David Megginson wrote:
Since YASim uses the atmosphere model from FGEnvironment (in JSBSim,
it's always 15degC and 29.92inHG at sea level), I tried some
experiments with different settings from KSFO:
Heh, I can just (barely) imaging a day in the Bay Area that peaks
around 95F. But the Bostonian in me is lauging hard at the thought of
these californians trying to deal with -13F. :) (Fahrenheit numbers
provided for those like me who can deal in Kelvin when it comes to
physics and material properties, but never Celsius for is it cold
outside determination).
1. Sea level 35degC, 28.5inHG
2. Sea level -25degC, 32inHG
I get a density ratio between these two condition of:
(273+35) 32.0
* = 1.394
(273-25) 28.5
The density altitude difference (a butchered term -- the density
altitude that corresponds to the same ratio vs. standard sea level
conditions) that this corresponds to is about 11000 feet MSL. That's
a pretty hefty difference in density! :)
The C172 barely climbs out of ground effect, and eventually manages an
anemic 200-300fpm climb at 70 KIAS
[vs.]
The C172 finishes the takeoff roll in seconds and shoots up like a
rocket at over 1500fpm at 70 KIAS.
These are the right types of effects, but I think that the magnitudes
are a little excessive, at least for a sea-level airfield.
It's actually hard to say. On the cold day, the aircraft will get 39%
more engine power at a given RPM, which will be able to turn the fixed
pitch propeller faster and thus actually produce more power at a
higher RPM than the hot day engine. The current propeller on the 172
model has a steeper thrust-vs-rpm curve than I think it should. It
can only turn the propeller at 2200RPM at 70 knots (on a standard
day), but gets to 2400 quickly by 85 knots or so. It's possible that
that cold day engine can get it into the good region, but the hot
one day can't. This would be a model bug.
Add to that the fact that climb rate is itself a complicated number.
It's not linear with density, but in fact with the difference between
available power and required power -- themselves dependent quantities.
Maybe you could try to test differences in maximum speed?
One does read horror stories about the effect of density altitude on
the unwary pilot. I'm a little surprised by your results too, but I'm
wondering if they might be closer to the truth than you would think.
I wonder if it's because I'm not changing the air density with the
other values,
No, YASim queries only for the pressure and temperature -- it
calculates the density for itself. So whatever bugs there are lie in
these two numbers only.
Andy
--
Andrew J. RossNextBus Information Systems
Senior Software Engineer Emeryville, CA
[EMAIL PROTECTED] http://www.nextbus.com
Men go crazy in conflagrations. They only get better one by one.
- Sting (misquoted)
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