise as it comes
  through but UNLIKE a barrier, will treat noise
  ONCE IT IS IN THE AIRCRAFT! (Like soaking up
  water already in the boat)

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10. What is this absorber and barrier stuff?

• This concept of barriers and absorbers is so
  important, Ill repeat it. If you have a known
  noise source (like a firewall or vibrating
  fuselage panel), placing a barrier between it and
  the interior is THE most effective way to treat
  noise since absorbers treat it only after it gets
  in! And from the previous graph, we know a
  barrier is much more effective.

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10. What is this absorber and barrier stuff?
(cont.)

• Absorbers are the lightest noise control
  materials around. They are also typically
  excellent thermal materials so they are the first
  material to consider lining the interior with.
  Absorbers MUST be very porous to allow air and
  thus noise in. These include foams and
  fiberglass batting. NOTE closed cell foams are
  more effective at low frequencies, open cell
  foams at wider frequencies. An added bonus of
  foam is that when upholstered, it adds a plush
  look.

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10. What is this absorber and barrier stuff?
(cont.)

• Barriers are the heaviest noise control
  materials. They MUST be non-porous since WHERE
  AIR CAN GO, NOISE CAN GO. Since noise is a form
  of vibration, to go through the barrier, it
  must cause the barrier to accelerate in a
  vibration, thus, the more massive the barrier,
  the less acceleration, (vibration). Barriers also
  should be limp to make waves traveling through it
  difficult.

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Why dont we use noise traps like Helmholtz
resonators and Bass traps?

• These tend to treat noise in a very narrow band
  of noise. They can be effective but only for
  specific frequencies say for frequencies of 120
  Hz plus or minus 2-3 Hz or less, and do nothing
  for any other frequency. Sound in an aircraft is
  typically broad band meaning covering many
  frequencies. The size of the trap also depends
  on the frequency with low frequency requiring a
  lot of depth.

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SUMMARY

• There are only two types of DIRECT noise control
  materials-ABSORBERS and BARRIERS. A 2 place
  aircraft insulated with only an absorber of .6
  lbs/cu. ft. may only have a total insulation
  weight of 4 lbs. (75 sq. ft. x .05 lbs/sq. ft.
  for 1 insulation). This would drastically
  improve thermal and marginally improve acoustics,
  (there would be less tinny high frequency sound
  but the overall noise would be only slightly
  less). But also remember that only 2/3 to ¾ of
  the aircraft can be acoustically treated since
  the canopy cant be insulated in any way.

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• END OF NOISE CONTROL?

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12. So is that all there is to noise control?

• Not at all! Quite the contrary, absorption and
  barriers may be the LEAST of your noise control.
  Most noise first begins as a structural
  vibration, so much of our efforts should be on
  controlling VIBRATION

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13. Vibration control?

• Right! Noise is waves moving through a fluid but
  sound doesnt travel through a non-porous solid.
  What it does is transfer its energy to a
  structure which can then transport that vibration
  elsewhere (it isnt noise when shaking the
  structure).

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14. Let me explain

• If you were in a spaceship outside the
  atmosphere, the engine firing would cause a lot
  of vibration but you wouldnt hear it outside the
  ship since sound doesnt travel in a vacuum.
  Once you were inside the capsule, you would hear
  the roar of the engine just fine. That roar is
  noise traveling in the structure then being
  converted to noise in the air of the capsule. So
  lets say you close your helmet. The noise is
  still there but its much quieter. Why? The
  noise now has to vibrate the space suit and
  helmet which then turns BACK to sound on the
  inside of your helmet.

14a
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More about vibration

• 14b

• Lets say you and the mission com-mander go
  outside for a space walk but then find the radios
  dont work. You shout at her but she cant hear
  you. So press your helmet against hers and find
  that by shouting, you can hear each other. That
  is awkward so you hands to see if that works but
  again, you cant hear each other. So whats
  going on?

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And

• Speaking inside your rigid helmet, the structure
  vibrates. When you press your helmet against
  hers, it transfers energy (vibration-NOT sound)
  into her helmet since as a rigid material, it
  transmits vibration easily. This shaking is
  transferred back to sound with the helmet acting
  like a speaker diaphragm. When you tried hearing
  by holding hands, the soft fabric being limp, is
  a poor transmitter of vibration, so you couldnt
  hear each other.

14b cont.
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15. So why does the plane vibrate?

