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Imagine a plane with high mounted horizontal wings (with no dihedral). This plane flies straight, with no roll. Weight force is hooked at the mass centre and directed vertically down, lift is hooked above the centre and directed vertically up.
Now some random cause rolled the plane, for example, 5 degrees left. If high mounted wings are expected to recover the plane to horizontal position i.e. make a rotation of plane, they MUST apply some torque on it. But what really happens here: weight is still hooked at the centre, thus it doesn't cause any torque (the arm is zero). Lift is hooked at some point which is nearly above the centre, but deviated by 5 degrees left. And lift is directed not vertically up, but 5 degrees left, and actually its direction is exactly the same as the direction of the arm (i.e. vector from the mass centre to hook of lift force). In other words, vector of lift force is parallel to vector of arm, and thus lift doesn't generate any torque, either. Thus, there is still no torque and the plane does not recover!
It's better understandable if drawing is seen, but I don't know how to attach a drawing in the discussion pages.
In fact, if a random left roll occurs, it causes a left sideslip, and the fuselage begins to create a lift force directed to the right. And this lift can generate a torque. The hook of this side lift is usually around the centre of fuselage, and thus lower than the mass centre of the whole plane (mass of the wings raises the mass centre). And thus, it generates a torque which makes even more left roll and worsens the situation!
It's true, however, that high mounted wings increase pitch stability.
-- Grzes 23:49, 26 Jan 2005 (UTC)
I have read several explanations of how dihedral produced roll stability and this is the first time I've heard anything about this sideslip theory. The theory has merit and may be part of the picture but is not the generally-accepted theory. If this is a personal theory and not an established one it should not be put forth as the actual explanation. I think this requires some discussion and possibly changing the article.
The mistake most people make is assuming that movement around the roll axis somehow affects how much lift a wing produces. It doesn't; it can't. In the absence of gravity an aircraft will fly the same way no matter what roll angle it is at. The other thing people don't take into account is that, like any other force that is spread over an area, all of the lift produced by the entire airframe acts as if it were concentrated at a single point--the center of lift.
If the center of lift is above the center of gravity the aircraft will act as if it were hanging from a string. If the aircraft rolls, barring any other external forces, it will swing like a pendulum back to the position where the center of lift is directly over the center of gravity. If the center of lift is below the center of gravity the aircraft will act is if it were siting on a pin, clearly an unstable position. This is why a high-wing aircraft, like a Cessna 172, has very little dihedral. It doesn't need it. The center of lift is above the center of gravity by virtue of having the whole wing above the center of gravity. A low-wing aircraft, unless it is designed for maneuverability (i.e. aerobatics or dog fighting), will usually have more dihedral than a high-wing aircraft. The extra dihedral is required to raise the center of lift. This is why a high-wing aircraft is inherently more stable than a low wing aircraft. Rsduhamel 17:38, 22 Mar 2005 (UTC)
Guys, I'm adding the cleanup tag because the explanation of the stability effect in this article is simply wrong. It's not about the amount of aerodynamic force, it's the direction that matters. When an aircraft with dihedral rolls, the wing that's closer to horizontal will generate the same lift as the other wing, but the lift vector is closer to vertical, opposing gravity more directly than that of the other wing. It's not related to the angle of attack, or any sideslip. The stabilizing effect is there as soon as the wingtips aren't at the same height relative to the horizon. I'll come up with some corrected diagrams and fix the text as well.
The problem with dihedral is, it has more than one effect. Sideslip results in side force, yaw and roll torques due to dihedral. The side force effect is of second order, but dihedral tends to roll the aircraft to cancel sideslip, and is significant in stabilising the spiral mode, but has a de-stabilising effect on the Dutch roll. It is usually difficult to make both stable. High wing aircraft generate a dihedral effect because the body modifies the local angle of attack, and this effect is modified by sideslip. There is a further dihedral effect arising from wing sweep, to the extent that anhedral is required for a stable Dutch roll on highly swept fighter configurations. Gordon Vigurs 09:39, 7 August 2006 (UTC)
I removed the sentence about "cathedral" having the same derivation as "dihedral". Cathedral comes from the Latin word Cathedra for "chair", referring to the chair in which the bishop sits when presiding over the liturgy in his cathedral. The derivation has nothing to do with the structure of the ceiling. See Cathedral and Catholic Encyclopedia: Cathedral Sbreheny ( talk) 04:42, 7 August 2008 (UTC)
This article was automatically assessed because at least one WikiProject had rated the article as start, and the rating on other projects was brought up to start class. BetacommandBot 09:48, 10 November 2007 (UTC)
This article muddles an important difference between Dihedral Angle and Dihedral Effect.
