Trefftz analysis for tapered wing??
 

I am performing a Trefftz plane analysis for a straight wing with a taper ratio of 0.4. Whenever I perform a surface integration of the Trefftz equation the drag force is 30 Newtons less than the actual value. I am using the equation for induced drag: .5*rho*Uinfinity^2*Area*Cdi(0.0087) but still nothing. Will anyone help me?????