Energy Changes of a Sailplane in Moving Air

Abstract

The energy state of a sailplane is described in terms of its total energy, i.e. the sum of potential and kinetic energy.  Total energy (TE) may be defined either in an air-fixed reference frame (TE_AIR) or in an earth-fixed reference frame (TE_EARTH).  Both definitions are of importance here.  The fundamental physical laws are deduced here and, depending on the actual application, one will opt for one or the other definition.  In a practical example, the optimum flight path of a straight-line flight through an ascending air current (updraft) is computed and discussed.  There are two types of mechanism that allow an energy gain of the sailplane: static soaring in updrafts, and dynamic soaring with changes of wind speed.  Even though during “normal” flights updrafts are the primary source of energy, the exploitation of dynamic effects is likely to make a noticeable improvement of performance.  In soaring practice, however, dynamic effects are seldom used in order to gain energy these days, and the main reason for this seems to be the absence of an appropriate measuring instrument.  Therefore, some aspects of the question of how an “ideal variometer” should be designed are briefly discussed.  It turns out that the creation of an ideal variometer also would open up the possibility to measure the magnitude of dynamic effects.