Speed To Fly - Instrumentation

Instrumentation

The minimal instrumentation required is an airspeed indicator and a variometer. The pilot will use the polar curve information for the particular glider to derive the exact speeds to fly depending on the lift and sink conditions in which the glider is flying. This is commonly done using a speed to fly ring (known as a 'MacCready Ring') which is fitted around the aircraft's variometer. The ring is usually calibrated in either knots or meters per second and its markings are based on the aircraft's polar curve. 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.

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 PDA. The computer is connected to sensors that detect the aircraft's airspeed and rate of sink. If linked to a 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.