Collision avoidance plays a crucial role in safe multirotor flight in cluttered environments. Even though a given reference trajectory is designed to be collision free, it might lead to collision due to imperfect tracking caused by external disturbances. In this work, we tackle this problem by computing the Forward Reachable Set (FRS), which is the set of positions and velocities that a multirotor can reach while following a reference trajectory due to tracking errors. Hence, if the FRS is computed before flight, we can utilize it to check the safety of a given trajectory in terms of collision avoidance. To compute a realistic FRS that covers an agile flight envelope, we consider first-order aerodynamic effects, which have the most salient influence on the vehicle. For computing FRS, we conduct a thorought stability analysis including these aerodynamic effects. Then, we present a FRS computation method which can easily be adapted to newly given reference trajectories. The presented method is validated by comparing the FRS with real flight data collected during agile and high-speed flight. In addition, we compare the FRS computed with and without compensating for firstorder aerodynamic effect to highlight their significance on the trajectory tracking performance. To the best of our knowledge, this is the first attempt to compute FRSs by incorporating firstorder aerodynamic effects for multirotors.