Dihedral Effect is called "Dihedral Effect" because increasing Dihedral Angle increases the rolling moment produced by sideslip.
BUT! A gazillion other things also change the rolling moment created by sideslip. Such as: Wing sweep, Vertical CG, height of sideforce-producing surfaces like fins, fuselages, etc., etc.
Such things should be made clear in the article. I will set out to do that.
-- Gummer85 ( talk) 03:57, 3 April 2009 (UTC)
This article (but mostly the discussion) digresses too much toward Lateral-Directional Stability. Dihedral Effect is a subset of Lateral-Directional Stability.
Perhaps a new article could be created for "Lateral-Directional Stability" if we really want to include the whole miasma. That would be the proper place where everyone could post all their MAD THEORIES! :-)
The article "Dihedral" should be kept and renamed "Dihedral (flight)" or something like that. It should be small with a few pretty pictures of dihedral and anhedral like it already has. It should have the distinction between angle and effect of course and the fact that effect is important for Lateral-Directional Stability which it refers to and be done with it! :-)
-- Gummer85 ( talk) 06:03, 3 April 2009 (UTC)
Yeah. I hear ya. I am usually moved powerfully to "make it correct" first. It's more immediate, if not more important. Actually, correctness is more important than citing, but not by as much as it is more immediate. The kind of material I am fixing here is "common knowledge" among experts in the field and any error should be corrected quickly enough by other experts. Although, given that glaring "confer
Longitudinal static stability to roll axis" error that sat for so long, I'm not so sure of this any more. :-).
Because it is "common knowledge among experts", it appears in many texts on the subject. I can easily cite one of the more prominent of them (Roskam) that I cited in another article. My problem is that so much conceptual error exists in the "Aero" subject areas that correcting calls louder than citing. It screams! :-). I haven't yet learned how to cite inline. It's not that obvious how to do it, and the screams keep on distracting me! :-) .
-- Gummer85 ( talk) 20:06, 3 April 2009 (UTC)
This is an explanation for Spiral Mode which is important, but it is a superset and it isn't dihedral anything. However, it does contain within it "How Dihedral Angle creates Dihedral Effect" which is useful. The section should be retitled "How Dihedral Angle creates Dihedral effect" and rejiggered to explain just only that. I'll do that later if someone doesn't get to it first...
Also perhaps some other sections would be good like "How [fill in the blank] creates Dihedral Effect" for some of the other effectors (like wing sweep and all that other stuff). Please help here! :-)
-- Gummer85 ( talk) 06:55, 3 April 2009 (UTC)
The explanation isn't referenced and isn't "common knowledge of experts". It's also inconsistent with the fact that there is SO MUCH ANHEDRAL on the "horizontal" tail. There is so much anhedral on the tail, it looks like a Viper from Battlestar Galactica!
The story I heard years ago was that they had to tip up the wing tips to provide wing clearance for landing on aircraft carriers (as the F-4 was originally to be both a Navy as well as Air Force airplane). The increase in polyhedral angle has the same effect as an increase in Dihedral Angle. That is, it increases Dihedral Effect. The airplane then had too much Dihedral Effect. To reduce it, they made Anhedral on the tail.
That's what I heard. It's much more plausible than the explanation in the article, but I don't have a reference (yet). The current entry really should be removed, but I don't want to "lose the space" and then forget to add the "correct" explanation after it is confirmed.
Also, I probably won't get around to confirming "my" story very soon. Can someone else look into it? (Is anyone out there? :-) )
-- Gummer85 ( talk) 18:36, 4 April 2009 (UTC)
Whoops! I checked out F-4_Phantom_II#XF4H-1_prototype quickly on Wikipedia and it corroborates the existing story. AND! It's referenced! I'll try to make the references a bit more clear. Chock one up for reference checking!
-- Gummer85 ( talk) 00:25, 5 April 2009 (UTC)
I plan on changing the name of this article pretty soon if there aren't any objections. It'll take me a while to be sure anything that references it isn't broken, and to figure out exactly how to do that.
-- Gummer85 ( talk) 16:27, 20 May 2009 (UTC)
Hi Mike52,
I'm glad to see someone's actually watching and actually cares! :-) This whole time I've been essentially having a conversation with myself. I reverted your edits though, for the reasons explained here.
Rolling moment caused by yaw rate is not Dihedral Effect, which by definition is Rolling moment due to Sideslip. Rolling moment caused by yaw rate is actually a different Stability Derivative called, well, "Rolling moment due to yaw rate" (funny how that is). These two ideas are very commonly mixed up.
