Forgotten a term and need help
 

Guys and Gals,
For some reason the word declinage keeps pooping in to my head, yet I know it is not the word that I am looking for!
In Biplanes, what is that name of the point where the wakes of the two wings cross behind the aircraft. This term is possibly also used when explaining that the lower wing often creates less lift and offers less drag due to a difference in A of I
Anyone able to second guess me here?
Regards
D'vay

This may/may not help but the term "decalage" is usually applied to the relative angles of incidence of the wing chord and horizontal stabilizer chord.

Meanwhile "declination" is the angular distance of a heavenly body from the celestial equator, measured on the great circle passing through the celestial pole and the body. Thanks to dictionary.com.

Magnetic "declination" refers to the offset of a VOR facility - which may or may not correspond to the local magnetic variation .....:hmm:

This was an exam question way back when I was a boy doing initial aircraft training.

Decalage is the difference in angle of attack/incidence of the two wings in a biplane.
(Cardinal says the term relates to the difference between main wing and stab - maybe but the term was originally used for bi or triplanes)

If the top wings chord line has a greater angle than the lower one then you have 'positive decalage' If the lower has more AOA/incidence then you have negative decalage.


As D'vay alludes to, an aircraft with positive decalage (i believe negative decalage aircraft are rare) will produce less lift from the lower wing than from the upper. (of course assuming steady level flight and the wings are of the same construction/profile/area/aspect ratio etc etc)

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....the term "decalage" is usually applied to the relative angles of incidence of the wing chord and horizontal stabilizer chord.

Isn't that longitudinal dihedral?

Yep, it´s the same.

"Caler" in French is propping up or setting up something.
"Décaler" is to shift something, or set up two things differently, relative to each other.
"Décalage" is the amount you shift something by.
(Here endeth the French lesson.)

Never seen it applied to aircraft rigging, but makes perfect sense, as long as you qualify whether it's between upper and lower wing on a biplane, or between wing and horizontal stabiliser.

I'm fascinated by this...can we develop the thread a little and ask someone to explain why it is normal, as it seems to be, to have either positive or negative decalage?

Why does it help to have one wing on a biplane providing more or less lift than the other at any given AoA? Could the same effect be achieved with a different airfoil section on each wing?

Are the centres of lift of the two wings in vertical alignment in level flight?

Well, okay!! I've dug out the old text books and come up with the following.

" If the chords of the upper and lower wings of a biplane are parallel, the downwash of the upper wing has the effect of decreasing the angle of attack of the lower wing. Setting the lower wing at a greater angle of incidence will more properly distribute the lift between the two wings.Since the upper and lower wings then have different angles of attack in flight and there will be a difference of pressure distribution between them. Positive decalage gives the upper wing an increase in load percentage, especially at high speed. Negative decalage gives the lower wing an increase in load percentage. Each of the wings will reach its burble point at a different angle of attack, with a result that a stall will be less abrubt. On lift curves, this condition is shown by a flatter peak"





The term for that would be "stagger". This is pretty much the amount which the leading edge of one wing is in front of the other. Same goes - if the upper wing is further forward than the lower you have positive stagger. If the lower wing is further forward then negative stagger it is !


From the old text book again;

" The aerodynamic advantages of stagger are small. A biplane may have stagger to improve the vision of the pilot, provide better access to the cockpit or provide better angles of fire for machine guns. Since biplanes are no longer used for combat, the last reason is historical rather than practical. In some types of biplanes, when the positive stagger is increased, the pilot can see better forward and downward."


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