Talk:Longitudinal stability

Creation of a Lateral-Directional Stability Page?

I'm surprised a lateral-directional stability page does not already exist, unless it does under a different name/is a subsection of another page? There is a generic directional stability page (mostly for cars it looks like), but it's probably not a good idea to place information about airplanes in there or link to it from here because it misleads/misses the critical interaction between directional and lateral stability. — Preceding unsigned comment added by 2601:601:9F80:6AB0:D87A:841F:8BA2:FC2B (talk) 17:15, 8 March 2020 (UTC)

Incorrect diagram

In all conventional aircraft, the center of gravity must be ahead of the center of lift of the main airfoil in order to achieve positive static stability. The diagram in this article incorrectly shows the center of gravity behind the center of lift.

As the center of gravity moves aft toward the center of lift, stability decreases and is lost when the center of gravity is behind the center of lift. Some modern aircraft do fly with reduced or even negative static stability because static stability is achieved with a drag penalty created by the drag produced by the negative lift of the horizontal tail. This penalty can be reduced in civil aircraft to reduce drag and thus fuel consumption. Static stability can be virtually eliminated in some military aircraft in the interest of extreme maneuverability. There is a cost for this. These newer aircraft can only be operated safely by relying on active computer assisted controls. The controls must counter every disturbance from trimmed flight, perhaps many times per second. Otherwise these disturbances may cause the aircraft to depart controlled flight altogether. —Preceding unsigned comment added by 69.255.237.10 (talk) 16:39, 5 February 2010 (UTC)

The above asserts that; 'the drag produced by the negative lift of the horizontal tail.' I am of the opinion that the tailplane in modern commercial aircraft produce a vertically upwards lift component, not downwards. Downwards lift would make the aircraft dynamically unstable.

58.8.235.159 (talk) 07:09, 8 March 2010 (UTC) R.A.P

Disagree with "Incorrect diagram"

It is common for cg to be aft of wing aerodynamic center. -Krebs — Preceding unsigned comment added by 2620:0:10F2:FD00:755D:E84A:3B5D:4449 (talk) 20:23, 17 August 2015 (UTC)

Either condition is common enough (it is a popular fallacy that the tail must always exert a down force), in fact the lowest-drag ideal is for the tail force to be zero and for the centres of wing lift and gravity to coincide. Real life often approximates this condition and sometimes passes through it. Another popular confusion is that between the centre of lift and the aerodynamic centre. The fact that one is usually in front of the centre of gravity does not mean that the other one necessarily is too. These confusions are all present in the above discussion: read the article carefully and you will see that all is explained. — Cheers, Steelpillow (Talk) 09:00, 18 August 2015 (UTC)

The center of gravity must be in front of the airplane's neutral point, which lies between the aerodynamic centers of the wing and horizontal tail. It is the location at which the airplane would be neutrally stable if its CG were there. As long as the CG is in front of the neutral point, the airplane will have positive static stability, even if it is behind the wing AC. The horizontal tail will produce lift in whichever direction is necessary to trim the airplane, and that can be positive or negative. The AC of a surface is the location at which the pitching moment is constant with angle of attack (but only for angles of attack within linear aerodynamics). The center of pressure (what you are calling 'center of lift') is the position where at a given angle of attack the integrated pressure over the surface of the wing results in a force with zero moment. Measuring the force anywhere else will also return an additional force couple moment, hence the existence of a pitching moment in airfoil data taken about the quarter chord. *EDIT- To clear this up the neutral point should be included in the diagram, so a change may be warranted. — Preceding unsigned comment added by 2601:601:9F80:6AB0:B140:D618:52F8:C3E5 (talk) 02:52, 4 March 2020 (UTC)

This also jumped out at me. CG should be ahead of the center-of-lift. While perhaps there are configurations where CG is aft of CL, most non-technical readers of this wikipedia page are going to be mislead by the diagram because nearly all, if not all, light aircraft are going to be configured with CG forward of CL. Potentially this could mislead people with catastrophic results. Consider that some newly minted pilot sees this and comes to the conclusion the POH is wrong because they saw aft CG on wikipedia indicating to them it's acceptable. Personally, for safety reasons, I think this diagram should be corrected. — Preceding unsigned comment added by 192.184.143.229 (talk) 16:25, 29 May 2023 (UTC)

Wikipedia is in harmony with reliable sources on this subject - none recognise a “center of lift”. The aerodynamic force on a wing acts through a point on the chord line momentarily called the “center of pressure”. (This point moves about as angle of attack changes so it is not a fixed point.) It is convenient to decompose the aerodynamic force into two components - lift and drag - and both these forces must act through the center of pressure. Wikipedia and other reliable sources do not use the expressions “center of lift” or “center of drag”.

On a symmetric airfoil the center of pressure is located at about 25% of the chord but on a cambered airfoil the center of pressure moves around when the angle of attack changes - as the angle of attack reduces, the center of pressure moves in the rearward direction. At very high speed the center of pressure lies behind the aircraft so it is impossible for the center of gravity to lie behind the center of pressure at that speed. (When the center of pressure lies behind the aircraft it exerts a large nose-down pitching moment which is counteracted by a similar nose-up pitching moment exerted by the horizontal stabilizer.)

