Fire wall Ti?

[Stitspb]

I have a homebuilt and am thinking of putting a titanium fire wall on. I am not sure what size or grade to use? Any help would be great thank you.

[WLIU]

I'll bite. What is the engineering rationale for use Ti? The alternatives are easier to machine and lend themselves better to being shaped if you need a non-flat forewall.

The FAA specs for farewalls dictate the thickness of material that you require. A firewall needs to be able to handle a 2000 degree heat source for several minutes. If you have seen a firewall burnt through due to a failure in an exhaust system (I have), you will believe the spec. So you have to calculate how thick your material needs to be to resist the high temperature erosion for that long. Look in FAR 23 and its derivatives for the exact spec.

Best of luck,

Wes
N78PS

[Dave Prizio]

Titanium is a great choice for an airplane firewall, exept for the cost, of course. However you might be able to find some scrap material for around $200 that would be big enough to do a firewall. You need a thickness of .016 to .020 in. If you go any thicker you start losing the advantage of titanium's lighter weight. Commercial pure titanium is your best bet if you can find it. It has the lowest tensile strength, which is not really important, and is therefore easier to work with -- cut, make holes in ,etc.

The 6AL-4V alloy seems to be much more readily available but it is twice as strong and thus harder to work with.

When cutting or drilling titanium be sure to go slow and provide lots of lubrication for cooling. It tends to gum up tooling because of its poor heat transfer qualities. In fact it conducts heat about 10 times less efficiently than steel, which makes it great as a firewall material.

A titanium firewall can save about 2 pounds as compared to stainless steel on the typical single-engine airplane.

[WLIU]

What would a sheet of Ti say 42"w x 30"h cost?

Regards,

Wes
N78PS

[CraigCantwell]

From Online Metals, Ti Grade 2 sheet, .028" thick, 48"x 36" runs about 400$ plus shipping. I don't know what they are calling grade 2 alloy wise, you would have to call. Stainless is going to be 1/5th or less in cost.

[Stitspb]

Thank you all for your input. The reason Ii want Ti is to reduce weight I am rebuilding a Stitts SA3-B Playboy and I want it lite. Also a Ti fire wall will look so clean. I won't be forming it, so I only have to cut it to size and drill holes.

[Dave Prizio]

Here is a piece of .016 thick Titanium 6AL-4V that is 36" x 41" for $251.69 plus shipping.

http://www.titaniumjoe.com/6al4v%20sheet.htm

Titanium Joe always seems to have a good selection of sheet in this alloy, but seldom has anything very thin in com'l. pure for some reason.

If someone can find a source of com'l pure titanium in thicknesses in the .016 to .020 range I would love to hear about it.

Dave Prizio

[Bob H]

Thermal conductivity of Ti is around 8 while stainless is around 12 so a more realistic value for heat conduction comparison is 50% less for Ti.
You are paying mucho bucks for a relatively small benefit. Primary use of Ti is for hot structure around jet engines or special aircraft skins seeing high stagantion temps like an SR-71.
It also has value for corrosion resistance in bad environments not containing chlorides which promote cracking.
I'd vote for stainless on a cost-benefit comarison.

[steveinindy]

Thank you all for your input. The reason Ii want Ti is to reduce weight I am rebuilding a Stitts SA3-B Playboy and I want it lite.

Rule #1 of homebuilding: Don't screw with the structural design unless you REALLY know what you're doing.


Also a Ti fire wall will look so clean.

As compared to what precisely?


I'd vote for stainless on a cost-benefit comarison.

But you're overlooking the "cool" factor of having titanium in your homebuilt! ;)

[Matt Gonitzke]

Thermal conductivity of Ti is around 8 while stainless is around 12 so a more realistic value for heat conduction comparison is 50% less for Ti.

Assuming this (http://www.engineersedge.com/properties_of_metals.htm) site is accurate, you've got those numbers turned around.

[Dave Prizio]

The coefficient of thermal conductivity for 301 and 304 Stainless Steel is 113 btu/in/hr/sq ft/degree F. The coefficient for com'l pure titanium is 9.5 to 11.0, and for 6AL-4V it is 4.4. It is thus conservative to claim that titanium conducts heat 10 times less than stainles steel.

Dave Prizio

[WLIU]

All of the numbers mentioned are pretty interesting.

So since I don't know a lot about Ti, some questions come to mind.

How resistant Ti is to the application of a flame for say 5 minutes. A firewall is not just to keep your feet from getting too warm. In the real world, when an exhaust system fails, it can apply a jet of 1700 deg F hot gas to a small area, and there is an erosion component to the mechanism as well as a heating component. Ti can handle heat, but what properties tell us how it handles the type of erosion described above?

The chlorine issue was mentioned. How much exposure to chlorine degrades Ti? Can I clean my Ti firewall with Simple Green? What happens if I splash brake cleaner (has chlorine in it) on that firewall?

