Force and energy characteristics of dynamic soaring in different reference frames

The force, work and energy characteristics of dynamic soaring in a path-related and an airspeed-related reference frame are dealt with. This is put in connection with total and kinetic energy concepts related to these reference frames. It is shown how the energy gain from the wind is accomplished with regard to both reference frames. For this purpose, appropriate mathematical relationships based on the equations of motion holding for each reference frame are developed. In an assessment of the force, work and energy characteristics, it is shown that there is equality of the net energy harvesting which is necessary for compensating for the irreversible energy loss due to the drag work so that energy-neutral dynamic soaring is possible. This holds though there are substantial differences between the path-related and the airspeed-related reference frames during a cycle. Thus, it can be concluded that results using one or the other reference frame are of equal value. As a consequence, the total energy and the kinetic energy concepts of the path-related and the airspeed related reference frames are equivalent.