The aerial robot presented here for the rst time was based on a quadrotor structure, which is capable of unique morphing performances based on an actuated elastic mechanism. Like birds, which are able to negotiate narrow apertures despite their relatively large wingspan, our Quad-Morphing robot was able to pass through a narrow gap at a high forward speed of 2.5m.s −1 by swiftly folding up the structure supporting its propellers. A control strategy was developed to deal with the loss of controllability on the roll axis resulting from the folding process, while keeping the robot stable until it has crossed the gap. In addition, a complete recovery procedure was also implemented to stabilize the robot after the unfolding process. A new metric was also used to quantify the gain in terms of the gap crossing ability in comparison with that observed with classical quadrotors with rigid bodies. The performances of these morphing robots are presented, and experiments performed with a real ying robot passing through a small aperture by reducing its wingspan by 48% are described and discussed. 1 Objective Flying through cluttered environments requires an outstanding level of agility, which often involves the ability to trigger aggressive maneuvers to quickly avoid obstacles or pass through gaps at high speed. In the living world, agility is not restricted to ying insects or even small birds such as hummingbirds. Larger birds such as goshawks  and budgerigars  are able to negotiate cluttered environments at high speed despite their relatively large wingspan. How do they manage to perform tasks of this kind? By morphing their shape dynamically and reducing their wingspan swiftly by tucking up their wings. Morphing abilities give a ying robot agility by momentarily reducing its wingspan while keeping a suciently high payload. Morphing does not require any aggressive maneuvers but fast embedded mechanisms for folding up the robot's structure, as described in  for a winged drone. In the eld of robotics, Unmanned Aerial Vehicles (UAVs) are being used increasingly in cluttered and indoor environments for various purposes such as search and rescue expeditions , mapping  and exploration . The latest ying robots therefore have to be able to avoid collisions and handle narrow gaps successfully. Quadrotors, with their hovering and Vertical Take-O and Landing (VTOL) abilities, are certainly among the best candidates for meeting these requirements. Here we focused on designing a narrow gap-crossing strategy which was implemented on a quadro-tor. One previous strategy, which has been widely studied consisted in performing aggressive maneuvers to make the quadrotor change its attitude swiftly in order to pass through a vertical or tilted window . Recent studies [8, 9] have succeeded in developing autonomous robotic gap crossing skills based on on-board sensing and computing processes. However, this aggressive attitude control approach has several limitations: the robots have to reach high velocities and angular accelerations which require low inertia of the robot's body as well as high sensor refresh rates, especially in the case of visual sensors so as to prevent blur motion and maintain accurate position estimation with respect to the gap to be crossed. To address this issue, a new approach was adopted based on morphological changes. Previous authors have presented various types of quadrotors in which the size of the structure can be adapted either passively or dynamically for dierent purposes. Non-actuated structures were used by  to ensure resilience to collision and by  to obtain a self-deployable system facilitating the robot's transport. Actuated structures were used by  to reduce the wingspan of a hovering robot or to reduce the robot's volume with a scissor-like foldable structure in . Simulated robotic platforms with morphing abilities have been endowed by  with full attitude control and by  with an interesting tilting rotor mechanism.