Since SPA doesn't allow pipes or retracts I've never really understood the 1975 cutoff. Planes in the late '70s and early '80s were getting bigger and heavier. Flying an EU1-A, a Brushfire or an XLT with a non-piped, side exhaust sixty is not an advantage over flying a smaller Curare or Dirty Birdy with the same engine. In fact, it would probably be a handicap since these later planes need the extra power of the hotter rear exhaust piped engines to fly their best. Does that make sense? I mean, if I had to use a side exhaust, non-piped sixty I probably wouldn't choose an 850 sq." design. Right? So, what practical effect does the '75 cutoff serve?

Just my own thoughts and opinion.

david