41
$\begingroup$

In this question there is a drawing of a 787 that shows that the tail cone is made of titanium.

Why is such a heavy-duty material like titanium required for the tail cone?

I do not see how there could be any significant loads or reasons for excess temperature there.

Improve this question
$\endgroup$
3
  • 1
    $\begingroup$ It is probably to increase stiffness of the rear control surfaces- the elevators and rudder can bear enormous forces in their design envelope. Failure at that point would be a guaranteed disaster. Note the statements about use of materials in the 787 are bit misleading it is all high grade aeronautical alloys each with incredibly high strengths. $\endgroup$
    – user2617804
    Dec 29, 2017 at 9:33
  • 1
    $\begingroup$ That is the stinger, not the entire tailcone, which extends up to about the location of the last window shown. And the graphic is not indicating that the stinger is made of titanium. Rather, it is giving the overall percentage of titanium used in the entire aircraft. Each arrow does point to locations where the material is used heavily, but other materials are also used in these regions. While I know about Cessna jets, not the 787, I can say with high confidence that there is likely far more Al in the stinger than Ti. Gypaets is correct about why Ti is used there. $\endgroup$
    – Paul Sinclair
    Dec 30, 2017 at 0:11
  • $\begingroup$ @PaulSinclair -- the color coding in the diagram does suggest that it's indicating the material used in each surface. For example, it indicates in purple 10% steel (mainly for landing gear), but since the landing gear is not visible, there's no purple on the aircraft diagram. $\endgroup$
    – Johnny
    Dec 30, 2017 at 21:10

1 Answer 1

Reset to default
75
$\begingroup$

Whenever you see titanium being used on aircraft, look for heat sources. The metallic alternative aluminum is usually lighter and much cheaper.

In most commercial aircraft the APU (Auxiliary Power Unit) is in the tail, behind the rear pressure bulkhead and the empennage.

enter image description here

(modified from aerospaceweb.org)

The APU is a small turbine and its exhaust gases are very hot (I don't have exact data and it depends on the APU model, but the exhaust temperature should be somewhere in the range between 400°C and 800°C). Aluminium has not only a low melting point (660°C), but its mechanical properties degrade fast once it gets hot. On the other hand, titanium has a higher melting point (1668°C) and still performs very well at high temperatures without significantly loosing strength.

While carbon fibers don't lose their mechanical properties at high temperatures, the resin types used as a matrix are very far from being able to withstand these temperatures. Even PEEK, probably the best resin for use at "high" temperatures, has a glass temperature of 143°C (melting point at 343°C). This makes resin-based composites inappropriate. An alternative would be using ceramic materials as a matrix (CMCs). However, the simple geometry of the tail cone probably makes the production with titanium cheaper than with CMCs.

Here's an image of the cone (source: flight.org):

http://www.flight.org/r/2012/06/320/Boeing-787-APU.jpg

Improve this answer
$\endgroup$
8
  • 25
    $\begingroup$ it is not necessarily only melting points - aluminium performs really poorly at the "lower" elevated temperatures (~400ish) , softening, grain growth / segregation of alloying materials, diminished fracture strength, and not to mention increased corrosion, as the oxide layer tends to "peel off". Titanium on the other hand, is a champion of high temperature performance. (material scientist nitpicking aside, +1 for you) $\endgroup$
    – Stian
    Dec 29, 2017 at 11:03
  • $\begingroup$ Great answer, thanks. I wonder how composites would handle these temperatures?? $\endgroup$
    – Fattie
    Dec 29, 2017 at 14:45
  • 3
    $\begingroup$ Nice. Some more composite nitpicking: Resins are precursors for polymers, and mostly thermosets at that. Although thermoplastic stuff (like PEEK) is also often put under the resin nomer, this is not correct. There are thermosets that work up to more than 350°C, which are used for exhaust parts (compositesworld.com/articles/…), but it seems that Ti is the better option. CMCs are way too expensive for such large parts in a commercial setting, and while tougher than pure ceramics, they are much more brittle and less tough than Ti alloy. $\endgroup$
    – yankeekilo
    Dec 30, 2017 at 7:38
  • 1
    $\begingroup$ Heat is not the only reason to use Ti. It's used for the leading edge of carbon fibre fan blades to give them impact resistance. $\endgroup$
    – Notts90
    Dec 30, 2017 at 9:28
  • 2
    $\begingroup$ Another plus for titanium is its much better electrochemical compatibility with the graphite-epoxy structure ahead of the tailcone. Being at the extreme end of the fuselage, it must be electrically connected with the fuselage structure for lightning protection. $\endgroup$
    – Peter Kämpf
    Dec 30, 2017 at 21:53

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .