Speed to fly
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Speed to fly is a technique used by soaring pilots when flying between sources of lift, usually thermals, ridge lift and wave. The idea is to minimize the amount of time spent in the sinking air that is found between the rising air. The idea is usually credited to Paul MacCready though an early version of the theory was first described by Wolfgang Späte in 1938.[1] However Späte may not have considered sinking air between thermals, and there is no mention of this until 1947 when Ernest Dewing and George Pirie independently included this aspect.[2] Paul MacCready, however, certainly invented the 'ring', which allowed an easy indication of flight of the optimal speed to fly.
[edit] 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. Using this technique, these speeds must be memorized for various combinations of lift and sink.
The next step up in instrumentation is 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. The pilot rotates the ring so that a marker is opposite the expected rate of climb on the vario scale. If the rate of climb is less than zero, the vario needle will indicate the speed to fly, marked on the ring, as well as showing the rate of sink, marked on the face.
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, 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.