• The engine, prop, hydraulics, starter,
  alternator, etc. are all moving and thus have
  some vibration. In addition, there is a lot of
  boundary layer vibration from aerodynamics that
  transfers energy to the structure. This
  vibration is doing EXACTLY what a stereo speaker
  doesEXACTLYexcept that it is a mechanical
  vibration, not electromagnetic waves moving a
  magnet, (did I say that this was important?)

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16. So what does this imply?
16a

• If you were to gasket ALL gaps and openings into
  the aircraft, there would be NO NOISE coming
  DIRECTLY into the aircraftNONE! So the ONLY
  noise you would hear could ONLY come from
  VIBRATION THROUGH THE STRUCTURE. So if you could
  perfectly isolate the engine/prop mount plus any
  other systems and had laminar flow over the
  aircraft, it would be COMPLETELY SILENT!

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16 Vibration Solutions

• This points directly at what the solutions to
  noise should be and your first treatments
• Seal the entire aircraft with good gaskets.
  NO GAPS!!! This is almost a NO WEIGHT solution
  here so it is CHEAP NOISE CONTROL in and
  weight. Particularly check all doors and
  canopies. Also check gear doors for tight fits.

16b
1st.
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16 Vibration Solutions

• Install the engine with the right mounts for
  your engine, prop and style of flying (an
  aerobatic mount for the same engine wont be as
  quiet as a cross country mount that is softer)

2nd
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16 Vibration Solutions

• Clean up the aircraft aerodynamically as much as
  possible. Use gap seals when possible and do
  tight fits for moving parts as much as possible
  (consistent with clearances needed for normal
  movement, ice buildup, etc.). Install the doors
  (including landing gear) carefully so as not to
  hang out in the slipstream.

3rd
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17. Can vibration (and noise) be stopped
completely?

• Not in a practical sense for aircraft. It takes
  weight (or more correctly mass) to stop
  acceleration and enough mass to practically stop
  vibration is very prohibitive. Ever wonder why
  you can get thicker and heavier windscreens to
  make quieter cabins? Its called the mass law
  that states every doubling of the mass drops the
  noise by about 6dB. Likewise, noise absorption
  requires thickness and barriers require mass.

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18. What is the next best solution in dealing
with the vibrations?

• Reduce the magnitude with materials that affect
  the way the fuselage, bulkheads and wing spars
  vibrate.

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19. How can the movement be reduced?

• There is another class of vibration control
  called DAMPERS or more specifically,
  VISCO-ELASTIC DAMPERS. The material used is
  half elastic like a rubber band and half like a
  putty. When stretched or moved, it recovers but
  slowly and out of phase with the vibrations in
  the panel being treated

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20. Is that what the aluminum foil covered gummy
rubber sheets are Ive seen?

• YES! That is a special type of damper (note that
  at no time have I called this a DAMPENER,
  dampening happens when you stand in the rain and
  has NOTHING to do with acoustics). If you know
  composite panel theory and about cored
  composites, the aluminum stiffens the panel but
  makes the visco-elastic material work harder
  increasing losses in skin vibration.

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• 21. Lets do a summary of the materials and what
  they do

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Summary (cont.)

• 22a. ABSORBERS Absorb NOISE (not vibration)
  once it has entered the aircraft.

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Summary (cont.)
22b

• BARRIERS Block NOISE (not vibration) from
  entering an area. A barrier behind the firewall
  is a GREAT place to install a barrier blanket
  since firewall vibration is converted to noise
  there typically in vast amounts. NOTE Many
  barrier blankets also have an isolation layer on
  one side of the barrier and an absorber layer on
  the other, (the isolation layer keeps the barrier
  from vibrating along with the structure just like
  any other isolation mount). Also a trim panel is
  a special type of barrier and like ALL barriers
  it MUST be tight against the fuselage to TRAP the
  noise!

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Summary (cont.)
22c

• ISOLATION MOUNTS, (decoupler) Reduce VIBRATION
  (not noise) into a structure. Typically a soft
  mount for engines, hydraulics, trim panels or
  anything else that generates noise. These are
  designed to vibrate out of phase with the source
  so that little energy is transmitted to the
  structure. NOTE Trim panels are VERY effective
  noise treatments EXCEPT if they are hard mounted
  and thus vibrate along with the fuselage.

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Summary (cont.)
22d

• DAMPERS A VIBRATION (not noise) control device
  that reduces the magnitude of vibration traveling
  through a structure. This is like gluing foam to
  the diaphragm on the back of a stereo speaker or
  touching a cymbal with your finger.

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23. Reducing noise in the cabin, a systems
approach

• Eliminate ALL gaps and openings into the
  aircraft! Use soft closed cell foam or silicone
  caulking to close EVERY opening since we now know
  that once air tight, noise CANNOT enter
  DIRECTLY into an aircraft!

