Dave Perry wrote: > I have continued to work with the dc3.xml for yasim and some of the > numbers in the original file seem inconsistent with the 3D model. > Here are my questions:
The model and YASim description don't agree exactly. The biggest difference is (I think) the location of the origin. YASim puts it at the tip of the nose, whereas the model uses a nominal c.g. instead. This can be fixed by adding offsets to all the YASim coordinates. I did this for a copy of the A-4 (that, er, I haven't checked in), and it made the model animations much better. What I did for the A-4 was pull it up in ac3d, find the coordinates of the nose (note that the axis conventions are different), and subtract those values from all the coordinates in the YASim file with a perl script. > 1. Shouldn't the z value for the wing be -0.95 as the z for the wing > is for the root mid cord. This is a low wing. This matches the tank > and the engines better also. This looks like a typo. Uh, yup. That looks pretty wrong. I'll fix this tonight; I think I still have the sheet of paper with all of my original 3-view measurements. :) > 2. The z value for the tail wheel, z=-1.5, seems to leave the tail > off the ground at rest in the 3D model with compression="0.2". I have > tried increasing the compression to 0.3 to 0.5 or increasing z to > -1.0. I don't think the tail wheel compressed very much at all -- just the compression of the tires. My original 20cm value was almost certainly too high already (I made it so for numerics reasons -- very short compressions lead to correspondingly high spring constants, which increase the likelihood of the tail "bouncing" unphysically due to aliasing interactions with the 120 Hz integration interval. I figured that no one would notice an extra 20cm of "squish" in the tail on hard landings.). The best thing to tune is almost certainly the tail position. But only do this once the coordinate origins match. > Then the over-the-nose visibility is terrible. In a real dc3 it was > not so great until the tail came up in the takeoff roll. Visibility issues really need to be addressed using a 3D panel, which the DC-3 doesn't have yet. Right now, the panel lives at the same place on the screen regardless of what the resting orientation of the aircraft is. The Right Thing is to figure out a position for the panel inside the aircraft's coordinate frame, and fix the panel there. This then begs for a 3D-modeled cockpit environment, though, to avoid the "panel floating in space" effect. :) > 3. Is there a way to add damping or make the stiffness of the > compression for the main gear? It is very easy to get an oscillation > the feels like a pre-bounce while accellerating on the mains or while > rolling out from a wheel landing. Um, no, although there's no good reason for this. The spring constant is currently computed automatically as sufficient to support a landing at 3x the vertical velocity of a normal 3 degree approach. The damping is sufficient to critically damp a normal landing. This works surprisingly well for most aircraft, so I never added the ability to tune these parameters. You're the first one to really worry about ground handling in such detail. How would "spring" and "damp" attributes on the <gear> tag sound? These would be unitless numbers, so spring="2" would indicate a spring constant twice as high as the normally computed one, etc... > 4. Does yasim report an actual Center of Gravity? I know that it > checks to see if the approach config can be achieve with trim. This > would tell me if I was going overboard with the ballast. There's a C.G. line at the bottom of the solution report. I believe it's for the approach configuration. Thanks for all your work. I'll try to get you an origin-consistent model ready tonight, with support for the gear spring tuning. 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
