What About Substituting Titanium Tubing for Aluminum Tubing in Fuselage?

Discussion in 'General Experimental Aviation Questions' started by Southron, May 25, 2012.

Help Support HomeBuiltAirplanes Forum by donating:

  1. May 25, 2012 #1

    Southron

    Southron

    Southron

    Well-Known Member

    Joined:
    May 5, 2012
    Messages:
    86
    Likes Received:
    5
    Location:
    Eatonton
    This is some of those "What If" questions from a beginner (I am considering building an airplane cause I already know how to fly them)

    What If I were to build a "Scratch Built" airplane that utilized a welded aluminum truss structure for the fuselage.

    What if I were to substitute titanium tubing (of the same size) for the aluminum tubing the plans called for?

    [1] I assume that the titanium fuselage would weigh a little less than the same fuselage constructed of aluminum tubing, is that right?

    [2] Can the titanium tubing be welded like aluminum tubing? If not welded, could stainless steel gussets and rivets join the members of the titanium truss work together? How strong would the stainless steel gussets be compared to welded joints?

    [3] Assuming that the aluminum tubing was replaced by titanium tubing of the same size-how much stronger would be the titanium fuselage?

    [4] What is the price difference between aluminum and titanium tubing of the same size?

    [5] Does anyone in the EAA ever use titanium tubing for aircraft construction?

    Your Answers will be Appreciated!

    THANK YOU!
     
  2. May 25, 2012 #2

    SVSUSteve

    SVSUSteve

    SVSUSteve

    Well-Known Member

    Joined:
    Aug 21, 2007
    Messages:
    3,879
    Likes Received:
    927
    Location:
    Evansville, Indiana
    Titanium tends to be tougher to work with and more expensive than aluminum. Those are the biggest reasons it is not commonly used in homebuilts. There are just few applications where the benefit is sufficient to overcome some of these issues (and others listed below).

    It depends on what alloys your specifically referring to.


    You have to be very careful using different metals together because of galvanic corrosion.

    As for how strong a gusset would be versus a bolted versus a welded joint would depended on the design, the alloys involved and other things that should be learned as part of the basic engineering knowledge base of anyone trying to design an airplane. It should be learned by anyone trying to build even if they are just "copying" someone else's design but a lot of people skip this step but it is best to know it so you can recognize potential issues that you might otherwise overlook.

    If you're not willing to put in the effort to thoroughly learn the vast amount of information necessary for even a "basic" design, then I strongly urge you to build from existing plans.
     
  3. May 25, 2012 #3

    Norman

    Norman

    Norman

    Well-Known Member

    Joined:
    Nov 29, 2003
    Messages:
    2,880
    Likes Received:
    918
    Location:
    Grand Junction, Colorado
    Pure titanium is comparable to mild steel but half the weight and doesn't need to be painted. There is a titanium alloy to stand in for every steel alloy if you have the budget for it but replacing aluminum with it will just make your frame a whole lot more expensive and probably not save much weight because the wall thickness of the tubing has more to do with the structural characteristics of the frame than the material. It also takes a gawdaful lot of shielding gas to weld the stuff. See if there is a custom bicycle shop near you. If the sky opens and my fortune falls out my dream plane will probably have a Ti motor mount. In preparation for that miracle I've located three cycle shops in Colorado specialize in titanium tube frames
    :beer:
     
  4. May 25, 2012 #4

    benrobuck

    benrobuck

    benrobuck

    New Member

    Joined:
    Apr 15, 2011
    Messages:
    2
    Likes Received:
    0
    Location:
    Burbank, IL
    When I was working as a machinist at an aerospace company, There were some 6" by 2" by 10" blanks of titanium that they told me were worth more than a nice sports car. Each block. All the engineers wanted to watch it get machined. I think that the cost of titanium is prohibitive for a homebuilt aircraft.
     
  5. May 25, 2012 #5

    Norman

    Norman

    Norman

    Well-Known Member

    Joined:
    Nov 29, 2003
    Messages:
    2,880
    Likes Received:
    918
    Location:
    Grand Junction, Colorado
    Kind of amazing that Russia makes submarine hulls out of the stuff, ain't it?
     
