This has important implications for Datcom /JSBSim users, as Datcom output is
in stability axes. It will also apply to FDM´s built from other data that was
supplied in stability axis form.
It is only significant for aircraft which are able to fly at high incidence –
typically delta wing types. At low angles of attack the difference between the
two axes systems is small, and can often be neglected.
To convert derivatives from stability axis (as output by Datcom) to body axis
(as needed by JSBSim) the equations given in table 8 of FDL-TDR-64-70
http://contrails.iit.edu/DigitalCollection/1964/FDLTDR64-070.pdf may be used.
This report was intended to have been part of the Datcom compendium.
JSBSim allows the user to select body or stability axes for the linear axes
(i.e. lift or Z, drag or X), but has no such option for the roll and yaw axes.
It is only necessary to do this conversion for the lateral derivatives.
I have taken the unusual step of cross posting this to JSBSim, Datcom ad
Flightgear developers lists as it is equally important to all three. Replies
would be best kept to the JSBSim list if possible.
Alan
From: Jon S. Berndt
Sent: Monday, April 22, 2013 4:43 AM
To: JSBSim Developer List
Subject: Re: [Jsbsim-devel] wind vs body axes
Alan,
Yes, JSBSim assumes rotational coefficients are expressed in body frame.
Jon
----- Reply message -----
From: "Alan Teeder" <ajtee...@v-twin.org.uk>
To: "Development issues" <jsbsim-de...@lists.sourceforge.net>
Subject: [Jsbsim-devel] wind vs body axes
Date: Sun, Apr 21, 2013 3:00 PM
I have just done a rather unscientific test in which I set up a simplified
flight case from my unflyable stability axis simulation into one with
lateral derivatives expressed in body axes. The result is flyable at all
incidences.
Could someone please confirm that JSBSim indeed uses body axes for roll and
yaw.
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