Talk:Stability derivatives

Disambiguation link repair: Expert knowledge needed!
This page contains a ilnk to the disambiguation page "equilibrium". Please edit it to point to the specific most relevant page, with a link like equilibrium, or remove the link. Thanks. --Coppertwig 16:02, 10 February 2007 (UTC)


 * I have changed the link to point to 'Mechanical equilibrium', which is what is meant in this context. Gordon Vigurs 11:00, 11 February 2007 (UTC)

Body Axes and Stability Axes
I see the article uses the term "principle axes" which I think is meant to be the same as "body axes" (an aeronautical S&C term), which are the axes that are always fixed to the body of the vehicle. I will add description along these lines (as it were :-) ).

Also the stability axes are said to be alligned with the velocity vector which is not quite correct. The stability axes are alligned with the oncoming air. The velocity vector is different than the oncoming air when the air is moving, particularly if the air is "bumpy". That is really critical to the nature of "stability axes". Heck, when an airplane receives a "bump" from below, the velocity vector moves up (due to extra lift) while the direction the air comes from moves down. I'll see if I can fix it..

By the way, AOA is just "the angle between the X stability axis and the X body axis projected onto the XZ body plane". I wish I could describe it that way in the angle of attack article.

-- Gummer85 (talk) 04:14, 17 March 2009 (UTC)

I made the corrections I said I would (above) and now I am skeptical of:
 * "The z axis lies in the plane of incidence (i.e. the plane containing the relative wind vector and the longitudinal axis), perpendicular to the x axis, such that in level flight, the z axis points downwards."

This is true when Beta (sideslip) is zero, but is it true when Beta is non-zero? As I mulled this geometry about in my dimensionally-addled brain, it looked to me like an airplane in sideslip with wings level would be defined as having a roll angle in these axes. And that "roll angle" would increase with Beta. Hmmmm.... Am I right? Or just tired? Or just too lazy to pull out my books and check?

-- Gummer85 (talk) 04:48, 17 March 2009 (UTC)

I believe, after some review, that the X stability axis is the projection of the "oncoming air vector" on to the plane made by the Z and X body axes. The Z stability axis is the cross of the X stability axis and the Y body axis. The Y stability axis is the same as the Y body axis. SO, essentially, the Stability Axes are the same as the Body Axes except rotated by AOA about the Y body axis. I am unsure whether the AOA used is the "trim AOA" or the instantaneous AOA. Still looking into it.

My question still remains as to the meaning of "Wind Axes" (generally with non-zero Beta) Are they exactly the same as "Stability Axes"? Or, are they truly aligned around the oncoming wind vector, that is, not projected onto the XZ body plane? Is "trim AOA" and "trim Beta" used or is instantaneous AOA and Beta used? Still looking into this too. Tricky.

I will correct the Z stability axis definition that I complained about above. I become more convinced it is incorrect because of the "roll angle" it creates with Beta.

-- Gummer85 (talk) 19:36, 17 March 2009 (UTC)

Trim AOA it is. After more review, Stability axes are body axes rotated by trim AOA about the y body axis and "re-fixed" to the body (from my 1979/1982 Roskam blue tomes). I made edits to reflect this. -- Gummer85 (talk) 21:59, 23 March 2009 (UTC)

Thumbed?
I see the diagrams got "thumbed". I'm new and unfamiliar with what's preferred and what's considered the "right kind of formatting". It seems though that the new size is too small. Yes, it is blow-up-able, but the text refers to parts of the diagrams in a way that is disruptive to flow if the items in the diagrams are too small to readily refer to. And yes, the original size was too big and the lettering in the diagrams is too small. A re-doing of the diagrams, or a middle size might be optimal, but in the absence of the optimum, the original size is better I think. I'll change it back (in due time) if I get no complaints. . :-)

-- Gummer85 (talk) 07:16, 20 March 2009 (UTC)

Done! I bigified them to 360px. --Gummer85 (talk) 07:10, 6 April 2009 (UTC)

Can we chuck the "no inline citations" tag?
Okay, now we have one (!) inline citation. I know it's not enough, but the tag says we have none. Can the tag be corrected or deleted?

--Gummer85 (talk) 06:59, 6 April 2009 (UTC)


 * This is a very common situation. The  tag should be replaced by either  or  .  Further detailed information on these tags and what they say is available at WP:CTT.  Dolphin51 (talk) 05:29, 7 April 2009 (UTC)


 * Nifty! Thanks. Done.   --Gummer85 (talk) 19:42, 7 April 2009 (UTC)

Serious errors in the diagrams - maybe more. Major deletions proposed.
I saw that Lift is misdefined in the diagrams and capital "U" is used unconventionally instead of "V" for velocity.

The misuse of lift is serious. I made a quick and cruddy "correction" by calling out the "error" in the caption. I haven't looked in detail yet, but I suspect other unconventionalities and errors may be prevalent.

I am an airplane guy. All this is generally not how stability derivatives are done with airplanes -- and airplanes are a pretty big application of stability derivatives. Is it done this way in the missile world? I strongly suspect not. Please correct me if you know that this article's "take" on the subject is familiar to "missile people". Also please correct me if the non-inline referenced books at the bottom talk about stability derivatives in this way.

Since the "suspect" material isn't well referenced, it really is hard to tell how valid it is. This is a good situation where lack of inline references make it actually susceptible to deletion - and soon. I think the majority of this article should be deleted to a 4-5 paragraph stub until it can be rewritten.

The current material is potentially useful for ideas of what to put in a rewritten article however (as are most deleted materials). It just needs to be conventionalized, simplified for a smart lay-reader (built up in steps), colloquialized, etc.

At a minimum, it should be reviewed for outright errors such as the "lift equals sideforce" error. I will do this (when I get around to it) regardless, unless someone else gets to it first.

--Gummer85 (talk) 18:19, 6 April 2009 (UTC)

Introduced S&C Derivatives' application to Flight Simulators.
I made a major rewrite of the "Linearization/Simplification" section. I renamed it "Uses" and put the "original stuff", now colloquialized, under "Uses". I introduced use in simulators for engineering analysis and flight training.

It's vastly improved, while still needing a lot of work. I did some engineering analysis of maneuvers using a simulator for the X-29 some years ago (1991) and we called the tables of S&C derivatives the "Aero Model". I wanted to confirm the term was still used that way. When I googled it I got a bunch of references to model airplanes. Can anyone confirm this usage?

I am glad it occurred to me that whole racks and banks of stability derivatives are used in everyday home flight simulators. Now the subject seems much more relevant for lay people. Before, I kinda felt it had notability problems. Now we're important! :-)

--Gummer85 (talk) 19:20, 7 April 2009 (UTC)