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