K

kcameron

Well Known Member
I've been thinking of ways to reduce the weight of my RV-4. The ideas so far range from the simple (install Odyssey battery) to medium (replace aluminum prop with composite version) to radical (build custom parts out of exotic materials). One idea in the radical column is installing a titanium exhaust system. If I go to the trouble, I'll replace the current 4-pipe system with a 4-1 collector.

It seems that titanium might be an ideal material for exhaust systems. Scratching about on the web, I found these numbers for the two materials. I chose 321 stainless and 6a1-4v titanium since they seem to be common exhaust system materials.

321 stainless 6A1-4V titanium Units Titanium/Stainless
density 8000 4429 Kg/m^3 0.55
tensile strength 515 896 MPa 1.74
modulus 193 114 GPa 0.59
Specific strength .064 .202 MPa/Kg/m^3 3.15
Specific modulus .024 .026 Gpa/Kg/m^3 1.08

Sorry for the formatting. I haven't been able to figure out how to create a decent table with this forum system.
References:
http://www.assda.asn.au/asp/index.asp?pgid=17971
http://www.rembar.com/Titanium.htm

So it seems that a titanium exhaust system would be 45% lighter than a stainless exhaust with the same dimensions. However, since the strength is so much greater, one might be able to use thinner-walled tubing and achieve even greater weight savings: maybe as much as 60%. Moreover, these numbers seem to be at "ambient" temperature. I'm thinking titanium's advantage at the high temps of an exhaust system might be even greater. Unfortunately, I haven't yet been able to find good numbers these metals at high temps.

From what I've gathered, many titanium alloys have excellent corrosion resistance; even in harsh environments. But, still, perhaps the longevity isn't as good as stainless? I've seen that some aftermarket performance companies are producing titanium exhaust systems for cars and motorcycles so it can't be completely inappropriate. Obviously, the cost would be higher (probably by a lot).

Does anyone here know of other reasons why titanium shouldn't be used in an aircraft exhaust? I'd like to gather some of the excellent knowledge generally available on this list before contacting exhaust vendors about it.

Thanks.
 
Titanium and Inconel have been used in race car exhaust systems for years. Both work. Inconel being the better material in this application. You can go thin with Inconel but only to the point of useful welding ability of the stuff. You have it right- cost (material and welding) is the big reason you don't see much of it in general aviation. You won't save a whole bunch of weight over a proper 321 SS exhaust.

Way better way to lose weight would be a composite prop or a PC680 battery if you don't already have those.
 
I believe the new Corvette uses Ti exhaust. I'm sure it would be an interesting engineering task to create a Ti system for your plane.

An easier weight savings might be to find a way to increase engine efficiency (EI, FI) to allow for less fuel to be carried. The weight savings on the Ti exhaust might only be 4 pounds, if that. That is equal to two thirds gallon of fuel. Cut fuel burn by a pound or two per hour, and you have saved the weight.
 
Ti exhausts were more or less standard on a lot of Russian stuff back when they were the Evil Empire. I have a brand new set of pipes for my M14 radial, but they're getting rare. TI was cheap in a closed market, but today they sell it to Boeing <g>.

Dan
 
Russian Ti

DanH said:
Ti exhausts were more or less standard on a lot of Russian stuff back when they were the Evil Empire. I have a brand new set of pipes for my M14 radial, but they're getting rare. TI was cheap in a closed market, but today they sell it to Boeing <g>.

Dan
Click to expand...
No kidding! They had the lions share of the world's titanium supply and built entire pressure hulls of submarines out of the stuff. Danged fast ones too!
 
Strange Bedfellows

Ti exhausts were more or less standard on a lot of Russian stuff back when they were the Evil Empire. I have a brand new set of pipes for my M14 radial, but they're getting rare. TI was cheap in a closed market, but today they sell it to Boeing <g>.
Click to expand...
mlw450802 said:
No kidding! They had the lions share of the world's titanium supply and built entire pressure hulls of submarines out of the stuff. Danged fast ones too!
Click to expand...
Titanium ore occurs worldwide, even in the U.S. Russian ore just happens to be the most plentiful, purest and most highly prized ore of all. Even through the Reagan "Evil Empire" days, ALL of America's F-15's were built using vast amounts of imported Russian titanium. It is difficult for me to describe the immense size and complexity of the 3 main wing spars, each intricately machined from a single titanium billet that is heart to each wing half. In addition, its massive inboard wing skins are titanium, manufactured with stiffeners and risers machined into them. Way too heavy to be carried by hand, a hoist is always used to move them about. Impressive stuff. Irony abounds in the world of international commerce. For 2 years, I singlehandedly reworked Israeli-made F-15 horizontal stabilizer assemblies (right out of the packing crates from the Sea of Galilee) by removing (with a die grinder!) a few thousandths thickness along the length of its titanium main spar so the assembly would nest into its mating part, the leading edge which was produced (to correct tolerances) in my building. Ironically, many F-15's though I suspect but cannot positively confirm coming from that block, were produced for and purchased by Saudi Arabia.
 
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Welding Ti

We use certain Ti alloys in the manufacture of our jet engines. Proper welding of titanium requires use of a vacuum chamber filled with an inert gas to prevent oxidation at the joint. Something not available from Avery or Spruce.

Steve
 
If you want all the good info on shaping and welding really thin-walled Ti tubing, go to the guys who make Ti bike frames.
 
As with all dynamic, high temp applications of thin walled tubing, you do this at your own risk and expense. I've been building performance turbo exhaust systems for many years and frankly the thin tubing used on aircraft breaks and cracks a lot. A friend of mine fixes this stuff every week. It is really too light in many cases, especially on turbocharged engines. For want of 2-3 more pounds added with thicker tubing, reliability would double.

Having done lots of mountain biking with 2 guys having Ti bikes- well both developed cracks within 2 years of not totally extreme riding. My 4130 and 6061T6 frames had no problems. People tend to push the engineering fudge factors when using more exotic materials sometimes, leading to failures.
 
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Another resource for Ti applications would be from companies that make racing exhausts for motorcycles. Ti has been used sucessfully for many years for extreem racing conditions (24 hour races, etc) on high strung superbikes and powerful twins. I have no doubt it could do the job. "M4" is a company/brand made in Texas that does custom work.

Mcstealth
 
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