    Topaz likes this.
  6. May 25, 2012 #6

    Head in the clouds

    Head in the clouds

    Head in the clouds

    Well-Known Member

    Joined:
    Mar 11, 2012
    Messages:
    1,983
    Likes Received:
    890
    Location:
    Gold Coast, East Coast of Australia
    Aluminum welds fine with a TIG for that kind of application but I would not recommend it. Most truss fuselage frames are welded up in two halves which are then each curved to pull them together at the tail and the internal members added to form the complete box truss.

    When you make the first halves the top (or centre) longeron is usually straight and the bottom one curved. As you heat and weld the interchordal members to the longerons, the curved longeron(s) will 'give', and turn the curve into a series of straights between the welded interchordal joints. The same will happen when you add the internal members to join the halves and form the box truss. Not only that but the aly will stay very soft (annealed) in the heat affected zones.

    Titanium is a horrible material to cut, machine etc and is very expensive. As Norman mentioned it is awkward to weld but does weld well for experienced welders. One of its best (aircraft) uses is for undercarriage legs, especially when machined into a taper. Its slow springing rate has excellent shock damping qualities. But as for machining it...I use 4130 CRMO heat treated and let back to Rockwell 46 instead.

    If you want to weld a truss fuselage the most suitable material in common use is tubular CRMO, 4130N (N is the normalised condition), there has been a recent thread about designing those trusses here on HBA, the search function works well. A well designed welded CRMO truss fuselage will be lighter, for the same strength than a welded aly truss and about the same as a gussetted and rivetted aly truss. Also, if you go about the welding carefully and avoid building up too much heat then the curved members will stay that way.
     
  7. May 25, 2012 #7

    Head in the clouds

    Head in the clouds

    Head in the clouds

    Well-Known Member

    Joined:
    Mar 11, 2012
    Messages:
    1,983
    Likes Received:
    890
    Location:
    Gold Coast, East Coast of Australia
    Even stranger when you consider that they do it on a ship that's designed to sink, to make it lighter...
     
  8. May 25, 2012 #8

    Topaz

    Topaz

    Topaz

    Super Moderator

    Joined:
    Jul 30, 2005
    Messages:
    13,628
    Likes Received:
    5,279
    Location:
    Orange County, California
    And also (and more importantly) to let it dive deeper and not have as much of a magnetic signature. All things designed to keep the hidden submarine effectively hidden. Titanium isn't magic, but it has some neat properties that can be exploited for particular benefits. Nobody has yet metioned (except Norman, obliquely) that one of the principle benefits in aircraft construction is that it resists high temperatures better than aluminum or steel. However, none of our aircraft are likely to ever fly fast enough to take advantage of that property. ;)
     
  9. May 25, 2012 #9

    Head in the clouds

    Head in the clouds

    Head in the clouds

    Well-Known Member

    Joined:
    Mar 11, 2012
    Messages:
    1,983
    Likes Received:
    890
    Location:
    Gold Coast, East Coast of Australia
    Yes, saw the SR-71 landing at Farnborough in the dusk of late evening having just crossed the Atlantic in 1hr 13mins IIRC, leading edges glowing red. I had to think about that for a while until someone told me the skins were titanium...
     
    Topaz likes this.
  10. May 25, 2012 #10

    Norman

    Norman

    Norman

    Well-Known Member

    Joined:
    Nov 29, 2003
    Messages:
    2,880
    Likes Received:
    918
    Location:
    Grand Junction, Colorado
    Really obliquely. :grin: I wasn't even thinking about heat issues. I don't think it gets hot enough in an engine compartment to affect any titanium alloy. You could probably bolt it right to the exhaust manifold and never see corrosion. :think: ooooo Ti headers:cool::roll::roll:
     
    Topaz likes this.
  11. May 25, 2012 #11

    orion

    orion

    orion

    Well-Known Member

    Joined:
    Mar 3, 2003
    Messages:
    5,800
    Likes Received:
    135
    Location:
    Western Washington
    OK, let's see if I can get you more direct answers:

    The strength of Titanium is a bit variable so you really need to know what alloy you're planning to use. But assuming it's stronger than Aluminum (which it will probably will be) and the gauges are the same, the frame will weigh a bit more than an Aluminum one. Titanium is about 50% heavier than Aluminum.

