Issue with non-dimensionality of CD - biplane
 

Hei,

I am new to this, for my work on analysis of multiwing aircraft i am trying to predict CDi at specific CL value.

I am using Edge solver in Euler mode as I am only interested in CDi. I first solved a reference wing having lets say 100m2 area. I found CDi for that which was near to what I calculated mathematically.

Secondly now I have divided this single wing into two wings having combined area of 100m2. The CL I get is almost the same in the same alpha range as for single wing above. but the CDi I get is exactly twice the value i expect from this biplane results.

Although i am using same reference values. As in this case wing area is used to non-dimensionalize the lift and drag and I am putting same wing area for both cases. CL seems to be fine but CDi just gets double for biplane.

(both wings are optimally twisted to produce some specific lift and drag at some alpha)

any suggestion what might be wrong. I am confused.

Regards
Fahad