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

[Steve Hosgood]

Jon S. Berndt wrote: I just *know* I'm going to get totally flamed for this, but can someone please tell me how the CG, Eyepoint, AERORP and VRP are interconnected? Yeah, I know - RTFM. I'd say that, but there really isn't much of one, yet! :-( You won't get flamed. It's not the easiest concept to figure out. Before I say anything, I'd like to commend you Jon, along with Dave Culp and Erik Hofman for your three replies to my original question. *None* of you flamed me or anyone else, and answered pretty much all my questions really neatly. As you say, Jon, there isn't much of a manual (yet) and discussions like this in the archives are often the next best thing. Maybe you (as prime author of jsbsim) and the other guys could clear up one or two associated questions for me please? Issue 1, Vol1 of the Quarterly Newsletter states that the sim tracks an aircraft by its CG. >From the replies I've read so far then it would appear that Issue 1, Vol 1 meant to say "jsbsim tracks the stated CG of the empty airframe". That makes a lot more sense as the article then goes on to say that the actual flying CG of a plane isn't a useful reference because it moves. Do you confirm that the "AERORP" is the point from which the "tail arm" and "rudder arm" are referenced? Since it doesn't seem possible to specify a "wing arm", do I assume that the "AERORP" must be sited at the 0.25 chord point of the main wing? Since it is largely the relative positions of various points on the aircraft which matter most, it's somewhat arbitrary how the "base" coordinate system is defined. The XYZ points in the JSBSim config file are specified relative to a coordinate system that has its X axis pointing backwards, the Y axis pointing out the right side of the aircraft, and the Z axis pointing upwards. The origin of the coordinate frame could be anywhere, but it is usually out ahead of the aircraft, and often the X axis is coincident with the aircraft centerline. This is the "structural frame" as defined by some aircraft manufacturers. It is fixed in the aircraft for all time. OK. 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. However (and you're probably going to prove me wrong pretty quick here) it seems that the metrics section merely lists the span and width (or area) of the main wing and the same things for the horizontal stabilizer on the tail, along with the "tail arm" length. Also the rudder dimensions and "rudder arm". There doesn't seem to be any way to specify dihedral (of the wing or of the horizontal stabilizer), or that the horizontal stabilizer airfoil isn't necessarily the same as the main wing airfoil. There doesn't seem to be any way to specify the angle of the wing chord w.r.t the aircraft centerline, or the angle of the horizontal stabilizer chord w.r.t the aircraft centerline. This comes back to my original question about orientation of the X axis. I know it runs "nose to tail" but without a way so specify the wing chord you could end up with a plane that flies rather nose up or nose down. Similarly, there doesn't seem to be a way to specify where the centre of lift of the rudder is placed vertically off the aircraft centerline. Most planes have the rudder above the centerline, and surely that generates a rolling moment on the plane when the rudder is moved? You'd seem to need some concept of "rudder vertical arm" to deal with that, but if there is one then I (for one) don't know about it. Don't feel bad about "interrogating". If you have questions, please feel free to ask either here or in the JSBSim list. Let us know if the above does not answer your question. Best regards, Jon Jsbsim is a fine bit of work, Jon. Just trying to make up for a slight lack of definitive manuals, that's all! Steve.