In this article, downwash effect on the tail surfaces of a UAV is studied first. A theory to capture this unique characteristic is built and implemented in a nonlinear simulation environment. Both the thrust and downwash effect models emerging from the same Momentum Theory used and isolated results are validated for the specific UAV before stepping into the full model analysis. Hand launch trim settings are studied using the trim analysis utilities over the full model which incorporates the developed downwash effect mode. Trim settings are improved by considering the possibility of stall and altitude profile using the transient responses obtained using the model.