    Some alloys can be welded better than others but as was pointed out above, it does require a bit more specialized technique so practice will be prudent. Using Titanium with other metals however is generally not a problem since the alloy is one of the least reactive on the metals table.

    Again, depends on the alloy. Some alloys are quite strong (well over 100 ksi) so much of your analysis will have to be based on the material you can get. Getting the material however can be a challenge since sources are a bit limited.

    Difficult to tell. Can be two to five times that of aluminum, even more for some alloys. Every now and then you can find some through an aerospace salvage yard but that requires a lot of time on the phone and then again, you have to know exactly what you're buying.

    Not that I know of. However there are some competition Russian aerobatic Sukhois that do use the alloy for their truss. As a matter of fact Russian aerospace tends to use the Titanium as the West uses aluminum in everything from fighters to the largest cargo aircraft.

    And Russia is pretty much the only major source of Titanium world-wide.
     
  12. May 25, 2012 #12

    Topaz

    Topaz

    Topaz

    Super Moderator

    Joined:
    Jul 30, 2005
    Messages:
    13,628
    Likes Received:
    5,279
    Location:
    Orange County, California
    Sorry for the off-topic, but one of the most fantastic sights I've ever seen was an SR-71 lighting into full afterburner at 50'AGL on a low pass at the former Norton AFB airshow, and climbing out to go meet a tanker for a ferry mission after the display. Your chest shook with the low-frequency sound, and there where shock diamonds on the 10-15' long J58 visible exhaust plume. An absolutely amazing airplane, all the more so since it's older than I am and designed with slide rules and paper. I was born not long after the original A-12 was making its first flights. The boldness and fearlessness of aircraft design in the '60's is something sorely missing in today's world, even in homebuilding. People were willing to try back then. The world wasn't nearly so risk-averse. One of my biggest regrets is never seeing an XB-70 flying. Nothing like that would ever be produced today. We don't have the guts.

    /rant
     
    bmcj and Head in the clouds like this.
  13. May 25, 2012 #13

    Dana

    Dana

    Dana

    Super Moderator

    Joined:
    Apr 4, 2007
    Messages:
    8,552
    Likes Received:
    2,965
    Location:
    CT, USA
    As Orion pointed out the most common titanium alloy is roughly 50% heavier and nearly twice as strong and stiff as aluminum alloy, so the same size tubing would be heavier, not lighter... you'd want to use smaller diameters and/or wall thicknesses. However, in a tube structure buckling is often the critical failure mode so the larger diameter and/or wall thickness of an aluminum tube of comparable tensile strength may well give aluminum the advantage. Then the cost, and difficulty working with it, and lack of availability in the hundreds of sizes aluminum is available in, pretty much relegates titanium to specialty applications.

    -Dana

    If we wish to "restore" respect for the law, a good start would be to pass only laws that people will respect.
     
  14. May 25, 2012 #14

    autoreply

    autoreply

    autoreply

    Moderator

    Joined:
    Jul 8, 2009
    Messages:
    10,732
    Likes Received:
    2,540
    Location:
    Rotterdam, Netherlands
    A good source might be the Russian GA aircraft. The SU29 and 31 (I think) use Ti landing gears and many others do so too, mostly in fuselage tubing.
     
  15. May 25, 2012 #15

    Southron

    Southron

    Southron

    Well-Known Member

    Joined:
    May 5, 2012
    Messages:
    86
    Likes Received:
    5
    Location:
    Eatonton
    THANKS EVERYONE FOR YOUR REPLIES!

    Regarding the "Off Topic" SR-71.....I think that the SR-71 was the first American aircraft to utilize large quantities of titanium, so it wasn't really that far 'off topic.'

    This past weekend my grown son and I went over to the Air Force Museum at Warner Robbins, GA. In one of the hangars they have a U-2 hung up above and slightly to one side of a parked SR-71.

    One irony is that the SR-71 was designed to replace the U-2, yet the U-2 is still flying and the SR-71 has been retired.

