Thanks for taking this on board.
The effect is only important at high incidences. With a very conventional
aircraft that stalls at 12 or so degrees incidence, and therefore operates at
rather less than that, the difference is academic, – given the accuracy of
measuring/estimating the lateral derivatives.
At the other extreme thin slender delta wings can keep working up to almost 30
degrees – look at Concorde on the approach needing that droop nose to give
forward (not just downward) visibility.
When performing stability analysis it is advisable to do the analysis using the
frame of reference and unit system in which the data was supplied. The reason
is that it is all too easy to make errors when converting the data. The average
JSBSim newbie has enough to think about without having to do all these
manipulations himself.
In general the aero coefficients and derivatives are slightly easier to
visualise in stability axes. On the other hand the forces moments and motions
are easier to visualise in body axes. As an example, we are all familiar with
the shape of the CL and CD vs alpha curves. Compare these with the CZ and CX
(these are called CN and CA in Datcom) curves which look quite strange. It is
usual for CX (the body axis equivalent of CD ) to change sign at quite small
angles (5 degrees) of incidence. You may be asked to explain away the concept
of negative drag. The same distortion of the data applies to many of the
lateral derivatives.
Thanks
Alan
From: Jon S. Berndt
Sent: Saturday, May 11, 2013 6:32 PM
To: JSBSim Developer List ; Flightgear-Devel
Cc: JSBSim Developer List
Subject: Re: [Flightgear-devel] [Jsbsim-devel] about fdm properties
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
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