On Jan 17, 11:40 am, terry wrote:
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

Yeah, the usual is Lift =~= density * (air speed)^2,
(all other things being equal).

However Mr. Logajan provided this chart,
http://www.engineeringtoolbox.com/ai...c-viscosity-d_...

and I was surprised to find a Viscosity diff of 8%
between 0C and 30C.
The reason I went to Quantum Theory is because it' s
sometimes easier to go down to the basement to
figure out why the house is sinking. QT can be easier
terms than high level *classical gas* physics.
((yes the song)).
Regards
Ken