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After a lot of reading and three engine ground runs, I’m applying some sound deadening to the aircraft.
It’s noisy even with my Bose headset. I’ve applied Dynamat super light to the lower firewall which is all I can reach. I plan on putting patches of it on the side panels back to the seat front beam.
I know Dynamat isn’t fireproof. However if I spring a leak in the Rotax 912 iS plumbing, the fuel supply and engine can be off in seconds.
I’ll put a piece of Dynamat on the back of the baggage bulkhead which I think reverberates like a drum. I’ve got some marine polypropylene liner for the cargo area and some fire retardant spray to apply to it. That should help as well.
What did you do? What am I missing? Too much noise is a fatigue factor.
I think firewall insulation should be primarily for fire retardation and sound reduction will be a bonus. However, for fire retardation, the insulation should cover the entire interior firewall. Aircraft Spruce and others sell aluminized blankets with Fiberfrax or similar insulation. No, they won't hold back a raging fire for more than a brief time, but that few seconds or minute might be the difference between being on the ground and getting out of the aircraft vs still in flight. They're not intended as sound insulation, but if you cover any large resonating flat surface with a blanket of most any material, it'll cut noise transmission somewhat. 3M makes a "Firestop" adhesive caulk that is also fire-rated that can be used to seal the edges of the blanket around firewall penetrations.
I agree with your observation that the rear baggage bulkhead wall resonates like a drum. I noticed this during the first engine runs when the interior was bare. I obtained some airline carpet - it's burn certified and looks like a thin, short nap carpet, but it's actually a textured plastic that just looks like carpet fibers - it's waterproof and nothing sticks to it! On the back side, it has a very dense, thin layer of sound-absorptive foam (that's unfortunately fairly heavy, too!). I lined the baggage bulkhead wall and floor with it and it indeed killed the reverberations, plus it is very durable - no worries about throwing gear in back there! I used a fire retardant double-stick carpet tape from Spruce to adhere it to the aluminum. Supposedly the tape will pull up cleanly if I ever wanted to remove the carpet.
To save weight for the remainder of the interior, I can't say enough good things about "GIC" foam insulation from Hi-Tech Foams. It is not specifically listed on their website, but call or email them - they have it in stock. GIC is a sound-absorbing foam insulation about 1/2" thick. It is skinned on the surface for durability - sort of looks like black wrinkled leather - and the back side has a very thick adhesive layer than sticks really well and conforms and hides protrusions such as rivet tails. The best part, it is almost weightless and fire retardant. I used it to line my side walls, front floor, and even the baggage area ceiling. It is attractive enough (you can have any color you want as long as it is black! Ha!) to leave exposed as the final interior finish. If you put it down on the floor, it really needs a floor mat to protect the surface - I used some of my airline carpet as mats.
The picture below shows the airline carpet and the GIC foam is in the background on the side wall (click on the picture and enlarge it and you can see the texture/surface finish of the materials much better):
The effort paid off to use these materials - I've had several people tell me my 750 has the quietest interior they've ever ridden in. The interior added a few pounds (mostly due to the airline carpet), but my STOL 750's empty weight still came in at about 800 lbs (Jabiru engine, no slats) despite full paint and many options such as heavy autopilot servos, etc.
Many Bonanza's and Baron's came from the factory with a rubberized coating on many of the panels (especially the belly skins) to cut down on noise. One of the more respected rebuilders of these airframes uses the following product when making repairs. He claims it is it is gas/oil/brake fluid proof. It can be put over bare aluminum as well as epoxy primer.
Beechcraft don't tin-can quite like a Zenith, so I can't say how well it would work there, but I think its worthy of consideration.
I wonder how heavy Linex is. For the baggage are. Firewall would need something fire resistant
Edit: .7 lbs/sqft