This video shows the experiments of inclined hovering with a tethered quadrotor and its application to take off and landing on/from inclined surfaces
See  for the paper describing the method used in the following video.
- , “Takeoff and Landing on Slopes via Inclined Hovering with a Tethered Aerial Robot”, in 2016 IEEE/RSJ Int. Conf. on Intelligent Robots and Systems, Daejeon, South Korea, 2016, pp. 1702-1707.