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Composite Sandwich Firewall

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Composite Sandwich Firewall

Old 05-24-2008, 12:31 AM
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Default Composite Sandwich Firewall

I would like to build my next airplane entirely with composites in lieu of the wood. I thought that honeycomb/carbon would work for the formers and servo trays. The BIG QUESTION is what do I use for the firewall? The kit comes with a 1/4 inch lite ply that is "doubled, one on each side of the fuse lip. I have heard that "end grain" balsa sandwich with carbon or maybe kevlar might work as a replacement to the wood. Are there other sandwich combos that would be better? Would you countersink or insert a "hard point" of wood where the engine mounting bolt passes through? I have heard that bolting an engine down tight to a composite firewall will compromise the structural integrity of the composites. BTW the kit is about 40 pounds and the engine is capable of about the same in thrust. FWIW firewall is about 12" x 12".
Old 05-24-2008, 02:00 AM
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Default RE: Composite Sandwich Firewall

Before you invest too heavily in the sandwitches there is something you should consider. A carbon epoxy composite is twice as stiff as Aluminum and over seven time stronger, so imagen how much more stiffer and stronger it is the plywood. The weight savings over aluminum is about 40%. It is kind of like comparing apples to oranges to compear either of the above to plywood as plywood, especially lite-ply as it is the weakest of the plywoods, and specification are hard to come by. Aerospace Composite Products sells a 1/4" think carbon and end grain balsa sandwich (LP-25) that would be work well, but yoss you will have to use compress tube. They recommend aluminum tubes and use a 1/8" plywood backing plate for you blind nuts to grab in. Not sure that in the end it would be worth it. It sure would be fail safe properly ettached in the fuselage, but not sure you would save weight.
Old 05-25-2008, 01:10 AM
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Default RE: Composite Sandwich Firewall

Hi ByLoudDesign, What is the compress tube? I see where a blind nut backing plate eliminates the "point load" on the composites. What about on the front side? [sm=confused.gif]
Old 06-11-2008, 04:07 AM
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Default RE: Composite Sandwich Firewall

A "compress tube" is a short piece of tube that is glued in as a liner in the hole, and ties the two skins togehter and prevents crushing the core.
If the firewall is 12" x 12", then you should consider attaching some strengthening ribs across the back of firewall, to make it stiffer. Otherwise you may end up with a thin "membrane" that vibrates like a drumskin. (Note that the firewalls on most ARF's are very small, as they are made up as an engine box. If your firewall is the full width of the fuselage, then it may vibrate.)
I have used 1/4" endgrain balsa with carbon facings for firewalls, but not at your size.

Magne
Old 06-11-2008, 10:46 AM
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Default RE: Composite Sandwich Firewall

Why not use wood as the core in your sandwich, at least for your firewall?
Old 06-11-2008, 11:44 AM
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Default RE: Composite Sandwich Firewall

If you use wood you are not going to save mush weight, and are going to a lot of trouble for nothing. Better to use a couple of 1/8" (3 mm) Carbon laminated sheets cut them out, one square to the sheet side and the other at 45° and epoxy them together!
Old 07-04-2008, 08:05 PM
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Default RE: Composite Sandwich Firewall

How does this sound. I start with a .50 End Grain Balsa/.028 Carbon Shell Sandwich roughly 12"x12" outside diameter to fit the Fuse. The center is then relieved some 6 inches (lighting hole), leaving a 3 inch "Former." This center hole is covered by a n engine box constructed of .25 End Grain Balsa sandwiched with 6oz. Carbon/Aramid Hybrid. The materials as well as the "box" design (box would be something less than 90 degrees to taper to a minimum engine mounting area)would stiffen up the area. I realize that the firewall will require some aramid stringers to distribute the load across the fuselage to the rear, additionally I would need to install aluminum sleeves with ply backplates for the engine bolts and blind nuts. I am not sure how many layers of 6oz Hybrid cloth to use for the box, additionally any recommendations on best epoxies for the sandwich would be appreciatedIs West laminate epoxy okay? I thought 3M DP-460 or BVMs Aeropoxy would do the trick with some aramid tape to glue the joints. This is a new experience for me so if I am way off, please steer me in the right direction.
Old 07-18-2008, 04:36 PM
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Default RE: Composite Sandwich Firewall

you only need the peremisis of the fire wall to be hard like a former (Balsa End
Grain).. the center can be Rohacell 45 with 120g T1 carbon cloth on both sides.

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