What is a bi plane?

Answer 1

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

The decalage is said to be positive when the upper wing has a higher total angle-of incidence than the lower wing, and negative when its total angle of incidence is lower. Positive decalage results in greater lift from the upper wing than the lower wing, the difference increasing with the amount of decalage.

Uncle Fudd