Editing Speed to fly (section)

==Instrumentation==
The minimal instrumentation required is an [[airspeed]] indicator and a [[variometer]]. The pilot will use the [[polar curve (aviation)|polar curve]] information for the particular [[Glider (sailplane)|glider]] to derive the exact speeds to fly, minimum sink or maximum L/D, depending on the lift and sink conditions in which the glider is flying.  A speed to fly ring (known as a 'MacCready Ring'), which is fitted around the aircraft's variometer, will indicate the optimum airspeed to fly between thermals for maximum crosscountry performance.  The ring is usually calibrated in either [[knot (unit)|knots]] or [[meters per second]] and its markings are based on the aircraft's polar curve.<ref>1 meter per second = approximately 2 knots (more precisely 1.944 knots).</ref>  During the glide between thermals, the index arrow is set at the rate of climb expected in the next thermal. On the speed ring, the variometer needle points to the optimum speed to fly between thermals.<ref>{{cite book
 | title = Glider Flying Handbook
 | year = 2003
 | publisher = U.S. Federal Aviation Administration
 | location = U.S. Government Printing Office, Washington D.C.
 | id = FAA-8083-15
 | pages = 4–8
 | url=http://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/glider_handbook/
 }}
</ref>

Electronic versions of the MacCready Ring are built into glide computers that will give audible warnings to the pilot to speed up or slow down. Similar facilities can also be built into a [[Personal Digital Assistant|PDA]].  The computer is connected to sensors that detect the aircraft's [[airspeed]] and rate of sink.  If linked to a [[Global Positioning System|GPS]], and using a computed or manual estimate of the windspeed, the glide computer can also calculate the speed and altitude necessary to glide to a particular destination.  This glide is known as the final glide because no further lift should be necessary to reach the goal.  During this glide, speed to fly information is needed to ensure that the remaining height is used efficiently.