Wilbur Wright Quote

Hawks are better soarers than buzzards but more often resort to flapping because they wish greater speed. A damp day is unfavorable for soaring unless there is a high wind. No bird soars in a calm.
The object of the tail is to increase the spread of surface in the rear when the wings are moved forward in light winds and thus preserve the centre of pressure at about the same spot. It seems to be used as a rudder very little. In high winds it is folded up very narrow.

Wright's Notebook (1900)