I agree that - at least - we should mention it in the documentation. We could
hypothetically also accept data in any of the supported frames. Unfortunately,
a lot of the data present in technical reports (NACA/NASA/AIAA) that I have
seen is ambiguous as to frame for the rotational coefficients. When I get aero
data it has usually already been reduced by the supplier. I do recall seeing
aero data being supplied in stability frame, once. By default, we'll always
assume rotational coefficients are expressed in body frame. We will think about
adding support for other frames, but another thing I've done is to do the
conversion inline as a function in the aerodynamics element.
Jon
----- Reply message -----
From: "Alan Teeder" <ajtee...@v-twin.org.uk>
To: "FlightGear developers discussions" <flightgear-devel@lists.sourceforge.net>
Cc: "issues Development" <jsbsim-de...@lists.sourceforge.net>
Subject: [Jsbsim-devel] [Flightgear-devel] about fdm properties
Date: Sat, May 11, 2013 9:28 AM
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
_______________________________________________
Jsbsim-devel mailing list
jsbsim-de...@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/jsbsim-devel
_______________________________________________
The JSBSim Flight Dynamics Model project
http://www.JSBSim.org
_______________________________________________
------------------------------------------------------------------------------
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
