Upwind dynamic soaring of albatrosses and UAVs

Abstract

Albatrosses have been observed to soar in an upwind direction using what is called here an upwind mode of dynamic soaring. The upwind mode is modeled using the dynamics of a two-layer Rayleigh cycle in which the lower layer has zero velocity and the upper layer has a uniform wind speed of W. The upwind mode consists of a climb across the wind-shear layer headed upwind, a 90° turn and descent across the wind- shear layer perpendicular to the wind, followed by a 90° turn into the wind. The increase of airspeed gained from crossing the wind-shear layer headed upwind is balanced by the decrease of airspeed caused by drag. Results show that a wandering albatross can soar over the ocean in an upwind direction at a mean speed of 8.4 m/s in a 3.6 m/s wind, which is the minimum wind speed necessary for sustained dynamic soaring. The main result is that an albatross can soar upwind much faster that the wind speed. The upwind dynamic soaring mode of a possible robotic albatross UAV (Unmanned Aerial Vehicle) is also modeled using a Rayleigh cycle. Maximum possible airspeeds are approximately equal to 9.5 times the wind speed of the upper layer. In a wind of 10 m/s, the maximum possible upwind (56 m/s) and across-wind (61 m/s) components of UAV velocity over the ocean result in a diagonal upwind velocity of 83 m/s. In sufficient wind, a UAV could, in principle, use fast diagonal speeds to rapidly survey large areas of the ocean surface and the marine boundary layer. Limitations to achieving such fast travel velocity are discussed and suggestions are made for further studies.