Editing Hang gliding (section)

=== Variometer ===
{{main|Variometer}}
[[Image:Gleitschirmvario.jpg|right|thumb|upright|Vario-altimeter ({{Circa|1998}})]]
Gliding pilots are able to sense the [[acceleration]] forces when they first hit a thermal, but have difficulty gauging constant motion. Thus it is difficult to detect the difference between constantly rising air and constantly sinking air. A [[variometer]] is a very sensitive vertical speed indicator. The variometer indicates climb rate or sink rate with audio signals (beeps) and/or a visual display. These units are generally electronic, vary in sophistication, and often include an [[altimeter]] and an airspeed indicator. More advanced units often incorporate a [[barograph]] for recording flight data and/or a built-in GPS. The main purpose of a variometer is in helping a pilot find and stay in the 'core' of a thermal to maximize height gain, and conversely indicating when he or she is in sinking air and needs to find rising air. Variometers are sometimes capable of electronic calculations to indicate the optimal [[speed to fly]] for given conditions. The [[Paul MacCready|MacCready]] theory answers the question on how fast a pilot should cruise between thermals, given the average lift the pilot expects in the next thermal climb and the amount of lift or sink he encounters in cruise mode.<ref>{{cite web|url=http://www.modelaircraft.org/museum/bio/MacCready.pdf|title=Biography of DR. PAUL MAC CREADY|publisher=Academy of Model Aeronautics|access-date=21 August 2007|url-status=dead|archive-url=https://web.archive.org/web/20070927004606/http://www.modelaircraft.org/museum/bio/MacCready.pdf|archive-date=27 September 2007}}</ref> Some electronic variometers make the calculations automatically, allowing for factors such as the glider's theoretical performance (glide ratio), altitude, hook in weight, and wind direction.