Talk:Decalage

Note

We need to add the purpose of decalage (not currently stated). From memory, with a biplane if the top wing is set at a higher angle it stalls first and pitches the nose down, improving stall recovery behaviour. Decalage between wings and tailplanes is often about 4 degrees, setting the wing at the optimum angle of attack for cruise flight with the fuselage level (minimum drag). Would add it but I can't find a reference just now. Nimbus (Cumulus nimbus floats by) 23:11, 12 February 2009 (UTC)

Might be able to find something amongst the multitude of NACA technical reports – a bit of a dry read, but such are the sacrifices we make for the better good (Lol)! ;-)--Red Sunset 19:52, 13 February 2009 (UTC)

Take a look at the end of p.10 and p.11 – the conclusions of this MIT report. D'ya reckon it could be used? --Red Sunset 21:02, 13 February 2009 (UTC)

That's interesting, I had a skim through it. Based on wind tunnel testing in 1926, seems to indicate that positive decalage made the model more prone to spinning. Will have a better read of it later. That report talks about decalage between biplane wings, still need to find one on tailplane/wing angle of incidence which I have also heard described as 'longitudinal dihedral'. Good find there. Nimbus (Cumulus nimbus floats by) 21:48, 13 February 2009 (UTC)

Can we find a citation showing Decalage = Longitudinal Dihedral?

The 1927 NASA Paper shows Decalage as between the wings of a biplane. It doesn't show Decalage as the difference between the incidence angles of the wing and tail (which is longitudinal dihedral). There is some discussion over in Talk:Dihedral (aircraft) as to whether this is true. It looks like "Decalage" is used that way by at least one aerodynamics expert, but I'm not sure if it's used that way commonly by knowledgeable practitioners. It probably is, but a citation for it here would sure be nice!  :-)

Gummer85 (talk) 01:09, 26 August 2009 (UTC)

Possibly not! I do remember spending some time on this article and querying it in my own head, often on WP we take as read what has been written before ('must be right' syndrome), I think it is only now that we are finding much mythology in articles and fixing it. I note there is no French article which might have helped. Will have a look again and see if I can find anything. Nimbus (Cumulus nimbus floats by) 01:30, 26 August 2009 (UTC)

I have taken out the questionable alternative meaning until someone can add it back with a reference. I had a good look through my books and I couldn't find mention of the second meaning. I did find a book to support both the meaning of longitudinal dihedral and decalage as two separate things, it is: Whitehead, Gordon. Radio Control Scale Aircraft for Everyday Flying. Littlehampton, West Sussex: Radio Modeller Books Ltd, 1980. ISBN 0 903676 08 7

Although this is an aeromodelling book the same principles apply, Gordon Whitehead is a very clever chap and a retired Royal Air Force engineering officer I believe. In his glossary of terms he explains both as:

• Decalage: The difference in incidence between the upper and lower wings of a biplane.
• Longitudinal dihedral: The difference in angle of incidence between wing and tailplane.

I think that he has it right, cheers. Nimbus (Cumulus nimbus floats by) 15:18, 26 August 2009 (UTC)

There is an article on the French WP 'Calage'[1] where it says: Calage d'une aile : angle formé par la corde du profil de l'aile et l'axe de référence de l'avion (généralement l'axe du fuselage). Which from my schoolboy French translates as The angle formed between the mean chord line of the wing and the reference plane of the aircraft (usually the fuselage) I would say that this is describing the angle of incidence. 'De calage' = The incidence? There is a drawing here [2] of a Belgian aircraft which is showing the incidence angles of both a wing and tailplane with the term 'de calage' being used. Interesting. Nimbus (Cumulus nimbus floats by) 20:53, 26 August 2009 (UTC)

Interesting response. I happen to agree that the definitions you cite above are most likely to be the way they are used by those knowledgeable professional practioners who actually need to use the terms (terms that are even obscure among "knowledgeable professional practioners"). I'm a "knowledgeable former professional practioner", but not one that had a need in my career to use the terms. It looks like (for now) that it's hard to pin it down. On one hand, the cursory (for me) searches show these definitions (above), but on the other hand we have at least one well known aerodynamics god (Dr. Mark Drela) who has used the "Decalage" to mean the difference in angle of incidence between wing and tailplane. It wouldn't be the first time a "knowledgeable professional practioner" used a term unconventionally in a such a way, i.e., in an unusual way that wasn't really that critical in the scheme of things. I'm thinking that Drela's "misusage", wasn't misusage, the terms are obscure and their meanings well-specified at the time of usage. So, I'm going to declare (in my mind) Drela's usage an anomaly, also that he's earned a right to redefine it if he wants. I'm going to leave the matter unsaid in the two articles because I don't know what to make of it.  :-)

Here are some links I copied over from Talk:Dihedral (aircraft):

Mailing List Posting
Diagram by Drela showing decalage (A good diagram! (even though it uses "decalage" for "longitudinal dihedral"))

--Gummer85 (talk) 18:08, 31 August 2009 (UTC)

I think the second meaning is out there, just need to find a good reference for it. Mark has an account on Commons perhaps we could ask him, I know of his work through RC gliding. I don't remember the term being used in my Royal Air Force engineering training but it's a very long time ago and I don't remember much else of the course either!! I've had a small brainwave, my boss is a French aerodynamicist, without priming him I could ask him for a definition of the word. Nimbus (Cumulus nimbus floats by) 22:17, 31 August 2009 (UTC)

