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which alloy for stainless steel heat shield on belly

Desert Rat

Well Known Member
Hey all-

I know that a lot of people have incorporated a insulating "sandwich" of fiberfrax and stainless steel or titanium on the hot side of the firewall and on the belly just aft of the firewall, A.K.A. the Dan Horton mod.

I'm knocking around the idea of doing the same and have a few questions.

-what specific stainless alloy did people use here?
-prime anything to prevent dissimilar metal issues?
-thickness of steel sheet & fiberfrax blanket?
-approximate dimensions of belly shield?
-would likely install belly shield with monel blind rivets & pro seal the edges. what's a good rivet spacing?

I've read all the threads I can find on this and have a good handle on the pros & cons overall plus the need for cutouts around engine mount, seams for outgassing and so forth related to the firewall barrier. That's all well down the road for me, but it seems like the belly portion would be easiest to do prior to flipping the canoe, so I'm starting to think about it now.

comments and links to appropriate pictures are welcome!
 
On the belly of my RV-3B fuselage, I used 1/16" Fiberfrax and covered that with .020 titanium, grade 2, which I got from McMaster. They are somewhat limited in sizes so for a side by side airplane, you might want another source.

I chose .020 because it was available and not much thicker than the .016 that the FAA approves for firewalls on certified planes. I chose grade 2 because it's plenty strong (slightly stronger than 2024-T3 or T4), pleasant to work with and available at a reasonable price.

I chose titanium because it's density is lower than stainless steel. Titanium is .16 pounds per cubic inch and stainless is about .286, varying slightly with alloy.

I mentioned pleasant to work with. I enjoy working with this material.

Dave
 
My install

I don't recall which alloy I used. I picked it up at Spruce ... they sell 301 and 304. As far as install I used a 1/8" fiberfrax between the (primed and painted) belly and the SS sheet. I ran it from the sides of the cowl outlet outboard at a 45 degree angle for a few inches (to pick up a floor stiffner), and then aft about 18 inches. Its a compromise between coverage an weight. I was hoping that this would also reduce the cabin noise some, but ... well if you have to take your headset off (putting on oxygen for example) it is still REALLY loud.
 
On the belly of my RV-3B fuselage, I used 1/16" Fiberfrax and covered that with .020 titanium, grade 2, which I got from McMaster. They are somewhat limited in sizes so for a side by side airplane, you might want another source.

I chose .020 because it was available and not much thicker than the .016 that the FAA approves for firewalls on certified planes. I chose grade 2 because it's plenty strong (slightly stronger than 2024-T3 or T4), pleasant to work with and available at a reasonable price.

Dave

I wasn't aware that the FAA had approved Titanium for firewall use. I looked at it but there is a temperature (achievable with a gasoline fire) that will ignite Titanium... which put me off.
 
Titanium

I work in the big jet world, and firewalls are titanium or inconel, mostly titanium. The Titanium is lighter. I almost made my -4 forward belly skin from it, but didn't feel the need..a fire that bad and I better be already out.
 
Another thing..

I have 1/8" silicon rubber (red high temp) foam sheet used in big jet high temperature areas subject to fire, with a .020" titanium overlay in my footwells for heat and vibration protection, but I dont rely on that much for a fire situation. I guess it could help.
 
I documented what I did on my blog - Fireproof Belly

Hello, I spent quite a bit of time researching and sampling materials. I decided on Titanium and FiberFrax material vs. SS mostly because it was lighter and I found an easy source: eBay.

It is documented in the link below, scroll down to the bottom of the page. Lots of pictures. http://elstien.us/wordpress/?page_id=1000

Ti 6Al-4V (Grade 5)
Known as the “workhorse” of the titanium alloys, Ti 6Al-4V, or Grade 5 titanium, is the most commonly used of all titanium alloys. It accounts for 50 percent of total titanium usage the world over.
Its usability lies in its many benefits. Ti 6Al-4V may be heat treated to increase its strength. It can be used in welded construction at service temperatures of up to 600° F. This alloy offers its high strength at a light weight, useful formability and high corrosion resistance.
Ti 6AI-4V’s usability makes it the best alloy for use in several industries, like the aerospace, medical, marine and chemical processing industries. It can be used in the creation of such technical things as:
• Aircraft turbines
• Engine components
• Aircraft structural components
• Aerospace fasteners
• High-performance automatic parts
• Marine applications
• Sports equipment