23a
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23. Reducing noise in the cabin, a systems
approach

• Add or replace foam or rubber cushions under
  any structures that can rattle like cowlings
  against the fuselage, oil filler doors, trim
  panels against the fuselage, etc.

23b
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23. Reducing noise in the cabin, a systems
approach

• Clean up aerodynamics as much as possible
  like add gap seals to flaps, wing-fuselage gap,
  etc. to reduce aerodynamic excitation.

23c
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23. Reducing noise in the cabin, a systems
approach

• Balance engine and prop (very expensive so this
  is rarely done) but balancing the engine is the
  first thing done on a business jet, typically to
  10 of its normal tolerance.

23d
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23. Reducing noise in the cabin, a systems
approach
23e

• Make certain you have the correct engine mounts
  for your engine, prop and style of flying, (a
  cross country plane should NOT have the same
  mounts as an aerobatic plane even though the
  engine and prop may be identical).

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23. Reducing noise in the cabin, a systems
approach

• Install absorber blankets either foam, (can be
  directly upholstered over and looks great!) or
  fiberglass and bagging film. Thermal will be
  great too at very little weight, 1 of .6 lb/cu.
  ft glass .05 lbs/sq. ft. so a typical 4 place
  plane with 125 sq. ft. of area might only weigh
  5-7 lbs.

23f
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23. Reducing noise in the cabin, a systems
approach

• Install barrier blankets behind firewall or any
  other noise source (with isolation decoupler
  layer, barrier and absorber) plus
  felt/barrier/felt blankets on floor and over nose
  wheel well, wing spar, etc. Total weight app. 20
  lbs for a 4 place aircraft, (30 sq. ft. _at_ .6
  lb/sq. ft).

23g
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23. Reducing noise in the cabin, a systems
approach
23h

• Install trim panel for interior (fiberglass
  insulation batting is OK behind this) but be
  CERTAIN to ISOLATE the panel with silicone
  isolation mounts. Add ¼ foam and upholstery
  over trim panel to add absorption on the outside
  while being a barrier on the inside. Trim panels
  can be VERY EFFECTIVE but are a fair amount of
  work. About 8- 15 lbs. for a 4 place but very
  dependant on area and material.

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23. Reducing noise in the cabin, a systems
approach
23i

• Install DAMPERS against the fuselage. It only
  needs between 50 and 75 of the area of
  treatment covered and dampers arent extremely
  heavy. So if you have a standard
  stringer/longeron construction and say you have a
  panel of 20 x 10, (200 sq. inches), attach a
  piece of damper of approximately 100 150 sq.
  inches in the center and ideally, of an
  elliptical shape. This is to cover the area of
  greatest vibration, the center of the panel, that
  is acting like a drum skin. Figure 5- 10 lbs.

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23. Reducing noise in the cabin, a systems
approach
23j

• Add a plush ¼ ½ thick felt cover over glare
  shield to absorb noise radiating from canopy.
  Looks great and absorbs noise. About .25 lb

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23. Reducing noise in the cabin, a systems
approach
23k

• Add blocks of fiberglass or foam around
  instruments and behind firewall (Leave enough
  area for cooling!!!) to absorb noise under dash
  plus any other areas with excess volume like
  maybe under seats or under baggage floor. About
  .5 lbs.

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23 And last but not least

• Install a thicker but not necessarily heavier
  blanket, in FRONT of the firewall to reduce
  energy hitting the firewall, (If you have not
  installed this blanket in your aircraft yet, I
  would definitely do this and make it one of the
  highest priorities for cheap performance
  enhancement). No change in weight.

23l
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• 24. The above describes a maximum acoustic
  performance system. What is there in-between?

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In-between solutions
24a

• MINIMUM WEIGHT, ABSORBER BLANKETS OVER ENTIRE
  INTERIOR primarily thermal and cosmetic
  (upholstered over looks great!) but some high
  frequency attenuation Add ½ 2 foam
  insulation absorber blankets everywhere in
  aircraft. Total weight may only be 2-4 pounds.
  .5 1 dB reduction

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In-between solutions
24b.

• LOW WEIGHT, BARRIER BLANKETS ADDED
  STRATEGICALLY Plenty of thermal, but
  drastically reduces engine noise Absorber as
  before but 3 element barrier behind firewall
  (isolator, barrier, absorber), ¼
  felt/barrier/felt under floor mats, nose wheel
  well and wing spar. Adds 15-20 lbs. Barrier
  blanket MUST-BE TIGHT-FIT! 3-6 dB reduction.

