Jim Wilson wrote: > Andy Ross said: > > The bad pilot technique theory is looking pretty good. If YASim can > > get to 1400fpm at 300 knots, then I move that it be declared > > officially innocent on all counts. :) > > Well maybe not yet...it is better with today's patch...but still run > out of steam in the 29000 range. It also may be time related. In > otherwords the situation gets worse over time.
It's not time related. First off, ditch the autopilot. It gets stuck behind the power curve every time I use it. When it does this, it very rapidly (in the space of 30 seconds or so) gets you trimmed all the way down to 200 knots, at which point the engines can't keep you in the air anymore and you have to dive to regain airspeed. Just trim for speed; this is really easy to do in an aircraft this big. You can verify that the climb performance drops slowly off toward zero as the altitude increases, and recovers as you descend. I've done a zillion of these this afternoon. :) That said, here are two more layers to this giant onion of a bug that won't die: First, the air pressures returned from the environment system don't agree with the standard atmosphere that YASim uses to do its calibration. They match pretty well at sea level, but diverge as altitude increase. At 35000 feet, they're too low by 20%, which is substantial. Could someone check and see where the environment system is getting its numbers? The ones YASim uses are typed in from McCormick, who got them from "The ARDC Model Atmosphere". I verified this afternoon that they are correct, so I'm pretty sure the bug lies elsewhere; some difference is fine, of course, but 20% difference at FL350 is a lower low pressure system than any hurricane. Ever. Replacing the new environment system with the old one yields significantly improved performance at altitude. Second, I realize now that the cruise performance numbers in the 747.xml file are tickling a problem. They are specified as a maximum speed of 530 KTAS, which I got out of a book somewhere. Problem is, this corresponds to 0.92 mach. At that speed, the plane should be seeing more drag than YASim calculates because of (as yet unmodelled) transsonic effects. But since YASim doesn't model this extra drag at high speed, it solves for an airplane with extra drag at all speeds. I've replaced the cruise setting in my .xml file with the long-range cruise numbers I found in David's link (the 75% throttle is just a guess), and this makes things better still. ... <cruise speed="490" alt="36000"> <control-setting axis="/controls/throttle[0]" value="0.75"/> <control-setting axis="/controls/throttle[1]" value="0.75"/> ... But, after all that, things still aren't quite right. The plane should, by the definition given in the solver output, fly at 490KTAS, FL360 with an AoA of 2.5 degrees. But it doesn't. At that altitude (which is attainable with the above modifications), it wants to fly much faster to stay in the air. I'm still working on it. And did I mention not to use the autopilot? If you must, watch it carefully. If it tries to trim the plane below 300 knots, shut it off! 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