The force couple (the difference in lift force on wings) is a moment as that is the definition of a moment. Moments and Couples are one and the same. I initially wrote "produces" myself, then I realized it was the moment. It's a small point, but more correct I think and makes it so we don't need to explain exactly how the couple produces the moment. Hmmm, maybe I didn't need to italicise the is, (less teachy that way). (I've rejiggered and improved this sentence to be a little clearer I think.)
The differential drag under sideslip conditions you speak of is a contributor to yet another stability derivative (Cn-Beta) called "Yawing moment due to sideslip". This is an important stability derivative which is also called "directional stability", but it is unrelated to Dihedral Effect except how they both contribute (in their own different ways) to the stability of the spiral mode.
The geometric definition of "dihedral" is covered elsewhere in wikipedia. The disambiguation note at the top points to the geometric definition. It would be good though to rename the article "Dihedral (aircraft)" to make it more clear.
-- Gummer85 ( talk) 15:50, 27 May 2009 (UTC)
I added a "Common Confusions" section. I see these misunderstandings all the time in my pilot world and even among flight instructors and other near-experts. I thought it would be good to head them off at the pass.
-- Gummer85 ( talk) 16:49, 27 May 2009 (UTC)
I changed the name from "Dihedral" to "Dihedral (aircraft)" and properly rewired the references and redirections. All non-specific references go to Dihedral (disambiguation) whereas before they went here. This is to help people see that there is more on dihedral than this page and to help direct them to the most suitable location for their particular POV on dihedral (be it geometric, aircraft, etc.)
-- Gummer85 ( talk) 18:26, 27 May 2009 (UTC)
Hi,
Please don't think I'm possessive about this page. I thank you for your comments about admin actions for the name change. Also, I did indeed wonder what to do regarding capitalizing "Angle" and "Effect" and other editorial things like that. I gratefully seek guidance and improvement in that regard. There were some important changes in meaning you made that would have been better discussed here before making them. I reverted because that is the best way to correct them with no fuss nor muss.
In the last few months, I made changes and corrections to this article and explained every major change along the way here on this discussion page (and in detailed 255-character Edit Summaries). It was a discussion with myself, no one seemed to be responding so I had no choice but to see the changes as "acceptable" and "good". I am an aeronautical engineer and the changes are correct. The distinction between Dihedral Angle and Dihedral Effect is important and often confused. The main citation given (Roskam) is a well known reliable source and it verifies my explanations. I know people always like to see more citations. I can and will easily include some. However, the Roskam citation does indeed cover the corrections I made. In my opinion, it is quite adequate to justify the removal of the refimprove tag (although, I recognize that these questions are judgement calls).
In the future, I would love to discuss proposed major technical changes here before risking introduction of technical errors. I'm up for "explanatory" improvements too. My technical explanations are, I think, usually much better than those made by other technically-oriented people, but it sure does help to have a layperson's point of view to spot possible confusions that we don't see because things are just too "obvious" to us. :-)
Gummer85 ( talk) 03:38, 8 June 2009 (UTC)
I looked over the changes more closely and I see the biggest change is the replacement of "Dihedral Angle" with "Dihedral" in a number of spots. The key problem with this article early on was the muddling of the two ideas of Dihedral Angle and Dihedral Effect. The ideas are muddled thoroughly in the minds of pilots and other people who brush up against this subject. Because of this, it is very important to separate the ideas crisply and cleanly and consistently. This is why the word "dihedral" must always be followed by "effect" or "angle". It's just that critical to understanding.
The other thing I noticed was the decapitalization of "angle", "effect", and "dihedral". I had given that some thought, and I capitalized them because I wanted to securely identify "Dihedral Effect" and "Dihedral Angle" as different concepts from each other. I also want them to be seen as entities in themselves. I thought that if "Effect" and "Angle" weren't capitalized, they could accidentally be seen as more associated with the word following it than with "Dihedral". The capitalization of both words was my invention to achieve those two goals. I know it's imperfect. I welcome other ideas here, are there prettier ways to achieve the same thing?
