Talk:Aircraft dynamic modes

Poor choice of title?

"Instability modes" implies that the modes are not stable. This is untrue, at least closed-loop, for every aircraft flying today, with the notable exception of the spiral mode (where on some aircraft the pilot closes the loop to stabilize the mode). "Aircraft dynamic modes" strikes me as a better description of the contents. I don't know if I'm allowed to move pages, but if I can I may if there's no objection, and if it doesn't disturb the proposed merges. Patrick O'Leary 19:11, 15 November 2006 (UTC)

I'll give this a little more time, but I do see the merge proposals have been dropped from the page. Patrick O'Leary 17:50, 27 November 2006 (UTC)

I'd suggest to call it (more precisely) "Aircraft rigid body modes", as elastic modes (bending / aeroelastics, etc.) are not considered. 138.246.2.175 (talk) 11:55, 27 April 2018 (UTC)

Merge w/ Flight Dynamics?

Now I'm wondering if this article should really exist at all, given that all the good math is already in the Flight dynamics article. Perhaps a merger is in order? Patrick O'Leary 14:54, 14 December 2006 (UTC)

The Flight dynamics article is long, and probably quite daunting to some readers. I think a separate, more accessible, article is in order. Gordon Vigurs 17:56, 15 December 2006 (UTC)

Perhaps, but is this article really it? I know I just moved it to this title, but I would guess that anyone looking for information on aircraft dynamics and using the word "mode" is probably at least somewhat knowledgable in the math. I think if there were to be separate "basic" and "advanced" flight dynamics articles, we'd have to re-edit both this and flight dynamics to pick and choose, and that the bulk of this article should be under the title flight dynamics and the bulk of that here--that is, get the math into the more technically titled article. Patrick O'Leary 01:59, 20 December 2006 (UTC)

I like this article and, though it needs improvement and expansion, it is notable in itself. The FD article is afflicted with math and pedantry and an over expansive subject. This has the potential to make the subject actually usable by lay readers. "Mathematics lets fools do what only geniuses could do without it." (http://www.uh.edu/engines/epi1534.htm)

--Gummer85 (talk) 05:46, 29 May 2009 (UTC)

The Pendulum Effect is a Myth

"Also, a non-aerodynamic force is imposed by the relative vertical positions of the c.g. and the lift, creating a roll-in leverage if the c.g. is above the centre of lift, as in a low wing configuration; or roll-out if below, as in a high-wing configuration (a pendulum effect)."

Chief author(s), please remove any reference to the "pendulum effect." Although, unfortunately, it's an all too common belief, it nevertheless is a myth.

When a symmetric airplane banks, the projections on the roll plane of the of the lift and weight lines-of-action must intersect near the CG. Therefore they cannot form a couple, cannot create a restoring rolling moment and cannot contribute to the dihedral effect.

The case of a pendulum is different and irrelevant. In a pendulum, the weight acts downwards at the bob and the reaction acts upwards at the point of attachment. When the pendulum is displaced, the weight and reaction stay parallel to each other but displaced from each other, thus forming a restoring couple. There is no attachment point and no reaction in a free-flying airplane, therefore the pendulum analogy is invalid.

Yet, an upper wing configuration does contribute, significantly, to the dihedral effect, but the reason is aerodynamic, not mechanical. It has to do with wing-fuselage flow interaction: the fuselage creates a "fence" effect on the windward wing root and a "shadow" effect on the leeward wing root so that a marked difference in wing lifting efficiency appears between the wings, providing a strong restoring moment.

Dov elyada (talk) 10:44, 26 January 2015 (UTC)

 Done True, very similar to pendulum rocket fallacy. By the way, overwhelming majority of articles on Wikipedia have no long-term editors at all, so I'm just fixing it. --Kubanczyk (talk) 23:12, 15 January 2016 (UTC)

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Horizontal short period oscillation

Perhaps it is correct as written but presented differently than I am used to, but it seems to me that the three horizontal modes described are not distinct or do not exist at the same time and that the third mode is the yaw analogue of the pitch short period oscillation, the other "static" stability rotational mode that does not strongly couple with center of gravity motion because of its short period. David R. Ingham (talk) 00:44, 30 May 2017 (UTC)

Instable Dutch role

To my knowledge, the Dutch role is mandatory to be stable and handling qualities only allow spiral mode and (for some conditions) the phygoid to be instable. Are there any sources for aircraft that have a instable Dutch role by design (no accidents, etc.)? 138.246.2.175 (talk) 11:58, 27 April 2018 (UTC)

Spiral Divergence

The section on spiral divergence is a good intuitive way to introduce the problem but has a couple of challenges. First, yaw rate arises only indirectly from slip. When unbalanced lateral force (Lift * roll perturbation) accelerates the aircraft rightward, sideslip develops, but this is not the source of turn rate. Next CNbeta yaws into the flow. CNb, the source of lateral stability, is the culprit here.

The yaw rate is what "causes the left wing to go faster", multiplies Clr, which is why the wing rolls in.

The discussion is missing CNr which is the counterpoint to CLr. CNr drives beta UP, activating Clb, stabilizing the divergence while CNb drives it down, decreasing stability. There's a well known (but not memorized, I'll go look it up) spiral stability characteristic involving Clb, CNr, CLr, CNb.

There is a tension between CNr and CNb which stabilize and destabilize the spiral mode, and not much you can do about it. "More vertical stabilizer" drives them both. The article discusses a nuance, the fuselage contribution to Clb when Clb is substantially from dihedral and sweep, but leaves out the important impact of yaw rate.

Moreover, at speed, there's less yaw rate for a given trim bank turn (g*sin(phi)/V) and thus low speed favors stability, again through CNr. I think that may be why the author cites powered flight as unstable? Maybe it's just because pilots fly slower when gliding? Markrkrebs (talk) 11:14, 6 November 2021 (UTC)

Found it: Nr - Nb*Lr/Lb gives the root. See how Nb*Lr are the destabilizers here, Nr and Lb are stabilizing. (we want negative roots, and usual the signs of those terms are -, +, +,- respectively. This is a crude approximation, but shows how the main terms matter. Markrkrebs (talk) 12:18, 6 November 2021 (UTC)