ABSTRACT
In his famous book, Stick and Rudder, Langewiesche described how functional boundaries or constraints that are important to safe ±ight are directly speci¤ed as a result of the laws of dynamic optical perspective (optical ±ow). One important invariant relation, described in the opening quote, is the fact that the optical position of the horizon is essentially constant over change of observation point. The horizon will remain essentially at the same optical position (e.g., optical in¤nity) over a wide range of altitudes, as illustrated in Figure 11.1. Thus, this becomes an important referent for specifying positions of objects relative to the observer’s eye height. It also helps to specify other important functional boundaries, such as the glide line, which is the furthest point that an aircraft can reach in a powerless glide.