Gummer85 ( talk) 06:59, 8 June 2009 (UTC)
Should this be noted in the article? 69.250.53.240 ( talk) 07:17, 11 July 2009 (UTC)
I dunno. I've never used the term, but it looks legitimate in a quick search, particularly as the difference between the angles of incidence of the two wings of a biplane. I had learned "longitudinal dihedral" in school and reviewed it in Roskam, so I added it in as another use of the word "dihedral" in the context of aircraft. Hmmm... Maybe "longitudinal dihedral" could be added to the entry on decalage? Would you like to do it? :-)
-- Gummer85 ( talk) 23:40, 7 August 2009 (UTC)
I've been looking further and it seems like "decalage" is used for biplanes and not as the difference between wing and horizontal stabilizer angles of incidence. "Longitudinal dihedral" in a general sense is the angle between two planes about the y-axis and so it could conceivably cover decalage, but I don't know if it is used that way. I think "longitudinal dihedral" applies to wing and stabilizer and that "decalage" applies to the two wings of a biplane. But, I'm just thinking out loud here. If you have a good reliable source about it, lets hear it! :-)
-- Gummer85 ( talk) 23:57, 7 August 2009 (UTC)
I'm having a hard time coming up with a concise reference. But I know from personal knowledge that decalage is used to describe wing and horz. stab. incidence in the model airplane hobby. I have also found a link to a mailing list posting where Dr. Mark Drela (MIT professor, aerodynamics) talks about decalage in that context. Googling for Drela and decalage turns up many other items. Drela is an expert in full scale aircraft aerodynamics and also well known for designing high performance model gliders.
I would love to help with the editing but I'm not sure what to do or how to approach it. I know there are lots of rules here on wikipedia and I don't want to break any of them or mess anything up. Are these references good enough or should I look for something more definitive?
Mailing List Posting
Diagram by Drela showing decalage
Dr. Mark Drela
--
69.250.53.240 (
talk)
05:05, 17 August 2009 (UTC)
Neato. I happen to be a member of the same RC club as Drela here in the Boston area (Charles River Radio Controllers). Those are some good references you found. It sure looks like Drela uses the term "Decalage" for what is called "longitudinal dihedral" here. As he is an aerodynamics god, I'd say that's a pretty reliable source. I wonder how much "decalage" is used in the "knowledgeable community". I'm a member of that community, but I'm knowledgeable enough to know where my knowledge ends. I think the term is pretty obscure, but I could be wrong. I've never needed to use it in my career. Maybe it's a "tail" vs. "empanage" kind of thing - just a different language. It's a good picture too, illustrating the difference between the chords and lines of zero lift. It's probably worth mentioning that "decalage is also used to describe longitudinal dihedral" and citing Drela. Go ahead and do it man! Make the change! Be bold! I can't babysit this article forever or by myself! Make the change and make the citations any cruddy old way you can, stick them right in the text if you need to. I'll neaten it up for you, you can see how that's done, then you'll know how to it next time! :-)
I think the strength of references need only be in proportion to the strength of the claim. The references show that Drela uses the term, not that "the term for X is Y" as in a textbook. So, instead of saying "Decalage is also a term for LD", it could be said "Decalage is also used as a term for LD". If Drela had written a textbook defining decalage as the longitudinal dihedral angle, I would say that's a pretty reliable source and would justify the "stronger" form ("Decalage is also a term for LD").
Now, if you happened to know for sure that Decalage is also a term for LD and is used and understood by knowledgeable practitioners, then in my opinion, it is okay to make the strong form of the statement and not fret about the citation too much, although it would be nicer to find a textbook reference as well to ward off 1) the unknowledgeable who think they're knowledgeable who think you're wrong, and 2) rabid citation demanders. In my opinion, in such a case, even with no citation, the "strong statement" would be verifiable simply because anybody can readily observe the fact that knowledgeable practitioners use and understand the word that way. That's my opinion and it's a point of contention however. I think it's okay to simply state such points as true and maybe find a good citation later. I think that's important because in Wikipedia Aerodynamics, the errors abound and the need for speedy correction of really awful mistakes trumps the need for simultaneous citation. Some disagree. Some think that if you say "the sky is blue" you need two reliable sources, and four pictures. In my opinion, when correctness is so wanting, it counts more than verification. Truth is self-verifiable (on testable "truths" such as these, not on untestable ones like how many angels can be stabbed with a pin).
Beware the use of aeronautical terms by RC guys and explanations from them. There are exceptions of course (like Drela). They're knowledgeable about some things, but there is a lot of misunderstanding among them that gets repeated so as to acquires an "air" of validity. RC "lore" is not a reliable source.
Gummer85 ( talk) 00:43, 26 August 2009 (UTC)
This article uses the term " polyhedral" to describe what is elsewhere (e.g. here) called a cranked wing. Is there any precedent for that, and if so then which should we standardise on? — Cheers, Steelpillow ( Talk) 20:48, 16 April 2012 (UTC)
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Reading this article was like having someone wave their finger at me and tell me how wrong I am about everything. I came here to learn about this stuff, not to be lectured. What's worse, some of these explanations of the stability imparted by a positive dihedral contradict what I have read in textbooks by reputable publishers (e.g., Jeppesen Sanderson), yet these strongly worded and condescending explanations (contradictions) are not sourced. I came to learn about anhedral wings, and I got what I came for, but I could have done without the lecture. Dcs002 ( talk) 06:29, 18 October 2019 (UTC)