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. Ross NextBus 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) _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
