Re: [Flightgear-devel] Frames of Reference in jsbsim

[Steve Hosgood]

Dave Culp wrote: On Thursday 11 May 2006 04:56 am, Steve Hosgood wrote One thing I've noticed about the aircraft.xml file (both the new version and the old) is that the "metrics" section (describing the physical layout of the plane) is rather "lightweight" compared with the vast other parts of the file that can provide lift and drag and stall characteristics of the airfoil. I think you're looking at JSBSim from a YASim-like point of view. It's a completely different animal. In JSBSim we use the "build-up" method to define the aerodynamic characteristics of an object. The build-up is done in the <aerodynamics> section of the config file. The important thing to know here is that if you don't put anything in the <aerodynamics> section, then your object has no aerodynamic characteristics at all. There are no inferred characteristics from the <metrics> section. You are right in that the <aerodynamics> section is the heart of the config file. It is only here that we define how the object reacts to the air. Fortunately, the ways in which airplanes react to air are pretty well known, so we only need about twenty or so entries in the <aerodynamics> section to get our object to behave like an airplane. Things are becoming a bit more transparent. Trouble is, an aircraft modeller usually starts by knowing just physical parameters (location of wings and things, location of tail, rudder etc along with amount of dihedral, angle of incidence of wing). Deriving the parameters like "yaw moment due to beta" and other such magic numbers from physical parameters is going to be pretty non-trivial. Hence the existance of Aeromatic I presume. And that's fair enough, having a "compiler" that takes "high level language" (a description of the physical layout of a plane) and compiles it down to "assembly languuage" (the 20 or so entries in the <aerodynamics> section). However, it doesn't seem possible to specify the things I was griping about to Aeromatic (wing incidence, dihedral, vertical dispacement of rudder w.r.t centerline etc). Steve.