On Jan 15, 10:24*am, quietguy wrote:
Your PA-to-DA calculations clearly differ from Cessna's, probably
because they used a different standard atmosphere. *There are plenty
to choose from: International SA, U.S. SA, ICAO SA (all revised over
the years) and some others, some of which are no longer used. *You'd
need to find out which SA was used by Cessna when the 172N was built.
Good luck with that project. *I would just plot some points from the
POH and draw a smooth curve connecting them; I'd be conservative in my
choices of data points and call that good enough.

Some very sensible suggestions quietguy, I know I could just be
conservative and plot a curve through the higher set of data on the
graph, but
the curiosity in me just wants to find the reasons. I think you are
probably right about the standard atmospehere and i have had no luck
yet finding out exactly what Cessna used, but I can tell you after
some manipulation of the data by just changing the correction factor
for pressure to density altitude of 120 feet to 80 feet per 1 degree
off isa standard atmosphere temp, the points revert to the single
smooth line of takeoff distance vs density altitude that I was
expecting.
Does that ring any bells with anyone re some other version of a
standard atmosphere?
Terry