In other news....
We've sent an email to the Rotary Club. We're explaining that our
current aluminium wheelchair prototype will not work if we don't
send it for heat treatment, and our budget doesn't allow it. So
we're asking them whether a proof-of-concept prototype can be built
instead of a fully-operating prototype.
If they say no, then we'll be forced to turn to mild steel. The
plus side of mild steel is that its mechanical properties don't
change with welding, and we can (somewhat) offset the increased
density with a reduction in tube thickness.
The bad thing is that we'll have to import the steel tubes from the
US - which will of course take time and money.
Right now I'm doing recalculations to make sure that the parts that
I'm designing do not fail under several modes of mechanical failure
(buckling, shear, combined tension/bending).
It's causing me one big uber headache. 
