On Jan 19, 10:34*am, "Peter Dohm" wrote:
"terry" wrote in message

...
On Jan 17, 5:39 am, Rajat Garg wrote: Guys,

I just came across this -

http://www.pilotoutlook.com/calculat...ude-calculator

It may be useful for other students.

They also have other flight planning tools
at -http://www.pilotoutlook.com/flight_planning/calculators

Thanks Rajat. Its fun to play with but *I dont know many pilots that
actually worrry too much about the moisture effect on density. Its not
that great except for very humid and hot days where it will be no more
than 10%. * *Its best to calculate the density altitude when you are
actually at the airfield ready to go, in which case you prollly wont
have a computer. *Easiest way is to just set the Kollsman window for
1013.2 ( or 29,92 if you are still behind the times :) *) .
which of course gives you your pressure altitude then just correct for
temp. *Add or subtract 120 ft for each 1 deg C above or below the ISA
temp for that pressure altitude. With a bit of practice you should be
able to do it in your head in less than a minute
eg *set 1013.2 *and say your altimeter reads 1500 ft that is your
pressure altitude. ISA temp would *be 15-2x1.5 or *12 deg C. *if your
actual temp read off the aircrafts outside temp gauge *is say 20 deg
C , then add 8 x120 or 960 ft to get 2460 ft density altitude.
Terry
PPL Downunder

---------------new post begins----------------

I agree that the effect of moisture is greatest at the highest temperatures,
when the air can hold the mosture and that the effect of moisture on density
altitude is admittedly small for aerodynamic purposes--notably lift.

However, I have heard that the effect on power is much greater. *For
example, I would expect the same amount of moisture to cause a 1 percent
drop in aerodynamic lift and approximately a 6 percent drop in power--at the
same temperature and pressure.

I do not have the real numbers available, and am not currently flying; but I
believe that they are well worth the research for those who are!

Thats interesting Peter, Water does have a very high specific heat so
perhaps it just absorbs a lot of the heat of combustion so there is
less to convert into mechanical energy.? Surprising then that
humidity isnt included in the take off charts, where power is
critical. But then there are lots of anomalies with take off charts.
Piper data shows take off distance to be a function of density
altitude whereas Cessna tables show that take off is also a function
of temperature, ie higher temperatures give better performance for the
same density altitude. ( or you could also say higher pressures at the
same density altitude are better) . the best explanation I had for
the latter was that at higher pressures there is more driving force to
get the air into the cylinder.
Hope you get your wings back soon.
Terry