When a duct needs to turn a corner, consider that the air is rushing along
the duct very fast, and the fan cannot create much air pressure behind it, so
corners must be as smooth as possible. For example:
Making a bend in a tube is easy, take the angle you want the bend to be, halve
this, and you will get the angle you need to cut at. See the diagram below.
A tube cut in this way will fit together snugly when you put the two parts
back together, with one rotated 180 degrees. Any duct with a bend more than
about an 8th of a turn will loose much of its efficiency, so consider putting
a fan on each end, and using two joints if you need to make a 90 degree duct
for example.