    This is a conversation I had with a museum docent about the SR-71:

    Docent: "This SR-71 holds the record for the fastest flight of any SR-71 in the fleet!"

    Me: "WOW! How fast did it go?"

    Docent:"That is Classified Information."
     
  16. May 25, 2012 #16

    TFF

    TFF

    TFF

    Well-Known Member

    Joined:
    Apr 28, 2010
    Messages:
    11,552
    Likes Received:
    3,234
    Location:
    Memphis, TN
    When the SR-71 was grounded it cost a million dollars a flight. Sadly that was about 20 years ago, but what would it cost now? They also destroyed all the tooling in the late 60s so spying on design could not be done from the inside by workers, original workers were loyal , but replacement workers, as years went by might not have been; we ran out of spare parts and used it longer than we were suppose to, no replacement thought of, just like everything else we do here, like road care.
    The biggest draw back is cost for the average person. About 10X cost of aluminum. What is funny is we have aluminum available in the US and the Russians have titanium. During the cold war guess where we had to get the titanium for our projects and guess where they got aluminum.
     
  17. May 25, 2012 #17

    bmcj

    bmcj

    bmcj

    Well-Known Member HBA Supporter

    Joined:
    Apr 10, 2007
    Messages:
    12,841
    Likes Received:
    4,799
    Location:
    Fresno, California
    Yep, I was there for that one too... definitely rattled your chest! Agree on the second point about the XB-70 too, an amazing plane. To stand underneath it and move around to different vantage points showed how big and amazing it was. I was sad when the Wright-Pat Air Force Museum moved it from outdoors to inside the hangar with other planes... you really can't step back to get appreciate the true scope of this plane now because it has so many other planes crowded around it. It belonged outdoors with room to move around it.


    Yep, we spied on Russia with a plane built out of Titanium bought from Russia. :gig: Prior to that, no one really used Titanium extensively for aircraft structures or knew how to do it. That's why Russia sold it to us so readily and cheaply (according to what Kelly Johnson told me). Lockheed had to develop techniques for working with the metal.



    Yes, Michael (forum member Toobuilder) uses Titanium rods for his tailwheel stingers. I know he did on his Starduster and think he did on his Hyperbipe and maybe even hi RV-8. You can see the stinger on his Starduster here: http://www.homebuiltairplanes.com/forums/sale-wanted/12831-starduster-too-project.html

    Bruce :)
     
    Topaz likes this.
  18. May 25, 2012 #18

    Dana

    Dana

    Dana

    Super Moderator

    Joined:
    Apr 4, 2007
    Messages:
    8,552
    Likes Received:
    2,965
    Location:
    CT, USA
    There are also some paramotors with titanium frames and cages. There, the extra strength seems to be an advantage against the kind of abuse they tend to incur. Repairing them can be a PITA though... my friend had a titanium frame, there was only one welder in the area who could fix it when he broke it (she did nice work though).

    -Dana

    I only drink to make other people more interesting.
     
  19. May 26, 2012 #19

    Capt.Crash'n'Burn

    Capt.Crash'n'Burn

    Capt.Crash'n'Burn

    Active Member

    Joined:
    May 26, 2012
    Messages:
    33
    Likes Received:
    0
    Location:
    Lompton, Ca
    Yes, TI is tricky to weld since any contact with oxygen at elevated temps (700F IIRC) will cause the weld to oxidize. The back side of the weld must be bathed in argon as well, often requireing expensive jigs to weld properly.

    In regards to heat treated 4130, many chassis in the NHRA have failed as a result of using this material and a couple of drivers killed as a result. Heat treated chrome moly tends to break at the edge of the heat affected zone of the welds. You are right that 4130N is the best material, because cracks at the edge of a weld is much less of a problem (although it doesn't eliminate it 100%).
     
  20. May 27, 2012 #20

    Head in the clouds

    Head in the clouds

    Head in the clouds

    Well-Known Member

    Joined:
    Mar 11, 2012
    Messages:
    1,983
    Likes Received:
    890
    Location:
    Gold Coast, East Coast of Australia
    Welcome to the forum CB.

