[Flightgear-devel] YASim and the atmosphere
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: 1. Sea level 35degC, 28.5inHG The C172 barely climbs out of ground effect, and eventually manages an anemic 200-300fpm climb at 70 KIAS (it's very easy to get stuck in ground effect if you're not careful). 2. Sea level -25degC, 32inHG 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. I wonder if it's because I'm not changing the air density with the other values, or because YASim's solver is a little off. Anyway, it's impressive that this stuff is working at all -- good stuff, Andy. I have to figure out why JSBSim doesn't want to work with the external atmosphere model now. All the best, David -- David Megginson, [EMAIL PROTECTED], http://www.megginson.com/ ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] YASim and the atmosphere
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) ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] YASim and the atmosphere
Andy Ross writes: > 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. FGEnvironment is now (as of a few minutes ago) calculating density from pressure, temperature, *and* humidity (through the dewpoint) -- you might want to consider grabbing the density as well instead of calculating it internally, so that we can set up classic scenarios like this: Field elevation: 6,000ft Temperature: 40degC Dewpoint: 35degC Altimeter: 28.0inHG I wouldn't want to bet much on my chances of clearing any obstacles near the end of the runway in a C172 under these conditions. All the best, David -- David Megginson, [EMAIL PROTECTED], http://www.megginson.com/ ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] YASim and the atmosphere
Andy Ross <[EMAIL PROTECTED]> writes: > David Megginson wrote: >> 1. Sea level 35degC, 28.5inHG >> 2. Sea level -25degC, 32inHG > 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. I have done a substantial amount of flying at high altitudes, including training for and actual missions doing search and rescue at high altitudes (up to 13000+ feet). From my experience, I would say that the effects you are seeing with a density altitude difference of 11000 feet are pretty much in line with reality, for a 172. When flying at those altitudes, climb rate is minimal, and rate of turn is huge -- i.e. if you try to reverse your direction by 180 degrees, it takes you *much* longer to make that turn at 11000 feet than it does at 2000 feet. (Do the various FDMs model that characteristic of density altitude well or at all?) Cheers, Derrell ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel