more confusion on cessna performance chart

On Jan 16, 9:55 pm, terry wrote:
On Jan 17, 8:18 am, "Ken S. Tucker" wrote:



On Jan 16, 12:22 pm, terry wrote:

On Jan 17, 4:16 am, "Ken S. Tucker" wrote:

On Jan 15, 5:59 pm, terry wrote:

On Jan 16, 7:05 am, "Ken S. Tucker" wrote:

Humidity feeds into "density altitude" because water
vapour molecule H2O has density ~ 10 compared to
Nitrogen N2 ~ 14 *at equal pressures*

Not quite. The density is proportional to molecular weight, which
would be in the ratio of 18 for water to 28 for nitrogen ( g /mol )
But of course we are really interested in the density ratio between
water and air which would be 18 to 28.9

Ths simply comes from rearranging the Gas Equation we all learn in
high school
PV =nRT
substiute n =m/M where m is mass and M molecular weight , you
rearrange to get
m/V = PM / RT
m/V of course = density
( assuming ideal behaviour exists which is a pretty good assumption at
the pressures and temperatures involved in flying light aircraft ).

I'm guessing: but I get the impression that the onset
of turbulence over wings was also dependant on temp-
erature, even when the density altitude is the same.
In Quantum Theory that makes sense.
To start, warm air is more chaotic than cold air at the
molecular level, and the chaos *seeds* the turbulence.
You know, hot fluids are less viscous than cold and so
less sticky. That's likely a secondary correction.
Regards
Ken- Hide quoted text -

So if warm air is more turbulent ( I think I can accept that ) wouldnt
that mean that at higher temperatures for the same density altitude
you would get less lift and require longer take off distance?

" As previously stated the results are the other way around."

Cheers
Terry

I checked what you "previously stated", and the words
"correction" and "difference" didn't have the usual "+/-"
in them.
Is the Cessna handbook online, that will save time,
I'm interested.

I coulnt find it on line, but I would be happy to scan and email the
page, or even email you the Excel spreadsheet with the data and my
calculations. Then you can do all the quantum mechanics, vector
analysis, euclid geometry and reverse differentiation your heart
desires, and report back to us. Just let me know if you want to
recieve this info by email.
terry

Let's see the posted info for all to see and then we all
may examine the data equally, otherwise, shut the ****
up. Don't waste our time.
Regards

Now no need to be rude Ken, its your choice how you spend your time,
its not me that is wasting it. I posted a question which I thought
would be of interest to pilots of real aircraft for whom understanding
(or lack thereof) of takeoff performance data can literally be the
difference between living and dying. I didnt post the data set
because it is large and it was not my intention to have others spend
hours analysing it. ( although anyone is welcome to it , and my full
analysis of it, but offf line) I found something that didnt gel with
my understanding, and sort reasons for the potential discrepancy. As
for some of your suggestions Ken, all I can say mate is that you need
to come out of the clouds a little, most things in life are not as
complicated as you seem to think. I am sure you could come up with a
thousand brilliant ideas, that even your Mensa friends would be
impressed by, as to the possible reasons why a car might go put put
splutter , splutter and then stop.. but just checking the gas first
makes a lot more sense. I like physics and maths too and whilst I do
have a PhD in science ( physical chemistry) I am certainly no genius.
I have spent many many hours mathematically analysing a lot what is
involved in flying, from navigation problems, density altitudes with
corrections for humidty, wt and balance, radius of a turn with
correction for crosswind ( thanks Cain Liddle) , I even wrote a
program for an air traffic controller who wanted to be able to predict
the wind speed and direction from radar tracks and flight plan
information for multiple aircraft..... But you know what, none of
this stuff has ever required anything I didnt learn in high school,
like a good grasp of trigonometry, solve a quadratic here and there,
Newtons Laws, the gas equations etc. For christ sake leave the
quantum mechanics and anti viscosity discussions to your Mensa
meetings.
Terry

Well heck, I didn't get a High School Diploma!
What *****es me off* is the take-off roll for the
C-152 is ~ 735', so although your using a C-172,
the variation of 300' between 0C and 40C at constant
density altitude is rather high.
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.

