more confusion on cessna performance chart

On Jan 15, 9:49*am, Dallas wrote:
On Mon, 14 Jan 2008 12:56:46 -0800 (PST), terry wrote:
So why would the takeoff distance required vary with temperature at
the same density altitude?

I confess that I'm lazy today and didn't read what you said for
comprehension, but I will address the above statement.

DA mantra:
Density altitude is pressure altitude corrected for non-standard
temperature.

I agree
Ok, so if DA has been corrected for temperature, then we're done. *

The "vary with temperature" part of your statement doesn't make any sense
if you are using DA, the temperature factor has been applied already. *You
wouldn't apply temperature again to come up with an answer.

No I havent corrected twice, the point of the post was that after
correcting pressure altitude for temperature to get density altitude,
different combinations of pressure altitude and temperature which
result in the same calculated density altitude give different
performance figures.

The statement above would make sense if you said: *The takeoff distance
required will vary with temperature at the same "pressure altitude".

yes it would make sense but then I would have had no need post the
problem, which is that the same density altitude calculated using
different combinations of pressure altitude and temperature do not
give the same performance. For example, Dallas if the presssure
altitude as 5000 ft and temp was 10 dec C, that would be a density
altitude of 5520 ft.
Now I could get the same density altitude of 5520 ft if I had a
pressure altitude of 3000 feet and a temperature of 30 C . Do we
agree on that? I I would then expect that the takeoff distance
required for either of these 2 scenarios would be the same. The point
of the post being that this not the case with the data set in the
table.

I could have explained it a lot better with graphs but I dont think
you can do that on usenet.

Terry

On Mon, 14 Jan 2008 16:20:34 -0800 (PST), terry wrote:

I could have explained it a lot better with graphs but I dont think
you can do that on usenet.

I've been known to use graphics to ask questions here.. like this:

http://img.photobucket.com/albums/v1...ofileChart.jpg

--------------
Oh, and this might interest you... as mentioned in an other post, the
algorithms to determine DA are not "fixed" they are a black art of sorts.
Here's a good example, two of the same model but different editions of an
electronic E6B with the same values entered yield two different answers for
DA & PA.

http://img.photobucket.com/albums/v1...SportysE6B.jpg

Go figure.
--
Dallas

On Jan 15, 11:20*am, terry wrote:

*For example, Dallas if the presssure
altitude as 5000 ft and temp was 10 dec C, that would be a density
altitude of *5520 ft.

DOH not a good example that should have been 5000 ft and 9.3 deg C to
give 5520 ft

Now I could get the same density altitude of 5520 ft if I had a
pressure altitude of * 3000 *feet and a temperature of 30 C . Do we
agree on that? *I I would then expect that the takeoff distance
required for either of these 2 scenarios would be the same. *The point
of the post being that this not the case with the data set in the
table.

I could have explained it a lot better with graphs but I dont think
you can do that on usenet.

Terry

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.

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.

Good point , perhaps that is the issue,the data I have do not specifiy
what standard atmosphere.

Thanks
Terry

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

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

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.

Just when I thought this was the correct reason. I have now further
analysed the data in the flight manual and looked at the landing
distance required data which was in exactly the same form , ie a table
of distance required as a function of different combinations of
pressure altitude and temperature. With this data table after
converting to density altitude, i get a nice smooth curve of landing
distance required vs density altitude ( as I would have expected with
the take off distance data). This would seem to eliminate the use of
a different standard atmosphere as the cause of the discrepancy.
Whilst I will certainly take your advice and use the conservative
line, my curiosity ( and stubboness) wont rest until I understand the
reason for this.

I am sure someone at Cessna would be able to explain it. Anybody know
who I should contact?

terry

terry wrote in news:11884665-471a-47c3-881b-
:

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.

Just when I thought this was the correct reason. I have now further
analysed the data in the flight manual and looked at the landing
distance required data which was in exactly the same form , ie a table
of distance required as a function of different combinations of
pressure altitude and temperature. With this data table after
converting to density altitude, i get a nice smooth curve of landing
distance required vs density altitude ( as I would have expected with
the take off distance data). This would seem to eliminate the use of
a different standard atmosphere as the cause of the discrepancy.
Whilst I will certainly take your advice and use the conservative
line, my curiosity ( and stubboness) wont rest until I understand the
reason for this.


Yeah, I can appreciate this.
I have seen figures run that come up with different figures at the end
before and what it appears to me to be is performance engineers using
differnet approaches.

I am sure someone at Cessna would be able to explain it. Anybody know
who I should contact?

Get on the phone and ask! Or e-mail them. They have an interest in
ensuring the flying public have confidence in how well their airplanes
perform. I believe if you ask five engineers to crank those figures you
will come up with five different appraoches and answers, though.

Bertie

So why would the takeoff distance required vary with temperature at
the same density altitude?, it goes against everything I understood
about peformance being a function of the air density.

Any help appreciated.
Terry
PPL downunder

I wonder if Cessna used formulas at all. I would think rather not.
They probably measured all of those values during the certification
process. I don't see how any aircraft could get its performance info
certificated based solely on mathematical calculations. You have to
test the plane for realiable data.

If I'm right and all those data points come from actual flight data
(and an average of that, too), then it's not a big surprise that
simple calculations regarding density altitude don't seem to make
sense.

Also density altitude calculations that consider only temperature are
at best approximations -- good enough ones for most conditions,
probably. But density altitude is also dependent on moisture content
of the air, which is perhaps even less known in a given air parcel
than temperature.

Has anyone on the list ever worked with or for Cessna who might know
how they generate their performance charts? Experimental measurement
-- or calculated "guess"?