|
|
Posted for daniel newland <compositeguy@sbcglobal.net>:
Hi Alan
I am a Lancir ES builder and aircraft interior designer (and also build
advanced composites like carbon parts and Colyn Case's custom breaker panel).
I saw this on the LML and thought I would respond.
I looked the stuff up and think it should do some good. It is an absorber
with a barrier bonded to it so it isn't anything really majic. The only thing
I am a little concerned about is the effectiveness for damping(not "DAMPENING,
it's "DAMPER" like a fireplace damper). Without bonding to the skin and
particularly on vertical surfaces, it can't be as effective as some other
materials that are called "visco-elastic dampers". You will get some but not
very much vibration damping since it floats on the surface. And most noise
you hear in an aircraft did not start as noise per se, but rather as vibration
so the control of vibration is a HUGE deal. An example is putting your finger
on a bell or chime. You aren't stopping the noise, you are stopping the
vibration that stops the noise. Using a barrier is like encasing the same
bell in a concrete box to keep noise contained (vs. stopping it with a finger
which is acting as a damper). It is a very important part of noise
control and WILL damp the skins to some degree, just not very effectively.
One class of noise control is a "visco-elastic damper". This is very
effective and only works by being very well bonded to a vibrating skin. Just
like the chime and the finger that must be in good contact to work. The way
it works is that just like a drum head, the vibrating parts are moving back
and forth and also like a drum head, the center is moving the greatest
distance, therefore by placing a damper at the center, the greatest effect is
achieved. For this reason, dampers only need to be bonded from 50% - 75% of
the area being treated since the edges really don't move much. This area on
an aluminum plane is typically the skins beween the longerons and the frames.
For example, a pair of longerons 10" apart between frames 20" apart (200 sq.
inches), a damper 7.5" x 15" would give an area of 112.5 sq. inches or about
56% of the area between structure so this is just about perfect (I generally
subtract 12.5% around the edges to get this figure). This saves weight
and cost.
The material works sort of like a cored composite but in reverse. Cored
composite like your Legacy (carbon or glass/honeycomb/carbon) work by
transfering bending stress to the outside skins. The thicker the core, the
more the outside has to bend alternately becoming either longer or shorter as
the surface deflects. To do this, the core must have good strength so that it
doesn't shear and also high modulus so that it effectively transfers the load
to the skins. Visco-elstic dampers sort of do this the other way around. The
"Visco" means it is almost gel-like so that it deforms easily and the
"elastic" part means it returns to shape. Being sort of a gummy foam, it
recovers elastically but out of phase with the vibration. The aluminum foil
backing ads to the effectiveness by stiffening the sides making the core "work
harder" by reducing deflection in the panel and absorbing more vibration which
is in turn converted to low grade heat.
I did Dave Morss's Legacy and several things made a huge difference on his
plane. The first is that the firewall, nose wheel well and floor make upi the
majority of noise entering the airplane. You want to spend your weight and
$$$ budget on these areas as much as you can. Also, damping these areas will
help more than hitting the other areas. I should mention that I tested this
at an acoustics lab and Dave and I did 27 flight tests in different
configurations to show the various effects of various materials.
Last I should mention that you REALLY REALLY want to spend time making your
seals PERFECT to eliminate as many noise and air leaks as possible. That is
because unlike stopping vibration that will then stop noise, a hole is a
DIRECT noise path into the aircraft plus it can add whistles as the air blows
through it.
So my recommendation is to use that material over a viscoelastic damper that
is bonded to 50 - 75% of the firewall and inside the nose wheel well (watch
your clearances!). Then use the barrier/absorber stuff you were looking at
over that. For the floor, I would use damper bonded to the underside between
it and the fuselage then but a massive barier over the top of Nomex felt pad.
The Nomex acts primarily as an isolator and absorber but it also is working
relatively effectively as a damper and a barrier (gravity keeps it in contct
with the floor thus making it damp vibration effectively unlike a vertical
surface such as the firewall). A barrier over that and covered in carpet then
tends to keep the noise from entering the cabin. Plus it makes your feet
really comfortable! The entire kit I did for this Harmon F-1 weighed 14 lbs
including 8.8 lbs for the floor covering and about 4 lbs for the damping.
I have all of the materials mentioned and I can make up a a kit for you.
The damper comes in a sheet 27" x 48" x .25". The weight is .22 lbs/sq. ft.
You would need 2 sheets for the firewall, floor underside and nose wheel
well. I sell those for $75 each. I can discuss making a kit for your
aircraft that you cut out but I am moving out of California in the next two
weeks and will be setting up my new business in Port Townsend, Washington. I
could get you the damper pretty quickly but I need to hear from you quickly.
Unfortunately all of the patterns I made for Dave's Legacy were tossed out
of the company I worked for when I was doing the testing. They are out of the
light aircraft business and only work on airline stuff (I designed all of
their airline kits and did the STC's for MANY aircraft). I also bought all of
their materials and will be taking them with me to Washington so I need to
know before I pack them into a trailer if you are inerested.
Attached are some pictures of a Harmon H-1 I just did. It shows the floor
barrier and Nomex felt pad, aluminum backed damper and some of the foam
insulation pressed into place.
I hope this helped!
Dan Newland
|
|