Definition
On a biplane, the difference in the angle of incidence between the upper and lower wings. Positive decalage means the upper wing is set at a greater angle of incidence than the lower wing; negative decalage means the lower wing is set at the greater angle.
Plain English
On an aircraft with two stacked wings, decalage is the small difference in how the upper and lower wings are tilted relative to the fuselage. One wing meets the air at a slightly steeper angle than the other.
Context Anchor
Seen in aircraft design, rigging, stability discussions, and biplane or tailplane alignment references.
Derivation
From the French décalage, meaning a shift, gap, or offset. The word captures the idea that the two wings are not set at exactly the same angle — one is offset from the other.
Why Pilots Care
Decalage affects how a biplane behaves at the stall. With positive decalage, the upper wing reaches its stalling angle before the lower wing, which generally produces a gentler, more predictable stall.
Analogy
Think of two shelves on the same wall. If one shelf is level and the other is tilted slightly upward, the difference between their tilt angles is like decalage.
Intuition Check
Decalage is not related to decals or markings on an aircraft. It means an angle offset between lifting surfaces.
Example Sentence 1
The biplane's positive decalage caused the upper wing to stall first, giving the pilot clear warning before the lower wing let go.
Example Sentence 2
Adjusting decalage during design improved the airplane's pitch stability at cruise speed.