Performance characteristic of an Unmanned Air Vehicle (UAV) is investigated using a newly developed heuristic approach. Almost all flight phases of any air vehicle can be categorized into trim and maneuvering flights. In this paper, a new envelope called trim-ability envelope, is introduced and sketched within the conventional flight envelope for a small UAV. Optimal maneuverability of the intended UAV is evaluated for minimum time pull-up and turn maneuvers. For both the trim and the maneuver problems, the nonlinear 6DOF dynamic models as well as the vehicle constraints are considered. A heuristic based constrained optimization approach is developed to solve both the trim and maneuver problems. Several interesting performance characteristics are extracted. The results are indicative of a good potential for the proposed algorithm to handle complex constrained optimization problems in aerospace engineering .