more confusion on cessna performance chart

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"?

To answer my own question: in the USA FAR part 23 describes in
excruciating detail how these data charts have to be created and I
excerpt for GA (a bit wily nily):

Sec. 23.45

General.

(a) Unless otherwise prescribed, the performance requirements of this
part must be met for--
(1) Still air and standard atmosphere; ...

(b) Performance data must be determined over not less than the
following ranges of conditions--
(1) Airport altitudes from sea level to 10,000 feet; and
(2) For reciprocating engine-powered airplanes of 6,000 pounds, or
less, maximum weight, temperature from standard to 30° C above
standard; ...

(f) Unless otherwise prescribed, in determining the takeoff and
landing distances, changes in the airplane's configuration, speed, and
power must be made in accordance with procedures established by the
applicant for operation in service. These procedures must be able to
be executed consistently by pilots of average skill in atmospheric
conditions reasonably expected to be encountered in service.

(g) The following, as applicable, must be determined on a smooth, dry,
hard-surfaced runway--
(1) Takeoff distance of Sec. 23.53(b);
(2) Accelerate-stop distance of Sec. 23.55;
(3) Takeoff distance and takeoff run of Sec. 23.59; and
(4) Landing distance of Sec. 23.75.
NOTE: The effect on these distances of operation on other types of
surfaces (for example, grass, gravel) when dry, may be determined or
derived and these surfaces listed in the Airplane Flight Manual in
accordance with Sec. 23.1583(p).

Note the word "determined", not "calculated" or "derived" for all
except the bit about types of surfaces, where "derivation" is allowed.

There are a LOT of variables in those rules that don't lend themselves
to mathematical expressions. My conclusion is that there is no simple
formula available to apply in an Excel spreadsheet that will reliably
predict the numbers from the chart, thus your calculations seem to
have contradictory results (eg, different performance for same density
altitude).

Of course, someone with more experience in aircraft certification /
performance data generation will probably post something right away
showing I don't know a damned thing and my conclusions are completely
wrong.

On Jan 15, 11:58*am, wrote:
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"?

good points, but the data looks too smooth to me to be entirely
based on experiment, although obviously some of it must be..
I dont think moisture is the issue. the effect of moisture on air
density really only becomes significant at higher temperatures and if
this were the factor in play
then you would expect the higher temperature data to have the worse
performance ( moisture lowers the density and even at 100% relative
humidity there is very little water in air at 0 dec C)
The data actually show the opposite effect. if you plot take off
distance vs density ht. you can see 4 distinct curves wtih from top to
bottom, density altitude calculated at 1, 10,20,30 and 40 deg C
respectively. They are all smooth curves which fit a binomial
equation quite nicely. No I think the use of a different method to
convert pressure altitude to density altitude seems like the best
explanation.

terry

On Jan 14, 8:57 pm, terry wrote:
On Jan 15, 11:58 am, wrote:



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"?

good points, but the data looks too smooth to me to be entirely
based on experiment, although obviously some of it must be..
I dont think moisture is the issue. the effect of moisture on air
density really only becomes significant at higher temperatures and if
this were the factor in play
then you would expect the higher temperature data to have the worse
performance ( moisture lowers the density and even at 100% relative
humidity there is very little water in air at 0 dec C)
The data actually show the opposite effect. if you plot take off
distance vs density ht. you can see 4 distinct curves wtih from top to
bottom, density altitude calculated at 1, 10,20,30 and 40 deg C
respectively. They are all smooth curves which fit a binomial
equation quite nicely. No I think the use of a different method to
convert pressure altitude to density altitude seems like the best
explanation.
terry

http://en.wikipedia.org/wiki/Density_altitude
http://en.wikipedia.org/wiki/Pressure_altitude

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

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

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

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.
Regards
Ken

"Ken S. Tucker" wrote:
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.

Your sudden invocation of quantum theory doesn't make any sense to me. At
no point does one need to utilize the Schrodinger, Dirac, or Klein-Gordon
equations or any of their related equations in order to model or understand
the onset of turbulence.

Jim Logajan wrote in
:

"Ken S. Tucker" wrote:
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.

Your sudden invocation of quantum theory doesn't make any sense to me.
At no point does one need to utilize the Schrodinger, Dirac, or
Klein-Gordon equations or any of their related equations in order to
model or understand the onset of turbulence.



When the molecules in your coke can start sonoluminescing from the
turbulence, prolly.

Bertie

On Jan 15, 10:05 pm, Jim Logajan wrote:
"Ken S. Tucker" wrote:

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.

Your sudden invocation of quantum theory doesn't make any sense to me. At
no point does one need to utilize the Schrodinger, Dirac, or Klein-Gordon
equations or any of their related equations in order to model or understand
the onset of turbulence.

Warmer atmospheric gas has a greater photon
exchange rate and that creates repulsion, that of course
is why a heated closed volume increases in pressure.
We may term that as "anti-viscosity", where viscosity
is similiar to "stickiness".

In brief, *warm things repel warm things* better than
*cold things repel colds things*, all other things being
equal.
Regards
Ken