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Assuming hypothetical biplane variant of a modern conventional glider, wingspan of 8m, 1m separation between the wings, no stagger, no struts, what should be the decalage between the wings assuming best L/D ratio is aimed for? From what I understand, it shouldn't be 0° since upper wing's downwash impacts lower wing's local angle of incidence.

I fail to find real or RC values for that decalage, and why it is adopted. I understand when lower wing is aft staggered from upper wing, decalage is positive so that upper wing stalls first for safety reasons. What if those safety reasons are set aside, and best glide performance is aimed for?

Assuming large lever arm/far aft away horizontal stabilizer, should decalage be negative? i.e upper wing is set at slightly negative incidence relative to lower wing's incidence? (as dumb as that question is, single wing being way more efficient)

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You talk about the upper wing affecting the lower wing -- but the lower wing affects the upper wing in essentially the exact same way.

I see no reason to expect that the wings should be anything other than 'parallel'.

If the wings are close together -- less than half a chord separation -- then you would see a 'channel' effect between the wings -- where the airfoil thickness would cause the local flow to accelerate substantially. This may take away from the lift on the upper wing and increase the lift on the lower wing, but that alone would not push one closer to stall than the other.

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