With neutral longitudinal stability the center of gravity marks a position called the neutral point. For positive longitudinal static stability, the center of gravity must lie forward of the neutral point. Dolphin (t) 14:23, 30 May 2023 (UTC)

I realize the diagram is technically accurate, but it can be accurate and also potentially confuse or mislead those of lesser expertise to make potentially unsafe decisions. I would suggest the addition of some qualifying remarks that would aid readers to understanding that CG must be forward of the NP for safe flight in light aircraft. 192.184.143.229 (talk) 03:16, 31 May 2023 (UTC)

The diagram was created by User:User_A1 and, as far as I'm aware, only that User can alter it. Only A1 has the software that defines the diagram so only A1 can change it. We could ask A1 to make the desired changes but this User has not edited at Wikipedia since September 2010 so I doubt our request would achieve anything. That leaves us with only two options - retain the diagram as it is; or remove it. Does the diagram add value to the article? What do you think? If we remove it we have another two options - remove without replacement; or replace it with another diagram.

You have written "CG must be forward of the NP for safe flight in light aircraft." This is actually incorrect. When the CG is at the neutral point the long stab is zero and that is unsafe - adequate long stab is necessary for safe flight in any civil aircraft and most military aircraft. If the CG is a small distance ahead of the NP the long stab is positive, but only weakly so, and that is also unsatisfactory. The aft-most CG at which safe flight has been demonstrated is called the "aft CG limit". The forward and aft CG limits constitute the permitted CG envelope, and these limits are published in the flight manual for every type certificated civil aircraft, and most military aircraft too. Most large aircraft have a loading system that informs pilots that their weight and CG are within the permitted envelope. Your edits here make me think you are a pilot. Every pilot is required by law to ensure the CG is within the permitted weight and CG envelope during flight, and therefore the CG should never be aft of the aft limit and should never reach the neutral point.

The concept of neutral point is not a concept that needs to be understood by most pilots. It becomes a part of the knowledge base for test pilots, aerospace engineers and possibly loadmasters. Pilots need to have a good understanding that the weight and CG limits published in the flight manual, or the loading system provided by the aircraft operator, define the permitted limits of weight and CG, and compliance with those limits is required by law. Dolphin (t) 12:50, 31 May 2023 (UTC)

Wrong axis

Longitudinal Stability describes what a layman would call "Roll." LATERAL Stability is Pitch..... —Preceding unsigned comment added by 71.194.185.138 (talk) 03:58, 13 January 2011 (UTC)

No. — Cheers, Steelpillow (Talk) 09:01, 18 August 2015 (UTC)

Cleanup Protocol?

There are comments below [above], all now either invalid or actioned (corrections implemented in the article), so I think the balance of this page is defunct, and would like to delete it. That feels capricious & unilateral so I won't. Instead I'll do some research to see what the habitual techniques of maintenance and housecleaning before presuming to delete. Meanwhile, any reading this be advised (one person) thinks the article's pretty good and the below [above] comments no longer relevant. Markrkrebs (talk) 13:29, 13 May 2012 (UTC)

All is as it should be. This talk page stands as a record of the discussions. If it ever gets too long, then the earlier discussions can be archived. I moved your comment down to the correct place - new topics go below earlier ones - and edited it accordingly. — Cheers, Steelpillow (Talk) 09:04, 18 August 2015 (UTC)

Suggested addition

This article is mostly very difficult I would say. I think it would benefit greatly with the addition of one image, ie pitching moment v. lift or alpha, and a few explanatory words. If someone wants to make up the image I'll supply the accompanying text. Giving readers this image and explaining what it shows is , I believe, the single most important bit of information worth imparting to those who don't know the subject. I say this because it actually enables them to make some sense of actual pitching behaviour of real aircraft since it is a common way of presenting such from wind tunnel results.

If anyone wants to do an image I can suggest a good one on which to base it.Pieter1963 (talk) 15:36, 28 September 2021 (UTC)

Recent changes

Cagliost: The first sentence in the lead says “... under steady flight conditions.”

The article about steady flight indicates that it cannot include motions of the kind considered in dynamic stability. Therefore the first sentence refers only to static stability but you have deleted the word “static”. Dolphin (t) 12:08, 29 June 2022 (UTC)

I'm going to make further changes. The article already had a section on longitudinal dynamic stability, so I moved the article from "Longitudinal static stability" to "Longitudinal stability". It doesn't make sense to have separate articles for static and dynamic. But I still need to clean up. The article was a mess anyway. cagliost (talk) 12:54, 29 June 2022 (UTC)

I’m not referring to the change of name of the article. I’m referring to removing the word “static” from the first sentence but retaining mention of “steady flight conditions.” The first sentence is now in error. It isn’t a good idea to introduce errors and then say you will fix them some time in the future.

