The problem is that it is not documented that JSBSim only accepts body axis derivatives/aero coefficients for the rotary axes. The linear axis has the choice of body or wind/stability axes. I had assumed (wrongly) that as it was not specified, the rotary axes used the same axis frame as the linear axes. The result was an aircraft that was unaccountably uncontrollable at high angles of attack.
By posting here I was trying to bring this matter to Flightgear developers attention. I cross posted the same message to JSBSim and Datcom mailing lists. Jon Berndt has replied on the JSBSim list saying "I don't recall seeing a set of rotational coefficients given in stability or wind frame." Unfortunately this is far from being the case. In my working life running the research simulator at BAC Weybridge the only body axis data that I can recall being given was for the Concorde, back in 1968/9. All other aircraft that I worked on used wind axis data. Datcom is of particular relevance to JSBSim FDM authors and gives its output in stability axes. Perhaps I should add this to the JSBSim bug list http://sourceforge.net/p/jsbsim/bugs/, but this still has open bugs from 2003/4. What is needed is at minimum is a documentation update. However, due to the number of Datcom based FDMs, it would be better to add full support for stability axes. I have cross-posted this to the JSBSim list. Alan -----Original Message----- From: Anders Gidenstam Sent: Saturday, May 11, 2013 3:41 PM To: FlightGear developers discussions Subject: Re: [Flightgear-devel] about fdm properties On Fri, 3 May 2013, Alan Teeder wrote: > It exposes a serious problem in JSBSim which affects all Datcom users. Isn't it rather a missing feature than a fault in JSBSim? The JSBSim documentation and axis names are fairly clear on that the moments are specified around the yaw, pitch and roll axes (but the origin is at AeroRP rather than at the body frame origin/CoG so additional corrections according to the parallel axis theorem are added based on the distance from AeroRP to the actual CoG). If the DATCOM -> JSBSim input translation is faulty (I have no professional grounding in aerodynamics and I cannot tell) the best place to get it addressed might be at the mailing list/group for Bill Galbraith's DATCOM+ program (I think it is the most commonly used way to generate a JSBSim configuration from DATCOM): http://tech.groups.yahoo.com/group/digital_datcom/ Cheers, Anders -- --------------------------------------------------------------------------- Anders Gidenstam WWW: http://gitorious.org/anders-hangar http://www.gidenstam.org/FlightGear/ ------------------------------------------------------------------------------ Learn Graph Databases - Download FREE O'Reilly Book "Graph Databases" is the definitive new guide to graph databases and their applications. This 200-page book is written by three acclaimed leaders in the field. The early access version is available now. Download your free book today! http://p.sf.net/sfu/neotech_d2d_may _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel ------------------------------------------------------------------------------ Learn Graph Databases - Download FREE O'Reilly Book "Graph Databases" is the definitive new guide to graph databases and their applications. This 200-page book is written by three acclaimed leaders in the field. The early access version is available now. Download your free book today! http://p.sf.net/sfu/neotech_d2d_may _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
