From: del@branta.demon.co.uk   
      
   On Thu, 24 Mar 2005, in sci.space.tech,   
   kitplane01@gmail.com said:   
      
   > wrote:   
   >>A titanium structure would weigh less...   
   >   
   >No, substantially more -- NASA estimated an extra 2t -- although that   
   >would be more than made up by lighter thermal protection.   
   >   
   >>NASA did study this option.  Why did they pick aluminum?  Was it just   
   >>the cost of titanium fabrication.   
   >   
   >Why would a titanium structre weight 2 tons *more* than an aluminum   
   >one?Titanium has a higher strength/weight ratio, and although aluminum   
   >has a better stiffness/weight ration, that does not seem as relevent.   
   >I would expect a titanium wing to weigh less, be stronger, but flex   
   >more.   
      
   Yes, but flexural stiffness is a very important function of an airframe,   
   much more important than mere tensile strength.  When you look out the   
   window of a plane, you want to see the wing sitting in place, not   
   flapping up and down (not more than it actually does, at least)   
      
   And when it comes to flexural stiffness, low density counts for a *lot*.   
   It's essentially the reason aluminium alloys are preferred over steel,   
   which is after all much stronger than Al.  The reason is similar to that   
   which makes girders the shape of a capital 'I' and bones hollow: to   
   combat flexing, you want to put as much material as far from the centre   
   of flexure as possible.  Low density metals actually have a big   
   advantage over stronger but denser metals in this regard.   
      
      
      
   PS I am currently sending my contributions to this newsgroup to   
   sci-space-tech@moderators.isc.org, and they are taking days to arrive,   
   and sometimes are never posted at all.  If someone can tell me how I can   
   get my posts up more quickly and reliably, I would be grateful.   
   Thanks.   
      
   --   
   Del Cotter   
   Thanks to the recent increase in UBE, I will soon be ignoring email   
   sent to del@branta.demon.co.uk.  Please send your email to del2 instead.   
      
   --- SoupGate-Win32 v1.05   
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