    You have a few things a little tangled there which might confuse some members who have decided to use chromoly in various ways for parts of their aircraft, so with your indulgence I would like to provide some clarification.

    All CRMO (chromoly) used in the aircraft and racecar industries is supplied as 4130N, whether in sheet, bar or tube form. The 4130 is the designation of the alloy (chrome, molybdenum, iron, carbon, among other things), N is the condition of it when suupplied and stands for 'Normalised' (Austenitic structure) which is a bit different from Annealed (about twice as hard, and that's not very hard).

    4130 is considered to be a 'high carbon steel' (although it actually has very low real carbon content compared with mild steel but that's another story) and as such its hardness, brittleness and ductility can be adjusted by various methods generally collectively referred to as 'heat treatment'.

    The undercarriage legs I referred to are simply lengths of 4130N tube which telescope into each other as sleeves and by adjusting the sleeve lengths the spring characteristics of the gear legs can be conveniently adjusted. Legs like those are not welded at all and cannot be used in the N condition they must be heat treated to adjust their properties so that they are similar to the springs of a car. To do that they are heated to 900C (cherry red) and allowed to soak at that temperature for some hours then they are cold quenched, that used to be by dropping them into water but these days a polymer is used. At that stage they will be very hard and extremely brittle (Martensitic structure).

    The hardness is then 'let back' by a process called tempering which involves reheating them to a much lesser temperature 450C/850F (straw) and then allowing it to cool slowly. The hardness will be reduced from around 65 Rockwell to about 46 and the properties of the material will be similar to those of 4140 steel when conditioned for vehicle axles, springs or torsion bar suspension. There are no welds to break.

    Initially the tensile strength of the N material was around 40,000psi, was raised to about 120,000psi and then let back to around 85,000psi.

    I learnt my welding while building space frame chassis for a well known/regarded race car equipe. Welded race car space frames need to be heat treated after welding because in use they are subjected to very localised stresses which often work the welds and after welding the tubing is left hard and brittle on the outer edge of the heat-affected zone. This kind of frame needs to be heat treated but quite differently from the gear legs discussed previously. Space frame treatment is called 'stress relieving' and is conducted to do just that. The whole frame is put into the oven and heated to blue temp (a bit above straw) and then allowed to cool slowly. The result is an even condition a little softer than the N condition. This part of race car chassis preparation is often neglected because ovens tend to be small or large and the small ones won't accommodate the size of the frame and the large ones are extremely expensive to fire. Some more cost-concious race car outfits have had catastrophic outcomes from cracks that developed into failures in areas which were hard to inspect, but the statistics suggest that this was a rare outcome rather than frequent (extremely rare these days since most race cars have been built using monocoque structures in the last 25yrs at least).

    Aircraft truss fuselages and aircraft welded wishbone suspension units are usually designed to avoid areas that work (i.e. areas with bending moments are avoided, the structures are fully resolved as far as possible). High stress areas like strut and wing attach points are designed so that there is no unresolved load at all and the actual attachment plates are not simply welded onto the tubing, they are shaped so that they go around the longerons and attach in alignment with, and to, a crossmember. The welds with which they are attached never go across the attachment itself, instead they are aligned with the load that the attachment will encounter in use i.e. the weld runs along the crossmember.

    Homebuilt structures are very rarely stress-relieved, for the previously-described reasons but they rarely suffer any failure if properly designed, because the welded areas are not subject to much load. Nonetheless a lot depends on the skill of the welder to not overheat the area while welding. The fuselage area close to the engine suffers from a fair bit of vibration and it can be beneficial to stress relieve those local welds individually. For that purpose I use a propane torch and carefully and evenly heat the whole cluster joint to straw. The removal of the stressed area can be watched and as it disappears the torch should be kept on the whole area and only slowly moved away so as to allow it to cool very slowly. It is important to conduct that operation in a sheltered area away from breezes.

    In general, unless specialised knowledge has been procured in the design and fabrication of a welded structure you would never harden and temper it because the welds would be unlikely to flex in unison with the rest of the structure and so it would fail if loaded to the extent to which the unwelded portions could support.
     
    Chlomo, delta and autoreply like this.

Share This Page

arrow_white