I wrote: > Jim Wilson wrote: > > It appears that the thrust/altitude curve is a bit too steep. [...] > > Also there seems to be a greatly exagerated "ram effect" (not sure > > of correct term). It seems that airspeed changes might be affecting > > the thrust value too greatly, but I don't have a feel for this at > > all. > > OK, this doesn't surprise me too much. I haven't examined the Jet > stuff very closely.
OK, I've examined the jet code a bit more closely, and it actually looks pretty good to me. Attached is a graph of available thrust vs. speed and altitude for the engines as modelled on the 747-400. I threw together a little program that looped over the Jet object, and played with gnuplot's really nifty contour feature a bit. (Apologies for the binary attachment -- it's only 9k, which puts it right at the border of acceptability, IMHO). All the features look about right to me: Thrust falls off linearly as speed increases; this is due to the reduction in exhaust velocity relative to the aircraft. But at speed, it starts increasing again due to the V^2 scaling of the static air compression as it enters the engine. The numbers might be different for the real thing, but the appearance of the graph is spot on. So let's fix the numbers. One thing I can see is that the CF6 engines on the 747 are listed as "60k lbs" of thrust. But they're also flat rated at 33 degrees C (306K) (that is, below this temperature, you can't use any of the extra thrust). As it happens, YASim calibrates to a standard atmosphere of 288K. Air at the flat rated temperature is 6% less dense than the air YASim is calibrating to. So if the engine can get 60k at the hotter temerature, it should really be listed as a 63706 lb engine in the configuration file. This will give you too much power at takeoff, as YASim doesn't yet implement an engine governor. But the performance at altitude might match what you want to see more closely. Try it out and see how it feels. 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)
