Abstract: Balancing an inverted pendulum on an unmanned aerial vehicle has been a topic of interest in recent literature. For example, a recent study  uses an LQR controller to balance the inverted pendulum on a quadrotor drone. However, these studies consider the length of the pendulum to be known a priori. Indeed, in certain applications this assumption might not hold true. For example, consider a quadrotor hoverboard being used by people of different heights. In such cases, an approach is required to estimate the length of the pendulum. This paper analyzes the linearized dynamics of the combined system of quadrotor and inverted pendulum. It is found that unknown length of pendulum causes the system to fall in the category of unmatched uncertain systems where the control input cannot be used to cancel the uncertainty. This paper formulates the problem in such a manner that the system is still controllable in presence of this unmatched uncertainty. A concurrent learning adaptive controller, which avoids the use of persistently exciting signals, is then utilized to estimate the unmatched uncertainty and hence the length of the pendulum. Simulation results validate the effectiveness of the adaptive controller for the proposed problem.