Thanks for your detailed answers.
First, John is right when he recalls that "Attack angle is around the
horizontal left-right axis". Probably Mike was rather suggesting
"platform_sideslip_angle". Is that it ?
Second, you all seem to agree that platform_yaw is "difference between
orientation and course" (degree). I personally can't say much on that
topic, I'm not a native english speaker.
But in the case described by John, I would rather call
"platform_drift_angle" the difference between the platform orientation
(which way it's pointing) and the actual direction of travel (track made
good over the ground).
I think that it corresponds to a static point of view.
Dynamicians will rather call "yaw" the rotation along the Z axis, and
thus will it "yaw_angle". You can find this kind of naming in satellite
dynamics. But I fear that it doesn't correspond to the norms, at least
for planes.
I agree it's difficult to find something definitive in this domain (I
have access to French norms, but not to english version of ISO
documents).
I would just like to mention that in "The Dynamics of Flight/The
Equations" by Jean-Luc Boiffier (John Wiley & Sons, 1998), the angle
between the platform axis and a fixed direction is called Azimuth, and
noted Psi.
The author states that the conventional range for Psi is ]-Pi, Pi], but
this is not required by ISO-1151 standard, it is only an ANSI
recommendation.
It is written that the azimuth angle is sometimes called the yaw angle.
And, when the fixed direction is the geographic North, the azimuth is
called true heading (and noted Psi_v); when ?fixed direction is the
magnetic North, the azimuth is called magnetic heading (and noted Psi_m)
(pages 27 and 28 of the book, equation 2.39)
So, if anybody with access to ISO-1151 standard can confirm this, I
think this could be considered definitive.
I had a quick look at RAF bulletin Nr 23, with no great success : ?Don
Lenschow and P. Spyers-Duran, are using the "Psi" name, saying it
correspond to "true heading" when using a meteorological and inertial
navigation frame of reference, and only us the word "yaw" in "yaw
maneuvers". They refer to Etkin, B., 1959: Dynamics of Flight. John
Wiley & Sons, editor.
Other references, anybody ?
Bruno.
Le jeudi 23 octobre 2008 ? 16:55 -0700, John Graybeal a ?crit :
> Platform_yaw_angle as defined per Bruno/CF makes more sense to me, but
> it appears to be incorrect with respect to the current definitions you
> can easily find for yaw angle. (hard to find something definitive
> here) Attack angle is around the horizontal left-right axis -- it is
> the difference e.g. between the airfoil angle and the line of movement
> of the airplane through air.
>
> I wish platform_orientation (which is 3D to me) could be
> platform_pointing_direction, by the way, since it can't be platform
> heading.
>
> But that aside, the difference in the original definitions is that the
> first (platform orientation) is measurement of aircraft orientation
> with respect to a fixed external reference (true north or magnetic
> north being the most common), so it's a number between 0 and 360;
> while the second (platform yaw angle) is the difference between the
> platform orientation (which way it's pointing) and the actual
> direction of travel (track made good over the ground), so it's
> typically a number between -10 and 10 for airplanes, and highly
> varying numbers for ships. (A slow ship in high currents may have to
> point almost totally upstream to go horizontally, which would make for
> a yaw angle near 90 degrees.)
>
> john
>
> On Oct 23, 2008, at 4:08 PM, Godin, Michael wrote:
>
> > Bruno,
> >
> > Like all decisions made by committee, the decisions on these names
> > seem rather odd now. I think I had originally proposed only
> > "platform_heading" to indicate the "forward looking" direction of a
> > platform, and Roy Lowry wisely pointed out the ambiguity between
> > longitudinal axis and direction of travel.
> >
> > I countered with "platform_forward_looking_direction".
> >
> > Nan Galbraith noted that on "ships, true_heading is the gyro
> > reading, corrected for magnetic declination. Course is the direction
> > of the ship's movement, generally acquired from gps."
> >
> > Chris Webster noted that for aircraft "track_angle is our direction
> > of travel."
> >
> > After a discussion of true vs magnetic heading, Nan asked "Is there
> > anything wrong with using platform_heading, platform_heading_true
> > (or platform_heading_corrected), and platform_course?"
> >
> > Jonathan Gregory noted and asked "The problem with "heading" is that
> > it means direction of travel as well as the direction you're
> > pointing in. That's what I understood it to mean when Michael
> > proposed it, and why I asked if it was the same thing as course.
> > Looking on Google, I find that half the definitions are direction of
> > travel.
> > Hence "heading" is too confusing, I'd say. How about
> >
> > platform_orientation (degree) for the direction the platform is
> > pointing in platform_course (degree) for the direction it's
> > travelling in?"
> >
> > And then (quite clearly out of my mind), I noted: "The one case I
> > can think of a yaw variable being useful and measurable is in the
> > case of towfish, in which yaw is typically defined as the difference
> > between orientation and course. I imagine there may be other
> > similar useful situations, so I'd support adding the variable:
> > platform_yaw ('difference between orientation and course')(degree)"
> >
> > I'm not sure why I inexplicably used the word "yaw" when I should
> > have used "attack angle".
> >
> > Anyway, everyone was sick of the discussion at that point in time,
> > and the variables were added with no further discussion as:
> >
> > platform_orientation ("pointing direction") (degree)
> > platform_course ("travel direction") (degree)
> > platform_yaw ("difference between orientation and course")(degree)
> >
> > However, the documentation for platform_yaw (wisely?) disagrees with
> > the above.
> >
> > If I were sane and looking at this today, I'd 1) declare
> > platform_yaw to be "travel direction", 2) make platform_course an
> > alias for platform_yaw, and 3) add a new standard name
> > platform_attack_angle -- which is a variable I'm going to need soon.
> >
> > Mike
> > _______________________________________________
> > CF-metadata mailing list
> > CF-metadata at cgd.ucar.edu
> > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>
>
> John
>
> --------------
> John Graybeal <mailto:graybeal at mbari.org> -- 831-775-1956
> Monterey Bay Aquarium Research Institute
> Marine Metadata Interoperability Project: http://marinemetadata.org
>
--
Bruno PIGUET
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Received on Fri Oct 24 2008 - 07:43:28 BST
