On Aug 7, 3:32 pm, Dallas wrote:
On Tue, 07 Aug 2007 18:31:41 -0000, Jim Logajan wrote:
So when Jeppesen said "assuming all other variables remain constant" it
was basically saying "assuming the density remains constant."

Are you pretty comfortable with that statement? I can't imagine why
Jeppesen would bother to publish the paragraph if the assumption was that
the density would remain constant, which is basically impossible outside
the laboratory, (at least, as far as I know) and makes whole statement of
no value to a pilot in the real world.

"Assuming all other variables remain constant". - I picture two barometers
a few miles apart on a consistent, flat surface. There is no wind and the
sky is overcast. A hole in the overcast opens up and heats the area around
the first barometer. If I correctly interpret what Jeppesen appears to be
saying, the pressure in the area of the heated barometer will rise above
the barometer in the shade.

OK, I think I understand what's going on. You are interpreting
something that, while technically correct, is aimed at non-physists.
All it is trying to say is that if you have the same mass of air
contained in the same volume, but with the air at different
temperatures, the pressures will be different. One of the main issues
of contention that I have is that we are talking about energy
transfers that affect multiple different component variables (e.g.
mass, volume, and temperature) in an open system, and trying to close
the system AND hold all but one of them constant (the old "ignoring
friction" routine).

I still don't like their use of "exerting pressure" on the surrounding
atmosphere, because I really don't think it is. However, I guess you
could demonstrate their statement by using an infinitely thin, capped,
and flexible column surrounding a mass of air (think condom shape).
if you heated the air inside of it, you would see the column walls bow
to the pressure changes. But the walls bowing is a demonstration of
the pressure differential and attempt to establish equalibrium more
than anything else, and handwaves the fact that pressure isn't defined
this way.

I think Jim is right; a more important relation is how a particular
temperature reading at a particular pressure reading relates to the
density of the air at altitude. This is probably one of those cases
where cursory hand wave...it's close enough about how it works is
less important than the effects that it has on an aircraft's
performance at a given altitude for different conditions.

(see also further example in r.a.s.)