Here is a useful hint. Copy and paste the article of interest into your personal sandbox and then work on it in your own time. When you have finished copy and paste back into the article. If the changes are extensive you can leave the new version in your sandbox and use the Talk page to alert interested Users and invite them to peruse it and make any comments. When the dust has settled you can copy from your sandbox and paste into the article. Cheers! Dolphin (t) 13:08, 29 June 2022 (UTC)

I see you have now solved the problem by removing mention of “steady flight conditions”. Thanks for making that repair. Dolphin (t) 13:14, 29 June 2022 (UTC)

Merge Static margin

I propose merging Static margin into this article. Repetition, substantially similar content, not clear why we need a separate article for what is basically a definition.

The only snag is the (unsourced) section in "Static margin" about missiles. cagliost (talk) 14:50, 29 June 2022 (UTC)

Done. cagliost (talk) 09:46, 6 July 2022 (UTC)

Rotation about center of gravity

User cagliost (talk) has added the statement that the aircraft will always pitch about its center of gravity, with the comment "that is crucial to the explanation". I have removed that statement, and just now, they have reverted my removal with the comment "Extraordinary claims require a source". They have since added two sources to support that statement.

I am sorry, but that statement is wrong. There is no physical law that requires an aircraft, or any object, to rotate about its center of gravity at all times. If you hold a pen by its tip, you can swish it around in between your fingers, and it will rotate about your fingers, not its CG. If (conceptually) you grab a 747 by the tail and swoosh it around you, it will rotate around you, not its CG. In a more realistic example: take a powered paraglider. Its CG lies close to the pilot and propulsion element. Yet casual observation will reveal that it will oscillate around a point close to the sail, far above the pilot.

Now, coming to sources. The first source given to support the statement, a web page on a university website, does not actually support the statement (unless I missed the corresponding sentence?). The second source is a recording of a university lecture. Neither of those are high-quality sources (e.g. content subjected to peer review or long-standing textbook).

I have looked in Phillips (ref 2 in the article and quite an authoritative reference) and nowhere does it state anything about the point of rotation of the aircraft. I have looked in two more lower-level books: Barnhard and Philpott (Aircraft Flight 4th ed) and Carpenter (Flightwise volume 2, aircraft stability and control). Neither says anything about rotating about the CG. I have also taken a look at a solid mechanics book (Meriam & Kraige, Dynamics 4th ed) and could not see anything about objects having to rotate around their CG.

I conclude that the statement should be removed. Thank you. Ariadacapo (talk) 14:47, 8 July 2022 (UTC)

Ariadacapo (talk), I think I see your point: that unlike a satellite in a vacuum, the aircraft is subject to drag which acts like a person gripping the tip of a pencil. Is that right?

We do need to explain *why* the center of gravity is important. Perhaps the article should say the aircraft will pitch about a point *near* its center of gravity? cagliost (talk) 15:56, 8 July 2022 (UTC)

Longitudinal static stability is defined for 1G flight. It is determined for static conditions so there must be no assumption or assertion that the aircraft is accelerating or pitching or experiencing lift greater than weight. Static stability is determined with no rate of change in angle of attack or body angle so it is incorrect for Wikipedia to say that, in the context of static stability, the aircraft is pitching or rotating or that any parameter is changing with time. The way the aircraft responds when parameters are changing with time is dynamic stability.

Wikipedia must not say things like “if the aircraft is subjected to a small change in angle of attack, if it is statically stable it will return towards its original angle of attack.” That is a matter of dynamic stability but in non-authoritative sources it is often used to illustrate what long stab means. Wikipedia must say things like “If the angle of attack is different to the angle associated with the trimmed speed there will exist a restoring force moment acting in the direction to return the angle of attack towards the trimmed value etc.” There is a restoring force moment; the aircraft is not pitching. Dolphin (t) 21:48, 8 July 2022 (UTC)

With respect to 1G flight, I am not so sure — there are many different sub-cases to consider (stability while pitching, while turning, while slipping, "stick-free" stability etc). Level flight is certainly the place to start, but I am not sure that this 1G is part of a definition.

With respect to the description language, I agree. It is always nice to have a plain-English introduction before we dive into the math. But in this article it has grown out of proportions. Most of the statements in there are just describing relationships that are much more precisely described and explained using the main balance equation (the moment derivative equation) further down. We should move those statements to accompany the equations, or to serve as subsequent commentary. --Ariadacapo (talk) 07:43, 9 July 2022 (UTC)

@Cagliost: The center of gravity is not important. It is just convenient to use this point as a reference when writing the moment and moment derivative balance equations, because this removes one term (weight) from the equations. You can write the balance equations from the nose or tail of the aircraft, too, reaching the same result (the static margin ends up being the key parameter). Your suggested statement ("near the CG") will not help, and we will be back to the same problem: making bold statements and then struggling to find sources to support them afterwards. Please proceed the other way around: catch one or two high-quality sources and cover the content the way they do it. --Ariadacapo (talk) 07:32, 9 July 2022 (UTC)