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 18, 6:10*am, "Ken S. Tucker" wrote:
On Jan 16, 9:55 pm, terry wrote:





On Jan 17, 8:18 am, "Ken S. Tucker" wrote:

On Jan 16, 12:22 pm, terry wrote:

On Jan 17, 4:16 am, "Ken S. Tucker" wrote:

On Jan 15, 5:59 pm, terry wrote:

On Jan 16, 7:05 am, "Ken S. Tucker" wrote:

Humidity feeds into "density altitude" because water
vapour molecule H2O has density ~ 10 compared to
Nitrogen N2 ~ 14 *at equal pressures*

Not quite. *The density is *proportional to molecular weight, which
would be in the ratio of *18 for water to 28 for nitrogen ( g /mol )
But of course we are really interested in the density ratio between
water and air which would be 18 to 28.9

Ths simply comes from rearranging the Gas Equation we all learn in
high school
PV =nRT
substiute n =m/M * where m is mass and M *molecular weight , you
rearrange to get
m/V = PM / RT
m/V of course = density
( assuming ideal behaviour exists which is a pretty good assumption at
the pressures and temperatures involved in flying light aircraft ).

I'm guessing: but I get the impression that the onset
of turbulence over wings was also dependant on temp-
erature, even when the density altitude is the same.
* In Quantum Theory that makes sense.
To start, warm air is more chaotic than cold air at the
molecular level, and the chaos *seeds* the turbulence.
You know, hot fluids are less viscous than cold and so
less sticky. That's likely a secondary correction.
Regards
Ken- Hide quoted text -

So if warm air is more turbulent ( I think I can accept that ) wouldnt
that mean that at higher temperatures for the same density altitude
you would get less lift and require longer take off distance?

" As previously stated the results are the other way around."

Cheers
Terry

I checked what you "previously stated", and the words
"correction" and "difference" didn't have the usual "+/-"
in them.
Is the Cessna handbook online, that will save time,
I'm interested.

I coulnt find it on line, but I would be happy to scan and email the
page, or even email you the Excel spreadsheet with the data and my
calculations. *Then you can do all the quantum mechanics, vector
analysis, euclid geometry and *reverse differentiation your heart
desires, and report back to us. *Just let me know if you want to
recieve this info by email.
terry

Let's see the posted info for all to see and then we all
may examine the data equally, otherwise, shut the ****
up. Don't waste our time.
Regards

Now no need to be rude Ken, *its your choice how you spend your time,
its not me that is wasting it. *I posted a question which I thought
would be of interest to pilots of real aircraft for whom understanding
(or lack thereof) *of takeoff *performance data can literally be the
difference between living and dying. *I didnt post the data set
because it is large and it was not my intention to have others spend
hours analysing it. ( although anyone is welcome to it , and my full
analysis of it, but offf line) *I *found something that didnt gel with
my understanding, and sort reasons for the potential discrepancy. *As
for some of your suggestions Ken, all I can say mate is that you need
to come out of the clouds a little, most things in life are not as
complicated as you seem to think. *I am sure you could come up with a
thousand brilliant ideas, that even your Mensa friends would be
impressed by, as to the *possible reasons why a car might go put put
splutter , splutter and then stop.. but just checking the gas first
makes a lot more sense. *I like physics and maths too and whilst I do
have a PhD in science ( physical chemistry) I am certainly no genius.
I have spent many many hours mathematically analysing a lot what is
involved in flying, from navigation problems, density altitudes with
corrections for humidty, wt and balance, radius of a turn with
correction for crosswind ( thanks Cain Liddle) *, *I even wrote a
program for an air traffic controller who wanted to be able to predict
the wind speed and direction from radar tracks and flight plan
information for multiple aircraft..... But you know what, *none of
this stuff has ever required *anything I didnt learn in high school,
like a good grasp of trigonometry, solve a quadratic here and there,
Newtons Laws, the gas equations etc. * * For christ sake leave the
quantum mechanics and anti viscosity discussions to your Mensa
meetings.
Terry

Well heck, I didn't get a High School Diploma!
What *****es me off* is the take-off roll for the
C-152 is ~ 735', so although your using a C-172,
the variation of 300' between 0C and 40C at constant
density altitude is rather high.
Regards
Ken- Hide quoted text -

You didnt get a high school diploma, now I am impressed! (i am not
being sarcastic either) but I still go with what i said about
searching for the simple answers first. The 300 ft variation between
0 and 40 C was at 8000 ft density altitude where the takeoff distance
was around 2600 ft, so percentage wise it is not huge but enough to be
concerned about.

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