Lets see... the 2 inner runners that enter the plenum wall are 1.25" in
diameter and the 2 outer runners entering the plenum wall are 1.5"
diameter. Add a radiused bellmouth to each of the 4 runners with the
bellmouth radius equal to 2D of the pipe. I come up with a plenum wall that
is 27.5" long. This may be ideal from an airflow standpoint but it sure as
hell doesnt fit my airplane.  :-)
Mike Wills

  2 X 1.25"=2.50" X 2= 5" + 2 X 1.5"=3" X 2=6" = 11"
Perhaps I was unclear. Lynn E. Hanover

 
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