From the FAA

Sec. 23.1191 — Firewalls.
(a) Each engine, auxiliary power unit, fuel burning heater, and other combustion equipment, must be isolated from the rest of the airplane by firewalls, shrouds, or equivalent means.
(b) Each firewall or shroud must be constructed so that no hazardous quantity of liquid, gas, or flame can pass from the compartment created by the firewall or shroud to other parts of the airplane.
(c) Each opening in the firewall or shroud must be sealed with close fitting, fireproof grommets, bushings, or firewall fittings.
(d) [Reserved]
(e) Each firewall and shroud must be fireproof and protected against corrosion.
(f) Compliance with the criteria for fireproof materials or components must be shown as follows:
(1) The flame to which the materials or components are subjected must be 2,000 ±150 °F.
(2) Sheet materials approximately 10 inches square must be subjected to the flame from a suitable burner.
(3) The flame must be large enough to maintain the required test temperature over an area approximately five inches square.
(g) Firewall materials and fittings must resist flame penetration for at least 15 minutes.
(h) The following materials may be used in firewalls or shrouds without being tested as required by this section:
(1) Stainless steel sheet, 0.015 inch thick.
(2) Mild steel sheet (coated with aluminum or otherwise protected against corrosion) 0.018 inch thick.
(3) Terne plate, 0.018 inch thick.
(4) Monel metal, 0.018 inch thick.
(5) Steel or copper base alloy firewall fittings.
(6) Titanium sheet, 0.016 inch thick.
[Doc. No. 4080, 29 FR 17955, Dec. 18, 1964, as amended by Amdt. 23–43, 58 FR 18975, Apr. 9, 1993; 58 FR 27060, May 6, 1993; Amdt. 23–51, 61 FR 5138, Feb. 9, 1996]
 
Yeah, to make it clear, if I'm on fire, my first priority is to get away from that airplane, but there was that one case where a RV10 had a fire and the guy was able to land it Fred Flintstone style with a hole in the floor where the skin had melted due to airflow pulling the fire out the bottom of the cowl.

I've flown jillions of factory airplanes that don't have anything like this on the belly and certainly don't have problem with that, especially since I'll be doing my own maintenance and I'm a pretty diligent guy. I'm still on the fence about the value added of the firewall portion vs. the chance of actually needing it, but adding the belly shield seems like a simple thing that might buy you a few minutes to get on the ground in that 1:1,000,000 time.
 
I'm copying you

Hello, I spent quite a bit of time researching and sampling materials. I decided on Titanium and FiberFrax material vs. SS mostly because it was lighter and I found an easy source: eBay.

It is documented in the link below, scroll down to the bottom of the page. Lots of pictures. http://elstien.us/wordpress/?page_id=1000

I'm in the process of doing mine, modeled off yours. Couple of things I'm doing differently are that the perimeter of my sheet will be riveted through structure on the backside. The sides are following the 2nd ribs in. I bought some rounded 1/8 inch bar that I'm planning to use for spacers but if I need to go shallower and feel the need to compress the fiberfrax I'll do that.

Spacing in your post between rivets isnt clear and the original poster asked what spacing to use so insight would be helpful. I'll be moving forward with what ever feels right, at the time. I plan to get to drilling holes next week. Did you have any trouble with primer/paint sticking to the metal after firebarrier 2000 was used? I know silicone messes up paint but haven't looked into the makeup of firebarrier yet. I plan to prime the back side of the titanium and both sides of the aluminum belly before install.
 
Feedback

I am not flying yet, so I have no data to go on.

I know the RV-10 owner who had the fire. Great guy. I see him a few times a week and often visit his hanger to gain some valuable wisdom. I have a fear of fire so I probably erred on the safer side.

I have not painted yet. Yes silicone will be a problem with paint. I tried to remove it on the edge but the painter will need to chemically and then physically clean the area. This is the bottom side and less visible.

Rivet spacing. If I was doing it again, I would use more rivets and add structure, as mentioned in my blog. I am using 1/8 rivets so spacing does not need to be as great as if using #40 rivets. It "feels" very solid. My rivet spacing was approximately 5-6 inches on center. I bit tighter along the firewall front to avoid any peel-back forces. Using a 2-3 inch on center would probably be better at very little penalty.
 
Stainless or titanium without fiberfrax?

If the goal is to avoid burn-through of the floor, why the need for a layer of fiberfrax between the stainless/titanium layer and the aluminum front floor? It does limit heat transfer, but not really as beneficial on the floor skin. Some risk I would think of the sandwiched fiberfrax absorbing moisture and promoting corrosion of the aluminum floor.
 
I documented the Fiberfrax and titanium installation on my RV-3B here in my VAF blog. It starts around post #365.

The reasons I added the Fiberfrax are that my feet rest on the belly skin and insulation would be good, and that since the titanium is the main fire barrier, a bit of insulation will help protect the floor in case of a fire.

Titanium fires seem to be more associated with powder or fine particles of the metal, rather than solid sheets. I had some .005 titanium foil so I cut a couple narrow strips and tried to ignite them with a MAP torch, with no joy. I did get them white/yellow hot, though. I'd read that adding water to hot titanium was a combustible combination, but some nearby snow had no effect other than cooling the metal. I think the blog mentions this in a later post.

Note that aluminum definitely will burn. Just ask any airplane salvage yard owner. Titanium is much less of a worry.

Grade 5 is considerably stronger than grade 2, and likewise, more difficult to work with. If you're using the titanium for structure, then go with grade 5. Otherwise I'd prefer grade 2.

Dave
 
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