This paper presents a strategy for real-time generation of formation trajectories using a leader-follower approach. A trajectory generator prescribes the motion of a group of virtual vehicles, using a Lyapunov-based nonlinear controller that stabilizes the position of the leader in the reference frame of the virtual vehicles at a predefined distance vector. This strategy differs from the standard approach of defining the desired distance vector in an inertial frame and can be used to obtain rich formation trajectories with varying curvatures between vehicles. By imposing adequate constraints on the motion of the virtual vehicles, the generation of valid formation trajectories is naturally guaranteed, bypassing the demanding task of obtaining complete path descriptions. The trajectories are generated online and provided to a trajectory tracking controller specifically designed for quadrotor vehicles. Simulation and experimental flight tests are presented to evaluate the performance of the solution proposed, applied to formation control of quadrotors.