As an aside. I know of at least one Red Bull Air Race plane that had a Ti tailwheel spring. The pilot tells me that it cost about $2000, about 10X the cost of a stock spring.

Best of luck,

Wes
N78PS

[Mike Switzer]

Stainless also has a problem with chlorine leaching - the cheaper alloys are worse. At a job in a previous life we had a plate heat exchanger on the flue of the boiler, we would get pinholes in 304 plates somewhere within 1-2 years, 316 & 321 cost more but lasted a LOT longer.

Unless it is exposed directly to seawater or exhaust gases it really wont be a problem.

[Eric Witherspoon]

As an aside. I know of at least one Red Bull Air Race plane that had a Ti tailwheel spring. The pilot tells me that it cost about $2000, about 10X the cost of a stock spring.

Y'all probably know a bunch more homebuilts with Ti parts and don't realize it. The gear legs (all 3) on a Sonex are 6AL4V Ti rods. The price used to be comparable to tapered steel rod gear, but I believe more recently, the price of Ti is leading more of the more economically-driven Sonex builders to go with the steel gear. (Steel gear legs for Sonex are not available from the factory, but from an aftermarket light-airplane wheel/brake/gear supplier.)

Why Ti? Do a little research in mountain bikes. There is (or was) a line of bikes that used Ti frames instead of separate moving parts and springs to provide suspension. One nice thing about Ti is it has a lot more natural damping than steel. Get a Sonex down on the runway and it's going to bounce less than the comparable steel or Al gear. Provides a "smoother" ride and sucks up the bumps. Makes your landings look and feel better than maybe your skills (or the conditions) might produce in another airplane.

Of course, one feature of the Sonex gear that keeps the price down is their simple design. The mains are 1.125 round with a single hole drilled through each end. The tailwheel is 0.625 round with a single hole drilled through each end. It would probably be more weight efficient if they were tapered and/or hollow, but that's adding a bunch of cost to what are already some quite expensive parts.

[Matt Gonitzke]

The coefficient of thermal conductivity for 301 and 304 Stainless Steel is 113 btu/in/hr/sq ft/degree F. The coefficient for com'l pure titanium is 9.5 to 11.0, and for 6AL-4V it is 4.4. It is thus conservative to claim that titanium conducts heat 10 times less than stainles steel.

Dave Prizio

I don't think that's at all conservative. According to MIL-HDBK-5J, the thermal conductivities of 301 SS and Ti-6Al-4V at room temperature, with units BTU/(hr-ft^2-F/ft) are, respectively, 8.5ish and 4.4. An order of magnitude different, certainly not.

[WLIU]

Question: The average firewall is not very heavy. Does the weight savings of Ti over galvanized (the lightest firewalls) exceed 2lb?

I will suggest that there are areas of the airplane where you can get larger weight savings with less $$. Example - Use a light weight battery, a light weight alternator, and lightweight starter vs lead acid battery and Prestolite alternator and starter. There are 32+ lbs right there.

A composite tailwheel spring will be 50% lighter and is not expensive to fabricate. Use a lighter tailwheel tire.

Make the wheel pants out of either aluminum, or carbon fiber, not fiberglass. The original carbon fiber Extra 300 wheel pants were something line 2lbs each.

Benny Davis makes wing attach fittings out of Ti. In that application you can take advantage of both the strength and lightness of Ti without buying a lot of material ($$).

Best of luck,

Wes
N78PS

[abrucehughes]

I have a large sheet of "Timetal" (whatever that is) that I bought at Boeing surplus.
I found it VERY difficult to work with. It is sitting in the corner of my shop. I don't
want to sell it because I don't really know that Timetal is.

[CraigCantwell]

Timetal is a trade name for a particular vendor's titanium. Without more of the coding id info, it's not worth much more than scrap value as there are at least 23 alloys and more than a dozen grades and heat treats with the Timetale name.

[Dave Prizio]

After some further research I found that my original number for the themal conductivity of titanium was out to lunch. The reference I had was rather old and apparently just plain wrong. The thermal conductivity of 304 stainless steel and com'l. pure titanium are roughly equal. The thermal conductivity of 6AL-4V titanium is quite a bit less, less than 1/2 the other materials. In any case thermal conductivity is not going to be the most compelling argument for using titanium, rather weight savings will be.

Interestingly, if you Google titanium thermal conductivity you will find that there is not exactly universal agreement on what the exact number should be, which is further complicated by the fact that it is different at different temperatures.

Dave Prizio

[Bill]

Eric:

Did you get my PM?

[Bob H]

One alternative to using a denser metal over aluminum is to make firewall from aluminum and coat it with an intumescent. They react with flame heat and immediately expand into an insulating foam to protect underlying structure from heat damage. Engine nacelles on 727s used it effectively and oil rigs use it over steel supports. And I used it on glass sandwich cowl structure on my Pulsar. Did a test first and was amazed at the insulating qualities. But it does put out some smoke when activated.