When developing an aircraft, one of several important aspects is to predict and properly design the dynamic behaviour of the aircraft. This holds for manned aircraft as well as for UAVs. The optimal dynamic behaviour for an aircraft depends on the mission or purpose: for a certain use an aircraft should be agile, other may require a more stable one. In aeronautics, the properties that describe the aircraft efficacy with respect to some task are known as flying qualities, and our goal is to study their dependence on some design parameters. As a test model we use an existing UAV. After deriving its 6-DOF dynamic model and assessing its baseline characteristics, we perform parametric studies.The strategy followed is divided in two steps: the first consists on analyzing flying qualities sensitivity to changes in model parameters. The second step studies how specific design changes affect model parameters. Because the first step only depends on the dynamic model form, we verify, by testing two other different aircrafts, that conclusions drawn from this step are valid to other configurations.Finally we show how results from parametric studies can be used to improve the UAV flying qualities regarding a certain mission, through the introduction of slight modifications on baseline design.