I remembered to ask my French colleague today, the answer was interesting and slightly different to my guess above:

• 'De' in French is the same as the English preposition 'De' [3] which means to reverse or undo (as in delamination, detox etc.).
• 'Calage' is the noun form of the verb 'Caler' (pronounced 'kallay'!, you can tell that I'm not a linguist!) which means to set, fix or adjust, so 'calage' is a 'setting'.
• Decalage literally means 'to unset' or more correctly 'deliberately offset'. For this article then it means that biplane wing incidences are 'deliberately offset' and so too are the wing and tailplane incidences through longitudinal dihedral. As a Frenchman he said that he would use the term for both.
• Another use is for a wheel chock (calage) or an aircraft chock (cale d'avions), more 'fixing' devices. 'Calibrate' is apparently related (to check a setting). All original research of course but at least we understand what the word actually means literally now (in French anyway!). Très bon! Nimbus (Cumulus nimbus floats by) 19:28, 5 September 2009 (UTC)

This is what you (I'm late to the party!) have been looking for, I think. It's the entry under "decalage/décalage" from the full Oxford English Dictionary:

1917 R. B. Matthews Aviation Pocket-bk. (ed. 5) xii. 267 Decalage, the difference in the angle of incidence between any two distinct aerofoils on an aeroplane; e.g., the main plane and the tail; or more usually between the chords of the upper and lower planes of a biplane.

1933 Flight (Suppl. Aircraft Engin.) 27 Apr. 396d/1 The term aerodynamic decalage: defined as the angle that the no lift angle of the upper plane makes with the no lift angle of the lower plane, positive when the upper plane is at greater incidence than the lower. [My italics]

So the second usage (≡ longitudinal dihedral) is citable (first example). From an admittedly small sample of examples, I'd say longitudinal dihedral is more common, though not much more self-explanatory. Once you know what it means, there is sense in it as both dihedral (bog-standard) and longitudinal dihedral are ways of producing stability, one around the roll axis and the other the pitch. Both involve angular differences. Longitudinal is confusing (to me) but is short for longitudinal plane, I guess, not axis.

I'd like to make a pitch for a refining of the definition, something like the following:

from

Decalage on a fixed-wing aircraft is the angle difference between the upper and lower wings of a biplane, i.e. the acute angle contained between the chords of the wings in question.

to

Decalage on a fixed-wing aircraft is the difference between the angles of incidence of the upper and lower wings of a biplane or multiplane or between the angles of incidence of forward and rear aerodynamic surfaces. Equivalently, it is the acute angle contained between the chords of the surfaces in question, as projected onto the plane containing the roll and yaw axes.

The reason for expanding the second sentence is not just pedantry: when I first saw the definition I thought of a Tiger Moth, with different sweep on upper and lower planes. There's a non-zero angle between the chord lines in the horizontal plane, too.

Decalage has been applied to tandem wing aircraft, which are like biplanes with staggeringly large negative stagger and so less, but not negligible, aerodynamic interaction between wings. With negative decalage, the front wing stalls first so you put the ailerons on the rear, unstalled wing to retain lateral (roll!) control and avoid the spin. Something similar, but complicated by the higher pressure above the lower wing of a biplane (even with 0° decalage) must be true for them.TSRL (talk) 21:26, 23 May 2011 (UTC)

Break

I like the 1917 definition. The original question posed in this thread was can we cite the term 'longitudinal dihedral' as a meaning of decalage, I don't think we can just yet. More common, with British aircraft at least, is the simple term 'tailplane incidence', an angle measured relative to the fuselage datum line. This will also result in an angle difference between the mean chord lines of the wings as they should be bolted to the fuselage at their own angle of incidence!! Have to be careful not to introduce over-complication or our own analysis into the definition, forgive me for repeating it but the phrase as projected onto the plane containing the roll and yaw axes confuses me completely as a pilot and aircraft engineer. Both tailplane incidence and wing decalage are design features that can only affect an aircraft's stability in pitch about the lateral axis in conjunction with the fore and aft position of the centre of gravity.

Out of interest I had to carry out full rigging checks (using an inclinometer) on our Tiger Moth last week as the tailplane has been rebuilt and the interplane struts have been removed for reconditioning. Despite the unequal dihedral, unequal sweepback and positive stagger the wing decalage is 0° at all rib stations, the wing incidence is 4° relative to the fuselage. Its tailplane incidence angle relative to the fuselage is 0° (so it is 4° relative to the wings).

Would be nice to add a diagram and expand this article as it could possibly be tagged as a dictionary definition as it is. Cheers Nimbus (Cumulus nimbus floats by) 09:41, 24 May 2011 (UTC)

Deletion

Back in January 2009 a lot of content was deleted by an IP without any rationale. [4]

Two different decalage definitions are commonly used:

Aerodynamic decalage is the angle difference between the zero lift lines of the wing's and stab's airfoils.

Geometric decalage is the angle difference between the chord lines of the wing and horizontal stab.

Decalage is often confused with incidence which is the angle between each of the surfaces (wing and horizontal stabilizer) and some other reference, usually the fuselage´s axis or the airflow.

Just FYI, in case anything is missing from current text. 220 of ^Borg 02:12, 27 November 2015 (UTC) @Nimbus227: as an active commenter here. 220 of ^Borg 00:54, 28 November 2015 (UTC)

Looks like the images at least were restored in 2012, there still seems to be disagreement with what decalage actually is. Nimbus (Cumulus nimbus floats by) 20:18, 28 November 2015 (UTC)

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