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In-between solutions
24c

• MEDIUM WEIGHT, ADD DAMPER TO FUSE SKIN AND
  FIREWALL More thermal and more attenuation at
  all frequencies As above but add .25 lb. sq.
  ft. damper over about half of aircraft. Adds
  about 7-10 lbs on 4 place and is most effective
  on aluminum panels, doesnt work well on wing
  spar. 4-7 dB reduction.

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In-between solutions
24d

• MOST EFFECTIVE AND COSMETICALLY BEST LOOKING
  POTENTIALLY As above but add trim panels to
  interior over damper, (isolation mount trim
  panels!) and over absorber blankets. Trim panels
  must be gasketed to fuselage to seal in noise and
  eliminate rattles. Add ¼ foam and upholster
  over panels. This is a triple whammy in that it
  is a barrier, gasketed against rattles and an
  absorber. Adds about 10 lbs. 6-12 dB reduction.
  A LOT would depend on fit, isolation, weight and
  how much noise is coming through sidewalls.

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In-between solutions
24e.

• The estimates above are very very approximate
  guesses as to noise attenuation! There are WAY
  too many variables to make any guarantees. Like
  if there are gaps, your noise will be dominated
  by that coming in, not being generated by the
  engine so all the damping, barriers and isolation
  mounts will HAVE NO AFFECT except for the
  absorber (they handle noise that has already come
  in, remember?)

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Lancair Legacy test results

• Our tests revealed that most noise (at least on
  the Lancair Legacy) was coming through the
  firewall followed by the floor, so the barrier
  blankets made a HUGE difference here. This is
  probably typical so insulating the firewall and
  floor alone with barriers can make a huge
  difference and your feet will LOVE it! The main
  wing spar can also transmit a lot of noise so it
  should also have a barrier over it (the mass and
  stiffness is too great to easily damp it).

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25. FINAL SUGGESTIONS AND THOUGHTS
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Suggestions Recommendations
26.

• I strongly suggest that all materials be
  certified to FAR 25.853/23.853 flammability
  standards. While not required by experimental
  standards, it is a very important safety issue.

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Suggestions Recommendations
27.

• As stated earlier, noise can be thought of like
  water. Like a boat, first try and keep it out,
  then once it gets in, soak it up, (barrier
  between you and the source of the noise and
  absorbers once it gets in).

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Suggestions Recommendations
28.

• Eliminate hard surfaces inside the aircraft.
  Hard surfaces will bounce sound off of it and
  radiate vibrations most effectively.

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Suggestions Recommendations

• Even if the absolute minimum amount of
  insulation were added, it would still add a LOT
  of comfort if only for thermal insulation.

29.
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Lancair Legacy Cross view
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Insulating the Legacy

• Note that these blankets were made for ease of
  installation, not for cosmetics. As such, they
  do look a bit Industrial. But imagine taking
  the same blankets and applying a layer of
  Ultrasuede to them!
• We were able to remove 7.8 dB for a weight of 22
  lbs, this is not all that could have been removed
  had we added trim liners, dampers and more
  massive barriers.

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View looking aft, Turtledeck and rear bulkhead
showing
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View looking forward, cockpit side, left showing
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Cockpit sides left, note tight fit and tape
closing slit around rudder pedal cable
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Nose wheel well with closed cell foam gasket
around wire bundle penetration.
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Nose wheel well blanket with closed cell foam
gaskets to close out cable run.
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View of the 21 blanket required for complete
coverage. Rear bulkhead in background to
firewall bulkhead in foreground. Glare shield,
cockpit floor and nose wheel well are black
Orcofelt in middle.
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Dave and his screamin plane!
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Misc. holes for penetrations, etc. ?
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NO HOLES IN AIRCRAFT!!!!!!!
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Seat rails and seat supports?
83
Felt and fabric wrap around seat support
84
Headliner?
85
Trim panels
86
Far side of cabin?
87
Damper if desired
88
Side panels?
89
Dampers for aluminum aircraft
90
Cabin side of rear bulkhead?
91
Upholster over, barrier probably not needed
92
Over wing?
93
Felt and barrier only
94
Below floor?
95
Nothing, floor mounted to sides
96
Inside of firewall?
97
Dampers on thin flat part
98
Engine side of firewall?
99
Thick, acoustically absorbent blanket
100
Top of floor?
101
Felt absorber (wear resistant)
102
Engine mount treatment?
103
Harmonic dampers