In this work we introduce an approach that allows a quadcopter to find the velocity which maximizes its flight time (endurance) or flight distance (range) while moving along a given path, using on-board power measurement. The proposed strategy is based on Extremum Seeking control and (a) does not require any model of the power consumption of the system, (b) can be executed on-line, and (c) guarantees adaptation to unknown disturbances. We show experimentally that hovering is not the most energy-efficient loitering strategy, and we demonstrate the proposed method's ability to adapt to different aerodynamic disturbances, such as payloads. The method may be especially useful in applications where a quadcopter carries an unknown payload, allowing it to adapt for improved range.