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